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  Leak Testing Large Containers
by Gerald L. Anderson

2006-19 American Gas & Chemical Co., Ltd. 
All rights reserved

We see the dramatic headlines: "RAIL CAR EXPLODES", "OSHA INVESTIGATES CHLORINE GAS LEAKAGE", "EPA REQUIRES UNEARTHING OF GAS STATION FUEL TANK", "LNG TANKER FAILS COAST GUARD CERTIFICATION". Government agencies have become tougher and legal liability claims grow higher. Yet, for every headline, there are probably a hundred such leaks that have not yet come to the public's attention.

Why are so many containers leaking?  With today's concern for resource conservation and public safety, it cannot be because no one cares.

A major reason is the very size and construction complexity of these vessels make them very difficult and expensive to test. Each vessel has special problems; it is double walled, it's leaking on the bottom which is firmly in the ground, it leaks only intermittently, the source of the leak is distantly removed from it's indication.

Another possible reason for so many large tank leaks lies in the variety of possible leak testing methods available. These methods have evolved partially because many of the codes have failed to keep up with current needs and technologies. Because of the variety of available methods, and the lack of adequate information on these methods, the NDT engineer may be frustrated in trying to find the appropriate method for his application. All too often he ends up relying on a leak testing method too cumbersome for the job. NDT engineers sometimes give up on leak testing altogether, relying on the better known flaw detection techniques to find both flaws and leaks.

The Alaskan Pipeline, is a dramatic illustration of the need for leakage testing. The pipeline was not leak tested, instead the most sophisticated methods of traditional flaw testing were relied upon to solve both problems - leaks and flaws. However, once the pipeline went into operation, leaks developed immediately, some large enough to cause the shutdown of the pipeline. The Alaska Pipeline is not the only project that has suffered, sometimes at great expense, from a failure to understand that flaw testing is not a substitute for leak testing. Disasters which can cause death and great loss dramatize the need for a better understanding. As a result of such problems there is a developing interest in leakage testing.

Concepts & Terms
Leakage testing defined in its simplest form is a branch of nondestructive testing used for the detection (location or measurement) of fluid leakage in either a pressurized or an evacuated system. The word "leak" refers to the physical hole and not-to the quantity of gas or liquid flowing through the hole. In other words, leaks are flaws which-affect the safety or performance of a system or which result- in environmental contamination or energy loss. The word "leakage" refers to the flow of a-fluid through a leak without regard to the physical size of the hole. Leakage typically occurs as a result of a pressure differential across the hole, however capillary effects can also be a cause of leakage. When fluid flows through a small leak the rate of flow depends upon the geometry of the leak, the nature of the leaking fluid and the prevailing pressure and temperature.

The characteristics of a leak are often referred to as the conductance of the leak. Because the leak hole can usually not be seen or measured, the quantity used to describe the leak size is the conductance or leakage rate of a given fluid through the leak under given conditions. The leakage rate used as a measure of leak size must have dimensions equivalent to pressure, temperature, time, and volume.

Units of Measure
Users and manufacturers are often unable to communicate effectively with each other or to make comparisons between various methods. It is difficult enough to think of a hole so small it cannot be seen by x-ray and must be defined by a mass flow through it, without having the added complication of a great variety of different measures used to define it. The most commonly used measure now is the standard cubic centimeters per second. What does a standard cm3/sec. look like? A leak of 1x1O-4 std. cm3/sec. is equivalent to the loss of  1 cm3 of air over a 3 hour period -- this does not sound like a relatively large leak. However, it is equivalent to losing a pound of freon over 27 years. Besides these two equivalents, there are many more leakage flow measures in use. Some of the more common are torr liters per second, bubble time, kilograms per year, standard liters per day, micron liters per second, and micron cubic feet per hour. The latest to appear is the standard international unit pascal-cubic-meters per second. S.I. units have been adopted by almost all of the technical organizations such as N.B.S., ASTM and ASNT   See Equivalent Leak Size

How sensitive should the test be?
Nothing can ever be completely free of leakage. Every container always has some leakage, even if those leaks are so minute that it would take 320 years for a cubic centimeter of gas to leak out. The choice of which method to use revolves around two questions:
-  What should be the standard of leak tightness be?
-  How can this standard be met most economically and reliably, so as not to dramatically increase the cost of the tank?

The increasing cost of finding smaller leaks must be balanced with to the functioning of the unit over its useful life. Leakage tight therefore has no meaning except in relation to the substance which is to be contained, its normal operating conditions, and the objectives with respect to safety, contamination, and reliability. Leakage tight is the practical leakage which is acceptable under normal circumstances, e.g., clearly a gravel truck need not be free of water leakage.

Leakage Tightness
LEAKAGE std. cc/sec
Gravel from a truck 1 x 103
Sand in hour glass 1 x 102
Air whistle around car window l x l0-1
Oil from a truck 1 x 10-2
Water 1 x 10-3
Gasoline from a storage tank l x l0-4 
Gas from a pipe line 1 x 10-5
Leakage in a tanker 1 x 10-6
Gas in heart pacemaker 1 x 10-10

Leak Testing Methods
Leak testing a large pressure vessel whose size or complexity does not allow it to be easily enclosed can be a very time consuming and frustrating experience.   A multitude of leak location and leak measurement devices exist. Some devices are pneumatic or electronic; some are chemical. Each method has its advantages and disadvantages. Each of the leak testing methods has an optimum sensitivity range, outside of which its operational abilities can be severely limited. The methods discussed above for testing large pressure vessels can be compared on the basis of sensitivity as follows.

Only a few methods are discussed here -- the testing of a large pressure vessel is the focus.
For other methods see the Leak Testing Primer

Colormetric Developer
This method is one of the most recent. Its use is rapidly expanding for very large vessels. ASTM is currently working on a standard method. This method makes use of a tracer gas, usually ammonia, on the inside of the vessel and a powder developer on the outside. This developer changes color as the tracer- -traverses the leak. Specific sensitivity rates can be calculated by varying the pressure differential, test -time, and tracer concentration. See Colormetric Developers

Advantages Disadvantages
  • Operator Independent
  • Inexpensive
  • Very sensitive with ability to adjust test sensitivity.
  • Positive leak indication.
  • Rapid.
  • Uses ammonia as a tracer, which can corrode brass or copper, deteriorate some woods and, in large quantities, ammonia can be toxic.
  • Requires cleaning after use.

Liquid Film
This method requires the pressurization of the vessel. As the pressurized gas escapes through the leak, it causes a thin film of liquid on the surface of the vessel to bubble. Since the ban by ASTM and ASME of soap solutions, there has been tremendous interest in synthetic bubble detectors for pressure and vacuum box testing. These methods can achieve a high reliable sensitivity. See Leak Tec Thin Film Liquid Leak Detectors

Advantages Disadvantages
  • Bubbles pinpoint leak and give indication of size.
  • Inexpensive
  • Safe procedure
  • Requires little operator training.  Easy to use.
  • Very sensitive and reliable
  • Can be used even when there is no access to the opposite side.
  • Requires moderate pressures
  • Leak size difficult to estimate
  • Requires cleaning after use.

Hydrostatic Testing
This method is one of the most common leak testing methods and still one of the most confusing. It usually involves filling the vessel with water under high pressure and looking for leaks to weep moisture. ASTM has developed a standard method for hydrostatic leak testing (EI003-84).   Read more about Hydrostatic Testing

Advantages Disadvantages
  • Relatively operator independent
  • Inexpensive
  • Pinpoints leaks
  • Procedure for leak testing not well understood
  • Significant clean-up
  • Time consuming
  • Requires high pressure
  • May obscure leaks if a more sensitive leak test is to be used afterwards.
  • Water alone is not very sensitive

Ultrasonic leak testing relies on the ability of instrumentation to pick up small acoustic vibrations caused primarily by the turbulent flow of a pressurized gas passing through the leak. The detector can also be used to hear the noise from a special sound generator as it penetrates the leak in an unpressurized vessel. It is often used as a preliminary gross leak test. ASTM has adopted a standard method of calibrating ultrasonic detectors and using them to locate leaks (EI002-84).  See Sonic 3000

Advantages Disadvantages
  • Detects vacuum and pressure leaks
  • Leaks can be detected up to fifty feet from source
  • Easy to learn
  • The least expensive electronic method
  • Ambient noise in the instrument's frequency range may interfere
  • Somewhat operator dependent
  • Lower sensitivity

Inspection Penetrants
Penetrants (usually fluorescent) can be substituted for water in a pressure test similar to hydrostatic testing. They can achieve a similar sensitivity.  Read more about Inspection Penetrants

Advantages Disadvantages
  • Produce an easily seen leak indication when developed under black light
  • Require only medium pressure
  • Procedure is messy and requires significant cleanup
  • Can be expensive for large containers
  • Fluorescent penetrant testing requires a dark test area
  • Difficult to dispose of in large quantities

Pressure Change
This method is one very commonly used. It requires pneumatic pressurizing of a closed container. After isolating the vessel, adjusting for temperature and water vapor, the initial pressure and the final pressure in the vessel are compared. The test usually requires a minimum of two hours and may run several days. Although the test sensitivity can be increased by extending the test time, it's practical sensitivity is not great. Nevertheless, it is often used to test nuclear containment buildings.

Advantages Disadvantages
  • Operator Independent
  • Safe
  • Records total system leakage
  • Difficult to keep accurate on large containers
  • Very time consuming
  • Requires trained personnel and computers
  • Does not give leak location

Electronic Sniffers
This class of leak detector (principally the thermal conductivity meter or the halogen detector), senses minute leakages of specific tracer gases. They are widely used to test total leakage from small objects, as well as to locate leaks in containers which already contain the appropriate tracer gas. When used as a detector probe to locate large system leaks, these detectors cannot always achieve a high practical sensitivity.

Advantages Disadvantages
  • Low pressure required
  • Quanitative readout
  • No cleanup required
  • High sensitivity when used to measure total leakage.
  • Expensive & often delicate equipment
  • Operator dependent
  • Slow (scanning speed should not be faster than one foot per minute)
  • Freon has high relative molecular weight and tends to-stratify in vessels with any significant height, helium is better but tends to accumulate at top
  • The detector can be easily damaged by leaks
  • Requires frequent calibration
  • Results in windy areas are often poor

Helium Mass Spectrometers
This instrument is considered the most sensitive method of measuring leakage from small objects, such as hermetically sealed devices. When used in an evacuated chamber, it can produce sensitive results. When a pressurized container is tested by the so-called probe methods, much of its sensitivity is lost. However, special types of containers, such as large vacuum chambers or cryogenic tanks, often use mass spectrometers with great success.

Advantages Disadvantages
  • Quantitative readout
  • Most sensitive way to test evacuated tanks
  • Clean & safe
  • Expensive to buy and maintain
  • Subject to damage from large leaks
  • Requires skilled operators
  • Requires frequent recalibration
  • Not easily portable

Immersion testing with water or special fluids is another common leak testing method. Water has proven to be an insensitive method but water with additives has proven to be one of the most reliable ways to locate leaks. By controlling pressure and the concentration of additives the immediately discernible leak rate can be changed.   See Immersit

Advantages Disadvantages
  • Locates small leaks
  • Fast
  • Inexpensive
  • Not operator dependent
  • Can be automated
  • May require clean-up
  • Test object must be small enough to immerse
  • Test object must be unaffected by the immersion fluid

Method Sensitivity Comparison
The methods discussed above for testing large pressure vessels can be compared on the basis of sensitivity as follows.

(Unit of measure = standard cm3/second)
In order of descending sensitivity.

Mass Spectrometer (evacuated systems) 10-3 to 1 x 10-9
Colormetric Developer 10-1 to 1 x 10-7
Immersion Test System 1 cc to 1 x 10-6
Liquid Film Bubble 1 cc to 1 x 10-5
Thermal Conductivity (He) 1 cc to 1 x 10-5
Halogen Detector 10-1 to 1 x 10-5
Mass Spectrometer (probe mode) 10-3 to 1 x 10-5
Hydrostatic Test Systems 1 cc to 1 x 10-4
Inspection Penetrant 1 cc to 1 x 10-4
Pressure Change 1 cc to1 x 10-3
Ultrasonic (Acoustic) Detector 1 cc to l x  l0-2




American Nuclear Society, La Grange Park, Illinois
ANSI 7.60 "Leakage rate testing of contaminant structures".

American Society of Mechanic Engineers, New York, N.Y.
Boiler & Pressure Vessel Code Section V, Leak Testing.

American Society for Testing & Materials, Philadelphia, PA.
Annual ASTM Standards, Part II.

American Vacuum Society, New York, N.Y.,
Leak Testing Standards.

King, Cecil Dr. - American Gas & Chemical Co., Ltd.
Bulletin #1005 "Leak Testing Large Pressure Vessels". "Bubble Testing Process Specification".

Marr, J. William, - NASA, Washington, D. C. 1968
Leakage Testing Handbook; NASA Contractor Report; NASA CR952

American Society for Nondestructive Testing, Columbus, OH - McMaster, R.C. editor, 1980. Leak Testing Volume of ASNT Handbook

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