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This article is intended as a brief

introduction to the NDT methods

used for the examination of welded

steel tanks. It will address the NDT

methods specified by the relevant

standards; where, how and when

they should be applied; and the basic

technology involved. Tanks may beinspected during construction to

gather baseline data or after a period

of operation. The examination of

tanks includes roofs, shells or walls

and bottoms (Fig. 1).

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The Steel Tank Institute/Steel Plate

Fabricators Association (STI/SPFA)

and the American PetroleumInstitute (API) publish the following

standards:

STI SP001, Standard for the Inspectionof Aboveground Storage Tanks [for

inspection of tanks with diametersof 9 m (30 ft) or less].

ANSI/API 510,Pressure VesselInspection Code: MaintenanceInspection, Rating, Repair andAlteration.

API 570,Piping Inspection Code:In-Service Inspection, Rating, Repairand Alteration of Piping Systems.

API Standard 620,Design andConstruction of Large, Welded,Low-Pressure Storage Tanks.

API Standard 650, Welded Steel Tanksfor Oil Storage.

API Standard 653, Tank Inspection,Repair, Alteration and Reconstruction.

API conducts examinations and

issues certifications to qualifying

inspectors in accordance with

API Standard 653. These certified

inspectors are qualified to perform

inspection of aboveground steel

storage tanks in accordance with the

requirements of API Standard 650

and API Standard 653. These

inspections include nondestructivetesting.

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API Standard 653 defines the

inspection requirements for external

only in-service inspections and

internal/external out-of-service

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Division I, explosion-proof cameras can

be effective in finding problem areas, as

well as providing a cost savings to the

owner, allowing for inspection while the

tank remains in service.

Magnetic Flux Leakage. In the

magnetic flux leakage (MFL)

technique, a section of the tank bottomis magnetized to, or near to, saturation

of flux between the poles of the bridge.

Any significant thinning of the bottom

plate will result in some of the magnetic

flux being forced into, or leaking into,

the air around the area of reduction

(Fig. 2a). Sensors to detect these flux

leakages are placed between the poles of

the bridge. MFL is a qualitative test

method, so indications located by this

technology require follow-up testing.Ultrasonic testing (UT) checks for soil

side indications, and pit gaging checks

for product side indications.

Small scanners, both manual and

motorized, provide additional coverage

for inspection adjacent to the internal

shell-to-bottom weld and beneath

steam coils, under internal piping, and

near other areas of obstruction that

might limit use of a full-size scanner

(Fig. 2b).

MFL is recognized as a separate test

method independent from other

techniques of the electromagnetic test

method byRecommended Practice No.

SNT-TC-1A (2006).

Saturated Low Frequency Eddy

Current. The saturated low frequency

eddy current (SLOFEC) scanner is

designed to inspect tank bottoms withreinforced coatings up to 8 mm

(0.30 in.) thick. The scanner uses eddy

current and other electromagnetic

technologies to inspect through thick

coatings. SLOFEC can detect metal

loss as well as locate the area of metal

loss (soil side or product side) on the

plates. This method of testing does not

disturb the coating.

Ultrasonic Thickness Testing.Ultrasonic thickness testing is used to

measure MFL indications for soil side

metal loss. Follow-up quantification is

commonly referred to as prove up.

The principles of ultrasonic thickness

testing are thoroughly covered in

training literature. Adjustment should

be made for coated surfaces, and

suitable transducers should be used for

pitted surfaces and for faster surface

coverage. Crawler systems can access theexterior tank shell/wall. Surface

preparation and proper technique are

both essential for a quality inspection.

Magnetic Particle Testing. Magnetic

particle testing (MT) is used for the

detection of surface or near-surface

discontinuities in ferromagnetic

materials. In carbon steel tanks, MT is

most often performed on, but is not

limited to, the internal shell-to-bottom

weld, sump welds, shell penetration

welds, vertical welds, junction welds

(shell horizontal-to-vertical welds),

bottom and shell weld overlays (puddle

welds), patch/base/bearing plate welds

and butt welded inserts. Surface



Liquid Penetrant Testing. Liquid

penetrant testing (PT) is used for the

detection of discontinuities open to the

surface of nonporous metals and other

materials. In aboveground tanks, PT is

most often performed on, but is not

limited to, the internal shell-to-bottom

weld, sump welds and shell penetration

welds. Surface preparation and propertechnique are both essential for a

quality inspection.

Vacuum Box Bubble Testing. A

technique of leak testing, vacuum box

bubble testing (VBBT) is the leak test

most often used in aboveground tanks.

VBBT is used for detecting small leaks

or pinholes in welds. There are several

acceptable bubble test solutions

available. Care should be taken to notuse common liquid detergents because

too many bubbles can affect test

interpretation. VBBT is most often

performed on, but is not limited to,

bottom plate-to-plate lap welds, the

internal shell-to-bottom weld, bottom

weld overlays (puddle welds) and

patchtobase plate welds. Surface



Mass Spectrometer Leak Testing. Alsoa technique under the leak testing

method, mass spectrometer leak testing

(MSLT) has proven to be very effective

in the detection of leaks in many

components including tank bottoms,

sumps, roof pontoons, roof drain

systems and heat exchangers.

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The most common tracer gas is

helium, a nontoxic, inert (nonexplosive)

gas. One of the smallest elements on the

periodic table, helium can pass through

apertures that liquid cannot. Thus, it is a

very effective medium for detecting

leaks. Surface preparation and propertechnique are both essential for MSLT.

Alternating Current Field Measurement.

Alternating current field measurement

(ACFM) is another noncontact

technique to detect and size surface

breaking cracks through coatings up to

5 mm (0.20 in.) thick. An ET

technique, ACFM is commonly used to

detect surface discontinuities in tank

shell-to-bottom welds, bottom lap

welds, sump welds and shell penetration

welds and to detect stress corrosion

cracking in ethanol tanks. Surface

preparation requirements are less than

for other NDT methods, although

proper technique is essential for quality

inspection.

Radiography. Radiographic testing (RT)

is performed on tank welds for the

detection of discontinuities.

Radiographic testing is generallyperformed on a specified number of new

construction welds and when a major

repair or alteration has been performed.

Gamma radiographic testing uses

isotopes, radioactive elements.

Preplanning and proper execution are

critical for safety when performing this

technique.

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API Standard 650, Appendix U, details

the rules for using the ultrasonic method

to test tank construction welds in lieu of

radiography. This requires that the

ultrasonic test method shall be

performed using automated,

computer-based acquisition. A manual

scan of adjacent base metal for laminar

flaws is permitted.

Examination must be conducted using

a preapproved strategy or scan plan, and

the procedure must be qualified using a

calibration block with known

discontinuities such as might be

encountered during the examination.

The flaw acceptance criteria are given

in Appendix U, Paragraph U.6.6 and

Table U-1.

Experience with ultrasonic testing in

lieu of radiographic testing has shown

excellent detection of planar flaws. In

addition, the amount of down timenormally alloted to radiography

exclusion periods is reduced, providing

significant cost savings.

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AAccording to the American Society of Civil Engineers (ASCE), theinfrastructure delivering America's drinking water has earned theabysmal grade of D. In the 2009 Report Card for America'sInfrastructure,ASCE reports that America's drinking water systems

face an annual shortfall of at least $11 billion to replace aging facilitiesthat are near the end of their useful life..." and that "Leaking pipes losean estimated seven bill ion gallons of clean drinking water a day."(ASCE Publications, 2009). America commits just 2.4% of its grossdomestic product to infrastructure. In Europe, that number is 5% andin China, its 9%. Chris Garrett explains how NDT plays anincreasingly significant role in managing assets for aging and outdateddrinking water systems.

Q: How did you get started in NDT?

A: Our parent company wanted to get going in the United Statesmarket and was looking for someone willing to dive into a smallcompany. I graduated a couple of years ago with degrees in

business and Chinese and was hired on in April of 2010 to helpwith the business side of it. The first 9 to 10 months or so of theprocess have been very much hands on and have involved megetting comprehensive training in how the inspections take place,how the data is analyzed, how the tools are built andmanufactured, and the research process.

Q: What are the structures you inspect?

A: We focus on three markets; infrastructure for drinking water,infrastructure for wastewater and nuclear plant cooling water

pipelines. We can inspect lines as small as 3 in. (7.6 cm) indiameter up to 78 in. (198 cm) in diameter. As far as water and

wastewater inspections are concerned, the target range is rightthere in the middle, 6 in. up to 24 in. (15 cm up to 61 cm).

Q: What NDT methods are you using?

A: Principally, were using RFT or remote field testing. Thepipelines we inspect are of ferrous materials, so you're looking at

ductile iron, cast iron steel. Our tools have two maincomponents that work together. The first is the exciter that emitsan AC current that travels out of the pipe wall, along the pipe

wall, and then back into the pipe where it's picked up by thesecond component, the detector module. The detector modulemeasures any sort of differences in thickness from a baseline for

what we call full pipe wall. Differences from the baseline readingtell us that there's some sort of anomaly. For example, the wallmight get thicker if you go underneath a joint or valve or, ifthere's an area of corrosion, whether it's a general area ofcorrosion or maybe a local through-hole, those differences will bedetected.

Q: How is the baseline established?

A: Ideally, our client exhumes a section of pipe from the actualpipeline to be inspected. That gives us the most accurate sample ofpipe because it's already been in the natural soil conditions. Wemachine defects into the sample in our shop. Then we do test runs

with those known defects in the pipe. We calibrate the tool basedon the collected data, and then put the tool into the pipeline to beinspected. Of course, it's not always possible for our client to digup a section of their pipe and be without it for a period of time. Inthat case, the company may have excess pipe sitting around fromthe original installation or that may have been broken andreplaced. Or, the operator may tell us the type of pipe that was

used and we will source that from one of our contacts. We try toget it as accurate as we can. Even if it's not the exact same pipefrom the ground, we're still able to calibrate it so that we come

very close.The tools themselves are tailored to the job at hand. Before we

go into a job, we get the details from the engineer or the operatorand then build in a bit of customization based on, for example, theamount of bends in the pipeline. Really we focus on two types ofapplications; a tethered application using a wire line with a winchand what are called free-swimming tools. Those are for longerinspections because the winched tools are going to be limited bythe length of the line.

We build a launch and receive system into the existing pipeline.For example, we recently did a 3 km (1.86 mi) inspection on a

pipe underneath a bay. On one side of the bay, where there wasland access, we built a launch chamber, kind of an adapter cut intothe pipeline. On the other side, we installed a receiving chamber.

We put the tool into the line, in this case it was a water line so weused water to propel the tool to the other side, and after about14-15 hours or so, the tool came out on the other side.

Q: Can the equipment tell if the corrosion is on the inside or the outsideof the line?

A: The tools don't differentiate between the two but they canmeasure the two. The end user of our data is concerned with

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overall wall loss. If your pipe wall is missing 50% of its wall, theadded value of knowing whether that corrosion is on the insideor the outside is minimal. Your big concern is how much of the

pipe wall is still effective.

Q: Does the equipment have to contact the wall of the pipe?

A: Our tool does not need to have direct contact with the pipe wall.We're able to have a level of liftoff measured between themodules of the tool and the actual interior diameter of the pipe.

Q: How does the equipment keep a record of where it is in the line?

A: If it's a winched application, the winches will have an odometer.Some of our tools have odometers on the tool that help recordthat distance. For certain applications, if the pipe isn't buriedtoo deep, you can use aboveground markers and abovegroundlocators to help track the tools progress. There are redundanciesbuilt in to help monitor the progress of the tool through theline and then that information is meshed with the inspectiondata to generate an overall map of the pipeline and itscondition.

Q: What plans do you have plans for additional training orcertification?

A: Right now I'm taking a course on asset management to help meget familiar with what the end user or operator is looking for asfar as their whole infrastructure is concerned and how theydecide which of their assets are most important or need to beinspected first.

Q: What advice do you have for NDT technicians considering work inthis field?

A: We've recently hired individuals who come from very much anNDT inspection background. It's impressive to see them pullfrom previous experience and apply that to what we do. Thatexperience is a huge asset to those individuals. If you're looking toget into the industry or if you're looking to advance, the morecertification you have, the better off you are going to be in termsof finding a job or progressing up the chain in your company.

Q: What's the most difficult part of your work?

A: I guess the logistical aspects are the most difficult part becauseyou're looking at taking multiple parties and getting them all onthe same page. You have to find ways to coordinate all thedifferent little parts just to get the inspection completed. Thatcan be challenging sometimes because you have different people

with different needs.

Q: What's the best part of your work?

A: By far, it's putting the tool in the line and having it come out andrealizing that you just inspected 600 m (1,968 ft) or maybe20 km (12.4 mi) worth of pipe. It's a pretty cool feeling to have

the tool come out and start the download and analysis processand know that you can make recommendations based on thework you just did.

You can reach Chris Garrett by phone at (800) 661-0127 or bye-mail at .

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