What will be the reason of 0.42xRPM peak
which is not steady.? At Axial 4 vane propeller pump.

Can this be because of recirculation (a blob blob noise can be heard randomly)?
or the reason can be, the operation away from best effiency point.

Any experience.

Direct drive or belt drive?

I'm guessing this is a vertical with water-lubricated sleeve bearings?

.42X sounds close to bearing "oil whirl" but if the pump is as Pete guessed, water lubed sleeve bearing, might be bearing "water whirl". Never seen that, but who knows, anything is possible.

0.42X subsynchronous frequency isnt frequent, but are difficult to cure when they appear.
Can be two sources :
1.- Operating conditions
2.- Machine clearances

. Trapped liquid inside a hollow can be the source of "blob-blob" noise listened due to physical configuration of this kind of axial flow pumps.

. Does the four vanes has enought reinforcement to avoid whirl flutter?

. When cantilevered rotor is destiffen due to assymetrical bearing clearance, it can produce backward conical whirl and raise the strength of the gyroscopic moment wich yield subsynchronous frequency (0,42X)

Regards and have a nice day.

Another possibility for vertical pumps with subsynchronous frequency is resonant vibration. The pump column and/or motor stand can have bending vibration modes that are excited by internal impacts as pump bearing or wear rings develop increased clearance. A natural frequency in the .45 to .49xSS range can cause oil whip in large pump motors that have journal bearings. The best way to confirm or eliminante natural frequencies is to conduct an impact-response vibration test with the machine shutdown or running.

Walt

Thank you for all

Patrick
-Belt drive,Motor was hang up the pump, which all together hang to pipe at the corner of the pipeline.

Pete
- No, it is horizontal pump with antifriction bearings

Miguel
- Gyroscopic moment looks like the reason.
So how the problem can be solve.?
They have not vortex cut plates.

Walt
- Bending modes looks another reason becuase of extra forces which comes from fluid dynamics.

Patrick guessed right belt drive. That opens some possibilities. Can you tell us motor speed and pump speed and frequency of the subsyncrounous peak? Does it show on motor at all? Have you checked belt frequency or looked for anomalies with a strobe?

A/F bearings...Have you checked bearing defect frequency? FTF often very near 0.4x. Any symptoms of bearing problem? Harmonics of the 0.42x visible on log scale? 0.42x on demod? Other families of non-sync peaks anywhere to be found?

I haven't encountered an axial flow horizontal pump before.

Is it inline? Horizontal pipe? Does the shaft exit the pipe horizontally or vertically? So the Between bearings or overhung? Do you have a photo or model number or link?

Is it flow dependent? (vibrates more at low flow).

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

Well, when you say axial flow propeller pump, I think of the big Lawrence (no relation) circulating pumps. Any work done recently (bearing change, shaft change, belt/sheave change, etc.)? Is prop tight on shaft? Do you have reason to believe it's not running where it should be on the curve? Those questions and the ones Pete asked and I think we may be on to something. When I hear "blob, blob, blob" on belt drives, I think belt problems.

Subsynchronous frequency is almost always the result of several causes combined, for example combination of angular movement quantity of the rotor plus axial force due to the flow yield precession angular speed lower than shaft speed, wich in adition of possible assymetric wear of bearings , it help to increase the amplitude of the conical motion of the shaft in precession.
I, in your place, would try to change the operating flow to change the axial pull of the flow on the impeller to observe possible change in amplitude ,or frequency, of 0,42X.
The other thing, when possible, change bearing and review assymetrical wear between both to record this data and, obviously, observe the new behavior of the pump.

There are 4 ea Axial Pump,which are all new.
At the process these are at the corners, elbows.
Single stage with 4 vanes.
Because of thermal expansions at the pipe line, the designer was hang up the Electric motor to the pump up and down at parallel position. The pumps are hang to pipe line from flange axially.

We received the subsenkronize peaks at all.
These are not fit to belt, and roller bearing frequencies.

We though, because of recirculation, the subharmonic peaks are occurs, may be because of design problem so the pumps are not working at design area.

Probably a bypass line will be the solution, but how it be apply to the axial pumps is the problem.

May be these frequency peaks are caracteristic at axial pumps.

What does the pump manufacturer say about the flow conditions? I don't remember seeing anything like that where I used to work--but I'll check with them.

We have vertical axial flow circulating water pumps with no subsync vibs identified. Likewise several axial flow fans with no subsync frequencies identified.

We do have some mixed flow multi-stage large vertical pumps with vibration at 0.35x that gets worse as flow decreases. We don’t fully understand the reasons. Worn sleeve bearings and wear rings were ruled out since condition did not change when we replaced these.

There are lots of possibilities and I doubt it will be an easy problem to diagnose. Walt’s suggested bump test would be fairly easy and may be useful. Also in looking for resonance it would not hurt to survey magnitudes of that frequency component along the pipe to see if any pattern emerges.

Was the pump originally designed for belt drive? Can you describe bearing configuration and rough dimensions from pulleys to bearings to impeller.

I would also say before you look for exotic uncommon sources to carefully rule out the common things. A double check of belt condition/frequencies and strobe inspection wouldn't hurt.

One thing to consider, if you do have shaft whirl at subsyncronous speed, it is potentially much more destructive than typical 1x vibration. Typical running speed shaft vibration rotates with the shaft so that the shaft does not flex. Subsyncronous whirl will cause the shaft to flex at a frequency of (runningspeed minus whirlspeed) which can produce fatigue cycles.

Without a whole lot of data to go on, my vague impression is that whipping and subsync whirl are much more common with turbomachinery and long-shaft vertical pumps. Some textbook reading (Shock and Vib Handbook, Chapter on self-excited vibration), leads me to believe there is a lot more mechanisms for subsycnronous whirl when rotor is above first critical and especially when rotor speed reaches/exceeds twice the speed of first critical. The whirl speed is almost always the rotor critical speed, so whirl at 0.42xrunning speed would corresponds to operation at 2.5x first critical. Also there is the possibility of the distinct overhung rotor whirl mentioned by Miguel in which case whirling does not occur at same speed as critical speed.

I am far out of my knowledge area to suggest any troubleshooting. But if you have the machine on a variable speed drive. you can determine rotor critical speed and onset frequency of this vibration. Also coastdown test for rotor critical speed. None of the measured critical speeds will capture the overhung rotor whirl that Miguel mentioned. That can only be determined by calculation. And of course checking flow dependence will be an important piece of the puzzle.

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