Recently Bearing Inspection and Dry Gas Seal replacement activity has been carried out on a Motor driven Centrifugal Compressor during a planned shutdown. The Compressor was operating smoothly with vibration values at all four probes well within 20 microns prior to shutdown.
The Operating Speed of Compressor is 15712 rpm. High Vibration Alarm is at 50 microns and Trip at 65 microns. Calculated 1st critical of Compressor is 5000-5700 rpm. The Compressor is equipped with Bently Nevada NSV type proximity probes and proximitors. The Shaft Diameter at Probe Target Area is 3 inches.
During Start up, Vibration values on both probes at Non Drive End reached 120-130 microns at around 4000 rpm causing the compressor to Trip.
Bode Plot for both Non Drive End probes showed nearly flat Vibration Vs RPM curve during startup and coast down. In fact during coast down from around 3000 to below 100 RPM, the vibration values at both Non Drive End probes remained nearly constant at 80-85 microns on one probe and 90-95 microns on other.
Waterfall plot revealed that vibration was almost entirely at 1X component during startup and coast down.
Further it was found that even with Compressor standstill, vibration values at both probes at Non Drive End was 12 microns. Vibration Values at all other Probes on Compressor Train in standstill condition is 1 micron. Historical vibration Data was checked and it was found that previously with Compressor standstill vibration was used to be 1-2 microns.
NDE Probes were connected to DE Proximitors and it was found that vibration values in Compressor Stand Still condition swapped
Further it was found that with Compressor Stand Still, whenever one probe is disconnected from Proximitor or removed from Compressor Body and connected to Proximitor, the vibration on other probe reduces to 1 micron.
With Compressor Standstill, all Compressor Bearings RTDs were disconnected and Probes and Proximitors were replaced with like to like but problem persisted.
With Compressor Standstill, one probe was replaced with 8 mm Dia probe without changing Proximitor (keeping NSV Proximitor), vibration values at both probes reduced to 1 micron with Compressor Stand Still.