Rotor Dynamics Verification

The API mechanical standards for the rotary and centrifugal compressor have a test specified for proof of rotor insensitivity. This would normally be the test invoked at this point to ultimately prove the rotor. The fifth edition of API 617 expanded on this test and changed the acceptance criteria from those based on amplification factor to an acceptance level based on internal seal clearances.

The results from these tests will have additional value if they are used to verify the mathematical predictions made. In this way, the accuracy of other mathematical predictions, which cannot be verified except during site operation, is confirmed.

The manufacturer will have carried out a full mathematical investigation into the system vibration response. One quality control requirement will be to establish whether the correct assumptions have been made. Rotor assembly and component weights should be obtained during man ufacture and verified against mathematical data used.

The determination of the first bending critical speed is well established; however, there is also concern with regard to the rotor support system's sensitivity to exciting forces. These come from unbalance and/or gas dynamic forces arising during operation in service. Operation with dirty corrosive gas will soon cause rotor unbalance. The rotor dynamics verification test is concerned with synchronous excitation, namely unbalance. The test must also verify that the separation margins are to specification.

System response studies should include rotor residual unbalance, which can be verified during manufacture. An additional study must be made to consider the application of a deliberate unbalance weight at a location chosen by the vendor. The amount and location of the unbalance must be used in the following test. Normally the location chosen is the coupling.

After the completion of the balanced acceleration and deceleration runs, the unbalance weight must be placed at the predetermined location and the acceleration and deceleration test repeated using the same recording instrumentation. The results should be compared to the appropriate computer predictions and the API acceptance criteria applied. Accep ranee criteria were added in the API 617 sixth edition.

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