Vibration velocity limits?

[edison123]

What are the standard alarm / trip levels of vibration velocity for 2 pole generators as per different standards (MIL, ISO, API etc.)? In one 2 pole generator, after balancing by one of the leading vibration analysts, the final vibration velocity on vertical direction was about 22 mm/sec (0.75 in/sec) and he says it is okay as per standards. Any comments ?

 

[electricpete]

That sounds very hi. I don't have any generator references handy, but for NEMA MG-1 new motors, the required vibration is <0.12ips if tested on rigid mount and <0.15ips if tested on flexible mount (bearing housing accelerometer measurents).

 

[plawrence]

I'm not sure about generators either, but most of our motors (900-3600 rpm, 25 hp to 800 hp) run below 0.15 ips--and that's coupled to equipment that's running from 0.2 to 0.4 ips. If the generator is running 0.75, what's the equipment around it running at? What's the spectrum look like?

Patrick

 

[edison123]

ISO 2372 states that for class IV machines (Turbo generators with light weight substructures), velocity of 10 to 25 mm/sec is &quot;fair&quot;. We also feel that the present vibration, which is mostly of 1 X RPM despite the balancing by the expert, is still high. But then, when he throws the book at you, then you cannot do much about that.

 

[plawrence]

Forget ISO. Anything turning that fast and vibrating that much just isn't right--especially if you're sure it's all balance. Tell your expert the controlling standard is _your_ standard and throw the book back at him. You don't want fair--you want excellent. Your equipment will thank you for it.

Patrick

 

[electricpete]

I agree with pl. I know you're looking for a standard (not opinion) to strengthen your case and unfortunately I don't have one.

I was wondering about your unit conversion.
22 mmsec.

Would be 22/25.4 = 0.866 ips IF both numbers expressed in peak/0.

My understanding is the international folks use velocity in mm/sec RMS units and US uses velocity ips in PK/0 units, which means the ips number would be even 1.41 times higher

22mm/sec rms => 1.22 ips pk/0.

If that is really what you have I don't see how anybody can look at you with a straight face and tell you it's ok. Compare it to the new motor (rigid base) NEMA standard above and you see a factor of 10 difference!

 

[edison123]

electricpete:

Thanks for the correction about the unit conversion. The velocity I mentioned was peak.

I have posted this query in reliability forum and got some good suggestions about relevant standards.

Thanks

 

[vib1]

The big question is whether this is shaft vibration or bearing cap. Most OEM publish limits of shaft vibration. If it is shaft, the allowable is much higher (3-5X cap). Most OEM set alarms at about 50 micrometers 0-peak and trip at 75 micrometers. For 2-pole generator(3600 rpm) producing 1X unbalance vibration this produces about 20 mm/sec 0-p and 30 mm/sec 0-p, roughly. Some European OEM have higher limits but this would be too high for the caps.