A full-load, full-pressure, full-speed (FLFPFS) test is often called for when the compressor train is in gas injection service or other services where the discharge pressure exceeds 2000 or 3000 psi. There is no hard-and-fast rule for the pressure level at which such a test should be specified. Much depends on the Company’s and the vendor’s experience. A compressor handling natural gas at a discharge pressure of 4000 psi should undergo a FLFPFS test. As mentioned previously in regard to performance tests, such a machine would also be a likely candidate for a modified ASME Class I test to be run concurrently with the FLFPFS test.
A FLFPFS test for a compressor delivering a hydrogen-rich gas at 3000 psi probably would not be justifiable. The appropriateness of the FLFPFS test is related to the density of the gas in the compressor casing. Extremely dense gases can cause excitation of the rotor resulting in destructive levels of sub-synchronous vibration. The high pressure level in the casing can cause the oil film seals to partially act as bearings resulting in a shorter effective length of the rotor. Thus, the frequency of the sub-synchronous vibration is usually higher than that of the first critical speed, but lower than the frequency of running speed. Sub-synchronous vibration has occurred on lower pressure machines when the rotor had an unusually long bearing span.
This sub-synchronous phenomenon is known as aerodynamic whirl, and is too complex to adequately cover in this manual. When high-pressure centrifugal compressor applications arise, it is recommended that a mechanical specialist become involved in the specifications for the machinery as well as for the testing.