Fuel Gas Screw Compressor with axial high vibration

[sjpark]

Hi all,

I have a screw compressor, which is fuel gas compressor. It has 4 female/6male screws no timing gear. The customer want to reduce vibration because they had to welding cracked pipe and structure caused by high vibration.

The highest vibration freq. is 4x rpm, it could be screw mesh freq. I guess. And, the direction is axial.
Hor. and ver. vibratin is about 4mm/sec but axial is 17mm/sec. Not to mention, dominant freq. is 4x rpm.
I also checked the structure vibration, exactly this 4x rpm is main vibration on the structure and pipe.

I need your help for reducing this vibration.
If you have any information and experience, Please tell me about anything.

 

[CESSNA1]

sjpark: A little difficult to respond to because some addiotnal inof is needed. High axial vibration is unusual. It is almost always horzontal/transverse. Some basic questions. Is it for certain that the high vibration is cracking the dischage pipe? Who took and analyzed the vibration data? Is the dischage on the side or the end of the compressor? Is the acceleration p-p or G?

The following is based on assumptions.

Asuming a rotational speed of 1800 RPM(?) the frequency is 4*1800/60 = 120 hz. This gives an acceleration about 1.2G and a displacement of about .05 mm (about .002"). That seems like a small displacement. Is the dischage pipe exactly matched up to the compressor so there is no strain on the compressor? Is there a resonance in the discharge pipe? Can the discharge pipe move freely in reponse to temperature changes and vibration levels? How was the cause of the pipe failure determined? Is the compressor on isolation mounts?

Regards
Dave

 

[sjpark]

First of all, I should say 'thank you CESSNA1'.
I suspect that the high vibration would be main cause for pipe crack. I checked vibration data from several points, on both side of Compressor and some points of pipe lines. Whole machine including structure is rattling. The source of that is 4x rpm(14325cpm). The running speed of Compressor is 3550 rpm.

Highest vibration is 5.411 G, Peak to peak on that freq.
It is equal to 23.588 um(PtP), 0.929 mils.
Autally, We had to fix the crack of some pipe lines and structure frames until now.


I also suspect the resonance in the pipe.
But, I would like to reduce the source of vibration. That is best way to avoid the resonance.

 

[MikeHalloran]

I'd be inclined to let the compressor dance around a little if it wants to, and connect it to the pipes with replaceable metal bellows.



Mike Halloran
Pembroke Pines, FL, USA

 

[sms]

Sounds like a pulsation problem. Does the compressor have a discharge pulsation damper? Is it operating properly?

Mike, with this class of machine I have had pretty bad experience with bellows. The pulsation tore the metal bellows up and ended up filling the enclosure with gas. I try to avoid bellows on screw compressors.

 

[MikeHalloran]

Yeah, bellows have a finite life, which should be predictable, since the vibration spectrum is known.

Is the vibration new? Without timing gears, I'd conjecture the rotors would scuff and otherwise misbehave as they approach end of life.




Mike Halloran
Pembroke Pines, FL, USA