This test comes after the four-hour spin test, and is conducted at maximum seal design pressure (usually settling-out pressure) at zero speed with the seal system operating. The purpose is to check for gas leaks at all joints and connections. Also, the test is applied to compressors handling hazardous or flammable gases. If the compressor has oil film seals, it is a good idea to have the seal oil leakage rate measured in this static condition, because this condition will often exist at the compressor installation during startups and shutdowns. The static leakage rate will
be somewhat higher than that when the rotor is turning since the inner seal ring could be in an eccentric position (leakage varies approximately with the square of eccentricity). Very high static leakage could indicate an O-ring problem or some other fault that might not be apparent when the shaft is rotating.
If during the four-hour spin test, the seal oil leakage rate was not measured, or if the complete contract seal assemblies (both inner and outer rings) were not installed, a low-speed seal leakage test should be conducted at maximum seal design pressure. The complete contract shaft seals should be used, and the rotating speed should be at least 1000 RPM. This test will provide more meaningful seal leakage rates than those that can be obtained using non-contract outer seal rings. In fact, the low-speed leakage test should be the acceptance test. It is often not possible to spin the machine faster than 1000 RPM for this test because of test stand power or discharge temperature limitations.
Alternatively the seal leakage can be measured during the full-pressure, full-speed test if this optional test is specified.