I have a 6 stage Delaval centrifugal pump that is exhibiting a high amplitude sub-synchronous vibration. The vibration occurs briefly at .5x running speed during lower speed operation then jumps to a frequency range of .6x to .62x at speeds above 4900 RPM. The vibration stays in the above % of running speed frequency range up to operating speed of 5500 RPM. I understand that whirl frequencies have been measured up to 130% of running speed. I also know stall occurs in this frequency range in fans. Any ideas or better yet has anybody seen this issue and know what the fix is?

Hi Mark
Going thru some old notes and a possible cause in the frequency range you indicate is hysteresis.
I have personally not observed this in my equipment.
What is the history of this pump?
Was it recently overhauled? If so, were the pieces builtup during assembly? I assume the impeller is secured with a key. Is there another part of the shaft that has interference fit that may becoming loose?

Not sure if this helps...but keep us posted if you learn cause.
Regards
Jim P

Yes, the last two barrel assemblies were recently rebuilt and have exhibited this vibration. One installed last April and another this January. This pump had never had this problem before.

Mark
I have no idea if this is a possible cause...but, whenever I see changes following maintenance it makes me look hard at what was done.
Can you dig into the repair specs/procedure and confirm inspections were SAT??
Pay particular attention to interference fits.

Regards
JP

Mark,

Can you be more specific about the size of the pump (hp) and the machine train? Is there a gearbox or fluid drive? Is there a induction motor, VFD motor, or steam turbine driving the pump? Can you confirm that the unusual frequency tracks with speed? Is the pump meeting performance requirements? Are there any other vibration symptoms or other anomolies present, even if they are of lower value?

Walt

Based on your description, I have observed the exact same behavior in a Delaval pump. You may have some harmonics of the subsynch vibes. The pump(s) had the same operating speeds, but I'm not sure of the number of stages.

The first time it happened, we replaced the element and it went away. The second time, replacing the element did not cure it, but it did reduce the amplitude of the problem.

I believe the vibration is caused by an instability in a seal or balance ring. I recommended rotordynamic modeling to determine the likely source of the instability, but the plant did not pursue the problem. In my opinion, there is possibly a design issue with the pump that makes it susceptible to the problem if it is not rebuilt with exact tolerances. If I am remembering correctly, we had the problem on two slightly different pump sizes. I am no longer with the company and can't give an updated status. I also believe that the instability was causing a high enough amplitude whirl to lead to rubbing.

You may be able to affect the vibration by changing seal water flows. If I remember correctly, too much seal water makes the problem worse.

There are seveal EPRI documents that discuss the symptoms in some detail. I'm not sure how helpful they will be as to solving the problem, but at least they might provide some additional insight. If you have access to the documents, you might want to review them. I'm on the road and can't give specific report numbers, but you can search their site.

If you want to discuss in more detail, you can email me your phone # and I will contact you when I get off this job. My email is michaeltitone at aol period com.

Michael Titone

Fluid related instability/hysteresis is possible. Usually this is more dangerous than running the machine close to the critical speed because the instability will not detach if the speed changed slightly.

Cannot you post some cascades and orbits?
Clearances were mentioned as critical issue here, I support.