Mechanical run test - rotor response measurements during coastdown 1

[BS26]

Hi,

API guidelines for turbo machinery (centrifugal compressor or steam turbine)mechanical running test mention that the amplitudes and phase angle of the shaft vibration to be recorded during coastdown (from trip speed to slow roll speed). Why these readings are not taken during ramp up (from trip speed to trip speed)? This may be more relevant due to residual unbalance but would think that would also apply during ramp up. Any thoughts?

 

[JJPellin]

During ramp up, the speed control would be programmed to avoid operation near rotor critical speeds. As a result, the ramp rate will be high in that speed range During coast down, the speed ramps down more gradually (in my experience) allowing for a more complete picture of the response of the rotor passing through critical speed(s). In addition, the ramp up data would be influenced by the driver. If the driver on test is a motor, the motor will produce different vibration characteristics when energized and ramping up. During coast down, the motor is deenergized so these influences would not be present. If you are primarily interested in the response of the driven machine and not the test stand motor, coast down will provide a more pure data stream for analysis.

Johnny Pellin