Hydrostatic test is done on a pump [ * ] to probe the integrity of its casting [ * ] and to verify that it can withstand its maximum allowable working pressure (MAWP). But hydrostatic testing may not detect minute, or microscopic, material defect that can result in leakage.
To detect such material defect, and to test the pump for possible damage to its mechanical seal, o-ring, or gasket, during assembly, it maybe necessary to air leak test the pump.
The term "air leak test" is generic and does not mean the test medium is limited to air. Other gases, such as nitrogen and halogen, can be used as well. Nitrogen, being a naturally occurring gas is also widely used in so-called nitrogen leak test, but the use of halogen gas has been restricted due to environmental issue. A commercial air compressor, or an industrial pressurized gas canister, can be used for the leak test.
Air leak test is an optional test that is done at extra cost, and only if specified by customer. It may be specified if the process liquid is toxic, flammable, hazardous, or dangerous. Air leak test is not a substitute to the standard hydrostatic test.
The following are typical procedure for conducting an air leak test. These are very general - some steps may not apply depending on pump type and size, or the applicable acceptance criteria:
SAFETY FIRST. Conduct the test in a secured and safe environment. Access to the test area shall be restricted to authorized persons only.
Pump to be air leak-tested must first pass hydrostatic test (hydrotest).
Drain the pump and blow air to completely dry its inside surface.
Assemble the pump, including any mechanical seal if needed.
Plug all openings with pipe plugs and cover the nozzles with securely fastened gasketed metal covers.
Use a convenient pump's piping connection (vent, drain, gauge, etc.) as inlet for air injection.
Inject air into the case cavity at constant pressure of [ ** ] PSIG to [ ** ] PSIG, depending on size, and hold for [ ** ] minutes. Do not over-pressure. Air is compressible and easily expands with increase in temperature. Any defect in casting, or assembly, can cause a part or projectile to be hurled and cause damage or injury.
Completely submerge the unit in a water pool. (This may not be practical for large unit.) Observe for any presence of bubbles.
ALTERNATE: Using a soap and water solution, spray or brush the solution on the entire casing, including the seal area, if applicable.
Check the wetted areas for leaks; bubbles will appear if leaks are present.
Due to compressibility of air, a slight pressure drop is acceptable provided no bubbles are produced indicating the presence of leak. A pressure drop of [ ** ] PSIG over [ ** ] minutes is acceptable for pump components, or [** ] PSIG over [ ** ] minutes for pump assembly.
Any indication of leak should be fixed by approved method. If no leak is found the pump is deemed acceptable and ready for the next step in the manufacturing process.
Some form of identification via tag, or stamp, shall be made and attached to the unit to indicate that it passed the test.
A test certificate shall be issued, if required by customer.
[ * ] For simplicity, the term "pump", as used herein, shall mean the complete pump assembly, or the pressure-containment component of a pump such as casing, cover, stuffing box, barrel, etc. The term "casting" shall mean a casting, a forging, a fabrication, or a combination of these.
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