Pump running off BEP

[Muinda]

I have an oversized pump that sees excessive vibrations mainly becauses it is running off its BEP. The pumps BEP is 1350 GPM and it runs bwteen 770 GPM and 960 GPM. We have been replacing the thrust bearings yearly, because of high vibration.
At this time we can't change the process, make modifications to the shaft or re-rate the impeller. Ideally we would want to get this pump back on track by either a recirculation valve, triming the impeller or decreasing the L3/D4 ratio.
For the next repair I would like to make some changes in the bearings. Currently I have 5214 NR/ C3 on the thrust and 5312 C3 on the radial bearing.

Any comments on going down to CN clearance instead of C3?
I read the following thread821-76627 with much interest.I have reading at when the ambient temperature was 29 the bearing temp was around 80 C. Under what conditions could I put in a CN bearing and not experience heat problems?
Any comments on using ACBB with a different angle? Because of the excessive thrust, would going to 45 degrees make more sense?



Any info is much appreciated,

 

[Artisi]

Easiest way is to re-rate the impeller, why try and re-invent the wheel by changing bearings or shafts size. I would assume this is a pump with bearings and shaft correctly designed to normal operation. If you have an idea to make such radical changes at some time in the future - look at the process and re-rate the impeller, alternately change pump - probably easier, cheaper with less operating problems

International College
Naresuan University
Phitsanulok
Thailand

 

[PUMPDESIGNER]

It would help me make more specific comments if I knew more about the pump and what the manufacturer says.

Specifically I would like to know the pump specific speed and suction specific speed. I know the American values well (Ns & Nss), but if you do not have that then let me have the BEP flow rate and head and I will calculate myself.

I would also like to know what the manufacturer says, for they are the authorities on the pump’s range of acceptable flow rates, and also the authorities on how much NPSHa is required to maintain smooth flow through the impeller. They do the testing, and only testing can determine these things for any specific pump.

If you know, I would also like to know the inlet angle of the impeller vanes, and how many vanes on the impeller.

First realize that you have two paths to take.
1 - Eliminate the source of the problem.
2 - Strengthen pump to withstand the problem.

We all probably agree that eliminating the problem would be preferred, Leaving the problem in place is undesirable and personally I would not allow myself to get sucked into that.

Go ahead, play with the L3/D4 ratio, but then the vibration changes pitch, moves into other places like the motor, and you still have very high power draw and wasted money if the pump is high Ns.

With the information you have given, and IF I accept your diagnosis, then that means you are playing with a very high Ns and/or Nss pump where energy transfer from impeller into the water is chiefly by diffusion. If that be the case, then you must not operate the pump at low flow rates.

Now you are living with the consequences of the situation possibly created by situations not under your control. But if you modify the pump to withstand the symptoms of the problem rather than solving the problem, you then may end up living with the consequences of your own actions (modifications), which is far more painful.

Also consider that as an engineer you are stepping out into uncharted territory. No one tries to make pumps work under incorrect conditions to the extent that you are trying to do, so you are on your own.

Better to solve the problem. Increase flow, or reduce speed, trim, or whatever, but by some means end the problem and you will save money and have nothing to come back at you later.

If you decide to eliminate the problem, put up some more info and I will make more comments.


PUMPDESIGNER

 

[tonyh]

Note the above comment "IF I accept your diagnosis".
It is possible that the problem is associated with running off BEP if you are dealing with high specific speeds, but there are lots of other possibilities for high vibrations in centrifugal pumps, and many run very adequately at well below 57% of BEP without untoward vibration.
Are you sure that you don't have major misalignment - coupling or pipework -don't have insufficient NPSH and are cavitating your pump, - have balanced your impeller so don't have major out of balance etc....?

As far as your bearing design goes, what lubricant are you using? and what is the mode of failure of the bearings? Without that information it is difficult to make any sensible reply.

 

[Artisi]

Just expanding on tonyh's comments re vibration, all are valid points, my previous comments assumed (rightly or wrongly) as this has been an ongoing problem you have checked - shaft straightness, mis-alignment,loose couplings, inadequate pump / motor baseplate mounting / design, mis-aligned pipework,NPSH etc.
Unbalanced impellers, both dynamic and hydraulic are an often overlooked problem area and should be looked at- as should the electric motor rotor balance and shaft straightness and the integretary of the mounting of the motor rotor / shaft -unlikly but not entirely unknown.

Another often overlooked cause of "pump vibration" is pipe work vibration, you could be exciting the pipe work and assuming it is the pump that is vibrating when it is actually the reverse problem of the pipework vibrating the pump.
A vibration analysis of the system would be worthwhile as it will give you the frequency/s and go a long way in identify the problem.

International College
Naresuan University
Phitsanulok
Thailand

 

[Artisi]

An after thought to the previous comments.
Pump discharge isolators / eliminators / expansion bellows etc, if fitted to your pump unit, can also be the source of vibration, if they pulse at a frequency that coincides with that of the pump or pipework this can reinforce that frequency and cause many problems.
From my experience, "The pump you sold me is vibrating" when applied to the conditions it was sold for usually turns out to be the system not the pump.

International College
Naresuan University
Phitsanulok
Thailand

 

[electricpete]

A quote from SKF Publication “Bearings in Centrifugal Pumps”

“Bearing internal radial clearnace greater than normal (C3) is recommended for bearings mounted with heavier than normal interference shaft first and if high bearing temepratures are expected either from operation at high speed or from heat conducted to the bearing from an external source.

Greater than nroaml (C3) internal radial clearance is recommended for radial bearings operating at greater than 70% of the catalog speed rating.

The API 610 Standard specified that bearings, other than angular contact ball bearings, shall have greater than normal (C3) internal radial clearance. Angular contact ball bearings are sensitive to excessive clearance, so special considerations apply to them.”

By the way, what type of bearing is 5312 (what does the 5 stand for?)


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[Muinda]

Thank you for all your valuable posts. I really appreciate your interest.

I have a Byron Jackson 6X 9X 15 1/2. With a 3 vane 14.5 inch impeller.

The system is rated for 954 GPM and 910 ft head. BEP is at 1350 with 820 ft head. NPSHR is 22 ft at BEP and 10 at rated. We have 80 ft at the pump suction. RPM is 3550 12600 and 835 for Nss and Ns respectively.

Typicaly, the vibrations gradually trends upward from rebuild to failure with peaks that correspond with low flows. At these peaks there is predominatly a high amplitude spike at vane pass frequencies. Unbalance and misalignment has been ruled out from vibration analysis. We do not have any issues with bent shafts, looseness, soft foot or motor imbalance. We installed a new base 3 years ago.

We do suspect problems with lubrication level and will be adding a constant level oiler. Current we just have a vented sight glass. We use mobil 626. We have inpro vbxx on the bearing housing with steam quench on the seal. We had the bearings analyzed on the last failure and NTN determine the heavy wear was mostly likely due to insufficient lubrication.

Our oil analysis shows that the water content, acid number, wear particles and additives are all on spec.

I have spoken with Flowserve and they have recommended an impeller re-rate and an API 9th edition upgrade. My process, and vibe data is to be sent to their engineering group later this week. In the meantime, I have excessive vibes on the thrust and I do not want to just replace the bearings on this repair.

Does pipe strain vibrations produce frequencies other than misalignment?

For electricpete - the 5 is the designation of NTN's bearing series.

Thanks a lot everyone, your comments are much appreciated.

I'll be in touch

Stephen Muinda
Junior Mechanical Reliability Engineer

 

[Artisi]

A few thoughts for consideration.
A frequency problem resulting in the bearing failure is not a bearing problem per se.
Changing bearings and or shaft does not remove the cause.
Poor lubrication is a problem of its own.
You indicate that the thrust bearing is the problem - is the impeller shunting back and forth, you may need to review the axial thrust / hydraulic balance.
Is there another impeller available from B.J.ie, 4 vane or 5 vane , this will change the blade pass frequency and might go a long way to easing the problem.
Investigate changing the shape of the cut-water.
Can the duty afford an impeller reduction, remachine the diameter at something other than at 90 degree to the vertical axis, this might change the frequency some-what.

Good luck



International College
Naresuan University
Phitsanulok
Thailand

 

[smckennz]

Going to CN sounds like a bad idea. C4 may be better if there are lube problems, and things are hot.
Normal practice when hot is to increase clearance, not decrease it.
You havent stated oil temp but your comments imply it is not low. Is an oil cooler an option? Sometimes we run a water hose on a bearing housing, but I would not do this with CN.
I think your immediate concern is to get some heat out of the bearings.
Then do the good stuff with the impeller as has been recommended.
A cost/benefit proposal may speed things up as you are obviously wasting a lot of money on poor pump efficiency.

Cheers

Steve