I am attending on a vibration problem in a PA fan-1 (Primary air fan). I have attached the report and vib signatures for your reference. Motor rating is 870 KW. Max rated speed of the machine 1480 rpm and it’s a variable speed drive. Depending on the load the fan speed varies from 500 to1480 rpm.
The problem is there is steep increase in vibration levels when the machine reaches 1000 rpm on Fan DE horizontal. A predominant 33.4 Hz (2XRPM) was seen in the spectrum with amplitude of 15 mm/sec. When the speed goes to 1100 rpm again there is a decrease in vibs (5.2 mm/sec).
I can see 2X even at 800, 900 and 1100 rpm. But what is causing only 2X to excite at 1000 rpm is what I am just wondering…???. I did the bump test on fan shaft, coupling cover, fan blade, fan and found all frequencies are well above 33.4 Hz. Not able to do bump test on coupling internals and also run the motor in solo condition because of production pressure
It was observed that the vibrations on the motor cooler (Heat exchanger) just above the motor was also vibrating at 33.4 Hz predominant frequency at 1000 rpm with amplitudes reaching 77 mm/sec. So I took the cooler out and made measurements on the Fan DE. Again the same vibs are repeating. So I concluded that there is no influence of the cooler on Fan DE.
Phase measurements indicate the following at 1000 rpm,
Fan NDE: H-0.42/118 deg, V-0.068/146 deg, A-0.24/185 deg
Fan DE: H-1.05/302 deg, V-0.36/220 deg, A-0.18/55 deg
Motor DE: H-0.22/84 deg, A-0.10/255 deg
There is a phase difference of around 200 deg between Fan DE and motor DE in axial direction. But I was thinking how the coupling becomes unstable only at 1000 rpm and not at higher speeds. Infact the theory says the machine fault amplitude increases with increase in speed whether it is unbalance or alignment problems.
I made one more measurements at 1000 rpm. Here at 1000 rpm I slowly loosened both the bolts of top half cover of the Fan DE plummer block. The vibs dropped from 14.5 to 12.2 mm/sec (at 33.4 Hz). When I put my hands on Fan DE I felt some impacting inside bearing. But the bearing is in good condition and no bearing fault frequencies seen. You can see the time waveform plot given in the last page of the report.
There are no such high vibs on PA fan-2 which is running in good condition at all speeds. (Readings and signatures attached). PA Fan-2 is running next to PA fan-1
The snap of the spare coupling being used to connect motor and fan assembly is shown in the report. It is 2 halves of the coupling connected by 2 springs. Are these 2 springs trying to distort or making the coupling assembly unstable at 1000 rpm which shows predominant high 2X on Fan DE and Fan NDE and also low amplitudes (again 2X) on motor DE
and motor NDE. Your inputs will help me...???
But one question still remains... why only at 1000 rpm...??, why not at higher speeds…???