/ \

PB98-917001NTSB/SIR-98/01

NATIONALTRANSPORTATIONSAFETYBOARD

WASHINGTON, D.C. 20594

SPECIAL INVESTIGATION REPORT

BRITTLE-LIKE CRACKING INPLASTIC PIPE FOR GAS SERVICE

6984

)

Abstract: Despite the general acceptance of plastic piping as a safe and economicalalternative to piping made of steel or other materials, the National Transportation Safety Boardnotes that a number of pipeline accidents it has investigated have involved plastic piping thatcracked in a brittle-like manner. This special investigation report concludes that the procedureused in the United States to rate the strength of plastic pipe may have overrated the strength andresistance to brittle-like cracking of much of the plastic pipe manufactured and used for gasservice from the 1960s through the early 1980s. As a result, much of this piping may besusceptible to premature brittle-like failures when subjected to stress intensification, and thesefailures represent a potential public safety hazard.

The safety issues discussed in this report are the vulnerability of plastic piping to prematurefailures due to brittle-like cracking; the adequacy of available guidance relating to the installationand protection of plastic piping connections to steel mains; and performance monitoring of plasticpipeline systems as a way of detecting unacceptable performance in piping systems.

As a result of this special investigation, the National Transportation Safety Board issuedrecommendations to the Research and Special Programs Administration, the Gas ResearchInstitute, the Plastics Pipe Institute, the Gas Piping Technology Committee, the American Societyfor Testing and Materials, the American Gas Association, MidAmerican Energy Corporation,Continental Industries, Inc., Dresser Industries, Inc., Inner-Tite Corporation, and MuellerCompany.

The National Transportation Safety Board is an independent Federal agency dedicated to promotingaviation, railroad, highway, marine, pipeline, and hazardous materials safety. Established in 1967, theagency is mandated by Congress through the Independent Safety Board Act of 1974 to investigatetransportation accidents, determine the probable causes of the accidents, issue safety recommendations,study transportation safety issues, and evaluate the safety effectiveness of government agencies involvedin transportation. The Safety Board makes public its actions and decisions through accident reports,safety studies, special investigation reports, safety recommendations, and statistical reviews.

Information about available publications may be obtained by contacting:

National Transportation Safety BoardPublic Inquiries Section, RE-51490 L’Enfant Plaza, S.W.Washington, D.C. 20594(202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from:

National Technical Information Service5285 Port Royal RoadSpringfield, Virginia 22161(703) 605-6000

BRITTLE-LIKE CRACKING INPLASTIC PIPE FOR GAS SERVICE

SPECIAL INVESTIGATION REPORT

Adopted: April 23, 1998Notation 6984

NATIONALTRANSPORTATION

SAFETY BOARD

Washington, D.C. 20594

iii

CONTENTS

INTRODUCTION ......................................................................................................................... 1

INVESTIGATION ........................................................................................................................ 4Accident History.............................................................................................................................. 4Strength Ratings, Ductility, and Material Standards for Plastic Piping .......................................... 6Century Pipe Evaluation and History............................................................................................16

Union Carbide DHDA 2077 Tan Resin ................................................................................. 17Information Dissemination Within the Gas Industry .................................................................... 19Installation Standards and Practices .............................................................................................. 20

Federal Regulations................................................................................................................ 20GPTC Guide for Gas Transmission and Distribution Piping Systems .................................. 21A.G.A. Plastic Pipe Manual for Gas Service ......................................................................... 22A.G.A Gas Engineering and Operating Practices (GEOP) Series ......................................... 22ASTM..................................................................................................................................... 22Guidance Manual for Operators of Small Natural Gas Systems............................................ 23Manufacturers’ Recommendations ........................................................................................ 24

Bending and Shear Forces............................................................................................... 24Pipe Residual Bending.................................................................................................... 25

MidAmerican Energy Installation Standards ......................................................................... 25Gas System Performance Monitoring ........................................................................................... 26

ANALYSIS ................................................................................................................................... 28General .......................................................................................................................................... 28Durability of Century Utility Products Piping............................................................................... 28Strength Downturn and Brittle Characteristics ............................................................................. 29Information Dissemination Within the Gas Industry .................................................................... 31Installation Standards and Practices .............................................................................................. 32

Federal Regulations................................................................................................................ 33A.G.A. Plastic Pipe Manual for Gas Service ......................................................................... 33A.G.A. Gas Engineering and Operating Practices Series....................................................... 34GPTC Guide for Gas Transmission and Distribution Piping Systems .................................. 34ASTM..................................................................................................................................... 34Guidance Manual for Operators of Small Natural Gas Systems............................................ 35Manufacturers’ Recommendations ........................................................................................ 35Installation Issues At Site of Waterloo Accident ................................................................... 36

Gas System Performance Monitoring ........................................................................................... 37

CONCLUSIONS ........................................................................................................................... 2

RECOMMENDATIONS .............................................................................................................. 3

iv

APPENDIXESAppendix A--Pipeline Accident Brief--Waterloo, Iowa ............................................................... 45Appendix B-- Organizations, Agencies, and Associations Referenced in this Report ................. 49

1

he use of plastic piping to transportnatural gas has grown steadily over theyears because of the material’s economy,

outstanding corrosion resistance, light weight,and ease of installing and joining. According tothe American Gas Association (A.G.A.),1 thetotal miles of plastic piping in use in natural gasdistribution systems in the United States grewfrom about 9,200 miles in 1965 to more than45,800 miles in 1970. By 1982, this figure hadgrown to about 215,000 miles, of which morethan 85 percent was polyethylene.2 Datamaintained by Office of Pipeline Safety (OPS),an office of the Research and Special ProgramsAdministration (RSPA) within the U.S.Department of Transportation (DOT), indicatethat, by the end of 1996, more than 500,000miles of plastic piping had been installed.Plastic piping as a percentage of all gasdistribution piping installed each year has alsogrown steadily, as illustrated in figure 1.

Despite the general acceptance of plasticpiping as a safe and economical alternative topiping made of steel or other materials, theSafety Board notes that a number of pipelineaccidents it has investigated have involvedplastic piping that cracked in a brittle-likemanner.3 (See table 1 for information on threerecent accidents.) For example, on October 17,1994, an explosion and fire in Waterloo, Iowa,destroyed a building and damaged otherproperty. Six persons died and seven wereinjured in the accident. The Safety Boardinvestigation determined that natural gas hadbeen released from a plastic service pipe thathad failed in a brittle-like manner at aconnection to a steel main.

1See appendix B for brief descriptions of theorganizations, associations, and agencies referenced in thisreport.

2Watts, J., “Plastic Pipe Maintains Lion’s Share ofMarket,” Pipeline and Gas Journal, December 1982, p. 19,and National Transportation Safety Board Special Study--An Analysis of Accident Data from Plastic Pipe NaturalGas Distribution Systems (NTSB/PSS-80/1).

3The body of the report will make clear the distinctionbetween brittle-like and ductile fractures.

The Safety Board also investigated a gasexplosion that resulted in 33 deaths and 69injuries in San Juan, Puerto Rico, in November1996.4 The Safety Board’s investigationdetermined that the explosion resulted fromignition of propane gas that had migrated underpressure from a failed plastic pipe. Stressintensification at a connection to a plastic fittingled to the formation of brittle-like cracks.

The Railroad Commission of Texasinvestigated a natural gas explosion and fire thatresulted in one fatality in Lake Dallas, Texas, inAugust 1997.5 A metal pipe pressing against aplastic pipe generated stress intensification thatled to a brittle-like crack in the plastic pipe.

A Safety Board survey of the accidenthistory of plastic piping suggested that thematerial may be susceptible to brittle-likecracking under conditions of stressintensification. No statistics exist that detailhow much and from what years any plasticpiping may already have been replaced;however, as noted above, hundreds of thousandsof miles of plastic piping have been installed,with a significant amount of it having beeninstalled prior to the mid-1980s. Anyvulnerability of this material to prematurefailure could represent a serious potential hazardto public safety.

In an attempt to gauge the extent of brittle-like failures in plastic piping and to assesstrends and causes, the Safety Board examinedpipeline accident data compiled by RSPA. Theexamination revealed that the RSPA data areinsufficient to serve as a basis for assessing thelong-term performance of plastic pipe.

4National Transportation Safety Board PipelineAccident Report--San Juan Gas Company, Inc./EnronCorp., Propane Gas Explosion in San Juan, Puerto Rico,on November 21, 1996 (NTSB/PAR-97/01).

5Railroad Commission of Texas AccidentInvestigation No. 97-AI-055, October 31, 1997.

T

INTRODUCTION

2

Lacking adequate data from RSPA, theSafety Board reviewed published technicalliterature and contacted more than 20 experts ingas distribution plastic piping to determine theestimated frequency of brittle-like cracks inplastic piping. The majority of the publishedliterature and experts indicated that failurestatistics would be expected to vary from onegas system operator to another based on factorssuch as brands and dates of manufacture ofplastic piping in service, installation practices,and ground temperatures, but they indicated thatbrittle-like failures, as a nationwide average,may represent the second most frequent failuremode for older plastic piping, exceeded only byexcavation damage.

The Safety Board asked several gas systemoperators about their direct experience withbrittle-like cracks. Four major gas systemoperators reported that they had compiledfailure statistics sufficient to estimate the extentof brittle-like failures. Three of those four saidthat brittle-like failures are the second mostfrequent failure mode in their plastic pipeline

systems. One of these operators supplied datashowing that it experienced at least 77 brittle-like failures in plastic piping in 1996 alone.

As an outgrowth of the Safety Board’sinvestigations into the Waterloo, Iowa, SanJuan, Puerto Rico, and other accidents, and inview of indications that some plastic piping,particularly older piping, may be subject topremature failure attributable to brittle-likecracking, the Safety Board undertook a specialinvestigation of polyethylene gas service pipe.The investigation addressed the following safetyissues:

• The vulnerability of plastic piping topremature failures due to brittle-likecracking;

• The adequacy of available guidancerelating to the installation andprotection of plastic piping connectionsto steel mains; and

Figure 1 -- Plastic pipe as a percentage of all piping used in gas distribution. (Source: Duvall,D.E., “Polyethylene Pipe for Natural Gas Distribution,” presented at the Transportation Safety Institute’s PipelineFailure Investigation course, 1997. Data from Pipeline & Gas Journal surveys.)

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• Performance monitoring of plasticpipeline systems as a way of detectingunacceptable performance in pipingsystems.

As a result of its investigation, the SafetyBoard makes three safety recommendations tothe Research and Special ProgramsAdministration, one safety recommendation tothe Gas Research Institute, three safety recom-mendations to the Plastics Pipe Institute, one

safety recommendation to the Gas Piping Tech-nology Committee, two safety recommendationsto the American Society for Testing and Materi-als, one safety recommendation to the AmericanGas Association, two safety recommendations toMidAmerican Energy Corporation, two safetyrecommendations to Continental Industries, Inc.,and one safety recommendation each to DresserIndustries, Inc., Inner-Tite Corporation, andMueller Company.

Table 1 -- Recent pipeline accidents involving brittle-like cracking

Accident Location PipeManufacturer

Year PipeManufactured

Year ofAccident

Waterloo, Iowa Amdevco/Century 1970 1994

San Juan, Puerto Rico DuPont 1982 1996

Lake Dallas, Texas Nipak 1970 1997

4

Accident Historyn October 17, 1994, a natural gasexplosion and fire in Waterloo, Iowa,destroyed a building and damaged other

property. Six persons died and seven wereinjured in the accident. The Safety Boardinvestigation determined that the source of thegas was a 1/2-inch-diameter plastic service pipethat had failed in a brittle-like manner at aconnection to a steel main.6

Excavations following the accidentuncovered, at a depth of about 3 feet, a 4-inchsteel main. Welded to the top of the main was asteel tapping tee manufactured by ContinentalIndustries, Inc. (Continental). Connected to thesteel tee was a 1/2-inch plastic service pipe.(See figure 2.) Markings on the plastic pipeindicated that it was a medium-densitypolyethylene material manufactured on June 11,1970, in accordance with American Society forTesting and Materials (ASTM) standard D2513.The pipe had been marketed by Century UtilityProducts, Inc. (Century). The plastic pipe wasfound cracked at the end of the tee’s internalstiffener and beyond the coupling nut.

The investigation determined that much ofthe top portion of the circumference of the pipeimmediately outside the tee’s internal stiffenerdisplayed several brittle-like slow crackinitiation and growth fracture sites. These slowcrack fractures propagated on almost parallelplanes slightly offset from each other throughthe wall of the pipe. As the slow cracks fromdifferent planes continued to grow and began tooverlap one another, ductile tearing occurredbetween the planes. Substantial deformation wasobserved in part of the fracture; however, theinitiating cracks were still classified as brittle-like.

Samples recovered from the plastic serviceline underwent several laboratory tests under the

6For more detailed information, see Pipeline AccidentBrief in appendix A to this report.

supervision of the Safety Board. Two of thesetests were meant to roughly gauge the pipe’ssusceptibility to brittle-like cracking. These testswere a compressed ring environmental stresscrack resistance (ESCR) test in accordance withASTM F1248 and a notch tensile test known asa PENT test that is now ASTM F1473. Lowerfailure times in these tests indicate greatersusceptibility to brittle-like cracking under testconditions. The ESCR testing of 10 samplesfrom the pipe yielded a mean failure time of 1.5hours, and the PENT testing of 2 samplesyielded failure times of 0.6 and 0.7 hours. Testvalues this low have been associated withmaterials having poor performance histories7

7Uralil, F. S., et al., The Development of ImprovedPlastic Piping Materials and Systems for Fuel GasDistribution—Effects of Loads on the Structural andFracture Behavior of Polyolefin Gas Piping, Gas ResearchInstitute Topical Report, 1/75 - 6/80, NTIS No. PB82-180654, GRI Report No. 80/0045, 1981, and Hulbert, L.E., Cassady, M. J., Leis, B. N., Skidmore, A., Field FailureReference Catalog for Polyethylene Gas Piping, AddendumNo. 1, Gas Research Institute Report No. 84/0235.2, 1989,and Brown, N. and Lu, X., “Controlling the Quality of PEGas Piping Systems by Controlling the Quality of theResin,” Proceedings Thirteenth International Plastic FuelGas Pipe Symposium, pp. 327-338, American Gas

INVESTIGATION

O

Figure 2 -- Typical plastic service pipeconnection to steel gas main. Manyconnections are protected against shearand bending forces by a plastic sleevethat encloses the service pipe-to-teeconnection on either side of thecou plin g nut.

5

characterized by high leakage rates at points ofstress intensification8 due to crack initiation andslow crack growth typical of brittle-likecracking.

In late 1996, the Safety Board began aninvestigation of a November 1996 gas explosionthat resulted in 33 deaths and 69 injuries in SanJuan, Puerto Rico. The investigation determinedthat the explosion resulted from ignition ofpropane gas that, after migrating under pressurefrom a failed plastic pipe at a connection to aplastic fitting, had accumulated in the basementof a commercial building. The Safety Boardconcluded that apparent inadequate supportunder the piping and the resulting differentialsettlement generated long-term stressintensification that led to the formation ofbrittle-like circumferential cracks on the pipe.

The Railroad Commission of Texasinvestigation of a fatal natural gas explosion andfire in Lake Dallas, Texas, in August 1997determined that a metal pipe pressing against aplastic pipe generated stress intensification thatled to a brittle-like crack in the plastic pipe.

The Waterloo, San Juan, and Lake Dallasaccidents were only three of the most recent in aseries of accidents in which brittle-like cracks inplastic piping have been implicated. In Texas in1971, natural gas migrated into a house from abrittle-like crack at the connection of a plasticservice line to a plastic main.9 The gas ignitedand exploded, destroying the house and burningone person. The investigation determined thatvertical loading over the connection generatedlong-term stress that led to the crack.

A 1973 natural gas explosion and fire inMaryland severely damaged a house, killedthree occupants, and injured a fourth.10

Association, Gas Research Institute, Battelle ColumbusLaboratories, 1993.

8Stress intensification occurs when stress is higher inone area of a pipe than in those areas adjacent to it. Stressintensification can be generated by external forces or achange in the geometry of the pipe (such as at a connectionto a fitting).

9National Transportation Safety Board PipelineAccident Report--Lone Star Gas Company, Fort Worth,Texas, October 4, 1971 (NTSB/PAR-72/5).

10National Transportation Safety Board Pipeline

The Safety Board’s investigation revealed that abrittle-like crack occurred in a plastic pipe as aresult of an occluded particle that created astress point.

The Safety Board’s investigation of anatural gas explosion and fire that resulted inthree fatalities in North Carolina in 197511

determined that the gas had accumulatedbecause a concrete drain pipe resting on aplastic service pipe had precipitated two cracksin the plastic pipe. Available documentationsuggests that these cracks were brittle-like.

A 1978 natural gas accident in Arizonadestroyed 1 house, extensively damaged 2others, partially damaged 11 other homes, andresulted in 1 fatality and 5 injuries.12 Availabledocumentation indicates that the gas line crackthat caused the accident was brittle-like.

A 1978 accident in Nebraska involved thesame brand of plastic piping as that involved inthe Waterloo accident. A crack in a plasticpiping fitting resulted in an explosion thatinjured one person, destroyed one house, anddamaged three other houses.13 The Safety Boarddetermined that inadequate support under theplastic fitting resulted in long-term stressintensification that led to the formation of acircumferential crack in the fitting. Availabledocumentation indicates that the crack wasbrittle-like.

A December 1981 natural gas explosion andfire in Arizona destroyed an apartment,damaged five other apartments in the samebuilding, damaged nearby buildings, and injuredthree occupants.14 The Safety Board’s

Accident Report--Washington Gas Light Company, Bowie,Maryland, June 23, 1973 (NTSB/PAR-74/5).

11National Transportation Safety Board PipelineAccident Brief--“Natural Gas Corporation, Kinston, NorthCarolina, September 29, 1975.”

12National Transportation Safety Board PipelineAccident Brief--“Arizona Public Service Company,Phoenix, Arizona, June 30, 1978.”

13National Transportation Safety Board PipelineAccident Brief--“Northwestern Public Service, GrandIsland, Nebraska, August 28, 1978.”

14National Transportation Safety Board PipelineAccident Brief--“Southwest Gas Corporation, Tucson,Arizona, December 3, 1981.”

6

investigation determined that assorted debris,rocks, and chunks of concrete in the excavationbackfill generated stress intensification thatresulted in a circumferential crack in a plasticpipe at a connection to a plastic fitting.Available documentation indicates that the crackwas brittle-like.

A July 1982 natural gas explosion and firein California destroyed a store and two resi-dences, severely damaged nearby commercialand residential structures, and damaged auto-mobiles.15 The Safety Board’s investigationidentified a longitudinal crack in a plastic pipeas the source of the gas leak that led to the ex-plosion. Available documentation indicates thatthe crack was brittle-like.

A September 1983 natural gas explosion inMinnesota involved the same brand of plasticpiping as that involved in the Waterloo andNebraska accidents.16 The explosion destroyedone house and damaged several others, andinjured five persons. The Safety Board’sinvestigation determined that rock impingementgenerated stress intensification that resulted in acrack in a plastic pipe. Available documentationindicates that the crack was brittle-like.

One woman was killed and her 9-month-olddaughter injured in a December 1983 natural gasexplosion and fire in Texas.17 The SafetyBoard’s investigation determined that the sourceof the gas leak was a brittle-like crack that hadresulted from damage to the plastic pipe duringan earlier squeezing operation to control gasflow.18

15National Transportation Safety Board PipelineAccident Brief--“Pacific Gas and Electric Company, SanAndreas, California, July 8, 1982.”

16National Transportation Safety Board PipelineAccident Brief--“Northern States Power Company,Newport, Minnesota, September 19, 1983.”

17National Transportation Safety Board PipelineAccident Brief--“Lone Star Gas Company, Terell, Texas,December 9, 1983.”

18Plastic pipe is sometimes squeezed to control theflow of gas. In some cases, squeezing plastic pipe candamage it and make it more susceptible to brittle-likecracking.

A September 1984 natural gas explosion inArizona resulted in five fatalities, seven injuries,and two destroyed apartments.19 The SafetyBoard’s investigation determined that a reactionbetween a segment of plastic pipe and someliquid trapped in the pipe weakened the pipe andled to a brittle-like crack.

During the course of the investigation of theaccident at Waterloo, Iowa, the Safety Boardlearned of several other accidents, notinvestigated by the Safety Board, that involvedcracks in the same brand of plastic piping as thatinvolved in the Waterloo accident. Three ofthese accidents, which occurred in Illinois (1978and 1979) and in Iowa (1983), resulted in fiveinjuries and damage to buildings.20 A 1995accident in Michigan also involved a crack inthis same brand of pipe.21 Availabledocumentation indicates that the cracks werebrittle-like.

Strength Ratings, Ductility, and MaterialStandards for Plastic Piping

During the 1950s and early 1960s, whenplastic piping was beginning to gain acceptanceas an alternative to steel piping for the transportof water and gas, no established proceduresexisted for rating the strength of materialsintended for use in plastic pressure piping.

In November 1958, the Thermoplastic PipeDivision of the Society of the Plastics Industryorganized a group called the Working StressSubcommittee.22 The subcommittee, in January1963, issued a procedure (hereinafter referred toas the PPI procedure) that specified a uniformprotocol for rating the strength of materials used

19National Transportation Safety Board PipelineAccident Report--Arizona Public Service Company NaturalGas Explosion and Fire, Phoenix, Arizona, September 25,1984 (NTSB/PAR-85/01).

20Illinois Commerce Commission accident reportsdated September 14, 1978, and December 4, 1979. IowaState Commerce Commission accident report datedAugust 29, 1983.

21Research and Special Programs AdministrationIncident Report—“Gas Distribution System,” Report No.318063, January 8, 1996.

22This subcommittee was subsequently made into apermanent unit and was renamed the Hydrostatic StressBoard.

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in the manufacture of thermoplastic pipe in theUnited States. In March 1963, the Thermoplas-tic Pipe Division adopted its current name, thePlastics Pipe Institute (PPI).

On July 1, 1963, the PPI established avoluntary program of listing the materialstrengths of plastic piping materials,specifically, those materials designed for waterapplications. To apply for a PPI listing,applicants sent strength test data to the PPI,often accompanied by the manufacturer’sanalysis of the data and a proposed materialstrength rating. The PPI would analyze the dataand, if warranted, list the material for thecalculated strength. The PPI did not certify orapprove the material received or validate thedata submitted, nor did it audit or inspect thosesubmitting data.23

In simplified terms, the PPI procedure,which is performed by the materialsmanufacturers themselves, involves recordinghow much time it takes stressed pipe samples torupture at a standardized temperature of 73 °F.The stresses used in the tests are recorded as“hoop stress,” which is tensile stress in the wallof the pipe in a circumferential orientation(hence the term “hoop”) due to internalpressure. Although hoop stress is expressed inpounds per square inch, it is a value quitedifferent from the pipe’s internal pressure.

The testing process involves subjecting pipesamples to various hoop stress levels, and thenrecording the time to rupture. For some samplesat some pressures, rupture will occur in as littleas 10 hours. As hoop stress is reduced, the time-to-failure increases. At some hoop stress level,at least one of the tested specimens will notrupture until at least 10,000 hours (slightly morethan 1 year). After the rupture data points (hoopstresses and times-to-failure) for this materialhave been recorded, the data points are plottedon log-log coordinates as the relationshipbetween hoop stress and time-to-failure. (Seefigure 3.) A mathematically developed “best-fit”

23As a result of Safety Board inquiries to the PPIabout its inability to verify the actual data submitted, theinstitute, in 1997, revised its policy document for its listingservice to require a signed statement from applicants thatdata accompanying applications for a PPI listing arecomplete, accurate, and reliable.

straight line is correlated with the data points torepresent the material’s resistance to rupturingat various hoop stress levels.

Once the best-fit straight line is calculatedto 10,000 hours, it is extrapolated to 100,000hours (about 11 years). The hoop stress levelthat coincides with the point at which the lineintersects the 100,000-hour time line representsthe calculated long-term hydrostatic strength ofthat particular material.

To simplify the ratings and facilitatestandardization, the PPI procedure groupedmaterials with similar long-term hydrostaticstrength ranges into “hydrostatic design basis”categories. For example, those materials havinglong-term hydrostatic strengths between 1200and 1520 psi were grouped together andassigned a hydrostatic design basis of 1250 psi.Those materials having long-term hydrostaticstrengths between 1530 and 1910 psi weregrouped together and assigned a hydrostaticdesign basis of 1600 psi.

To help ensure the validity of themathematically derived line, the PPI procedurerequired the submission of all rupture datapoints. It further specified the minimum numberof data points and minimum number of testedlots. The procedure employed statistical tests toverify the quality of data and quality of fit to themathematically derived line. These measuresexcluded materials when the data demonstratedexcessive data scatter due to either inadequatequality of data or deviation from straight linebehavior through 10,000 hours.24

The PPI procedure, after some refinement,was issued as an ASTM method in 1969 (ASTMD2837). The PPI adopted a policy document25

for PPI’s listing service in 1968, whichremained under PPI jurisdiction.

24The PPI procedure also had restrictions on thedegree of slope of the straight line so that the material’sstrength would not excessively diminish beyond 100,000hours.

25Plastics Pipe Institute, Policies and Procedures forDeveloping Recommended Hydrostatic Design Stresses forThermostatic Pipe, PPI-TR3-July 1968.

8

When polyethylene pipe fails duringlaboratory stress rupture testing at 73 °F, it failsprimarily by means of ductile fractures, whichare characterized by substantial visibledeformation (see figure 4). During stress rupturetests, if hoop stress on the test piping isdecreased, the time-to-failure increases, and theamount of deformation apparent in the failuredecreases.26 In pipe subjected to prolongedstress rupture testing, slit fractures27 may begin

26Mruk, S. A., “The Ductile Failure of PolyethylenePipe,” SPE Journal, Vol. 19, No. 1, January 1963.

27Because of the frequent lack of visible deformationassociated with them, slit fractures are also referred to asbrittle-like fractures.

to appear at some point (depending on thespecific polyethylene resin material). Figure 5shows a slit fracture that resulted from a stressrupture test. The PPI procedure did notdifferentiate between ductile and slit failuretypes, and, based on most available laboratorytest data (at 73 °F),28 assumed that both types of

28Kulhman, H. W., Wolter, F., Sowell, S., Smith, R.B., Second Summary Report, The Development ofImproved Plastic Pipe for Gas Service, Prepared for theAmerican Gas Association, Battelle Memorial Institute,covering the work from mid-1968 through 1969. Stressrupture tests were performed using methane and nitrogen asthe internal pressure medium and air as the outsideenvironment. Some experts have advised the Safety Boardthat stress rupture testing showing time-to-failure in the slitmode may vary with different pressure media and

Figure 3 -- Stress rupture data plotted as best-fit straight line and extrapolated todetermine long-term hydrostatic strength. (Derived from A.G.A. Plastic Pipe Manual for GasService.)

9

failures would be described by the sameextrapolated (straight) line.

In 1963-64, the National SanitationFoundation29 amended its standard for plasticpiping used for potable water service to requirethat manufacturers furnish evidence of havingan appropriate strength rating in accordancewith the PPI procedure. Manufacturers thendecided to utilize the PPI listing service, havingdetermined that this was the most convenientway to furnish the required evidence.

environments and that Battelle Memorial Institute’s choicesfor these fluids may have contributed to the slowrecognition in the United States of a downturn in the stressrupture line.

29Now known as NSF International.

In 1966, the ASTM issued ASTM D2513,the society’s first standard specificationcovering polyethylene plastic piping for gasservice.30 ASTM D2513 made reference to long-term hydrostatic strength and hydrostatic designstress and included an appendix defining theseterms in accordance with the PPI procedure.31 Italso required that polyethylene pipe meet certainrequirements of ASTM D2239 (a polyethylenepipe specification for water service), which alsoincluded references to the PPI procedure. ASTMD2513 did not explicitly require materials tohave a PPI listing.

30This standard also included plastic piping materialsother than polyethylene.

31Although adherence to ASTM appendixes is notmandatory, the PPI procedure was the only industry-accepted mechanism to determine long-term hydrostaticstrength and hydrostatic design stress.

Figure 4 -- Ductile fracture resulting from stress rupture test. Note substantial deformation(ballooning) at the failure.

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Even without an explicit requirement, somemanufacturers voluntarily obtained PPI listingsfor their resin materials32 intended for gas use,and some others,33 as noted above, obtained PPIlistings for their resins that were intended forwater use (but were similar to their resinsintended for gas service) as a way of meetingNational Sanitation Foundation requirements.

In 1967, the United States of AmericaStandards Institute B31.8 code,34 GasTransmission and Distribution Piping Systems,for the first time recognized the suitability of

32Resins are polymer materials used for themanufacture of plastics.

33For example, E. I. du Pont de Nemours & Company,Inc., and Union Carbide Corporation.

34Now known as ASME B31.8.

plastic piping for gas distribution service andincluded requirements for the pipings’ use. The1966 issuance of ASTM D2513 and the 1967inclusion of plastic piping within B31.8 clearedthe way for the general use of plastic piping forgas distribution.35 B31.8 included a designequation (see discussion below), and althoughthe code, like the ASTM standard, did notexplicitly require a PPI listing, it did require thatmaterial used to manufacture plastic pipeestablish its long-term hydrostatic strength inaccordance with the PPI procedure.

35A.G.A. Plastic Pipe Handbook for Gas Service,American Gas Association, Catalog No. X50967, April1971.

Figure 5 -- Slit fracture resulting from a stress rupture test conducted at 100 °F. Note lackof deformation visible in the fracture. This pipe was manufactured by DuPont in 1977. Afterfailing Minnegasco’s incoming inspection tests, the pipe was subjected to stress rupturetesting. (Source: Henrich, R.C., and Funck, D.L., “Effects of ESCR Variation on Some OtherProperties of Plastic Pipe.” Proceedings, Eighth Annual Plastic Fuel Gas Pipe Symposium, 1983.)

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On August 12, 1968, the Natural GasPipeline Safety Act was enacted, requiring theDOT to adopt minimum Federal regulations forgas pipelines. In December 1968, the DOTinstituted interim Federal regulations byfederalizing the State pipeline safety regulationsthat were in place at the time. The DOT, havingconcluded that the majority of the Statesrequired compliance with the 1968 version ofB31.8, adopted that version of the code for theFederal regulations covering those States not yethaving their own natural gas pipeline safetyregulations.

Most of these Federal interim standardswere replaced in November 1970 by 49 Code ofFederal Regulations (CFR) 192; however, theinterim provisions concerning the design,installation, construction, initial inspection, andinitial testing of new pipelines remained ineffect until March 1971. At that time, 49 CFR192 incorporated the design equation for plasticpipe from B31.8 and also required that plasticpiping conform to ASTM D2513.36

The 1967 version of B31.8 introduced fixeddesign factors37 (subsequently incorporated into49 CFR 192) as a catch-all mechanism toaccount for various influences on pipeperformance and durability. These influencesincluded external loadings, limitations of andimprecision in the PPI procedure, variations inpipe manufacturing, handling and storageeffects, temperature fluctuations, and harshenvironments.38 A design equation was used todetermine the allowable gas service pipepressure rating based on the hydrostatic designbasis category, pipe dimensions, and designfactor.39 The design basis for plastic pipe thus

36RSPA reviews revised editions of ASTM D2513 foracceptability before referencing them in 49 CFR 192.

37A design factor is similar to a safety factor, exceptthat a design factor attempts to account for other factors notdirectly included within the design equation thatsignificantly affect the durability of the pipe.

38Reinhart, F. W., “Whence Cometh the 2.0 DesignFactor,” Plastics Pipe Institute, undated, and Mruk, S. A.,“Validating the Hydrostatic Design Basis of PE PipingMaterials.”

39The design equation (with the current design factor,0.32) can be found in 49 CFR 192.121, although 192.121erroneously references the long-term hydrostatic strengthinstead of the hydrostatic design basis category. RSPA is

used internal pressures as a design criterion butdid not directly take into account additionalstresses that could be generated by externalloadings, despite the fact that field failures inplastic piping systems were frequentlyassociated with external loads but were rarelyattributable to internal pressure effects alone.40

Kulmann and Mruk have reported that nodirect basis was established to design forexternal loads because:

• The industry had no easy means ofquantifying external loads and theireffects on plastic piping systems;41

and

• Many in the industry believed thatplastic piping, like steel and copperpiping, behaved as a ductilematerial that would withstandconsiderable deformation beforeundergoing damage, thus alleviatingand redistributing local stress con-centrations that would crack brittlematerials such as cast iron. This be-lief resulted from short-termlaboratory tests showing that plasticpiping had enormous capacity to de-form before rupturing.42

Because of plastic piping’s expected ductilebehavior, many manufacturers believed it safe tobase their designs on average distributed stressconcentrations generated primarily by internalpressure and, within reason, to neglect localizedstress concentrations. They believed such stresswould be reduced by localized yielding, ordeformation. Mruk and Palermo have pointedout that design protocols were predicated on theassumption of such ductile behavior.43

currently conducting rulemaking activities to correct thiserror.

40Kulmann, H. W., Wolter, F., Sowell, S.,“Investigation of Joint Performance of Plastic Pipe for GasService,” 1970 Operating Section Proceedings, AmericanGas Association, pp. D-191 to D-198.

41Kulmann, Wolter, and Sowell.42Mruk, S. A., “Validating the Hydrostatic Design

Basis of PE Piping Materials.”43Mruk, S. and Palermo, E., “The Notched Constant

12

In contrast, cast iron piping has recognizedbrittle characteristics. The design basis for castiron therefore does not assume that localizedyielding or deformation will reduce stressintensification. As a result, the design protocolfor cast iron includes the quantification anddirect input of external loading factors that cangenerate localized stress intensification.44

Failures in polyethylene piping that occurunder actual service conditions are frequently

Tensile Load Test: A New Index of the Long TermDuctility of Polyethylene Piping Materials,” summary ofpresentation given in the Technical Information Sessionhosted by ASTM Committee F17’s task group on Project62-95-02, held in conjunction with ASTM CommitteeF17’s November 1996 meetings, New Orleans, LA.

44Mruk and Palermo and Hunt, W. J., “The Design ofGrey and Ductile Cast Iron Pipe,” Cast Iron Pipe News,March/April 1970.

slit failures; ductile failures are rare.45 Figure 6shows a slit (brittle-like) fracture in a pipe thatwas found leaking and had to be replaced. Arock pressing against the plastic pipe generatedlong-term stress intensification that led to theformation of the brittle-like crack. Slit failuresin polyethylene, whether occurring during stressrupture testing or under actual serviceconditions, result from crack initiation and slowcrack growth and are similar to brittle cracks inother materials in that they can occur with littleor no visible deformation.46

45Mruk, S. A., “Validating the Hydrostatic DesignBasis of PE Piping Materials,” and Bragaw, C. G.,“Fracture Modes in Medium-Density Polyethylene GasPiping Systems,” Plastics and Rubber: Materials andApplications, pp. 145-148, November 1979.

46Mruk and Palermo have quantified and discussed thedeformation in brittle-like failures in: Mruk, S. andPalermo, E., “The Notched Constant Tensile Load Test: ANew Index of the Long Term Ductility of Polyethylene

Figure 6 -- Slit fracture on a polyethylene pipe manufactured by DuPont that was foundleaking and removed from a gas piping system.

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Figure 7 illustrates brittle-like cracking thatwas found in a plastic pipe involved in the fatalpropane gas explosion in San Juan, Puerto Rico,in November 1996. That pipe was manufacturedin 1982 by E. I. du Pont de Nemours & Com-pany, Inc., (DuPont) at its Pencador, Delaware,plant. Apparently, differential settlement re-sulting from inadequate support under thepiping generated long-term stress intensificationthat led to the formation of brittle-like cracks inthe pipe.

Figure 8 shows a brittle-like crack that wasfound in a plastic pipe involved in the fatalnatural gas explosion and fire in Lake Dallas,

Piping Materials,” summary of presentation given in theTechnical Information Session hosted by ASTMCommittee F17’s task group on Project 62-95-02, held inconjunction with ASTM Committee F17’s November 1996meetings, New Orleans, LA, and Mruk, S. A., “Validatingthe Hydrostatic Design Basis of PE Piping Materials,”pp. 202-214, 1985.

Texas, in August 1997. That pipe wasmanufactured in 1970 by Nipak, Inc. A metalpipeline pressing against the plastic pipegenerated long-term stress intensification thatled to the crack.

During the 1960s and 1970s, some expertsbegan to question the validity of the PPIprocedure’s assumption of a continuing, gradualstraight-line decline in strength (figure 3). 47 Bythe late 1970s and early 1980s, the plasticpiping industry in the United States realized that

47The 1971 A.G.A. Plastic Handbook for Gas Servicenoted that the cause and mechanisms of brittle fracturessometimes found with long-term stress rupture testing wasnot yet well established. Two of the pioneering papers inthe United States to suggest a downturn in long-termhydrostatic strength with brittle-like failures or in elevatedtemperature testing were: Mruk, S. A., “The Ductile Failureof Polyethylene Pipe,” SPE Journal, Vol. 19, No. 1,January 1963, and Davis, G. W., “What are Long TermCriteria for Evaluating Plastic Gas Pipe?” ProceedingsThird A.G.A. Plastic Pipe Symposium, American GasAssociation, pp. 28-35, 1971.

Figure 7 -- Interior of polyethylene pipe from San Juan pipeline accident showing brittle-like crack with no visible deformation.

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testing piping materials at elevated temperatureswas a way to accelerate failure behavior thatwould occur much later at lower temperatures(such as 73 °F). Based on data derived fromelevated-temperature testing, the industryconcluded that the gradual straight-line declinein strength assumed by the PPI procedure wasnot valid. Instead, two distinct failure zoneswere indicated for polyethylene piping in stressrupture testing. (See figure 9.) The first zone ischaracterized by the gradual straight-line declinein strength accompanied primarily by ductilefractures. The first zone gradually transitions tothe second zone, which is characterized by amore rapid decline in strength accompanied bybrittle-like fractures only. The time andmagnitude of this more rapid decline in strengthvaries by type and brand of polyethylene. Pipingmanufacturers have worked to improve theirproducts’ resistance to slit-type failures and thusto push this downturn further out in time. ThePPI procedure did not account for thisdownturn, and the difference between the actual

falloff shown in figure 9 and the projectedstraight-line strengths shown in figure 3 forlisted materials became more pronounced as thelines were extrapolated beyond 100,000 hours.

As manufacturers steadily improved theirformulations to delay the onset of the downturnin long-term strength and associated brittle-likebehavior, PPI and ASTM industry standardswere upgraded to reflect what the major manu-facturers were able and willing to accomplish.48

Accordingly, and because a consensus of manu-facturers recognized the relationship between

48Both the PPI and the ASTM work on a consensusprinciple, meaning that requirements are put into place onlywhen a consensus of voting members is reached. The PPI isa manufacturers’ organization. With respect to the ASTMtechnical committee that generates requirements for plasticpiping, the major piping manufacturers participate activelyin the committee and are in a position to influence ASTMstrength rating requirements.

Figure 8 -- Brittle-like crack in pipe involved in August 1997 accident in Lake Dallas, Texas.The crack extends from the left to upper right of the area defined by the ellipse.

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improved elevated-temperature properties andimproved longer term pipe performance, the PPIin 1982 recommended that ASTM D2513specify a minimum acceptable hydrostaticstrength at 140 °F. In 1984, ASTM D2513 in-cluded a statement in its non-mandatoryappendix that gas pipe materials should have aspecified long-term hydrostatic strength at140 °F. In the 1988 edition, this requirementwas moved to the mandatory section of thestandard. This strength at 140 °F was calculatedthe same way that the 73 °F strength was calcu-lated—data demonstrating a straight line to10,000 hours was assumed to extrapolate to100,000 hours without a downturn.

Gradually, more manufacturers obtained PPIlistings for their resins intended for gas service,and by the early to mid-1980s, virtually allresins used for gas service had PPI listings. Atthat time, a consensus of manufacturerssupported a change within ASTM D2513 torequire PPI listings. In 1985, ASTM D2513 wasrevised to require that materials for gas servicehave a PPI listing.

By 1985, manufacturers reached a consen-sus to exclude materials that deviated from the73 °F extrapolation before 100,000 hours. ThePPI adopted this restriction and advised theindustry that, effective January 1986, allmaterials not demonstrating straight-line per-formance to 100,000 hours would be dropped

Figure 9 -- Stress rupture data plotted as best-fit straight line transitioning to downturn instrength. (Derived from A.G.A. Plastic Pipe Manual for Gas Service.)

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from its listing.49 In 1988, ASTM D2837 alsoincluded the restriction.50 The new PPI andASTM requirements had no effect on pipeinstalled prior to the effective date of therequirements.

On August 20, 1997, after manufacturersreached a consensus, the PPI issued notice that,effective January 1999, in order for materials toretain their PPI listings for long-term hydrostaticstrength at temperatures above 73 °F (forexample, at 140 °F), these materials will have todemonstrate (mathematically, via elevated-temperature testing) that a downturn does notexist prior to 100,000 hours or, alternatively, if adownturn does exist before 100,000 hours, thestrength rating will be reduced to reflect thepoint at which the calculated downturn instrength intercepts 100,000 hours. An ASTMproject has been initiated to incorporate thisrequirement within ASTM D2837. The SafetyBoard also notes that the PPI has endorsed aproposal to have ASTM D2513 requirepolyethylene piping to have no downturn instress rupture testing at 73 °F before 50 years, asmathematically determined in elevated-temperature tests.

All available evidence indicates thatpolyethylene piping’s resistance to brittle-likecracking has improved significantly through theyears. Several experts in gas distribution plasticpiping have told the Safety Board that a major-ity of the polyethylene piping manufactured inthe 1960s and early 1970s had poor resistance tobrittle-like cracking, while only a minority ofthat manufactured by the early 1980s could beso characterized.51 Several gas system operatorshave told the Safety Board that they are awareof no instances of brittle-like cracking with theirown modern polyethylene piping installations.

49Mruk, S. A., “Validating the Hydrostatic DesignBasis of PE Piping Materials.”

50A.G.A. Plastic Pipe Manual for Gas Service,American Gas Association, Catalog No. XR 9401, 1994.

51A number of these experts considered material tohave poor resistance to brittle-like cracking if the materialwas shown to have a downturn in strength associated withbrittle-like fractures in stress rupture testing (at 73 °F)before 100,000 hours.

Century Pipe Evaluation and HistoryThe Safety Board’s investigation of the

Waterloo, Iowa, accident determined that thepipe involved in the accident had beenmanufactured by Amdevco ProductsCorporation (Amdevco) in Mankato, Minnesota.Amdevco’s Mankato plant first began producingplastic pipe in 1970, with plastic piping for gasservice as its only piping product. Amdevcomade the pipe from Union Carbide’s BakeliteDHDA 2077 Tan 3955 (hereinafter referred toas DHDA 2077 Tan) resin material. CenturyUtility Products, Inc., marketed the pipe to IowaPublic Service Company,52 and Century’s namewas marked on the pipe. Century and Amdevcoformally merged in 1973. The combinedcorporation went out of business in 1979.

Because Amdevco/Century no longer exists,Safety Board investigators could locate norecords to indicate the qualification stepsAmdevco may have performed before Centurymarketed its pipe to Iowa Public ServiceCompany. A plastic pipe manufacturer wouldnormally have obtained documentation from itsresin supplier indicating that the resin materialhad a sufficient long-term hydrostatic strength.Code B31.8 required and ASTM D2513recommended that polyethylene pipemanufacturers perform certain quality controltests on production samples, including twice-per-year sustained pressure tests.

Like many gas operators of that time, IowaPublic Service Company (now MidAmericanEnergy Corporation), which had installed theWaterloo piping in 1971, had no formalprogram for testing or evaluating products.According to MidAmerican Energy, thecompany accepted representations from aprincipal of Century, a former DuPontemployee, who portrayed himself as beingintimately involved with the development andmarketing of DuPont’s polyethylene piping.MidAmerican Energy has reported that theserepresentations included assertions that Century

52Because of a series of organizational changes andmergers, the name of the owner/operator of the gas systemat Waterloo, Iowa, has changed over the years. In 1971,Iowa Public Service Company installed the gas service thatultimately failed. At the time of the accident, the gas systemoperator was Midwest Gas Company. The current operatoris MidAmerican Energy Corporation.

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plastic pipe met industry standards and had thesame formulation as DuPont’s plastic pipe. In1970, according to MidAmerican Energyofficials, Century offered Iowa Public ServiceCompany attractive commercial terms for itsproduct, with the result that, in 1970, whenAmdevco first started to manufacture pipe, IowaPublic Service Company began purchasing all ofits plastic pipe from Century.53

Before the Waterloo accident, a previousaccident involving Century pipe had beenreported in the Midwest Gas (the operator at thetime of the accident) system. That accidentoccurred in August 1983 in Hudson, Iowa, andresulted in multiple injuries. Midwest Gas,attributing this accident to a rock pressing intothe pipe, considered it an isolated incident.During 1992-94, the company had twosignificant failures with pipe fittings involvingbrittle-like cracks in Century pipe. Sections ofthe failed pipe were sent to the two affected pipefitting manufacturers, and one responded thatnothing was wrong with the fitting, suggestinginstead that the problem might rest with thepiping material.

MidAmerican Energy reported that, as aresult of these two failures, Midwest Gasdirected inquiries to other utilities operating inthe Midwest and, in May 1994, learned of oneother accident involving Century pipe. In June1994, Midwest Gas decided to send samples ofCentury polyethylene piping to an independentlaboratory for test and evaluation. The samplecollection was in process at the time of theWaterloo accident. In August 1995, MidwestGas issued a report, based on the laboratorytesting, concluding that the Century samples hadpoor resistance to slow crack growth.

Subsequent to the accident, Midwest Gasworked to determine if its installations withCentury plastic piping had had higher rates offailure than those with piping from other

53Iowa Public Service Company continued to purchaseDuPont plastic piping fittings until fittings were availablefrom Century. MidAmerican Energy made technicalprocurement decisions via a Gas Standards Committee.According to company officials, the company hasimplemented a process to ensure that it continues to receivequality products once the products have passed an initialqualification process.

manufacturers. After analyzing the data,Midwest Gas concluded that the pipinginstallations with Century piping had failurerates that were significantly higher than thoseinstallations with plastic piping from othermanufacturers. Based on this analysis, as well ason other factors—including the severity andconsequences of leaks involving Century piping,the laboratory test results, recommendationsfrom two manufacturers of pipe fittingscautioning against use of their fittings withCentury pipe because of the pipe’s poorresistance to brittle-like cracking, and interviewswith field personnel—MidAmerican Energy(the current operator) has replaced all its knownCentury piping with new piping, completing thereplacement program in 1997.

Safety Board investigators found little addi-tional documentation regarding qualificationtests of Century plastic pipe by other gas systemoperators having Century pipe in service. Areference was found to a 1971 Northern StatesPower Company Testing Department progressreport stating that Century pipe complied withASTM D2513, and that the pipe was acceptablefor use with DuPont polyethylene fittings. Theactual progress report and records of any teststhat may have been performed were notlocated.54

Union Carbide DHDA 2077 Tan Resin --The resin used to manufacture the pipe involvedin the Waterloo accident was DHDA 2077 Tan.To examine how Union Carbide qualified thismaterial requires some background.

During the late 1960s, several companiesmanufactured plastic resin and plastic pipe forthe gas distribution plastic piping market. Atthat time, Union Carbide began a process ofmodifying its DHDA 2077 Black resin (forwater distribution) in order to create a DHDA2077 Tan resin for the gas distribution industry.

Before Union Carbide could market itsDHDA 2077 Tan resin material for natural gasservice, it needed to generate stress rupture data,in accordance with the PPI procedure, thatwould support the long-term hydrostatic

54Northern States Power is based in St. Paul,Minnesota.

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strength rating it was assigning to the material (arequirement of the interim Federal regulationseffective at that time).55 The company had threeresources to draw upon to support thehydrostatic design basis category: (1) internalstress rupture data on its DHDA 2077 Tan resin,(2) a PPI listing already obtained on its similarblack resin, and (3) additional internal stressrupture data on its black resin.

On June 11, 1968, Union Carbide beganstress rupture testing on specimens of pipe madefrom a pilot-plant batch of its newly developedDHDA 2077 Tan resin. The results of thistesting supported Union Carbide’s declaredhydrostatic design basis category for DHDA2077 Tan. The number of data points generatedby these stress rupture tests for the DHDA 2077Tan was less than that required by PPIprocedure; however, Union Carbide began tomarket the product for use in gas systems basedon these tests and on additional testingperformed on the company’s black resinmaterial.

Because Union Carbide had not developedthe PPI-prescribed number of data points on itsDHDA 2077 Tan resin before marketing theproduct, Safety Board investigators reviewedthe data the company developed on its blackresin. A review of Union Carbide’s laboratorynotebooks revealed that a number of adversedata points Union Carbide developed for its blackresin were not submitted to the PPI when thecompany applied for a PPI listing for the blackmaterial.56

Union Carbide first made a commercialversion of its DHDA 2077 Tan resin during thespring of 1969, and in April 1970, a first

55The company was required to follow the PPIprocedure in developing the necessary stress rupture data,but no requirement existed for those data to be submitted tothe PPI or for the PPI to assign a listing before the testedmaterial could be marketed.

56Although the PPI procedure required the submissionof all valid data points for statistical analysis, the UnionCarbide employee who managed the data indicated that hebelieved he could discard data that, in his judgment, did notadequately characterize the material’s performance. UnionCarbide has contended that the non-submitted data mayhave been invalid because of experimental error,uncompleted tests, or other reasons.

shipment of 80,000 pounds of DHDA 2077 Tanresin was shipped to Amdevco’s Mankato plant.The next shipment of the material to Amdevcowas not until 1971. Based on Amdevco’sJune 11, 1970, manufacturing date for theWaterloo pipe, Union Carbide manufactured,sold, and delivered the resin used to make theWaterloo pipe between the spring of 1969 andJune 11, 1970, and the resin used to make thepipe involved in the Waterloo accident probablywas included in the April 1970 shipment.

Union Carbide began, on December 3, 1970,additional stress rupture tests on its commercialDHDA 2077 Tan resin. These tests generatedthe results to further support its claimed long-term hydrostatic strength and also provided thenumber of data points required by the PPIprocedure. Additional stress rupture tests on thecommercial DHDA 2077 Tan resin beginningon December 28, 1970, and again onJanuary 6, 1972, further supported the material’slong-term hydrostatic strength.

During the late 1960s and 1970s,Minnegasco, a gas system operator based inMinneapolis, Minnesota, routinely employed a1,000-hour sustained pressure test at 100 °Fdetailed in ASTM D2239 and a 1,000-hoursustained pressure test at 73 °F detailed inASTM D2513 to qualify plastic piping for usein its system. Minnegasco went beyond therequirements of ASTM standards by continuingboth versions of the testing beyond 1,000 hoursuntil eventual failure occurred. The companyused this information to evaluate the relativestrengths of different brands of piping.

In 1969-70, Minnegasco began a series oftests on samples from five different suppliers ofplastic piping made from DHDA 2077 Tanresin. On March 3, 1972, Minnegasco’slaboratory issued an internal report thatcontained the results of its latest tests on pipingmade from the resin and referenced earlier testson several brands of piping (includingAmdevco/Century) that were also made from it.Based on this report, Minnegasco rejected foruse in its gas system the DHDA 2077 Tan resin.According to the report, the company rejectedthe material because (1) none of the pipesamples made from this resin could consistentlypass the 1,000-hour sustained pressure test at

19

100 °F, and (2) the pipe samples had lowerperformance in 73 °F sustained pressure teststhan similar plastic piping materials already inuse in the company’s gas system.

In 1971, Union Carbide acknowledged to apipe manufacturer that piping materialmanufactured by DuPont had a higher pressurerating at 100 °F than did its ownDHDA 2077 Tan. Union Carbide laboratorynotebooks examined by the Safety Boardshowed test results for the DHDA 2077 Tanmaterial that generally met the 1,000-hoursustained pressure test value at both 100 °F and73 °F, although, in the case of the 100 °F test,not by a wide margin. The notebooks alsoshowed that the material had an early ductile-to-brittle transition point in stress rupture tests.57

Information Dissemination Within theGas Industry

The OPS reports that more than 1,200 gasdistribution or master meter system58 pipelineoperators submit reports to the OPS.Additionally, more than 9,000 gas distributionor master meter system pipeline operators aresubject to oversight by the States.

As noted earlier, a frequent failuremechanism with polyethylene piping involvescrack initiation and slow crack growth. Thesebrittle-like fractures occur at points of stressintensification generated by external loadingacting in concert with internal pressure andresidual stresses.59

57The data from the laboratory notebooks suggest thatthis material’s early ductile-to-brittle transition would nothave met today’s standards.

58Master meter system refers to a pipeline system thatdistributes gas to a definable area, such as a mobile homepark, a housing project, or an apartment complex, wherethe master meter operator purchases gas for resale to theultimate consumer.

59Kanninen, M. F., O’Donoghue, P. E., Popelar, C. F.,Popelar, C. H., Kenner, V. H., Brief Guide for the Use ofthe Slow Crack Growth Test for Modeling and Predictingthe Long-Term Performance of Polyethylene Gas Pipes,Gas Research Institute Report 93/0105, February 1993.Because, after extrusion, the outside of the pipe coolsbefore the inside, residual stresses are usually developed inthe wall of the pipe.

A 1985 paper60 analyzed, for linear (straightline) behavior up to 100,000 hours, the stressrupture test performance (by elevated-temperature testing) of six polyethylene pipingmaterials. The results were then correlated withfield performance. This paper found that thosematerials that did not maintain linearity through100,000 hours had what the author characterizedas “known poor” or “questionable” fieldperformance. On the other hand, those materialsthat maintained linearity through 100,000 hourshad what the author characterized as “knowngood” field performance through their 20-yearhistory logged as of 1985.

By the early to mid-1980s, the industry haddeveloped a method to mathematically relatefailure times to temperatures and stresses duringstress rupture testing.61 In the early 1990s, theindustry developed “shift functions,” anothermathematical method to relate failure times totemperatures and stresses.62

One study63 pointed out that usingmathematical methods to calculate theremaining service life of pipe under theassumption that the pipe would only be exposed

60Mruk, S. A., “Validating the Hydrostatic DesignBasis of PE Piping Materials.”

61Bragaw, C. G., “Prediction of Service Life ofPolyethylene Gas Piping System,” Proceedings SeventhPlastic Fuel Gas Pipe Symposium, pp. 20-24, 1980, andBragaw, C. G., “Service Rating of Polyethylene PipingSystems by the Rate Process Method,” Proceedings EighthPlastic Fuel Gas Pipe Symposium, pp. 40-47, 1983, andPalermo, E. F., “Rate Process Method as a PracticalApproach to a Quality Control Method for PolyethylenePipe,” Proceedings Eighth Plastic Fuel Gas PipeSymposium, pp. 96-101, 1983, and Mruk, S. A.,“Validating the Hydrostatic Design Basis of PE PipingMaterials,” and Palermo, E. F., “Rate Process MethodConcepts Applied to Hydrostatically Rating PolyethylenePipe,” Proceedings Ninth Plastic Fuel Gas PipeSymposium, pp. 215-240, 1985.

62Popelar, C. H., “A Comparison of the Rate ProcessMethod and the Bidirectional Shifting Method,”Proceedings of the Thirteenth International Plastic FuelGas Pipe Symposium, pp. 151-161, and Henrich, R. C.,“Shift Functions,” 1992 Operating Section Proceedings,American Gas Association.

63Broutman, L. J., Bartelt, L. A., Duvall, D. E.,Edwards, D. B., Nylander, L. R., Stellmack-Yonan, M.,Aging of Plastic Pipe Used for Gas Distribution, FinalReport, Gas Research Institute report number GRI-88/0285, December 1988.

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to stresses of internal operating pressures wouldresult in unrealistically long service-lifepredictions. As noted earlier, polyethylenepiping systems have failed at points of long-termstress intensification caused by external loadingacting in concert with internal pressure andresidual stresses; thus, to obtain a realisticprediction of useful service life, stresses fromexternal loadings need to be acknowledged.

Over a number of years, the Gas ResearchInstitute (GRI) sponsored research projectsinvestigating various tests and performancecharacteristics of polyethylene piping materials.Among these projects was a series of researchinvestigations directed at exploring the fracturemechanics principles behind crack initiation andslow crack growth. These investigations led tothe development of slow crack growth tests. Theresearch studies frequently identified the pipingand resins studied by codes rather than byspecific materials, manufacturers, or dates ofmanufacture.

In 1984, the GRI published a study64 thatcompared and ranked several commerciallyextruded polyethylene piping materialsproduced after 1971. Again, the materials testedwere identified by codes. Stress rupture testswere performed using methane and nitrogen asthe internal pressure medium and air as theoutside environment. Several stress rupturecurves showed early transitioning from ductileto brittle failure modes.

The A.G.A.’s Plastic Materials Committeeperiodically updates the A.G.A Plastic PipeManual for Gas Service, which addresses anumber of issues covered by this Safety Boardspecial investigation. In 1991, the committeeformed a task group to gather and thendisseminate to the industry informationregarding the performance of older plasticpiping systems. The task group disbanded in1994 without issuing a report.

In 1982 and 1986, DuPont formally notifiedits customers about brittle-like cracking

64Cassady, M. J., Uralil, F. S., Lustiger, A., Hulbert,L. E., Properties of Polyethylene Gas Piping MaterialsTopical Report (January 1973 - December 1983), GRIReport 84/0169, Gas Research Institute, Chicago, IL, 1984.

concerns with the company’s pre-1973 pipe.Safety Board investigators could find no recordof either Century/Amdevco, Union Carbide, orany other piping or resin manufacturer formallynotifying the gas industry of the susceptibility topremature brittle-like failures of their products.Nor does any mechanism exist to ensure that theOPS receives safety-related information frommanufacturers.

Regarding Federal actions on this issue, theOPS has not informed the Safety Board of anysubstantive action it has taken to advise gassystem operators of the susceptibility topremature brittle-like failures of any olderpolyethylene piping.65

Installation Standards and PracticesThe discussion in this section is intended to

present a “snapshot” of the regulations and someof the primary standards, practices, andguidance to prevent stress intensification atplastic service connections to steel tapping tees.The appendix to this report includes adescription of the connection in the Waterlooaccident, and figure 10 provides a close-up viewof the failed fitting.

Federal Regulations -- The OPSestablishes, in 49 CFR 192.361, minimumpipeline safety standards for the installation ofgas service piping.

Paragraph 192.361(b) reads as follows:

Support and backfill. Each service linemust be properly supported onundisturbed or well-compacted soil….

Paragraph 192.361(d) reads:

Protection against piping strain andexternal loading. Each service line mustbe installed so as to minimizeanticipated piping strain and externalloading.

65The Safety Board asked the OPS for informationabout its actions in regard to older piping, after which, in1997, the OPS notified State pipeline safety programmanagers of several issues regarding Century pipe andsolicited input on their experiences with this particularpiping.

21

Subsequent to the Waterloo accident,personnel from the Iowa Department ofCommerce, after discussions with OPSpersonnel, stated that the Waterloo installationwas not in violation of the Federal regulation.They further stated that, while they agree thatthe installation of protective sleeves66 at pipelineconnections is prudent, a specific requirement toinstall protective sleeves is beyond the scope ofPart 192 and is inconsistent with theregulation’s performance orientation.

The Transportation Safety Institute (TSI),part of RSPA, conducts training classes forFederal and State pipeline inspectors. TSI

66Protective sleeves are intended to help shield thepipe at the connection point from bearing loads and shearforces and to limit the maximum pipe bending.

instructors advise class participants that many ofthe performance-oriented regulations within Part192 can only be found to be violated if the gassystem fails in a way that demonstrates that theregulation was not followed. The TSIacknowledges the difficulty of identifyingviolations under paragraph 192.361(d). A TSIinstructor told the Safety Board that, in the caseof the failed pipe at Waterloo, an enforcementaction faulting the installation would be unlikelyto prevail because of the poor brittle-like crackresistance of the failed pipe and the length oftime (23 years) between the installation andfailure dates.

GPTC Guide for Gas Transmission andDistribution Piping Systems -- After theadoption of the Natural Gas Pipeline Safety Actin August 1968, the American Society ofMechanical Engineers, after discussions with

Figure 10 -- Close-up view of failed plastic pipe connection to steel tapping tee from site ofWaterloo, Iowa, accident. A portion of the fractured plastic service line (light-colored material)remains attached to the tee.

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the Secretary of Transportation, formed the GasPiping Standards Committee (later renamed theGas Piping Technology Committee) to developand publish “how-to” specifications forcomplying with Federal gas pipeline safetyregulations. The result was the GPTC Guide forGas Transmission and Distribution PipingSystems (GPTC Guide). The GPTC Guide liststhe regulations by section number and providesguidance, as appropriate, for each section of theregulation.

In its investigation of the previouslyreferenced 1971 accident in Texas, the SafetyBoard determined that protective sleeves weretoo short to fully protect a series of serviceconnections to a main. The Safety Board notedthat a protective sleeve must have the correctinner diameter and length if it is to protect theconnection from excessive shear forces. As aresult, and in response to a Safety Board safetyrecommendation,67 the 1974 and later editions ofthe GPTC Guide included guidance that “aprotective sleeve designed for the specific typeof connection should be used to reduce stressconcentrations.” No guidance was included as tothe importance of a protective sleeve’s length,diameter, or placement.68

The GPTC Guide does not includerecommendations to limit bending in plasticpiping during the installation of service linesunder 49 CFR 192.361. Although the guidereferences the A.G.A. Plastic Pipe Manual forGas Service, and this manual does providerecommendations on bending limits, the GPTCGuide does not reference this manual in itsguidance material under 49 CFR 192.361.

A.G.A. Plastic Pipe Manual for GasService -- The most recent edition of the A.G.A.Plastic Pipe Manual for Gas Service69 identifiesthe connection of plastic pipe to service tees as“a critical junction” needing installation

67Safety Recommendation P-72-64 from NationalTransportation Safety Board Pipeline Accident Report--Lone Star Gas Company, Fort Worth, Texas, October 4,1971.

68The correct positioning of the protective sleeve has abearing on its effective length.

69A.G.A. Plastic Pipe Manual for Gas Service,American Gas Association, Catalog No. XR 9401, 1994.

measures “to avoid the potentiallyhigh…stresses on the plastic at this point.” Themanual recommends proper support and the useof protective sleeves. Although the manualrecommends following manufacturers’recommendations, no guidance is included onthe importance of a protective sleeve’s properlength, diameter, or placement. The manualincludes, without elaboration, the followingsentence:

Installation of the tee outlet at angles upto 45° from the vertical or along the axisof the main as a ‘side saddle’ or ‘swingjoint’ may be considered to furtherminimize…stresses.

The 1994 edition adds that manufacturers’recommended limits on bending at fittings maybe more restrictive than for a run of pipingalone.

A.G.A. Gas Engineering and OperatingPractices (GEOP) Series -- The preface to thecurrent Distribution Book D-2 of the GEOPseries states that the intent of the books is tooffer broad general treatment of their subjects,and that listed references provide additionaldetailed information.

Figure 11 reproduces an illustration fromBook D-2. This figure shows a steel tapping teewith a compression coupling joint connected toa plastic service. The illustration shows aprotective sleeve and includes a note to extendthe protective sleeve to undisturbed orcompacted soil or to blocking. But the figurealso shows the blocking positioned so that eitherthe edge of the blocking or the edge of theprotective sleeve might provide a fixed contactpoint on the plastic service pipe if the weight ofbackfill were to cause the pipe to bend down.Additional illustrations within this GEOP seriesbook show this same positioning of the blockingwith respect to the plastic pipe.

ASTM -- The most recent ASTM standardcovering the installation of polyethylene pipingwas revised in 1994.70 This standard addresses

70ASTM D2774-94, Standard Practice forUnderground Installation of Thermoplastic PressurePiping, American Society for Testing and Materials, 1994.

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the vulnerability of the point-of-serviceconnection to the main.

This standard, advising consultation withmanufacturers, recommends taking extra careduring bedding and backfilling to provide forfirm and uniform support at the point ofconnection. In addition, the documentrecommends minimizing bends near tapconnections, generally recommending thatbends occur no closer than 10 pipe diametersfrom any fitting and that manufacturers’ bendlimits be followed. Similar recommendations foravoiding bends close to a fitting can be found inthe forward to a water industry standard.71

This ASTM standard further recommendsthe use of a protective sleeve if needed toprotect against possible differential settlement.Currently, manufacturers that provide protectivesleeves have their own criteria for designingsleeve lengths and diameters for their fittings.

71Forward to American Water Works AssociationStandard C901-96, AWWA Standard for Polyethylene (PE)and Tubing, ½ In. (13 mm) Through 3 In. (76 mm) forWater Service, effective March 1, 1997.

Some manufacturers’ criteria are based onlimiting stress to a maximum safe value,72 whileone manufacturer has advised the Safety Boardthat its sleeve is not designed to limit bending,but only to guard against shear forces at theconnection point.

Guidance Manual for Operators of SmallNatural Gas Systems -- The OPS/RSPAGuidance Manual for Operators of SmallNatural Gas Systems notes that plastic pipefailures have been found at transitions betweenplastic and metal pipes at mechanical fittings.The manual states the need to firmly compactsoil under plastic pipe, advises followingmanufacturers’ instructions for proper couplingprocedures, and shows protective sleeves onconnections of plastic services to steel tappingtees. The manual indicates that a properlydesigned protective sleeve should be used. Themanual does not caution against bending thepiping in proximity to a connection.

72Allman, W. B., “Determination of Stresses andStructural Performance in Polyethylene Gas Pipe andSocket Fittings Due to Internal Pressure and External SoilLoads,” 1975 Operating Section Proceedings, AmericanGas Association, 1975.

Figure 11 -- Reproduction from A.G.A. GEOP series illustrating application of protectivesleeve. (Hand-scribed notation from the original.)

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Manufacturers’ Recommendations -- Asnoted earlier, both the A.G.A. Plastic PipeManual for Gas Service and ASTM D2774specifically refer the reader to manufacturers forfurther guidance on limiting shear and bendingforces at plastic service connections made tosteel mains via steel tapping tees.

Bending and Shear Forces -- Safety Boardinvestigators contacted representatives of thefour principal companies that marketed plasticpiping for gas service to determine to whatextent plastic piping manufacturers wereproviding recommendations for limiting shearand bending forces at plastic serviceconnections to steel mains via steel tapping tees.The four manufacturers contacted were CSRPolyPipe, Phillips Driscopipe, Plexco, andUponor Aldyl Company (Uponor).

Three out of four of these manufacturershad published recommendations addressingthese issues. These three manufacturers havehistorically emphasized heat fusion fitting sys-tems73 instead of field-assembled mechanicalfitting systems. Representatives of these manu-facturers indicated that mechanical fittingsmanufacturers should provide installation in-structions covering their systems. Accordingly,one of the manufacturers’ published literaturereferred the reader to the manufacturers of me-chanical fittings for installation instructions.Nonetheless, these three major polyethylenepipe manufacturers did, in fact, provide recom-mendations to limit shear and bending forces,and these recommendations can apply to plasticservice connections to steel mains via steeltapping tees.

With respect to the specific issue of limitingbends, DuPont, in January 1970, issued recom-mendations to limit bends for polyethylene pipe.DuPont/Uponor74 later published bend radiusrecommendations that differentiated betweenpipe segments consisting of pipe alone and thosewith fusion fittings. The recommendationsspecified much less bending for pipe segments

73Heat fusion fittings are used to make piping joints byheating the mating surfaces and pressing them together sothat they become essentially one piece.

74Uponor purchased DuPont’s plastic pipe business in1991.

with fusion fittings; however, DuPont/Uponordid not provide bend limits for mechanicalfittings. Two of the other major manufacturers(Phillips Driscopipe and Plexco) provide bendlimits and differentiate between pipe alone andpipe with fittings, without specifying the type offittings. None of the manufacturers’ literaturediscusses bending with or against any residualbend remaining in the pipe after it is uncoiled.(See “Pipe Residual Bending” below.)

Of these four major polyethylene gas pipemanufacturers, only CSR PolyPipe had nopublished recommendations for limiting shearand bending forces at plastic serviceconnections to steel mains via steel tapping tees.Although the company does not manufacturesteel tapping tees with compression ends forattachment to plastic services, it doesmanufacture pipe that will be attached to steeltapping tees via mechanical compressioncouplings. The company has been supplyingpolyethylene pipe to the gas industry since the1980s75 and is thus relatively new to thatbusiness compared to the other three majormanufacturers. When CSR PolyPipe entered themarket, plastic materials were vastly improvedcompared to earlier versions with respect toresistance to crack initiation and slow crackgrowth. For this reason, according to CSRPolyPipe personnel, the company saw less needto publish installation recommendations.

The Safety Board attempted to identifyevery U.S. steel tee manufacturer that currentlymanufactures steel tees with a compression endfor plastic gas service connections.76 The SafetyBoard identified and contacted representativesof Continental Industries (Continental), DresserIndustries, Inc. (Dresser), Inner-Tite Corp.(Inner-Tite),77 and Mueller Company (Mueller).

75CSR Hydro Conduit Company purchased PolyPipein 1995. PolyPipe began supplying polyethylene pipe to thegas industry in the 1980s.

76J. B. Rombach, Inc., which manufactures M. B.Skinner Pipeline products, told the Safety Board that it nolonger manufactures or markets its “Punch-It-Tee” line ofsteel tapping tees. Chicago Fittings Corporation told theSafety Board it no longer manufactures or markets its lineof steel tapping tees. The Safety Board therefore made nofurther inquiry with these companies.

77Inner-Tite did not manufacture steel tees; itpurchased them, affixed its own compression connections,

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Only Continental and Inner-Tite offeredprotective sleeves to their customers as anoption. None of these manufacturers haspublished installation recommendations to limitshear and bending forces on the plastic pipe thatconnects to their steel tapping tees.

On another issue related to protectivesleeves, Safety Board examination of aprotective sleeve offered by Continental to itscustomers revealed that the sleeve that did nothave sufficient clearance to allow theapplication of field wrap (intended to protect thesteel tee from corrosion after it is in the ground)to that portion of the steel tee under the sleeve.This observation was confirmed by aContinental representative.

Pipe Residual Bending -- The service involvedin the Waterloo accident was installed with abend at the connection point to the main. (Seeillustration in appendix A.) The plastic servicepipe leaving the tee immediately curvedhorizontally. The pipe was cut out and broughtinto the laboratory, at which time the bend had ameasured horizontal radius of approximately 34inches. Based on field conditions and photos,MidAmerican Energy estimated the originalinstalled horizontal bend radius to have beenabout 32 inches. This bend is sharper than thatallowed by current industry installationrecommendations for modern piping adjacent tofittings.

An issue related to recommended bendradius is residual pipe bending. Plastic pipeoften arrives at a job site in banded coils. Afterthe bands are released, the coiled pipe willpartially straighten, but some residual bendingwill remain. The water industry alreadyrecognizes that bends in the direction of theresidual coil bend should be treated differentlythan bends against the direction of the bend;78

however, gas industry field bend radiusrecommendations do not address residual coilbending.

A former Iowa Public Service Companyemployee stated that Iowa Public Service

and marketed the complete assembly.78Forward to American Water Works Association

Standard C901-96.

Company, in an effort to reduce stress atconnection points, generally attempted to installpolyethylene services at an angle to the main tomatch the residual bend left after uncoiling thepipe. This former employee stated that no settime was specified to allow for completerelaxing of the pipe, but that the pipe would beplaced in the ditch, and the crews would weldthe tee at what they judged to be the appropriateangle.

MidAmerican Energy InstallationStandards -- As a result of the Waterlooaccident, Safety Board investigators examinedsome of MidAmerican Energy’s constructionstandards for minimizing shear and bendingforces at plastic service connection points tosteel mains. Specifically, Safety Boardinvestigators examined MidAmerican Energy’sstandards pertaining to providing firm support,using protective sleeves, and limiting bends atplastic service connections to steel mains.

According to the company, MidAmericanEnergy no longer installed steel tapping teeswith mechanical compression ends to connect toplastic service pipe. Instead, it employed steeltapping tees welded at the factory to factory-made steel-to-plastic transition fittings. It thenfield-fused the plastic ends from the transitionfittings to the plastic service pipe.

MidAmerican Energy advised the SafetyBoard that it had no standard calling for firmcompacted support under plastic serviceconnection points to steel mains.

MidAmerican Energy designed, constructed,and installed its own protective sleeves forinstallation on its purchased steel tappingtee/transition fitting assemblies. MidAmericanEnergy required its protective sleeves to be aminimum of 12 inches long; however,MidAmerican Energy could provide no designcriteria for this length. MidAmerican Energy hasreported that the company’s unwritten fieldpractice was to install the smallest diametersleeve that will clear the field wrapped fitting,but MidAmerican Energy had no written re-quirements or design criteria for the diameter ofits protective sleeves. The company’s standardshowed the sleeve as approximately centeredover the steel-to-plastic transition, and no

26

criteria or instructions were provided for thecorrect positioning of the sleeves.

The Safety Board notes that manufacturersthat provide factory-made steel-to-plastictransition fittings will also provide protectivesleeves along with the transition fittings and willprovide positioning guidance for their use.

Effective January 27, 1997, MidAmericanEnergy instituted minimum bend radiirequirements that differentiated between pipesegments consisting of pipe alone and pipe withfittings.

Gas System Performance MonitoringThis section examines gas system perform-

ance monitoring largely in the context of theWaterloo accident.

Federal regulations (49 CFR 192.613 and192.617) require that gas pipeline systemoperators have procedures in place formonitoring the performance of their gas sys-tems. These procedures must cover surveillanceof gas system failures and leakage history,analysis of failures, submission of failed sam-ples for laboratory examination (to determinethe causes of failure), and minimizing the possi-bility of failure recurrences.

Prior to the Waterloo accident, Midwest Gashad two systems for tracking, identifying, andstatistically characterizing failures. The firstsystem was the leak data base, which tracked thestatus of leak reports, documented actions taken,and recorded almost all gas system leaks. Thedata base received input from two primarysources: leak reports from customers and leaksurvey results. The data base parametersclassified the general type of piping materialthat leaked (such as “plastic,” “cast iron,” “baresteel”), and indicated whether the leak occurredin pipe or certain fittings. The parameters didnot include manufacturers, manufacturing orinstallation dates, sizes,79 or failure conditionscommonly found with plastic piping (forexample, poor fusions, bending force failures,

79While sizes of the piping, along with a drawing ofthe piping assembly, were normally written or drawn on theforms, piping size was not captured in the data basegenerated by these forms.

insufficient soil compaction, rock impingementfailures, and lack or improper use of protectivesleeves). The data base indicated that theperformance of plastic piping overall wascomparable to other piping materials.MidAmerican Energy stated that the parameterschosen for this data base were those required forreporting to the DOT. The company said theparameters were also chosen on the premise thatpipe meeting industry specifications wouldperform similarly.

The second system used by Midwest Gas fortracking failures was the company’s materialfailure report data base, which was intended foruse in evaluating the quality and performancehistories of products installed in the company’sgas system. Input to the data base was by way ofa form (or, in some cases, a tag) filled out byfield personnel. The form included categoriessuch as the manufacturer, size, and an internalmaterial identification number of the affectedpipe or component. It also included areas for anarrative description of the failure. The form didnot include dates of manufacture or installationdates or failure conditions commonly found onplastic piping. Field personnel sent the faileditem, along with the completed form or tag, toengineering personnel, who examined the itemand accompanying information to determine theneed for corrections. Midwest Gas personnelthen transcribed the narrative description of thefailure word-for-word into the data base withoutattempting to determine and categorize causes offailure. Engineering personnel compiled theavailable data into periodically issued materialsummary reports. The company said engineeringpersonnel from time to time sorted availabledata fields to determine trends.

The material failure report data baseincluded only a portion of the leaks in theMidwest Gas system. For example, if MidwestGas field personnel corrected a leak byreplacing an entire line segment without diggingup the leaking component (which the companysaid was a frequent occurrence with bare steel,cast iron, and certain plastic piping that wasdifficult to join), the material failure report database system was not used. Also, field personnelwere not required to use the reporting system ifthey determined that the failed item was relatedto an operating problem, such as excavationdamage, rather than to a material problem.

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Additionally, the company indicated that thesystem did not enjoy full participation fromfield personnel.

When, after the Waterloo accident, MidwestGas attempted to determine if installations withCentury plastic piping had higher rates of failurethan those with piping from othermanufacturers, it found that its material failurereport data base’s incomplete coverage of gasleaks made that data base unsuitable for thepurpose. The company decided instead to usethe leak data base, which the company believedincluded almost all leaks. But because the leakdata base did not list the manufacturers ofplastic piping, Midwest Gas took several monthsto correlate entries in the leak data base withrecords showing the manufacturers of plasticpiping. Midwest Gas, in 1995, concluded thatpiping installations with Century piping hadfailure incidence rates that were significantlyhigher than the balance of its plastic pipingsystem. The company did not correlate entrieswith the years of installation.

Since the Waterloo accident, the currentWaterloo gas system operator, MidAmericanEnergy, in addition to replacing all its Centurypipe, has added parameters such as piping size,installation date, and pressure to the forms usedfor input into its leak data base. Also since theaccident, MidAmerican Energy has addedparameters such as installation date, pressure,and component location and position to its formfor input into its material failure report database. The company has also worked todetermine if any other plastic pipingmanufacturers can be linked to piping withunacceptable performance.

The current (1994) edition of the A.G.A.Plastic Pipe Manual for Gas Servicerecommends the use and provides a sample of aform for recording information on plastic pipingfailures. The manual recommends collecting thisinformation and then performing a visualexamination or, in some cases, a laboratoryanalysis, to determine the type and cause offailure.

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Generalhe common thread in a series of plasticpipeline accidents investigated by theSafety Board and others since the early

1970s—as well as in a number of reports ofother, non-accident, plastic pipeline leaks—isthe indicated presence of brittle-like crackingleading to eventual pipe failure. The number andsimilarity of these brittle-like failures seem toindicate that the long-term durability of plasticpiping, which was premised on the pipe’sductility, may have been overstated by themethod used to rate the long-term strength ofplastic piping materials.

Based on the available evidence, any publicsafety threat posed by possible premature failureof plastic piping appears to be limited to loca-tions where stress intensification exists. Thisspecial investigation examines in detail one in-stallation configuration—plastic pipe mechani-cal connections to steel mains via steel tappingtees—where great potential exists for thegeneration of stress intensification. At theseconnections, certain poor installation practiceshave been known to create stress that is greaterthan the pipe can withstand. Thus, inadequate orimproper installation of piping connections, incombination with brittle piping, represents oneidentifiable public safety hazard associated withthe thousands of miles of older plastic pipingnow in service nationwide.

Gas system operators need to have aneffective surveillance and data analysis(performance monitoring) program to determinethe extent of the possible hazard associated withtheir pipeline systems, including plastic piping.Such a program must be adequate to detecttrends as well as to identify localized problemareas, and it must be able to relate poorperformance to specific factors such as plasticpiping brands, dates of manufacture (orinstallation dates), and failure conditions.

The major safety issues developed duringthis special investigation are as follows:

• The vulnerability of plastic pipingto premature failures due to brittle-like cracking;

• The adequacy of available guidancerelating to the installation andprotection of plastic pipingconnections to steel mains; and

• Performance monitoring of plasticpipeline systems as a way ofdetecting unacceptable performancein piping systems.

The remainder of this analysis addresseseach of these major safety issues, as well as anumber of other issues affecting the safety ofplastic piping for gas service.

Durability of Century Utility ProductsPiping

Iowa Public Service Company, the companythat installed the Century pipe involved in the1994 Waterloo, Iowa, pipeline accident, beganpurchasing all of its plastic pipe from Century in1970, when Amdevco/Century had just startedto manufacture plastic pipe. These purchaseswere made without Iowa Public Service Com-pany’s having a testing or technical evaluationprogram and without Century/Amdevco havinga successful track record. Iowa Public ServiceCompany decided on the Century product be-cause Century offered favorable commercialterms for a product it claimed was virtuallyidentical to the DuPont plastic piping that hadpreviously been used.

The Safety Board has investigated two otherpipeline accidents, one in Nebraska in 1978 andone in Minnesota in 1983, that involved Centurypiping. The Safety Board is also aware of fourother accidents that it did not investigate thatinvolved the same brand of piping. Moreover,laboratory testing of Century product samplesfrom the Waterloo accident determined that thematerial had the same brittle-like crackproperties that have been associated withmaterials having poor performance histories.

ANALYSIS

T

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Laboratory examination also revealed evidenceof slow crack growth typical of brittle-likecracking.

The Century pipe involved in the Waterlooaccident was made from Union Carbide’sDHDA 2077 Tan resin. Although UnionCarbide’s laboratory data indicated that thematerial had the strength required by existinggovernment and industry requirements, theSafety Board’s review of the same data showedthat the material had an early ductile-to-brittletransition, indicating poor resistance to brittle-like fractures.

In the early 1970s, a Minnesota gas systemoperator tested a number of piping productsmade from DHDA 2077 Tan resin, includingthose marketed by Century, as part of itscomprehensive specification, testing, andevaluation program. The company rejectedpiping made from the Union Carbide product foruse in its system based on the results ofsustained pressure tests. Union Carbide, in1971, acknowledged that its DHDA 2077 Tanresin material had a lower pressure rating at100 °F than did DuPont’s polyethylene pipematerial.

Midwest Gas, the Waterloo, Iowa, gasoperator at the time of the explosion and fire,had experienced at least three other significantfailures involving Century pipe. The most recentfailures, occurring between 1992 and 1994,prompted the company to collect samples of theCentury material for independent laboratorytesting. Samples were being gathered for testingat the time of the Waterloo accident. Thesubsequent laboratory report indicated that theCentury piping had poor resistance to slowcrack growth.

Midwest Gas’s subsequent analysis of thecompany’s leakage history concluded that itsinstallations with Century piping had failurerates significantly higher than those with pipingfrom other manufacturers. Midwest Gas hadreceived warnings from two pipe fittingmanufacturers against use of their products withCentury pipe because of Century pipe’ssusceptibility to brittle-like cracking. Thecurrent operating company in the Waterloo,Iowa, area, MidAmerican Energy, has, since the

accident, replaced all the identified Centurypiping in its gas pipeline system.

The Safety Board concludes that plasticpipe extruded by Century Utility Products, Inc.,and made from Union Carbide’s DHDA 2077Tan resin has poor resistance to brittle-likecracking under stress intensification, and thischaracteristic contributed to the Waterloo, Iowa,accident.

The Safety Board believes that RSPAshould notify pipeline system operators whohave installed polyethylene gas piping extrudedby Century Utility Products, Inc., from UnionCarbide Corporation DHDA 2077 Tan resin ofthe piping’s poor brittle-crack resistance. TheSafety Board further believes that RSPA shouldrequire these operators to develop a plan toclosely monitor the performance of this pipingand to identify and replace, in a timely manner,any of the piping that indicates poorperformance based on such evaluation factors asinstallation, operating, and environmentalconditions; piping failure characteristics; andleak history.

Strength Downturn and BrittleCharacteristics

While Century piping has been identifiedspecifically as being subject to brittle-likecracking (slow crack growth), evidence suggeststhat much of the early polyethylene piping,depending on the brands, may be moresusceptible to such cracking than originallythought and thus may also be subject topremature failure.

The principal process used in the UnitedStates to rate the strength of plastic pipingmaterials has been, and remains, the procedurethis report has referred to as the PPI procedure.The PPI procedure, which was developed in theearly 1960s, involved subjecting test piping todifferent stress values and recording how muchtime elapsed before the piping ruptured. Theresulting data were then plotted, and a best-fitstraight line was derived to represent thematerial’s decline in rupture resistance as itstime under stress increased.

To meet the requirements of the PPIprocedure, at least one tested sample had to be

30

able to withstand some level of hoop stresswithout rupturing for at least 10,000 hours, orslightly more than 1 year. The straight lineplotted describing the data for this material wasextrapolated out by a factor of 10, to 100,000hours (about 11 years). The point at which thesloping straight line intersected the 100,000-hour point indicated the appropriate hydrostaticdesign basis for this material.

A key assumption characterized theassignment of a hydrostatic design basis underthe PPI procedure: The procedure assumed thatthe gradual decline in the strength of plasticpiping material as it was subjected to stress overtime would always be described by a straightline. In the early 1960s, the industry had hadlittle long-term experience with plastic piping,and a straight line seemed to represent theresponse of the material to laboratory stresstesting. With little other information on which tobase strength estimations, the straight-lineassumption appeared valid.

As experience grew with plastic pipingmaterials and as better testing methods weredeveloped, however, the straight-line assump-tions of the PPI procedure came to bechallenged. Elevated-temperature testing indi-cated that polyethylene piping can exhibit adecline in strength that does not follow astraight line path but instead describes a down-turn, as shown in figure 9. The differencebetween the actual (falloff) and projected(straight line) strengths became even more pro-nounced as the lines were extrapolated beyond100,000 hours. The timing and slope of thedownturn varied by pipe formulation and manu-facturer.

Piping manufacturers addressed this issueby improving their formulations to delay onsetof the downturn in strength. At the same time,the PPI procedure was improved to reflect thefact that elevated-temperature testing, byaccelerating the fracture process, provided agood representation of the true long-termstrength of the tested material at 73 °F. By 1986,the PPI adopted a requirement to exclude anymaterials that deviated from the straight-linepath to at least 100,000 hours at 73 °F.

The combination of more durable modernplastic piping materials and more realistic

strength testing has rendered the strength ratingsof modern plastic piping much more reliable.Unfortunately, much of the early plastic pipingwas sold and installed with expectations ofstrength and long-term performance that, be-cause they were based on questionableassumptions about long-term performance, maynot have been valid. This is borne out by datafrom a variety of sources. The history ofstrength rating requirements, a review of thepiping properties and literature, and observa-tions of several experts with extensiveexperience in plastic piping, all suggest thatmuch of the polyethylene pipe, depending uponthe brands, manufactured from the 1960sthrough the early 1980s fails at lower stressesand after less time than originally projected. TheSafety Board therefore concludes that the pro-cedure used in the United States to rate thestrength of plastic pipe may have overrated thestrength and resistance to brittle-like cracking ofmuch of the plastic pipe manufactured and usedfor gas service from the 1960s through the early1980s.

Another important assumption of the designprotocol for plastic pipe involved the ductility ofthe materials. It was assumed, based on short-term tests, that plastic piping had long-termductile properties. Ductile material, by bending,expanding, or flexing, can redistribute stressconcentrations better than can brittle material,such as cast iron. Notable from results of testsperformed under the PPI procedure was thatthose short-term stress ruptures in the testingprocess tended to be characterized by substantialmaterial deformation in the area of the rupture.This deformation described a material withobvious ductile properties. Under prolongedtesting, however, as time-to-failure increased,some stress ruptures in some materials occurredas slit failures that, because they were notaccompanied by substantial deformation, weremore typical of brittle-like failures. These slit orbrittle-like failures were characterized by crackinitiation and slow crack growth. The PPIprocedure did not distinguish between ductilefractures and slit fractures and assumed thatboth failures would be described by the samestraight line.

The assumption of ductility of plastic pipinghad important safety ramifications. For example,a number of experts believed it was safe to

31

design plastic piping installations based onstresses primarily generated by internal pressureand to give less consideration to stressintensification generated by external loading.Ductile material reduces stress intensification bylocalized yielding, or deformation.

As noted previously, laboratory datasupported the strength rating assigned to DHDA2077 Tan resin by the process used at the timeto rate strength; nevertheless, the materialshowed evidence of early ductile-to-brittletransition. The fact that the process used tomeasure the long-term durability of pipingmaterials did not reveal the prematuresusceptibility to brittle-like cracking of theDHDA 2077 Tan material highlights theweaknesses of the process in use at the time.More significantly, it calls into question thedurability of other early materials that wererated using the same process and that remain inservice today. This concern is heightened by thefact that, in addition to the Waterloo accidentinvolving Century pipe and DHDA 2077 Tanresin, numerous other accidents investigated ordocumented by the Safety Board have suggestedthat brittle-like cracking occurs in older plasticpiping at significant rates.

Stress intensification has been an elementcommon to many plastic gas pipeline accidentsinvestigated by the Safety Board. The prematuretransition of plastic piping from ductile failuresto brittle failures appears to have littleobservable adverse impact on the serviceabilityof plastic piping except in those instances inwhich the piping is subjected to externalstresses. Rock impingement, soil settlement, andexcess pipe bending are among the potentialsources of stress intensification, and thecombination of brittle piping and externalstresses can lead to significant rates of failures.These failures can, in turn, lead to seriousaccidents. The Safety Board therefore concludesthat much of the plastic pipe manufactured andused for gas service from the 1960s through theearly 1980s may be susceptible to prematurebrittle-like failures when subjected to stressintensification, and these failures represent apotential public safety hazard.

The Safety Board believes that RSPAshould determine the extent of the susceptibilityto premature brittle-like cracking of older plastic

piping (beyond that piping marketed by CenturyUtility Products, Inc.) that remains in use for gasservice nationwide. RSPA should then informgas system operators of the findings and requirethem to closely monitor the performance of theolder plastic piping and to identify and replace,in a timely manner, any of the piping thatindicates poor performance based on suchevaluation factors as installation, operating, andenvironmental conditions; piping failurecharacteristics; and leak history. Becausematerials other than polyethylene have beenused in plastic pipe for gas service, and eventhough the Safety Board has not examined thosematerials in depth, RSPA would do well toaddress those other plastic piping materials stillin gas service.

The Safety Board further believes thatRSPA should immediately notify those Statesand territories with gas pipeline safety programsof the susceptibility to premature brittle-likecracking of much of the plastic pipingmanufactured from the1960s through the early1980s and of the actions that RSPA will requireof gas system operators to monitor and replacepiping that indicates unacceptable performance.

Information Dissemination Within theGas Industry

As noted earlier, much of the polyethylenepipe, depending upon the brands, from the1960s through the early 1980s may besusceptible to premature brittle-like failureswhen subjected to stress intensification. Poorresistance to crack initiation and slow crackgrowth in the face of stress intensification cantranslate into a higher incidence of leaks and adecrease in public safety.

Premature brittle-like cracking in plasticpiping is a complex phenomenon. Thosepipelines operators who wish to study thephenomenon can gain a basic understanding ofbrittle-like cracking by researching the technicalliterature, but without direct and straightforwardcommunication to pipeline operators aboutbrands of piping and conditions that increase thelikelihood of brittle cracking, many pipelineoperators may not have the knowledge to makegood decisions affecting public safety. Some ofthese key decisions include how often to

32

conduct leak surveys and whether to repair orreplace portions of pipeline systems.

Frequently, piping manufacturers, becausethey can receive feedback from a number ofcustomers, are the first to learn of systemicproblems with their products. For smalloperators, contact with a manufacturer may bethe major source of outside communicationabout poorly performing products.Unfortunately, while manufacturers have a highdegree of technical expertise regarding theirproducts, they may also tend to aggressivelypublicize the best performance characteristics oftheir products while only reluctantlyacknowledging weaknesses. The Safety Board isaware of only a very few cases in whichmanufacturers of resin or pipe have formallynotified the gas industry of materials havingpoor resistance to brittle cracking.

Thus, although reputable manufacturerscommonly provide essential technical assistanceand serve as partners to pipeline operators,operators are still responsible for evaluating anddetermining which products are most likely tomaintain the integrity of their pipeline systems.Furthermore, perhaps because the possibility ofpremature failure of plastic piping due to brittle-like cracking has not been fully appreciatedwithin the industry and the scope of thepotential problem has not been fully measured,the Federal Government has not providedinformation on this issue to gas systemoperators. The Safety Board concludes that gaspipeline operators have had insufficientnotification that much of the plastic pipemanufactured and used for gas service from the1960s through the early 1980s may besusceptible to brittle-like cracking and thereforemay not have implemented adequate pipelinesurveillance and replacement programs for theirolder piping.

In the view of the Safety Board,manufacturers of resin and pipe should do moreto notify pipeline operators about the poorbrittle-crack resistance of some of their pastproducts. The PPI is the manufacturers’organization that covers most of the major resinand pipe producers, many of whom havemanufactured resin and pipe for several years.Although manufacturers of some of the worstperforming materials and piping products may

not have survived and therefore may not becurrent members of the PPI, the currentmembers of the PPI have produced much, if notmost, of the plastic piping and materials used inthe manufacture of plastic piping over manyyears. The Safety Board therefore believes thatthe PPI should advise its members to notifypipeline system operators if any of their pipingproducts, or materials used in the manufactureof piping products, currently in service fornatural gas or other hazardous materials indicatepoor resistance to brittle-like failure.

In the interest of public safety and in orderfor the Federal Government to fully exercise itsoversight responsibilities, the Safety Boardbelieves that RSPA should, in cooperation withthe manufacturers of products used in thetransportation of gases or liquids regulated bythe OPS, develop a mechanism by which theOPS will receive copies of all safety-relatednotices, bulletins, and other communicationsregarding any defect, unintended deviation fromdesign specification, or failure to meet expectedperformance of any piping or piping productthat is now in use or that may be expected to bein use for the transport of hazardous materials.

Over a number of years, the GRI hasdeveloped a significant amount of data on olderplastic piping, but it has published the data incodified terms. Without a way to associatecodes with specific products, the average gaspipeline operator could not make effective useof the data. The Safety Board concludes that,even though the GRI has developed a significantamount of data about older plastic piping usedfor gas service, because the data have beenpublished in codified terms, the information isnot sufficiently useful to gas pipeline systemoperators. The Safety Board believes that theGRI should publish the codes used to identifyplastic piping products in previous GRI studiesto make the information contained in thesestudies more useful to pipeline system operators.

Installation Standards and PracticesBecause of the large safety factor80 used in

the design equation, even many of the materials

80Technically, this term should be “design factor.”

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having early downturns in strength appear,absent stress intensification, to have the capacityto provide good service. Unfortunately, stressintensification, which can take many forms, hasbeen found in a number of gas piping systems.

Almost all of the plastic pipeline accidentsthe Safety Board has investigated involvingbrittle-like cracking have been linked to stressintensification generated by external forcesacting on the pipe. Examples of conditions thatcan generate stress intensification includedifferential earth settlement, particularly atconnections with more rigidly anchored fittings;excessive bending as a result of installationconfigurations, especially at fittings; and pointcontact with rocks or other objects.

As discussed below, much of the guidanceavailable to gas system operators for limitingstress intensification at plastic pipelineconnections to steel mains is inadequate orambiguous. It is particularly significant thatnone of the steel tapping tee manufacturers hadpublished recommendations to safely limit shearand bending forces at connections where theirproducts are used. Based on its review of thisguidance and on the history of the plasticpipeline accidents it has investigated, the SafetyBoard concludes that, because guidancecovering the installation of plastic piping isinadequate for limiting stress intensification atplastic service connections to steel mains, manyof these connections may have been installedwithout adequate protection from shear andbending forces. The specific limitations ofexisting guidance are addressed in the sectionsthat follow.

Federal Regulations -- RSPA acknowl-edges that the regulation that requires gasservice lines to be installed so as to minimizeanticipated piping strain and external loadinglacks performance measurement criteria. TheSafety Board pointed out in a previous accidentinvestigation report81 that, although the OPSconsiders many of its pipeline safety regulationsto be performance-oriented requirements, many

81National Transportation Safety Board PipelineAccident Report--Kansas Power and Light CompanyNatural Gas Pipeline Accidents, September 16, 1988, toMarch 29, 1989 (NTSB/PAR-90/03).

are no more than general statements of requiredactions that do not establish any criteria againstwhich the adequacy of the actions taken can beevaluated. The Safety Board has further statedthat regulations that do not contain measurablestandards for performance make it difficult todetermine compliance with the requirements.The Safety Board therefore previously recom-mended that RSPA:

P-90-15Evaluate each of its pipeline safetyregulations to identify those that do notcontain explicit objectives and criteriaagainst which accomplishment of theobjective can be measured; to the extentpractical, revise those that are soidentified.

As a result of this safety recommendation,the OPS asked the National Association ofPipeline Safety Representatives liaison com-mittee to review the 20 regulations deemed to bethe least enforceable due to lack of clarity. TheSafety Board has encouraged RSPA to makesuch a review a periodic effort so that all of theregulations, not just the specified 20, arecontinually clarified. The last correspondence tothe Safety Board from the OPS regarding thisrecommendation was on March 8, 1993, and therecommendation has remained classified “Open--Acceptable Response.” In an October 31, 1997,letter to the OPS, the Safety Board inquired asto the status of 28 open safety recommendationsto RSPA, including P-90-15. The OPS has notyet provided a written response to the requestfor the status of P-90-15. The Safety Board willcontinue to follow the progress and urgecompletion of this recommendation. In themeantime, other elements of the gas pipelineindustry can take steps to enhance the protectionof vulnerable piping at connections, as outlinedbelow.

A.G.A. Plastic Pipe Manual for GasService -- A protective sleeve helps to shieldthe pipe at the connection point from bearingloads and shear forces, and controls themaximum bending. The A.G.A. Plastic PipeManual for Gas Service recommends installingprotective sleeves at connections of plastic pipe,but it does not directly address designing thesleeve to have the correct inner diameter andlength, or the need to position the sleeve

34

properly. Instead, it includes a sentencerecommending that manufacturers’ instructionsbe followed carefully. Such advice presumesthat the manufacturers’ instructions address de-signing the sleeve to have the correct innerdiameter and length, as well as positioning thesleeve properly, in order to limit the shear andbending forces at the connection. Unfortunately,since none of the steel tapping tee manufactur-ers recommend any precautions to limit shearand bending forces at the connection point, gaspipeline operators may not realize theimportance of determining these parameters.

The A.G.A. Plastic Pipe Manual for GasService does not provide an explanation for thefollowing sentence:

Installation of the tee outlet at angles upto 45° from the vertical or along the axisof the main as a ‘side saddle’ or ‘swingjoint’ may be considered to furtherminimize…stresses.

This sentence is subject to differentinterpretations and does not explain howstresses might be reduced. Moreover, many gassystem pipeline operators recognize thatinstalling services 90° from the main helps withfuture locating of the pipe and reduces thelikelihood of excessive bending, which couldgenerate excessive stress. In the view of theSafety Board, this sentence does not provideuseful guidance as it is written, and the A.G.A.Plastic Materials Committee would be welladvised to either expand on or delete thissentence.

A.G.A. Gas Engineering and OperatingPractices Series -- Illustrations from the GEOPseries show protective sleeves extending toundisturbed or compacted soil or to blocking.But these figures show the blocking positionedso that, under some conditions, either the edgeof the blocking or the edge of the protectivesleeve might provide a fixed contact point onthe service pipe. The Safety Board notes thatB31.8 and ASTM D2774 discourage supportingplastic pipe by the use of blocking. In the viewof the Safety Board, these illustrations wouldprovide better guidance if they were revised toeliminate showing the possibility of blocking orother fixed contact point supporting plastic pipe.

The Safety Board believes that the A.G.A.should revise its Plastic Pipe Manual for GasService and the Gas Engineering and OperatingPractices series to provide complete andunambiguous guidance for limiting stress atplastic pipe service connections to steel mains.

GPTC Guide for Gas Transmission andDistribution Piping Systems -- The SafetyBoard has previously noted that a protectivesleeve’s correct inner diameter and length areimportant to protect the piping from excessiveforces. The Safety Board even issued a safetyrecommendation that the GPTC Guide bemodified accordingly. As a result of this safetyrecommendation, the GPTC Guide now includesguidance under 49 CFR 192.361 to installprotective sleeves “designed for the specificconnection…to reduce stress concentrations.”Designing protective sleeves for the specificconnection is presumed to include designing thesleeve for the correct inner diameter and length,and may also include positioning the sleevecorrectly, since positioning the sleeve affects itseffective length. However, if steel tapping teemanufacturers do not address the parameters forsleeve design and positioning, gas pipelineoperators may not realize the importance ofdetermining these parameters. The guidancewould be much more useful to gas pipelineoperators if the GPTC included in the guide aspecific statement of the need to designprotective sleeves so that they will have thecorrect inner diameter and length, as well as theneed to properly position the sleeves.

Although the guide references the A.G.A.Plastic Pipe Manual for Gas Service in variouslocations, and this manual providesrecommendations on bending limits, the guidedoes not reference this manual under the guidematerial under 49 CFR 192.361. Therefore, theSafety Board believes that the GPTC shouldrevise the guide to include complete guidancefor the proper installation of plastic service pipeconnections to steel mains. The guidance shouldemphasize the need to limit pipe bending andshould include a discussion of the proper designand positioning of a protective sleeve to limitstress at the connection.

ASTM -- ASTM D2774 recommends the useof a protective sleeve, if needed to protectagainst possible differential settlement. The

35

standard practice additionally advisesconsultation with manufacturers, which wouldpresumably address designing the sleeve with aproper diameter and length, as well aspositioning the sleeve correctly. However, asnoted previously, none of the steel tapping teemanufacturers has recommended precautions tolimit stresses at the service to main connection;therefore, gas pipeline operators may not realizethe importance of determining these parameters.Consequently, the Safety Board believes that theASTM should revise ASTM D2774 toemphasize that a protective sleeve, in order to beeffective, must be of the proper length and innerdiameter for the particular connection and mustbe positioned properly.

Currently, manufacturers that provideprotective sleeves have their own criteria forsleeve lengths and diameters. Somemanufacturers’ criteria are based on limitingstress to a maximum safe value,82 while onemanufacturer has advised the Safety Board thatits sleeve is not designed to limit bending butonly to guard against shear forces at theconnection point. A published common criteriawould better motivate a wider spectrum ofmanufacturers and gas operators to applyscientific reasoning to their decisions onprotective sleeve use. A published commoncriteria would additionally provide guidance togas operators who provide their own sleevesrather than using manufacturer-supplied sleeves.The Safety Board therefore believes that theASTM should develop and publish standardcriteria for the design of protective sleeves tolimit stress intensification at plastic pipelineconnections.

Guidance Manual for Operators of SmallNatural Gas Systems -- The expressed purposeof RSPA’s Guidance Manual for Operators ofSmall Natural Gas Systems is to assistnontechnically trained persons who operatesmall gas systems. However, the manualprovides no caution against bending close to aplastic service connection to a steel main. Themanual recommends following manufacturers’

82Allman, W. B., “Determination of Stresses andStructural Performance in Polyethylene Gas Pipe andSocket Fittings Due to Internal Pressure and External SoilLoads,” 1975 Operating Section Proceedings, AmericanGas Association, 1975.

instructions and indicates that a properlydesigned sleeve should be used at thisconnection, which would address designing thesleeve with the proper diameter and length.However, as noted previously, none of the steeltapping tee manufacturers has recommendedprecautions to limit stresses at the service tomain connection; therefore, nontechnicallytrained persons may not realize the importanceof determining these parameters.

Because manufacturers’ recommendationsin the above areas are also currently inadequate,the Safety Board believes that RSPA shouldrevise its Guidance Manual for Operators ofSmall Natural Gas Systems to include morecomplete guidance for the proper installation ofplastic service pipe connections to steel mains.The guidance should address pipe bendinglimits and should emphasize that a protectivesleeve, in order to be effective, must be of theproper length and inner diameter for theparticular connection and must be positionedproperly.

Manufacturers’ Recommendations --Reliance on manufacturers’ recommendations isa common theme running through many of theprimary published sources of industry guidancefor limiting stress intensification on plasticpiping. CSR PolyPipe was relatively new toproviding polyethylene pipe to the gas market.When CSR PolyPipe entered the market, thethree other major polyethylene pipingmanufacturers had already published installationrecommendations to limit stress intensification,and plastic materials were vastly improvedcompared to earlier versions with respect toresistance to crack initiation and slow crackgrowth. CSR PolyPipe therefore saw less needto develop extensive recommendations. Andalthough CSR PolyPipe does not manufacturesteel tapping tees with compression ends forattachment to plastic services, it doesmanufacture the pipe that will be attached tosteel tapping tees via mechanical compressioncouplings. To facilitate the safe use of plasticpiping, the Safety Board believes that the PPI, ofwhich all four of the major piping producers aremembers, should advise its plastic pipemanufacturing members to develop and publishrecommendations for limiting shear and bendingforces at plastic service pipe connections to steelmains.

36

Compared to plastic piping manufacturers,steel tapping tee manufacturers may have muchless technical expertise regarding the strengthand failure modes of plastic pipe; however, steeltapping tee manufacturers, who have designedtheir rigid steel tees to connect to flexible plasticgas pipe, have a responsibility to provide recom-mendations for the safe use of their products. Ifa steel tee manufacturer believes thatinstallation options are dependent on the type ofplastic to be connected and that these optionscan be addressed only by the pipe manufacturer,the tee manufacturer has a responsibility to statethat in its literature and to provide the gassystem operator with direction for best using itsproduct safely.

The Safety Board therefore believes thatContinental, Dresser, Inner-Tite, and Muellershould develop and publish detailedrecommendations and instructions for limitingshear and bending forces at locations wheretheir steel tapping tees are used to connectservice pipe to steel mains. While gas systemoperators have the option of not acceptingmanufacturers’ recommendations, many gassystem operators rely on manufacturers toprovide installation recommendations for thesafe use of their products. With publishedrecommendations, gas system operators may befar less likely to overlook prudent constructionpractices, such as providing proper compactionand support, limiting bends, and usingprotective sleeves. Tee manufacturers may wishto make these published recommendations evenmore effective by packaging them with each teeshipped, thus ensuring that the gas operator orthe tee installer, or both, will have ready accessto them.

A Continental representative told the SafetyBoard that the protective sleeve it provides tocustomers as an option does not providesufficient clearance to allow field wrap to beapplied to the metallic portion under the sleeveas a way to prevent corrosion. The Safety Boardconcludes that the use of Continental tappingtees with Continental protective sleeves mayleave the tapping tees susceptible to corrosionbecause the sleeves do not provide sufficientclearance for the application of field wrap to themetallic steel tapping tee. The Safety Boardtherefore believes that Continental shouldprovide a means to ensure that use of

Continental-designed protective sleeves with thecompany’s steel tapping tees at plastic pipeconnections to steel mains does not compromisecorrosion protection for the connection.

Installation Issues at Site of WaterlooAccident -- Safety Board examination of thefracture surface and the failed pipe from theWaterloo accident revealed evidence of stressintensification. For example, the upper portionof the inside of the pipe showed the impressionof the edge of the tee stiffener, indicating thatthe top of the pipe had been pressed down. Thefailure of the pipe can be directly associatedwith this stressed area, which was characterizedby several brittle-like slow crack growthfractures that originated on or near the pipeinner wall just outside the depression associatedwith the tip of the tee stiffener. These slowcrack fractures propagated through the wall ofthe pipe.

The stress intensification noted in theWaterloo pipe was consistent with the pipe’shaving been subjected to shear and bendingforces generated primarily by soil settlement.83

Soil settlement is a common source of stressintensification for buried plastic pipelines, and itcan occur and contribute to a piping failure eventhough no observable voids are noted during asubsequent excavation. Ultimate settlement ofbackfill can take many years, and sometimes itonly occurs after periods of heavy rains (such asthe area experienced the previous year) or underadditional external loading (such as thatrepresented by truck traffic over theconnection).

The accident investigation could notdetermine whether the ground settlement atWaterloo occurred because of inadequatecompaction and support under the connection atthe time it was installed, or whether it occurreddespite initial adequate compaction and support.Nor could it be conclusively determinedwhether the amount of soil settlement was slightand generated relatively low stresses over a long

83The failed pipe also showed signs that the installedhorizontal curve may have generated horizontal bendingforces. Other factors contributing to stress at the connectionincluded the pipe’s internal pressure and may haveincluded residual stresses inside the wall of the piperesulting from the manufacturing process.

37

period of time, or whether the soil settlementwas substantial and generated relatively highstresses over a relatively short period of time.Because of these uncertainties, investigatorscould not determine how much more resistanceto crack initiation and slow crack growth thepipe would have needed to have successfullyresisted the stresses to which it was subjected.

MidAmerican Energy, at the time of thisaccident investigation, had no installationstandard that called for firm compacted supportunder plastic service connection points to steelmains. MidAmerican Energy connected plasticservice pipe to mains via factory-joined plastic-to-steel transition fittings. As noted previously,the manufacturers for these specialty fittings,unlike steel tapping tee manufacturers, haveprotective sleeves available. AlthoughMidAmerican Energy designed its ownprotective sleeves for this application, it did sowithout a design criteria for length or innerdiameter, or for positioning the protectivesleeves. Without such criteria, MidAmericanEnergy may reduce the sleeve’s effectiveness inlimiting stress intensification. The Safety Boardconcludes that, because MidAmerican Energy’sgas construction standards do not establish well-defined criteria for supporting plastic pipeconnections to steel mains or for designing orinstalling its protective sleeves at theseconnections, these standards do not ensure thatconnections will be adequately protected fromstress intensification. The Safety Board believesthat MidAmerican Energy should modify its gasconstruction standards to require (1) firmcompacted support under plastic serviceconnections to steel mains, and (2) the properdesign and positioning of protective sleeves atthese connections.

The service involved in the Waterlooaccident was installed with a horizontal bendthat was sharper than that recommended bycurrent gas industry guidance recommendations;however, the bend may have been installed inthe direction of the residual coil bend. Gasindustry recommendations do not addressresidual bending in the pipe, even though plasticpiping is often delivered to job sites in bandedcoils, which leaves some residual bending in thepiping even after the bands are removed.Installing coiled pipe with any necessarybending in the direction of the residual bend

may be a good practice to limit stresses.Conversely, bending pipe against the directionof the residual coil bend, even if the resultingbend is in accordance with gas industryrecommendations, will induce greater stresses.

Plastic piping manufacturers continue tohave the best combination of technical expertiseand practical knowledge for determining bendradius recommendations. Therefore, the SafetyBoard believes that the PPI should advise itsplastic pipe manufacturing members to revisetheir pipeline bend radius recommendations asnecessary to take into account the effects ofresidual coil bends in plastic piping.

Gas System Performance MonitoringFederal regulations require that gas pipeline

system operators have in place an ongoingprogram to monitor the performance of theirpiping systems. Before the Waterloo accident,Midwest Gas developed only a limitedcapability for monitoring and analyzing thecondition of its gas system. For example, thecompany did not statistically correlate failurerates to the amounts of installed pipe providedby specific manufacturers. The design of theprogram meant that the relatively few areas withhigh failure rates (for example, those withCentury pipe) were aggregated with andtherefore masked by the large number of plasticpiping installations that had low failure rates.Thus, the Midwest Gas surveillance program didnot reveal the high failure rates associated withCentury pipe. Only after the accident didMidwest Gas identify the Century pipe withinits pipeline system as having high failure rates,even though the company could have collectedand processed the same type of data and reachedthe same determination before the accident. IfMidwest Gas had further correlated its data toyears of installation, it may have also been ableto examine the effects of its changinginstallation methods or changes in performancewith different manufacturers through the years.

The Safety Board concludes that, before theWaterloo accident, the systems used by MidwestGas Company for tracking, identifying, andstatistically characterizing plastic piping failuresdid not permit an effective analysis of systemfailures and leakage history. The Safety Boardfurther concludes that if, before the Waterloo

38

accident, Midwest Gas had had an effectivesurveillance program that tracked and identifiedthe high leakage rates associated with Centurypiping when subjected to stress intensification,the company could have implemented areplacement program for the pipe and may havereplaced the failed service connection before theaccident.

Since the accident, MidAmerican Energyhas revised its systems, adding parameters toprovide the company with added capability tosort failures. However, MidAmerican Energyhas not chosen parameters that will allow anadequate analysis of its plastic piping systemfailures and leakage history. For example, thegeneric “improper installation” is a parameter tobe linked to leaks; however, no parameters havebeen added for the presence, lack, improperdesign, or improper placement of a protectivesleeve. And no parameters have been added tolink leaks to squeeze locations, improperjoining, or items to differentiate betweeninsufficient support and excessive installedbending. The Safety Board therefore concludesthat MidAmerican Energy’s current systems fortracking, identifying, and statistically charac-terizing plastic piping failures do not enable aneffective analysis of system failures and leakagehistory.

The Safety Board believes thatMidAmerican Energy should, as a basis for thetimely replacement of its plastic piping systemsthat indicate unacceptable performance, reviewits existing plastic piping surveillance andanalysis program and make the changesnecessary to ensure that the program is based onsufficiently precise factors such as pipingmanufacturer, installation date, pipe diameter,geographical location, and conditions andlocations of failures.

An effective surveillance program wouldinclude the data base inputs that would allow thecompany to adequately monitor and characterizethe types and causes of plastic piping fieldfailures. The A.G.A. Plastic Pipe Manual forGas Service recommends the use of a form forrecording necessary information on plasticpiping failures; this form may be helpful toMidAmerican Energy as it decides which datafields would be necessary to provide for anadequate analysis of its plastic piping system

failures and leakage history. The A.G.A. PlasticPipe Manual for Gas Service furtherrecommends collecting this information, thenperforming visual examinations of the type andcause of failure and, in some instances, alaboratory analysis. The above steps may helpMidAmerican Energy comprehensively monitorand address parts of its plastic pipelinesystem—other than those installations withCentury pipe—that may also indicateunacceptable performance.

In a previous accident investigation report,84

the Safety Board pointed out that manyoperators had not established procedures tocomply with Federal regulations requiringsurveillance and investigation of failures. TheSafety Board recommended that RSPA:

P-90-14Emphasize, as a part of OPS inspectionsand during training and State monitor-ing programs, the actions expected ofgas operators to comply with the con-tinuing surveillance and failureinvestigation, including laboratory ex-amination requirements.

In a letter to the Safety Board, RSPAresponded that the TSI had increased emphasison gas surveillance and failure investigation inthe operations block of its industry seminarsheld across the country. The letter stated that theTSI would incorporate a discussion of accidentanalysis into a new hazardous liquids seminarthat was to be presented for the first time in FY1992. Additionally, RSPA noted that it plannedto place additional emphasis on continuingsurveillance and failure investigationrequirements in its new inspection forms at thetime of the next revision. Based on thisresponse, the Safety Board classified SafetyRecommendation P-90-14 “Closed—AcceptableAction.”

Despite the RSPA response to this safetyrecommendation, for a variety of reasons—in-cluding the inadequate performance monitoring

84National Transportation Safety Board PipelineAccident Report--Kansas Power and Light CompanyNatural Gas Pipeline Accidents, September 16, 1988, toMarch 29, 1989 (NTSB/PAR-90/03).

39

programs found at Midwest Gas/MidAmericanEnergy, the susceptibility to brittle cracking ofmuch of the polyethylene piping installedthrough the early 1980s, deficiencies noted ingas industry communications regarding poorlyperforming brands of polyethylene piping, anddifferences noted in the performance of dif-ferent types and brands of polyethylenepiping—RSPA may need to do more. Gassystem operators may need to be advised onceagain of the importance of complying withFederal requirements for piping systemsurveillance and analyses. As is the case witholder piping, an effective general pipelinesurveillance program would be based on factors

such as piping manufacturer, installation date,pipe diameter, operating pressure, leak history,geographical location, modes of failure (such asbending, inadequate support, rock impingement,or improper joining), location of failure (such asat the main to service or at pipe squeezelocations), and other factors such as the presence,absence, or misapplication of a sleeve. Aneffective program would also evaluate past pipingand components installed, as well as pastinstallation practices, to provide a basis for thereplacement, in a planned, timely manner, ofplastic piping systems that indicate unacceptableperformance.

40

1. Plastic pipe extruded by Century UtilityProducts, Inc., and made from UnionCarbide’s DHDA 2077 Tan resin has poorresistance to brittle-like cracking understress intensification, and this characteristiccontributed to the Waterloo, Iowa, accident.

2. The procedure used in the United States torate the strength of plastic pipe may haveoverrated the strength and resistance tobrittle-like cracking of much of the plasticpipe manufactured and used for gas servicefrom the 1960s through the early 1980s.

3. Much of the plastic pipe manufactured andused for gas service from the 1960s throughthe early 1980s may be susceptible topremature brittle-like failures whensubjected to stress intensification, and thesefailures represent a potential public safetyhazard.

4. Gas pipeline operators have had insufficientnotification that much of the plastic pipemanufactured and used for gas service fromthe 1960s through the early 1980s may besusceptible to brittle-like cracking andtherefore may not have implemented ade-quate pipeline surveillance and replacementprograms for their older piping.

5. Even though the Gas Research Institute hasdeveloped a significant amount of dataabout older plastic piping used for gasservice, because the data have beenpublished in codified terms, the informationis not sufficiently useful to gas pipelinesystem operators.

6. Because guidance covering the installationof plastic piping is inadequate for limitingstress intensification at plastic serviceconnections to steel mains, many of theseconnections may have been installed

without adequate protection from shear andbending forces.

7. Because MidAmerican Energy Corpora-tion’s gas construction standards do notestablish well-defined criteria for supportingplastic pipe connections to steel mains or fordesigning or installing its protective sleevesat these connections, these standards do notensure that connections will be adequatelyprotected from stress intensification.

8. Before the Waterloo, Iowa, accident, thesystems used by Midwest Gas Company fortracking, identifying, and statisticallycharacterizing plastic piping failures did notpermit an effective analysis of systemfailures and leakage history.

9. If, before the Waterloo accident, MidwestGas Company had had an effective surveil-lance program that tracked and identifiedthe high leakage rates associated withCentury Utility Products, Inc., piping whensubjected to stress intensification, the com-pany could have implemented a replacementprogram for the pipe and may have replacedthe failed service connection before theaccident.

10. MidAmerican Energy Corporation’s currentsystems for tracking, identifying, andstatistically characterizing plastic pipingfailures do not enable an effective analysisof system failures and leakage history.

11. The use of Continental Industries, Inc., tap-ping tees with the company’s protectivesleeves may leave the tapping teessusceptible to corrosion because the sleevesdo not provide sufficient clearance for theapplication of field wrap to the metallicsteel tapping tee.

CONCLUSIONS

41

As a result of this special investigation, theNational Transportation Safety Board makes thefollowing safety recommendations:

--to the Research and Special ProgramsAdministration:

Notify pipeline system operators whohave installed polyethylene gas pipingextruded by Century Utility Products,Inc., from Union Carbide CorporationDHDA 2077 Tan resin of the piping’spoor brittle-crack resistance. Requirethese operators to develop a plan toclosely monitor the performance of thispiping and to identify and replace, in atimely manner, any of the piping thatindicates poor performance based onsuch evaluation factors as installation,operating, and environmental condi-tions; piping failure characteristics; andleak history. (P-98-1)

Determine the extent of thesusceptibility to premature brittle-likecracking of older plastic piping (beyondthat piping marketed by Century UtilityProducts, Inc.) that remains in use forgas service nationwide. Inform gassystem operators of the findings andrequire them to closely monitor theperformance of the older plastic pipingand to identify and replace, in a timelymanner, any of the piping that indicatespoor performance based on suchevaluation factors as installation, oper-ating, and environmental conditions;piping failure characteristics; and leakhistory. (P-98-2)

Immediately notify those States andterritories with gas pipeline safetyprograms of the susceptibility topremature brittle-like cracking of muchof the plastic piping manufactured fromthe 1960s through the early 1980s andof the actions that the Research andSpecial Programs Administration willrequire of gas system operators to

monitor and replace piping thatindicates unacceptable performance.(P-98-3)

In cooperation with the manufacturersof products used in the transportation ofgases or liquids regulated by the Officeof Pipeline Safety, develop a mecha-nism by which the Office of PipelineSafety will receive copies of all safety-related notices, bulletins, and othercommunications regarding any defect,unintended deviation from designspecification, or failure to meetexpected performance of any piping orpiping product that is now in use or thatmay be expected to be in use for thetransport of hazardous materials.(P-98-4)

Revise the Guidance Manual forOperators of Small Natural GasSystems to include more completeguidance for the proper installation ofplastic service pipe connections to steelmains. The guidance should addresspipe bending limits and shouldemphasize that a protective sleeve, inorder to be effective, must be of theproper length and inner diameter for theparticular connection and must bepositioned properly. (P-98-5)

--to the Gas Research Institute:

Publish the codes used to identifyplastic piping products in previous GasResearch Institute studies to make theinformation contained in these studiesmore useful to pipeline system opera-tors. (P-98-6)

--to the Plastics Pipe Institute:

Advise your members to notify pipelinesystem operators if any of their pipingproducts, or materials used in themanufacture of piping products,currently in service for natural gas or

RECOMMENDATIONS

42

other hazardous materials indicate poorresistance to brittle-like failure. (P-98-7)

Advise your plastic pipe manufacturingmembers to develop and publish recom-mendations for limiting shear andbending forces at plastic service pipeconnections to steel mains. (P-98-8)

Advise your plastic pipe manufacturingmembers to revise their pipeline bendradius recommendations as necessary totake into account the effects of residualcoil bends in plastic piping. (P-98-9)

--to the Gas Piping Technology Committee:

Revise the Guide for Gas Transmissionand Distribution Piping Systems toinclude complete guidance for theproper installation of plastic servicepipe connections to steel mains. Theguidance should emphasize the need tolimit pipe bending and should include adiscussion of the proper design andpositioning of a protective sleeve tolimit stress at the connection. (P-98-10)

--to the American Society for Testing andMaterials:

Revise ASTM D2774 to emphasize thata protective sleeve, in order to beeffective, must be of the proper lengthand inner diameter for the particularconnection and must be positionedproperly. (P-98-11)

Develop and publish standard criteriafor the design of protective sleeves tolimit stress intensification at plasticpipeline connections. (P-98-12)

--to the American Gas Association:

Revise your Plastic Pipe Manual forGas Service and your Gas Engineering

and Operating Practices series toprovide complete and unambiguousguidance for limiting stress at plasticpipe service connections to steel mains.(P-98-13)

--to MidAmerican Energy Corporation:

Modify your gas construction standardsto require (1) firm compacted supportunder plastic service connections tosteel mains, and (2) the proper designand positioning of protective sleeves atthese connections. (P-98-14)

As a basis for the timely replacement ofyour plastic piping systems that indicateunacceptable performance, review yourexisting plastic piping surveillance andanalysis program and make the changesnecessary to ensure that the program isbased on sufficiently precise factorssuch as piping manufacturer, installationdate, pipe diameter, geographicallocation, and conditions and locations offailures. (P-98-15)

--to Continental Industries, Inc.:

Provide a means to ensure that the useof your protective sleeves with yourtapping tees at plastic pipe connectionsto steel mains does not compromisecorrosion protection for the connection.(P-98-16)

--to Continental Industries, Inc. (P-98-17):

--to Dresser Industries, Inc. (P-98-18):

--to Inner-Tite Corporation (P-98-19):

--to Mueller Company (P-98-20):

Develop and publish recommendations andinstructions for limiting shear and bendingforces at locations where your steel tappingtees are used to connect plastic service pipeto steel mains.

43

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

JAMES E. HALLChairman

ROBERT T. FRANCIS IIVice Chairman

JOHN A. HAMMERSCHMIDTMember

JOHN J. GOGLIAMember

GEORGE W. BLACK, JR.Member

April 23, 1998

APPENDIX A 45

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Pipeline Accident Number: DCA-95-MP-001Type of System: Gas distributionAccident Type: Explosion and FireLocation: Waterloo, IowaDate and Time: October 17, 1994; 10:07 a.m. localOwner/Operator: Midwest Gas Company1

Fatalities/Injuries: Six fatalities and seven non-fatal injuriesDamage: $250,000Material Released: Natural GasPipeline Pressure: 25 pounds per square inch, gauge (psig)Component Affected: 1/2-inch plastic pipe at steel tapping tee mechanical

compression connection to steel main

1Because of a series of organizational changes and mergers, the name of the owner/operator of the gas

system at Waterloo, Iowa, has changed over the years. In 1971, Iowa Public Service Company installed the gasservice that ultimately failed. At the time of the accident, the gas system operator was known as Midwest GasCompany, while the current operator’s name is MidAmerican Energy Corporation.

The Accident

At 10:07 a.m. central daylight savings time on Monday, October 17, 1994, a natural gasexplosion and fire destroyed a one-story, wood frame building in Waterloo, Iowa. The force ofthe explosion scattered debris over a 200-foot radius.

Six persons inside the building died, and one person sustained serious injuries. Threepersons working in an adjacent building sustained minor injuries when a wall of the buildingcollapsed inward from the force of the explosion. The explosion also damaged nine parked cars.A person in a vehicle who had just exited the adjacent building suffered minor injuries.Additionally, two firefighters sustained minor injuries during the emergency response. Two othernearby buildings also sustained structural damage and broken windows.

Site Information

The destroyed building was a neighborhood tavern known as Buzz’s Bar. Adjacent to andeast of the bar was Woodland Pattern Company, which was provided gas service by a 1/2-inch-diameter plastic polyethylene service pipeline. The service pipeline was installed by Iowa PublicService Company on September 3, 1971, and was operated at a maximum pressure of 25 psig.

APPENDIX A46

The underground pipeline connected with the steel gasmain and entered the Woodland Pattern Companybuilding between Buzz’s Bar and the Woodland PatternCompany.

The area between Buzz’s Bar and WoodlandPattern Company was unpaved and, according to thosefamiliar with the location, was regularly used by beertrucks making deliveries to Buzz’s Bar and bysemitrailers delivering materials to Woodland PatternCompany. These trucks had been seen to drive over thearea of the piping assembly that cracked. At varioustimes, beer trucks servicing Buzz’s Bar had beenobserved to park directly over the location of the pipebreak. One witness stated that a beer delivery truck hadbeen parked over the area of the pipe break atapproximately 7:00 a.m. on the day of the accident.

Excavations following the accident uncovered a4-inch-diameter steel main at a depth of about 3 feet.Welded to the top of the main was a steel tapping tee with markings indicating that the tee hadbeen manufactured by Continental Industries, Inc. (Continental). Connected to the steel tee was a1/2-inch-diameter plastic service pipe leading to Woodland Pattern Company. Markings on theplastic pipe indicated that it was a medium-density polyethylene material manufactured on June11, 1970, in accordance with American Society for Testing and Materials (ASTM) standardD2513, and marketed by Century Utility Products, Inc. (Century). A circumferential crackthrough the plastic pipe was found at the tip of the tee’s internal stiffener that protruded beyondthe tee’s coupling nut. A 1- to 2-foot-diameter “hard ball” surrounded the cracked pipe.2

Because Safety Board investigators did not arrive at the accident site until afterexcavation of the failed pipe, investigators had to consult several sources to determine thecondition of the piping at the time of excavation. Photographs of the excavation, a Waterloo FireDepartment video tape, and several witnesses all indicated that the downstream portion of theplastic pipe was found broken off and vertically displaced below the plastic pipe portion stillattached to the steel tee. However, an Iowa State Fire Marshall’s Office investigator, whodirected and participated in the excavation, reported that the pipe was displaced by theexcavation activities. That investigator also reported no observed voids in the soil under thefailed assembly.

Service-to-main connection at site of Waterloo accident.

MidAmerican Energy estimated that the steel tee on the steel main was installed so thatthe polyethylene pipe exited the tee at an approximate 30° angle to the steel main. (See figure.)

2A “hard ball” is a term used in the gas industry for a soil condition where leaking natural gas over a period

of time dries and hardens the soil adjacent to the leak.

4 “ Dia m e te r Ste e lM a in

Ste e l Ta p p ing Te e

1 /2 “ Pla stic Se rvic ePip e

3 2 In c h Be nd Ra d iu s

~30

o

Service-to-main connection at siteof Waterloo accident.

APPENDIX A 47

The plastic service pipe leaving the tee immediately curved horizontally. After a portion of thepipe was taken to the laboratory for testing, the bend radius was measured at about 34 inches.Based on field conditions and photos, MidAmerican Energy has estimated the original installedhorizontal bend radius to be approximately 32 inches.3 This bend is sharper than currentlyrecommended by industry guidelines for modern piping adjacent to fittings. However, a formerIowa Public Service Company employee stated that Iowa Public Service Company, in an effort toreduce the stress at the connection point, often attempted to install polyethylene services at anangle to the main to match the residual bend left after uncoiling the pipe.4 This former employeestated that no set time was prescribed to allow for complete relaxing of the pipe, but that the pipewould be placed in the ditch, and the crews would weld the tee at what they judged to be theappropriate angle, in consideration of the natural bend of the pipe.

Also immediately from the tee outlet, the polyethylene bent downward. The tee outlet didnot have a protective sleeve to reduce shear and bending forces at the connection.

Tests and Examination

Samples recovered from the plastic service line underwent several laboratory tests underthe supervision of the Safety Board. Two of these tests were meant to roughly gauge the pipe’ssusceptibility to brittle-like cracking. These tests were a compressed ring environmental stresscrack resistance (ESCR) test in accordance with ASTM F1248 and a notch tensile test known asa PENT test that is now ASTM F1473. Lower failure times in these tests indicate greatersusceptibility to brittle-like cracking under test conditions. The ESCR testing of 10 samples fromthe pipe yielded a mean failure time of 1.5 hours, and the PENT testing of 2 samples yieldedfailure times of 0.6 and 0.7 hours. Test values this low have been associated with materialshaving poor performance histories5 characterized by high leakage rates at points of stressintensification due to crack initiation and slow crack growth typical of brittle-like cracking.

To facilitate identification, the fracture surfaces were divided into two regions, A and B,around the circumference of the failed pipe. If a cross section of the pipe, looking toward the tee,were superimposed on a clock face, region A would extend from approximately the 9:00 positionup across the top and down to about 1:30, with the center of the region at about 11:15. Region Btook up the remainder of the pipe surface, extending from about the 1:30 position down acrossthe bottom and up to 9:00.

3Polyethylene pipe installed with a bend often, over time, permanently deforms in the direction of the bend.

This permanent deformation partially reduces the stresses generated by the bending forces. When the pipe is releasedfrom its installation configuration, the pipe can straighten to some extent.

4MidAmerican Energy has indicated that Iowa Public Service’s plastic service pipe was received in coilsfrom Century. After uncoiling the pipe, some residual bending remains. The amount of residual bending depends onthe factory coiling conditions.

5Uralil, F. S., et al., The Development of Improved Plastic Piping Materials and Systems for Fuel GasDistribution—Effects of Loads on the Structural and Fracture Behavior of Polyolefin Gas Piping, Gas ResearchInstitute Topical Report, 1/75 - 6/80, NTIS No. PB82-180654, GRI Report No. 80/0045, 1981, and Hulbert, L. E.,Cassady, M. J., Leis, B. N., Skidmore, A., Field Failure Reference Catalog for Polyethylene Gas Piping, AddendumNo. 1, Gas Research Institute Report No. 84/0235.2, 1989, and Brown, N. and Lu, X., “Controlling the Quality of PEGas Piping Systems by Controlling the Quality of the Resin,” Proceedings Thirteenth International Plastic Fuel GasPipe Symposium, pp 327-338, American Gas Association, Gas Research Institute, Battelle Columbus Laboratories,1993.

APPENDIX A48

The fracture in region A was located immediately outside the tee’s internal stiffener. Thecrack was perpendicular to the pipe wall and directly in line with the end of the tee’s internalstiffener. The inside surface of the pipe throughout region A was characterized by acircumferential impression from the tip of the tee’s stiffener. A similar impression was not foundin region B. This impression was only found on the pipe segment that was still attached to the steeltee, and was not evident on any part of the pipe segment that was detached from the steel tee.Region A was characterized by several brittle-like slow crack growth fractures, each of whichinitiated on or near the pipe inner wall just outside the depression associated with the tip of thetapping tee’s stiffener. These slow crack fractures propagated on almost parallel planes slightlyoffset from each other through the wall of the pipe. As the cracks from different planes continuedto grow and began to overlap one another, ductile tearing occurred between the planes, whichproduced a jagged appearance in parts of the overall circumferential crack in region A Thus, eventhough substantial deformation was observed in part of the fracture, the initiating cracks werestill classified as brittle-like.

Region B contained two brittle-like crack growth sections that initiated from each end ofregion A. Cracks from each end of region A propagated through region B on approximate 45°planes towards the tee (partially exposing the tee’s stiffener) and met at the bottom (the 6:00position). The remaining ligament tore with visible deformation at the bottom.

Laboratory comparisons showed that the fractures that initiated and grew in region Awere consistent with fractures generated by long-term shear and bending forces at the end of thestiffener. The fractures in region B were consistent with a continuation of the same loadingsystem described for region A but occurred subsequent to those in region A. The last ligamentthat fractured at the 6:00 position in region B was consistent with ductile tearing. Examinationcould not determine whether the last remaining ligament tore because of concentrated stressesprior to the excavation or because of excavation activities after the accident.

Other Information

Flooding was reported in the area during the summer of 1993. Midwest Gas’s most recentleak surveys, performed in March 1994, did not detect a leak in this area. Records of odoranttests performed in September 1994 and on October 17, 1994 (two and a half hours after theaccident), show odorant levels that met the level required by Federal standards.6

Probable Cause

The National Transportation Safety Board determines that the probable cause of thenatural gas explosion and fire in Waterloo, Iowa, was stress intensification, primarily generatedby soil settlement at a connection to a steel main, on a 1/2-inch polyethylene pipe that had poorresistance to brittle-like cracking.

6Federal standards require the odorant in natural gas systems to be detectable at one-fifth of the lower

explosive limit, which is typically at gas/air concentrations of 0.9 to 1.0 percent and above.

APPENDIX B 49

Organizations, Agencies, and Associations Referenced in this Report

American Gas Association (A.G.A.)An organization dedicated to promoting and protecting the interests of its member natural gaslocal distribution companies. The A.G.A. has approximately 300 members, of which about 250are natural gas local distribution companies.

American Society for Testing and Materials (ASTM)An organization that provides a forum for producers, users, consumers, and others with acommon interest, including representatives of government and academia, who come together towrite standards for materials, products, systems, and services.

Gas Piping Technology Committee (GPTC)An organization dedicated to the development of the GPTC Guide for Gas Transmission andDistribution Piping Systems (GPTC Guide). The purpose of the GPTC Guide is to provideassistance to gas pipeline system operators in complying with Federal regulations addressing thetransportation of natural and other gases by pipeline.

Gas Research Institute (GRI)A research, development, and commercialization organization dedicated to the interests of thenatural gas industry. The organization’s mission is to discover, develop, and deploy technologiesand information that benefit gas customers and the industry.

Plastics Pipe Institute (PPI)A manufacturers organization, the PPI is an operating unit of the Society of the Plastics Industry.Members of the PPI share a common interest in broadening market opportunities through theeffective use of plastic piping in water and gas distribution, sewage and wastewater transport, oiland gas production, and in industrial, mining, power, communications, and irrigationapplications.

Office of Pipeline Safety (OPS)The Research and Special Programs Administration (see below) acts through the OPS toadminister the U.S. Department of Transportation’s national regulatory program to ensure thesafe transportation of natural gas, petroleum, and other hazardous materials by pipeline. The OPSdevelops regulations and other mechanisms to ensure safety in design, construction, testing,operation, maintenance, and emergency response of pipeline facilities.

Research and Special Programs Administration (RSPA)A part of the U.S. Department of Transportation, RSPA has responsibility for emergencypreparedness, research and technology, and transportation safety. The agency’s safety mandate isto protect the Nation from the risks inherent in the transportation of hazardous materials by alltransportation modes, including pipelines. RSPA carries out its pipeline safety and trainingprograms through the Office of Pipeline Safety (see above).