An Old Quality Lesson From Failure of a New Heat Exchanger

8/3/2019 An Old Quality Lesson From Failure of a New Heat Exchanger

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AN OLD QUALITY LESSON FROM FAILURE OF A NEW HEAT EXCHANGER

Scott J. Whitlow, P.E.

Sr. Consulting Engineer

Gulf Regional Office

E.I. DuPont de Nemours

Hwy 347 South

Beaumont TX 77704Phone: 1 (409) 727-9722

Fax: 1 (409) 727-9970 (Fax)

e-mail: [email protected]

ABSTRACT

A Gulf Coast manufacturing unit has three heat exchangers in its reaction overhead off-gas system.

All the heat exchangers are once-through condensers with carbon steel shells, zirconium tubes anddouble tube sheets. There are four reaction trains in the manufacturing unit for a total of 12

condensers. All have many years of satisfactory service except the newest condenser which failed afteronly four months in service (typical service life is 8 to 15 years). Initially, one tube failed but

subsequent inspection found numerous leaks at the rolled joint between the zirconium tube and thezirconium tube sheet. The failure mechanism is stress corrosion cracking initiating at pits due to the

presence of ferric ions. Iron was embedded in the tube expansion process during fabrication and

subsequent iron corrosion produced ferric ions that lead to SCC and pitting. The fabrication problemoccurred despite very detailed and specific quality control plans. This failure reiterates the need for

thorough implementation of quality control plans during fabrication.

KEYWORDS

Corrosion, stress corrosion cracking, pitting, ferric chloride, hydrochloric acid, fabrication, rolling,iron, zirconium, heat exchanger, tubing.

BACKGROUND

The failed heat exchanger is a condenser in the off-gas system of a batch polymerization process

(Figure 1). HX1 and HX2 are condensers and HX3 is a liquid cooler. The failed heat exchanger, HX2,condenses the bulk of the vapors in the off-gas stream.. The remaining vapor is sent to a gas recycle

loop (not shown). The batch cycle has portions with oxidizing conditions and other portions with

reducing conditions.


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Figure 1. Process schematic for the off-gas system from a batch polymerization process. The heatexchangers have zirconium tubes and carbon steel shells with a double tube sheet design. HX1 andHX2 are condensers and HX3 is a cooler. The knock-out pot is fabricated from Zr 702. The reactorand piping are glass-lined. The failed heat exchanger is HX2.

The specific process conditions lead to a greater amount of bad actors absorbing into the liquid phase

in the failed heat exchanger than the first off-gas condenser. During polymerization, the condensingliquid includes organic chlorides, a few weight percent HCl and an occasional excursion up to 50 ppm

H2SO4. Between batches, the system is swept with chlorine, nitrogen and then air. There is very little

water in the failed exchanger during operation or between batches (on the order of a few ppmcondensing from the air or nitrogen during sweeps). The service conditions are aggressive to most

materials of construction, even zirconium shows moderate attack over time. Service history datingfrom the 1970s shows the 12 zirconium off-gas condensers (in four reaction trains) have a typicalservice life ranging from eight to fifteen years.

Figure 2. Schematic of the failed heat exchanger showing the double tube sheet design.


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The double tube sheet design (Figure 2) has proven very satisfactory for this service. Tubes are only

rolled into the tube sheets because service experience shows seals welds fail from stress corrosion

cracking. The rolled portion ends at least 3/8 away from the back end of the tube sheet. The heatexchangers are mounted vertically, with the off-gas entering the tube side at the top. The shell side is a

glycol mixture entering at the bottom. The failed heat exchanger had numerous leaking tubes in the

bottom zirconium tube sheet after only four months of service. No leaks were detected at the top

zirconium tube sheet or the carbon steel tube sheets (Figure 2). The leaking tubes are predominantlyon one-half of the tube sheet (Figure 3). No leaks were found on the other three tube sheets in the heat

exchanger.

Figure 3. Bottom tube sheet. The plugs denote numerous leaking tubes that were found byhydrotesting, distributed over one-half of the tube sheet.

ANALYSIS

The failed exchanger was removed from service because a leaking tube was detected. The heat

exchanger was sent to the original fabricators shop for assessment. Subsequent hydrotesting foundnumerous tube leaks. Before plugging tubes, random tubes were measured for the roll depth. Most

holes measured the specified 3/8, the minimum distance between the end of the rolled portion and the

back of the tube sheet was .


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A number of samples were removed from the failed heat exchanger, both from the bottom and top

zirconium tube sheets (Figure 4). In most cases, corresponding portions of the same tube from the top

and bottom were removed. On the leaking tubes, all crack-like indications were at the transitionbetween rolled and unrolled portions of the tube. Tables I and II contain chemistry and mechanical

property information for the OD, 0.065 wall ASME SB-523 tubes.

Table I.Chemical Analysis(Weight Percent)

ASTM B523 Sample

Element UNS R60702 (Ingot Analysis)

Zirconium + Hafnium 99.2 >99.2

Hafnium 4.5 max 1.3

Iron + Chromium 0.2 max 0.08

Hydrogen 0.005 max 0.00055Nitrogen 0.025 max 0.0047

Carbon 0.05 max 0.01

Oxygen 0.16 max 0.1370

Table II.Mechanical Properties

ASTM B523

Property UNS R60702 Sample

Tensile Strength ksi (MPa) 55 min (379 min) 76 (524)Yield Strength ksi (MPa) 30 min (207 min) 51.6 (356)

Elongation in 2 (%) 16 34.5

Sample 2 (Figure 5) was not initially reported as a leaking tube. However, there are longitudinal

cracks in the transition region between rolled and unrolled portions of the tube. This damage pattern is

characteristic of all samples showing cracks. The number of longitudinal cracks per tube varies from a

Figure 4. Sample removed from bottomtube sheet, row 4, tube 1. This is onesample removed in two pieces due to theheat exchanger configuration.


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few to numerous with no discernable pattern. Sample 2 has circumferential indications, (Figure 5),visible predominantly in the transition region.

Figure 5. ID surface at the transition between the rolled and unrolled portion of Sample 2. (16.5X).

Figure 6. SEM micrograph of Sample 2 showing the same area as the previous figure. Corrosion inthe rolled area is more apparent as are a few additional longitudinal cracks. The circumferentialindications in the previous figure are not visible because these indications are only in the oxide layer.(17X).

Scanning electron microscopy (SEM) reveals corrosion in the rolled area (Figure 6) that was not

visible in Figure 5, but the circumferential indications are not observable in SEM photo macrograph

(ensuing discussion will show the circumferential indications are only in the oxide). SEM will notimage the circumferential indications because the oxides are thin and electrically insulating so the

Rolled UnrolledTransition

Longitudinal Cracks

Circumferential

Indications

Rolled UnrolledTransition

Scanning Artifact(Not Relevant)

General

Corrosion

Longitudinal Cracks


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oxides will not produce sufficient secondary electrons at lower magnifications to be imaged withSEM. Figures 5 and 6 emphasize the need to perform both light optical microscopy as well as SEM.

Figure 7. Longitudinal crack fracture surface showing ID origination. (16.5X)

The fracture surface of a longitudinal crack shows ID origination (Figure 7). Comparing the fracturesurfaces of the service and laboratory fractures provides a qualitative check for the zirconium

brittleness (Figures 8 and 9, respectively). The service fracture (Figure 8) shows a very brittle

appearance while laboratory fracture (Figure 9) has the dimpling characteristic of a ductile fracture.

This quick check shows the zirconium tube has ductility in the regions not affected by cracking.

Figure 8. Cleavage facets on the surface of a Figure 9. Dimpling on the laboratory fracture

longitudinal crack showing brittle failure. showing the tube material was not initially brittle.(2500X). (2500X).

Service Fracture(Figure 8)

Lab Fracture(Figure 9)

Crack Initiationat ID


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.

The cracking morphology is transgranular with minor branching (Figures 10 and 11). Another

important observation is the internal oxide disruption in the unrolled area compared to the rolled area

(Figures 12 to 15). The circumferential indications in Figure 5 are macro features of the disruptedoxide seen in Figures 13 and 15. The other samples from the bottom tube sheet had very similar

damage patterns to sample 2.

Figure 10. Cross-section of alongitudinal crack from Sample 2showing ID origination (upper, rightcorner) with transgranular morphologyand very little branching. (100X,Unetched)

Figure 11. Cross-section of anotherlongitudinal crack in Sample 2 originatingfrom the ID surface at the right of themicrograph. The crack is transgranularwith a slightly higher degree of branching.(400X, Unetched).


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Sample 2G, from the top zirconium tube sheet, is the complementary portion of Sample 2. The internal

surface of sample 2G shows pitting and accelerated corrosion (Figure 16 and 17).The interior surface

of sample 2G is typical of all the tubes from the top of the tube sheet: Corrosion and pitting, but no

Figure 12. Oxide on the interior,unrolled surface of sample 2. Comparewith the following micrograph. (400X,Unetched).

Figure 13. Oxide on the interior, rolledsection of sample 2. Compare with the

previous micrograph. (400X, Unetched).

LongitudinalAxis

Figure 14. A secondary electron image of thesurface oxide on sample 8 in the unrolledregion. The longitudinal tube axis is parallelto the length of the figure. (1000X).

LongitudinalAxis

Figure 15. A secondary electron image of thesurface oxide on sample 8 in the rolled region.The circumferential (up and down) crackswere caused by rolling. The longitudinal axis is

parallel to the length of the figure. (1000X).


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cracking. Contrast the severe corrosion on the rolled portion of sample 2G with minor corrosion on theunrolled portion (Figure 16).

Figure 16. Internal surface of sample 2G from the top zirconium tube sheet, the complementaryportion of sample 2 from the bottom zirconium tube sheet. Corrosion is on the rolled portion of thetube. (7.5X).

Figure 17. Cross-section of pits in the rolled section of sample 2G. (400X, Unetched).

The most significant analysis for this failure is provided by the SEM Energy Dispersive Analysis byX-rays (EDAX) results (Figures 18a to 19b). EDAX of the sample 2 in the rolled region shows an iron

peak while the unrolled area has no iron (Figures 18a and 18b). Sample 2G has iron on the interior

surface in the rolled region (Figure 19a) and the unrolled region does not have iron (Figure 19b). Ironwas 3 wt% to 6 wt% on various rolled tube areas by semi-quantitative EDAX.

The rolled region of sample 2G also has nickel, chromium and calcium. The calcium is probably fromsample handling, however the iron, nickel and chromium are probably from the tube rolling process.

Sample 2 may also have trace amounts of nickel and chromium, but the excitation voltage for sample

2G EDAX was 25 keV while the sample 2 EDAX was only 10 keV, hence the EDAX on sample 2G

Rolled Unrolled

Transition


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can detect slightly smaller amounts of contaminants. The tube rollers are reported to have austeniticstainless steel components.

Figure 18a. Sample 2EDAX outside of tubesheet in an unrolledarea. (10keV).


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Figure 18b. Sample 2 EDAXfrom inside tube sheet in arolled area. (10 keV).Samples from the bottom

tubesheet showed from 3 wt%to 6 wt% iron.

Figure 19a. Sample 2GEDAX from outside the tubesheet in an unrolled area.Ca contamination is mostlikely from sample handling.(25 keV).

Figure 19b. Sample 2GEDAX from inside the tubesheet in a rolled areashowing Fe, Cr, Ni and Ca.The first three contaminantswere caused by the rolling

process, and the Ca is mostlikely from sample handling.(25 keV). Samples from thetop tube sheet had between0.3 wt% and 3 wt% Fe.


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DISCUSSION

Examining the condenser operation explains the differences in corrosion damage at the top and bottom

tube sheets. Implicit in the condenser design, most of the condensate forms below the top tube sheet asthe process travels from top to bottom. At some point in the body of the exchanger the temperature is

low enough for larger concentrations of bad actors like HCl absorb in the liquid on the tube walls. The

bottom tube sheets are exposed to higher concentrations of these bad actors than tubes in the top tube

sheet. It is also possible that there is less iron in the top tube sheet than in the bottom tube sheet. Evenwith challenging process conditions, the secondary condensers usually provide years of satisfactory

service as long as impurities like iron are not present.

The leak pattern on the bottom tube sheet prompted a check of the pipe stress to determine if stresses

from external loads contributed to the failure. The pipe stresses were within acceptable limits. The

glass-lined piping is very sensitive to misalignment and excessive pipe stresses would likely have alsocaused spalling and damage of the glass-lined pipe.

The failure mechanism is stress corrosion cracking initiating at pits in the protective oxide coating.Zirconium is very resistant to organic chlorides and HCl, even though this is a reducing environment

[1]. The presence of oxidizing impurities like ferric ions causes pitting and stress corrosion cracking(SCC) of zirconium because ferric ions polarize the zirconium surface to a potential exceeding the

pitting potential [2, 3]. Corrosion of the embedded iron by the condensing process stream createdferric ions that lead to pitting and SCC. Iron on the rolled tube surface is sufficient to cause pitting and

stress corrosion cracking. Differences in the corrosion attack in the tubes at the top and bottom tube

sheet is attributable to the differences in the amount of HCl absorbing at the two locations.

All potential sources of iron, chromium and nickel contamination were investigated. The exchanger

has never been hydroblasted, so there is no possibility of iron pick-up from the hydroblasting lance.The glass-lined reactor and glass-lined piping are inspected periodically. After the heat exchanger

failure, the inspections of the reaction train equipment were very thorough. The reactor has several

tantalum patches that were intact and the remaining glass surface was in very good condition. Theglass-lined piping has chips on the flange face, outside the gasket area, which is never in contact withthe process. All the instruments and piping components are Teflon lined and all were intact. The raw

materials are periodically tested for iron and the final product is always tested for iron because thiscontaminant has a strong impact on product quality. By a process of elimination, the only potentialsource of iron contamination is the fabrication process.

Supporting evidence for a fabrication problem is the other seven exchangers currently in this service

have no leaks or exchanger failures. The replacement for the failed exchanger has been in service formore than one year without any leaks. A leak would be readily detectable by an operator on his rounds

because the susceptible location on the tubes is open to the atmosphere between the two tube sheets.

An interesting side note is the condition of the protective oxide in the rolled regions. The tubes are

purchased with an enhanced oxide to improve scratch resistance during tube handling and bundle

insertion. The tube rolling process cracks this thicker oxide (compare Figures 14 and 15). In order tominimize the risk of trapping contaminants in the oxide cracks, the enhanced oxide should be removed

by pickling after the rolling process. A post rolling thermal treatment may reestablish the oxide film

but the reduction in residual stress may cause loss of seal integrity.

Pickling the tubes after tube expansion will be beneficial for removing contaminants. As mentioned

previously, this will also eliminate cracked oxide in the rolled regions. The oxide layer will be


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diminished, but as long as there is at least 10nm oxide, then this will be sufficient initial protection [4]until service conditions thicken the oxide.

There is a portion of the batch polymerization cycle where conditions are oxidizing and the protectiveoxide can reestablish. This is probably why the off-gas condensers have long service life even in the

aggressive process environment. In the case of the failed heat exchanger the cracked oxide lessened

the general corrosion resistance and the embedded iron accelerated pitting and cracking.

FABRICATION PRACTICES AND QUALITY CONTROL

Service history shows the performance of these zirconium heat exchangers is strongly dependent on

fabrication practices. Very comprehensive fabrication specifications have been developed over the

years to address numerous details needed for successful service. These address the unique issuesinvolved in fabricating zirconium equipment[5]. Tube sheet hole diameter, tube ID and tube OD have

very tight tolerances. The percent expansion of every tube is measured and recorded. The average

expansion on all the tubes in the failed heat exchanger is 6.05%, which is within the acceptable rangefor zirconium tube expansion. For reference the hardness in the rolled area ranges from 200 HV to 280

HV, the unrolled area hardness ranges from 170 HV to 200 HV. An inspector follows the fabricationto ensure the specification and quality control details are followed.

The fabricator has provided several zirconium heat exchangers for DuPont with acceptable service

lives. The failed heat exchanger was purchased on an order for two zirconium heat exchangers. The

vendor has two shops and decided to fabricate one heat exchanger at each location. The other heatexchanger on this order has been in service for several years with no reported leaks.

Numerous communication and scheduling problems occurred during the failed heat exchangerfabrication. Both shops were supposed to use clean rooms for fabricating these exchangers. One heat

exchanger on this order was build in a clean room. The failed heat exchanger was built in the open

shop. It is reported that the second shop adamantly refused to build a clean room and successfullydelayed fabrication until it was too late to transfer the job to another facility, thereby forcingfabrication to proceed without a clean room.

The job was quoted with hydroswage tube expansion, but the vendor subsequently changed to rollers.Both techniques are reported successful, but hydroswaging is preferred because this method has less

risk for embedding impurities, a more uniform tube expansion and decreased work hardening of the

zirconium tube. The vendor completed the portions of tube rolling without the required and specified

oversight. Judging by the embedded iron in the zirconium tubes, the rolling was apparently done in adirty environment and/or with dirty rollers. The shop had no special control on the rollers, so they

could have been used on another job with stainless steel tubes (based on impurities detected by

EDAX). The leaking tube pattern (Figure 3) suggests the embedded iron problem may have occurredon one or two shifts. This failure highlights that equipment owners need to be aware of the vendors

fabrication processes and must strictly enforce fabrication hold-points to ensure long service life of the

equipment.

This failure analysis touched on all the key corrosion control factors for a successful zirconium

application: a proven design, operation, maintenance (not an issue for this failure) and fabrication [6].

Generally, the service performance of the zirconium tube/carbon steel shell, double tube sheetexchangers has been very good. The failure analysis demonstrates the crucial role of fabrication for a


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long, successful service life. The unfortunate consequence of relearning this lesson is a very expensivereplacement condenser.

CONCLUSIONS

1. The failure mechanism is stress corrosion cracking initiating at pits. The failure mode is iron

corrosion. The iron corroded in a process containing organic chlorides and hydrochloric acidthereby creating ferric ions that initiated pitting.

2. Iron is present only in the rolled sections of the tubes which was most likely embedded during

fabrication.3. Chromium and nickel are also present on some samples, but these contaminants did not contribute

to the failure.

4. An interesting by-product of this failure analysis is the condition of the enhanced oxide in therolled portions of the tubes. Cracks in the enhanced oxide are potential locations for contaminants

to collect. Cracks in the enhanced oxide occur during rolling, resulting in portions of the surface

having lower corrosion protection until process conditions reestablish a thicker oxide.5. The heat exchanger service conditions are challenging service for many materials of construction

including zirconium. Successful service performance requires strict adherence to all the subtlefabrication and quality control details.

RECOMMENDATIONS

1. Maintain a very high level of inspection monitoring during fabrication to ensure vigilant adherence

to the fabrication and quality control procedures and requirements.

2. Only use hydroswaging for tube expansion. Hydroswaging will lessen the risk for embedding

contaminants into the tube ID, provide a more uniform tube expansion and decrease the amount of

work hardening in the tubes.

3. Consider pickling tubes after expansion to provide a uniformly clean surface and also to removethe cracked oxides in the expanded regions.

ACKNOWLEDGEMENTS

The author thanks Jack Tosdale, Wah Chang, Dr. Te-Lin Yau, Te-Lin Yau Consultancy and Dr. Brian

Saldanha, DuPont for assistance with the failure analysis and many useful discussions during the

preparation of this paper.


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REFERENCES

[1] T.L. Yau, Zirconium Meeting the Challenges of the New Millenium, Paper 01331, NACE

International, Houston, TX, 2001, p.18.

[2] R.T. Webster and T.L. Yau, Zirconium, Process Industries Corrosion, edited by B.J. Moniz and

W.I. Pollock, NACE, Houston, TX, 1986, p.537.

[3] T.L. Yau and R.T. Webster, Corrosion of Zirconium and Hafnium, Metals Handbook, 9th

ed.,

vol. 13, L.J. Korb and D.L. Olson eds., ASM International, Metals Park, OH, 1987, p.710.

[4] T.L. Yau, private communication

[5] G.J. Lentz and B.J. Sanders, Managing a Zirconium Project, Reactive Metals in CorrosionApplications, Wah Chang, Sunriver, OR, 1999.

[6] R.A. Clapp, J.J. Kvochak and B.J. Saldanha, Corrosion of Titanium and Zirconium in OrganicSolutions, Paper 95243, NACE International, Houston TX, 1995, p.11.

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An Old Quality Lesson From Failure of a New Heat Exchanger