Dear all
I want to open a discussion about some differences between static and dynamic mode of some problems such piping forces,cracks, misalignment and so on. As you know there are some cases that a problem in mentioned categories does not exist at static condition, but in dynamic condition (e.g. after start of motor) we encounter it. As a case we have a utility pump that have excessive vibration at 1xRPM on pump bearing housing in axial and vertical direction. We check all things such alignment, balance, looseness, bearings, shaft bending, impeller ,... and anything is ok. But the vibration still is present. So I suspected to piping force, dynamic misalignment, casing distortion and such problems. we checked the piping flanges connected to pump suction and discharge and found some problems. Unfortunately after correcting these problems, the high vibartion still is present. I still suspect to piping forces (and casing distortion due it). because when we loose one of the suction flange of the pump at running, the vibration decease about 40%.
I appreciate anyone who has experience in this field and could help me to more understand this situation ,how to confirm it with vib. analysis and how we can treat piping forces that is corrected in static mode but in dynamic mode it come back!

Regards.
Farahani

This message has been edited. Last edited by: Farahani,

ın my opinion;it can be cavitation.may be NPSH is not suitable points for pumps.that's get pumps force about press liquid to pressure line.

Just some thoughts on possible sources of movement off-line to on-line:

If the fluid system is hot, there can of course be thermal changes between off-line and on-line.

Static pressure the system can apply forces. If the system acts like a closed pressure vessel system, then there are equal/opposite forces which put the pressure boundary walls in tension but don't tend to move anything. But if you happen to have an expansion joint, that expansion joint will not effectively transmit any tension force (will deform significantly) and now the equal and opposite force from static pressure appear on two separate components... the pump and the pipe.

Location of piping supports can be important. In general you want a pipe support near the pump so the pump isn't carrying the piping load.

There may be other forces generated in the pump and piping from fluid flow.

If you have access to a system like "Permalign" from Ludecca, you can measure the actual movement from off-line to on-line.

Maybe others have more commetns.

To verify nature of 1x did you perform phase analysis across the coupling? What kind of coupling do you have?

Farahani,

You did not mention whether you tested for a natural frequency near 1xSS. An axial rocking mode would have high amplification in both vertical and axial directions. If vertical vibration at each pump bearing is out-of-phase, then that would be an indication. Conduct an impact-response (hammer) test to measure natural frequencies.

Walt

Thank to all!
It is unlikely to be a cavitation problem. Because there is no cavitation sound nor impeller damage, but I check again with higher Fmax to include cavitation region in spectrum.

Dear Pete
The temperature is low. The pump have a not well designed support near discharge piping. We aligned the flange of pump with its flanges of suction and discharge attached piping and after that we welded the flange to pipe. So I am sure that there is no piping force due to flange misalignment. But I think after start of pump the flow exert some forces (due to turbulance, static pressure you mentioned or ...) to pipe that lead to casing distortion of pump and probably the existing support don't do any work! In one unit of our plant we install Exp. Joint between pump and discharge piping (see below phot) but the piping still have high vibration and the vibration of pump decreased only 30% .
The phase readings are below and the pump has flexible coupling (2=motor inboard, 3=pump inboard, 4=pump outboard):

2A=2 mm/s rms (150 degree)
2H=1.5/76
2V=1.7/148
3A=9.5/177
3H=5.7/260
3V=6/352
4A=10.4/177
4H=2.4/270
4V=5.5/355

The spectrum have almost pure 1xRPM. There is some symptom of misalignment but in static mode we check it and alignment is in good condition.

any advise?

This message has been edited. Last edited by: Farahani,

What is wrong with your picture. only a small amount at the top opens, or is it something wrong with my server and not letting it all in?

Same here Ralph. Maybe the government (either one) is censoring it. LOL

Dave

Hi!
I think it is because of file size and so I replace it with a smaller one.
Thanks!
Farahani