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Excerpts from the
Leak Testing Primer
by Gerald L. Anderson
©2006-2019 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 |
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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.
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|
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|
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 |
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
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 |
|
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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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