High Axial levels in a Lobe type Compressor

Hi guys,

We have some Process Gas Compressors here which are Positive Displacement, Lobe Type Compressors from AERZENER. Everything was running smooth for a long time but after Overhauling, the compressors are showing high Axial vibration in the DE side. The Compressors have 2 lobes and 106 gear teeths in the timing gears and are running at 510RPM. Acceleration reading are not so high but the axial velocity reading is too much. Can anybody give some inputs in this problem?


Original Post
Check the repair history, was the bearing turning on the shaft? Was the shaft sprayed to fit the size? Was that a proper job? Have seen cases where the machining of the shaft that was made to enable spraying to size introduced a crack propagation set off point that eventually cracked the shaft. Turning of the shaft gave a very sharp edge, perfect to start off cracks. Only an example. Do you find any hi freq. contents indicating lobe contact or axial lobe contact w. wall? Is the gear worn? Is it running the regular direction not hanging upside down turning backwards? Those that do that are worse. I have seen 10 times higher levels than yours on such. Producer also have the same measurement system as you, send in the data and ask for advice.
Remedy if the above problems are found? Replace shaft and or gear. 3-lobe systems go smother. Olov
I am sorry that I am calibrated in velocity mm/s RMS vs. Hz could you rescale? So hi up a suspicion is a resonance excited by something possibly a rub or as this looks, from the looseness present. You could do a bump test when it´s not running to confirm suspicion. Olov

If you enter Bump in the Find droplist at the top of this window, you will get a lot to read. In this case, there is no need for any especially fancy stuff. Just hit it with 2"x4" in the axial direction, either you get response in the freq. area or not. My bet is that you would get that but you never know. I also think that when you fix the basic problem that freq. will also disappear. Don´t wait to long to fix your basic problem, don´t let the levels go up to the 30-50mm/s range I have seen, then it will stop by itself and you will need a larger supply of spare parts around to fix it quickly when it does stop. If your downtime is expensive, I would advice acquiring some parts already in this situation. I am not a sales guy for those parts. I have done work for other endusers of these machines only. Work of the machine and the cyclic nature of the loads do put hi demand on how the device is assembled after overhaul and that it´s tight as a fiddle and no play or looseness whatever in the bearing mount neither shaft fit nor outer ring mount and not to worn gear assy. There are several failure modes that I have seen including axial looseness that give the rotors or one rotor freedom to hit the bearing cover and end cover, gear wear that make loobes interfere and hit, and the faulty repair problems previously described. Repair guy may never know what is beneth the shiny new surface until it has cracked. I have seen some interesting procedures in the handbook and they are the only way to do it to get normal operating hours as explained by repair guys that only work with those. If you are operating at or above the upper range of the device or are considering increasing production, go for a 3-lobe device. Those I did that work for did that, partly on advice and on production increase and that saved me from having a Xmas in Bankok some years ago. Olov

A "Bump"(impulse-response)test can, in many situations, be performed with machine running. Two methods are possible:
1) Simply measure averaged spectrum with and without rapid hammer hits. A broad peak around the frequency of interest can indicate presence of natural frequency.
2) If your analyzer has "Negative Averaging" capability, then follow it's procedure. Measure averaged spectrum with hammer hits, and then stop hammer hits and continue average to remove the effect of operating vibrations, leaving just the response to the hammer hits.

No disrespect to Olov, but I would not use a 2x4 piece of wood to impact a large heavy blower. A 3 to 5-lb hammer with rubber tip or impact pad would be more suitable, if available.

If Machinery Information is important, then make the effort to measure or calculate it!

Walt, not to be grumpy, it will be hard to do a bump test on a resonance while it´s excited during operation. What will be averaged away, what will remain, who knows. I don´t think you can excite the resonance more effective with the hammer than the machine do by itself if that´s what it´s doing. If this is the type of machine I think it is and I have reason to believe so, it is neither large nor sturdy, nothing like a Nash design as a comparison and it normally sits on a beam framework on top of the expansion damper chamber. 2"x4" bumping works well as I have had people do that for me on such a device, no offence, just to clearify. Olov
Jenish, you mention that you are unable to do a bump test because if this blower shuts down, the plant shuts down. Based on the harmonic content I have a feeling that this blower may shut down anyways but just not at your convenience and probably with a bit more damage than if you were to shut it down to investigate. Sounds to me like the overhaul was bloched and you may have to get back into this machine. What are the economic differences, lost productio with a repairable blower or lost production and no blower? Good luck
Ok, my mistake, didn´t know there was 2 lobe that size and that speed, I am always happy to learn. Description was so similar to the things I have seen before so I fooled myself into believing it was the same. Do you have a silencer tank beside the device then? That size put the higher levels you have in a more alarming perspective in my view. Do you have access to sample oil to confirm if there are problems that can be verified from oilanalysis? Anyway I have made bump tests on full size papermachines and shaker tests on 800MW generators so size is not always a limiting factor, sometimes the other way around, getting true data across a turbine from the signal of a tiny hammer may sometimes be a challenge but I agree give much more information if you have use for it. I like a quick whack with the 2"x4" for quick answers as I rarely have a FEM model to input the results from hammer tests into. I would anyway look first at the primary looseness problem, have you looked at the feet and mounting so there is no problem like that? Olov

I have a lot of experience with large 2-lobe blowers. My current project involves structural modifications to a Sutorbilt Series 8000 blower rated 600-hp with 16" diameter by 48" long rotors. The blower could have been rebuilt incorrectly. The blower could have been installed incorrectly including:
1) Shaft Alignment and shaft end gap
2) Insufficient shim size, soft-foot and base bolt tightness
3) Suction and discharge pipe interferences

The foundation and pipe supports could have degraded over time or an internal fault developed prematurely.

My project involves all of the above plus weak and resonant foundation. My recommendation is take a very systematic approach to fault identification, or you may chase your tail on these machines for a long time.

Olov must swing a 2x4 much harder than I can!

w_f_strong [at] msn {dot} com
Walt, I just adjust the tools to the task, for papermachines I recommend one nearly empty tambou in the crane swung by 2 germans using 2 ropes, I prefere others to do the hard work, is that why you prefere the tiny hammer? :-) I know, you want the data, just joking. Seriously I have seen severe pipe cracking and weld cracking around these things when the silencer has been miscalculated or completely missing and pipe resonances also giving very hi vibration levels in the pipes also giving cracks so the pulsating nature of the flow give problems sometimes. I am more worried about this one now knowing it´s a big one. Olov

On the Sutorbilt blowers, I don't measure axial vibrations on the housing near the drive shaft, because there is only a seal there. The bearings are supported by the main casing, so I measure there. If you were measuring on the lightweight housing, then your data may be affected by a locally resonant or loose structure. Anyway, just be sure that you have a good representitive axial measurement before tearing apart the blower. I believe that the thrust bearing is at the other end from the drive shaft. The timing gear housing is at the far end and the shafts thermally grow away from that end toward the DE. The incidences of rotor-housing rubs were at the timing gear end of the casing. I would NOT recommend a blower overhaul based on one set of data that you posted.

Thatz really a good information for me.. I was measuring the axial vibrations on the housing near the drive shaft. Maybe that is the problem. A locally resonant or loose structure!! The casing is big enough that you may not get good reading. That made me to take it from near the shaft. Ok.. Let me check on the Main casing and come to you later!!!
Hi guys,

This compressor is giving a big headache for me..
As Walt suggested, i started taking one axial reading on the main casing. But the reading from the outer housing is alarming me. Itz around 50mm/s with an increasing trend where my main case reading is just around 10mm/s. I took some acceleration readings both vertical and horizontal direction which is in the attachment. Can anybody give a comment on it? Is it indicating mis-alignment-the one sided waveform in the vertical direction? The second signature is giving a clear indication of the peaks which is bothering me(24.36). I'm getting similar thing in two other compressors also!!

Add Reply

Likes (0)