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Leak-Testing Methods

There are many different methods for pressure and leak testing in the field. Seven of these are:

1. Hydrostatic testing, which uses water or another liquid under pressure
2. Pneumatic or gaseous-fluid testing, which uses air or another gas under pressure
3. A combination of pneumatic and hydrostatic testing, where low pressure air is first used to detect leaks
4. Initial service testing, which involves a leakage inspection when the system is first put into operation
5. Vacuum testing, which uses negative pressure to check for the existence of a leak
6. Static head testing, which is normally done for drain piping with water left in a standpipe for a set period of time
7. Halogen and helium leak detection

HYDROSTATIC LEAK TESTING
Hydrostatic testing is the preferred leak-testing method and perhaps the most often used. The most important reason for this is the relative safety of hydrostatic testing compared to pneumatic testing. Water is a much safer fluid test medium than air because it is nearly incompressible. Therefore, the amount of work required to compress water to a given pressure in a piping system is substantially less than the work required to compress air, or any other gas, to the same pressure. The work of compression is stored in the fluid as a potential energy, which could be released suddenly in the event of a failure during a pressure test.

A calculation of the potential energy of air compressed to a pressure of 1000 psig (6900 kPa) compared to the potential energy of the same final volume of water at 1000 psig (6900 kPa) shows a ratio of over 2500 to 1. Therefore, the potential damage to surrounding equipment and personnel resulting from a failure during a pressure test is far more serious when using a gaseous test medium. That is not to say that there is no danger at all in a hydrostatic leak test. There can be substantial danger in a hydrostatic test due to air trapped in the piping. Even if all air is vented from the piping before pressurizing, workers are well advised to conduct any high-pressure test with safety in mind.

PNEUMATIC LEAK TESTING
The fluid normally used for a pneumatic test is compressed air, or nitrogen if the source is bottled gas. Nitrogen should not be used in a closed area if the possibility exists that the escaping nitrogen could displace the air in the confined space. Persons have been known to become unconscious under such circumstances before realizing they were short of oxygen. Because of the greater danger of injury with a gaseous test medium, the pressure that may be used for visual examination for leaks is lower for some piping codes than is the case for a hydrostatic test. For example, for pneumatic tests, ASME B31.1 permits the pressure to be reduced to the lower of 100 psig (690 kPa) or the design pressure during the examination for leakage.

COMBINATION PNEUMATIC AND HYDROSTATIC TESTING
A low air pressure, most often 25 psig, (175 kPa) is first used to see if there are major leaks. This low pressure reduces the danger of personal injury but still enables major leaks to be quickly located. Repairs, if needed, can then be done before the hydrostatic test. This method can be very effective in saving time, particularly if it takes a long time to fill a system with water only to find leaks on the first try. If leaks are found in a hydrostatic test, it will take longer to remove the water and dry the piping sufficiently to make repairs.

Hydrostatic-pneumatic leak testing is different from the two-step test in the preceding paragraph. In this case the pressure test is conducted with a combination of air and water. For example, a pressure vessel designed to contain a process liquid with a vapor phase or air above the liquid may have been designed to support the weight of liquid to a certain maximum-expected height of liquid. If the vessel was not designed to support the weight when completely filled with liquid, it would be possible to test this vessel only if it was partially filled with process fluid to a level duplicating the effect of the maximum-expected level.

INITIAL SERVICE LEAK TESTING
This category of testing is limited by the codes to certain situations. For example, ASME B31.3 limits the use of this technique to category D fluid service. Category D fluid services are defined as nonhazardous to humans and must operate below 150 psi (1035 kPa) and at temperatures between -20 and 366°F (-29 and 185°C). ASME Code B31.1, section 137.7.1, does not allow initial service testing of boiler external piping. However, that same section of ASME B31.1 permits initial service testing of other piping systems if other types of leak testing are not practical. Initial service testing is also applicable to inspection of nuclear power plant components by Section XI of the ASME Boiler and Pressure Vessel Code. As indicated, this test is usually run when the system is first put into operation. The system is gradually raised to normal operating pressure as required in ASME B31.1 or design pressure as required in ASME B31.3. It is then maintained at that pressure while an examination for leaks is conducted.

VACUUM LEAK TESTING
Vacuum leak testing is an effective way to determine whether or not there is a leak anywhere in the system. This is normally done by drawing a vacuum on the system and trapping the vacuum within the system. A leak is indicated if the trapped vacuum rises toward atmospheric pressure. A manufacturer of components quite often uses this type of leak test as a production leak test. However, it is very difficult to determine the location or locations of a leak if one exists. Smoke generators have been used to determine the piping location where smoke is drawn into the piping. This is very difficult to utilize unless the leak is sufficiently great to draw all or most of the smoke into the pipe. If there is substantially more smoke generated than can be drawn into the pipe, the smoke that dissipates into the surrounding air can easily hide the leak location. Obviously, this method is not suitable for testing the piping at or above the operating pressure unless the piping is to be operated at a vacuum.

STATIC-HEAD LEAK TESTING
This test method is sometimes called a drop test because a drop in the water level in the open standpipe, added to the system to create the required pressure, is an indication of a leak. Once the system and standpipe is filled with water, the standpipe level is measured and noted. After a required hold period, the height is rechecked and any decrease in level and the hold period are recorded. Any leak location is determined by visual inspection.

HALOGEN AND HELIUM LEAK TESTING
These test methods use a tracer gas to identify leakage location and leakage quantity. In the case of halogen leak detection, the system is charged with halogen gas. A halogen detector probe is used to sense leakage of the tracer gas from any exposed joint. The halogen leak detector, or sniffer, consists of a tubular probe which sucks a mixture of leaking halogen gas and air into an instrument sensitive to small amounts of halogen gas.

This instrument employs a diode to sense the presence of halogen gas. The leaking halogen gas is passed over a heated platinum element (the anode). The heated element ionizes the halogen gas. The ions flow to a collector plate (the cathode). Current proportional to ion formation rate, and thus to leakage flow rate, is indicated by a meter. The halogen detector probe is calibrated using an orifice that passes a known leakage flow. The detector probe is passed over the orifice at the same rate that will be used to examine the system for leakage. The preferred tracer gas is refrigerant 12, but refrigerants 11, 21, 22, 114, or methylene chloride may be used. Halogens should not be used with austenitic stainless steels.

Helium leak testing may also be done in the sniffer mode, as explained above for halogens. However, in addition, helium leak testing may be performed using two other methods that are more sensitive in detecting leakage. These are the tracer mode and the hood or closed system mode. In the tracer mode a vacuum is drawn on the system, and helium is sprayed onto the outside of joints to be inspected for leakage. The system vacuum draws helium through any leaking joint and delivers it to a helium mass spectrometer. In the hood mode, the system to be tested is surrounded by concentrated helium.

The hood mode of helium leak testing is the most sensitive method for detecting leaks and the only method accepted by ASME Code Section V as quantitative. Manufacturers of components requiring a hermetic seal will use the hood method of helium leak detection as a production leak test. In these cases, the component may be surrounded by helium in a chamber.A connection to the component is made to a helium leak detector, which attempts to draw the internals of the component to a vacuum close to absolute zero.

Any leakage of helium from the surrounding chamber into the component will de drawn into the helium leak detector by the vacuum it is producing. The helium leak detector contains a mass spectrometer configured to sense the presence of helium molecules. This closed-system testing method is capable of sensing leaks as small as 1X10-10cc/sec (6.1X10-12 cubic in/sec), standard atmospheric air equivalent. The closed-system method is not appropriate to measuring a large leak that would flood the detector and render it useless for further measurement until every helium molecule could be withdrawn from the detector.

The closed-system method is not appropriate to a piping system in the field because of the large volumes. Also it does not show the location of the leak or leaks. Finally, the sensitivity of leak detection, using the closed system, is many orders of magnitude greater than normally required. The helium sniffer is the least sensitive method and is subject to false indications if helium from a large leak at one location in the system diffuses to other locations.

A large leak can also flood the detector, temporarily rendering it useless until all the helium is removed from the mass spectrometer. The helium pressure used in all these methods is normally one or two atmospheres, which is sufficient to detect the presence of very small leaks. The low pressure also serves to reduce the amount of helium required for the test. Helium leak testing is rarely, if ever, used to demonstrate that the system can safely withstand the design pressure rating.

Helium leak detectors will not be successful in finding leaks unless the component or piping system is completely dry. Liquid contained in a small leakage path, due to capillary action, may seal the leak because of the low pressure of the helium and the surface tension of the liquid. Therefore great care is required to use this approach under completely dry conditions. Otherwise this system may be even less sensitive in detecting a leak than a high-pressure hydrostatic test. Furthermore, the helium leak detector is easily contaminated by oils and other compounds and rendered inaccurate. Field conditions are normally not free of the possibility for contamination of the leak detector.