Centrifugal Compressor Four-Hour Mechanical Test

The four-hour mechanical spin test is a standard test for most vendors. It is also specified in API 617 and in Company Specification CMP-MS-1876. The purpose of this test is to:

1. Check the vibratory behavior of the rotor-bearing system during acceleration and at maximum continuous speed.
2. Check for proper assembly and running clearances.
3. Prove that the bearings and seals operate satisfactorily under running conditions.

This test is considered to be strictly “mechanical.” No part of the aerodynamic performance of the compressor is measured during the test. The test is usually conducted with the compressor operating in a closed piping loop at a relatively low pressure (100-200 psi discharge is common). An open piping loop could also be used. Although tests with the rotor running in a vacuum in its casing are sometimes proposed, the Company’s Specification CMP-MS-1876 prohibits the practice of testing without flow through the casing. Vacuum-type tests are disallowed because:

1. The lack of significant gas density inside the casing can have an influence on rotor dynamics.
2. The casing heats up abnormally from the churning of residual gas in casing (vacuum is not perfect).
3. The operation of oil-film type seals cannot be tested concurrently with the vacuum-type spin test.

For “flexible” rotors (those operating above the first critical), the location of the first critical speed is verified during acceleration and/or deceleration when possible. If the rotor system is highly damped, and if the rotor is dynamically balanced to an extremely low level of residual unbalance, it is not always possible to discern the first critical speed from plots of X or Y filtered vibration amplitude versus speed. Plots of phase angle versus speed also may not provide a reliable indication of the frequency of the first critical for the same reasons. For these rare occasions, the vendor’s calculations are usually relied on for the location of the first critical. If other unusual phenomena were observed during the test, it might be advisable to deliberately unbalance the rotor or coupling to make the critical more distinguishable. Engineering judgment must be employed in such cases, and it is strongly suggested that a mechanical specialist be consulted.

The inner seal leakage of the contract oil-film seals should be measured during the four-hour spin test. As pointed out in API 617, it may not be possible to use the contract outer seal bushing(s) during the test if the pressure in the test loop is too low to cause an adequate oil flow rate through these bushing(s). In this case, special test bushing(s) with greater clearance is used. The inner seal ring, however, should have proper differential pressure such that the inner seal leakage may be evaluated. CMP-MS-1876 includes some acceptance limits, the principal one being that one seal cannot have a leakage rate greater than 70% of the combined leakage of both seals.

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