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  Excerpts from the Leak Testing Primer
by Gerald L. Anderson

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

A complete copy with graphs and tables is available from:
American Gas & Chemical Co., Ltd., 220 Pegasus Avenue, Northvale, NJ 07647.
Request The Leak Testing Primer

 
Leak Testing
Leak Testing is the branch of nondestructive testing that is concerned with the escape of liquids, vacuum or gases from sealed components or systems. This article will cover the reasons for leak testing and some of the technology behind the science. The major leak testing methods will be surveyed on the basis of how to select the proper method. A brief description of how to establish a leak test specification is included.

Leak Testing Objectives
Like other forms of nondestructive testing, leak testing has a great impact on the safety or performance of a product. Reliable leak testing saves costs by reducing the number of reworked products, warranty repairs and liability claims. The time and money invested in leak testing often produces immediate profit.

The three most common reasons for performing a leak test are:

MATERIAL LOSS - With the high cost of energy, material loss is increasingly important. By leak testing, energy is saved not only directly, through the conservation of fuels such as gasoline and LNG but also indirectly, through the saving of expensive chemicals and even compressed air.

CONTAMINATION - With stricter OSHA and environmental regulations, this reason for testing is growing rapidly. Leakage of dangerous gases or liquids pollutes and creates serious personnel hazards.

RELIABILITY - Component reliability has long been a major reason for leakage testing. Leak tests operate directly to assure serviceability of critical parts from pacemakers to refrigeration units.

Terminology
Leaks are most commonly thought of as specialized flaws. However, not all leaks are flaws. Leaks can just as easily result from poor seals and connections, as well as from inadequate welds.

Technically, a leak is a hole or porosity in an enclosure capable of passing a fluid from the higher pressure side to the lower pressure side. Leaks are often conceived of being simply a round hole, however, this is almost never the case. A leak normally has an involved geometry sometimes extending quite a distance from beginning to end. As a result, leakage repair may require locating both the start and end of the leak.

Leaks do not always operate in the same manner. Leaks tend to grow over time and they tend to operate differently under different conditions. For instance, many leaks are open only intermittently due to temporary clogging by water vapor or unusual pressures. High temperature or vacuum leaks often disappear when operating conditions are removed. As a result, simulating actual operating conditions is best when testing for critical leaks.

The word leakage refers to the flow of a fluid through a leak without regard to the physical size or shape of the hole. Leakage typically occurs as a result of a pressure differential across the hole. However, capillary effects can also be causes of leakage flows. When a fluid flows through a small leak the rate of flow depends upon the geometry of the leak, the nature of the leaking fluid, the pressure differential, and the prevailing temperature.

The flow characteristics of a leak are often referred to as the conductance of the leak. Because the hole cannot usually be seen or measured, the quantity used to describe the leak 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.

It is difficult to think of a hole so small it cannot even be seen by x-ray and must be defined by a fluid flow through it. Unfortunately, a great variety of ways of expressing this flow have developed, making method sensitivities and leak tightness standards often difficult to compare.

The two most commonly used units of leakage rates are standard cubic centimeters per second (std.cc/s) for pressure leaks and torr liters per second for vacuum leaks. The recent adoption of the international system of units (SI) has generated a new measure intended to replace both, pascal cubic meters per second (pa.m3/s).

Leakage Tightness
Leakage tightness is a relative term. Too often specifications are given in unrealistic terms such as leak tight or no leakage However, nothing made by man can ever be completely free of leakage and, in most cases, it is uneconomic to even try. A balance must be struck between the increasing cost of finding smaller leaks and their importance 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, i.e., clearly a gravel truck need not be free of water leakage.

Once the useful life of a unit has been determined (i.e., about 5 years for a can of Coca-Cola and 50 years for a pacemaker), the allowable leakage which will not cause a unit to fail can be calculated.

For example, most refrigeration systems will continue to operate efficiently with 10% less refrigerant than originally charged. Studies indicate the useful life of these units to be about 10 years. Thus, on a refrigeration system containing a normal Freon charge of 20 lbs., one might find leakage smaller than 3 ounces uneconomical to repair.

Once the acceptable leakage has been determined, it must be translated into standard terminology, by comparing its flow characteristics to that of air. For every flow of a substance there is a theoretical relationship between it and air. When this relationship has been established, the leakage rate in std.cc/s can be developed. Often a safety factor of half of a decade is added to this leakage rate to compensate for error.

Gas Flow In Leaks
Gas flows are an integral part of leak testing. A knowledge of gas flows is important for comparing flows of liquids to air, as well as for determining the best conditions for a leak test.

There are four predominant pneumatic flow modes through leaks:

Flow Mode
Turbulent 10-2 std.cc/s.
Laminar 10-1 - 10-7 std.cc/s.
Transition 10-4 - 10-7 std.cc/s.
Molecular 10-6 std.cc/s.
As can be seen, the laminar flow mode corresponds very closely to the area of greatest concern in most leak testing. The two most important features of laminar flow leaks are:

an increase in the pressure difference across the leak causes the flow rate through the leak to increase. Therefore, the easiest method of increasing detection sensitivity is to supply increased pressure.
the leakage rate is inversely proportional to the test gas viscosity. Therefore, the less viscous the tracer gas, the more sensitive the test. The following table shows the viscosity in centipoises of the most common tracer gases.

 
 
Gas Viscosity Molecular
Weight
As can be seen, except for ammonia, there is little appreciable advantage to be gained by using one tracer gas over another until the leakage rate is less than 10-7. At this point, we are out of the area of laminar flow.
In a molecular flow, low molecular weight, not viscosity, increases gas flow and therefore, test sensitivity. In this area Helium has the edge.
Air 0.0169 30
Ammonia (NH3) 0.0092 17
Freon 12 0.0118 121
Helium (He) 0.0178 4
Nitrogen (N2) 0.0168 28
Nitrous Oxide (N2O) 0.0133 44

Vacuum Leaks

VACUUM LEAKS

Vacuum

Pressure Range

Low

760 - 50 torr

Medium

50 to 1x10-3 torr.

High

1x10-3 to 10-6 torr.

Very High

1x10-6 to 10-9 torr.

Ultra High

1x10-9 to 10-17 torr.

Method Selection
There are a great number of leak testing methods. Each method has its own advantages and disadvantages. Each has its own optimum sensitivity range. However, not all methods are useful for every application. By applying a number of selection criteria, the choice can often be narrowed to two or three methods with the final choice being determined by special circumstances or cost effectiveness. The Leak Testing Method Selection Guide illustrates a beginning decision process. As the chart illustrates, the first selection question which should be asked is what is the purpose of the test. Is it to locate every leak of a certain size? (If the test object is valuable, it is usually repairable and this answer is yes. Is it to measure the total leakage from the test object without regard to leak number or location? (Inexpensive test objects are often not worth the reworking cost and a simple accept-reject criteria is chosen). With mass produced objects it may not matter if every unit is tested, the primary test objective being simply to monitor production machine wear).

After the purpose of the test has been defined, the selection criteria most often utilized is whether the test object is under pressure or vacuum. Many methods are reliable only under one of these conditions. There are, however, other selection criteria which can be applied to narrow the field even more. If the test object already has a useable tracer gas incorporated in it, such as Ammonia in food refrigeration systems, one test method may be optimized.

Another important criteria is the sheer size of the test object. As the size of the object expands electronic methods for locating leaks in pressurized units become increasingly impractical due to stratifying of the tracer and the slowness with which the detector probe must be moved. The corollary of the large container is the small sealed test object. This category of test object is usually a mass produced, hermetically sealed unit which is accessible only on its external surface. In this area electronic methods predominate. The only test purpose is to accept or reject the test object.

Leak Testing Methods

  • Ultrasonic Leak Testing
  • Mass Spectrometer
  • Electron Capture
  • Colormetric Developer
  • Bubble test - thin film
  • Hydrostatic test
  • Pressure Change
  • Liquid Tracer
  • High Voltage
  • Halogen
  • Thermal Conductivity (He)
  • Gauge
  • Radioactive tracer
  • Infrared
  • Acoustic
  • Electronic Gas Detector
  • EQUIVALENT LEAK SIZE ( to Std. cc/sec)

    Std.cc/sec= One cubic centimeter of gas flow per second at 14.7 psi of pressure and a temperature of 77o.

    Std cc/sec Time for lb of
    freon to leak
    Torr
    Liters/sec
    Time for
    cc to leak
    Bubble Time
    in Water
    10-1 10 Days 7.6x10-2 10 seconds 1.3 seconds
    10-2 3 Months 7.6x10-3 100 seconds 13.3 seconds
    10-3 2.7 Years 7.6x10-4 16.67 minutes 14.5 minutes
    10-4 27 Years 7.6x10-5 2.78 hours 24 minutes
    10-5 270 Years 7.6x10-6 27.8 hours 4 hours
    10-6 2,700 Years 7.6x10-7 11.57 days  
    10-7 27,000 Years 7.6x10-8 3.86 months  
    10-8 270,000 Years 7.6x10-9 3.22 years  
    10-9 2,700,000 Years 7.6x10-10 32 years  
    10-10 27,000,000 Years 7.6x10-11 320 years  

     

    BIBLIOGRAPHY

    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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