Vertical Turbine Pump: Balancing correction vs Components wear out 6

[xrmntech]

Is balancing correction an acceptable practice to extend motor-pump service life when upper vibration increase is caused by down pump components wear?


On our vertical turbine pumps (Filtered water @ 80°F, 15 stages, 3600 rpm, 1000 HP, 30000 BPD, TDH= 1500 psi) we check motor uncoupled readings and take typical vibration baseline (motor-pump coupled) to confirm dynamic condition is acceptable after a unit replacement or corrective maintenance: i.e. Upper and lower cross/inline overall vibration below 0.1 ips, shaft radial movement below 3.0 Mils, acceleration in pump casing below 1.0 G's and others parameters like pressures, temperatures, motor amps, etc.

We have noticed that after some runtime (say 1 year) the 1X increases and then an inpection is scheduled to check basics at the surface level(loose components, upper bearing assembly, coupling, misalignment, throat bushing clearance, etc). If all is find OK and there isn't an appreciable change in performance (pump flow / head, motor amps), we consider it caused to pump shaft deflection due to higher clearances on pump wear rings and bearings. Then a trim balance is performed to return system (motor-pump) to above indicated overall vibration levels.

As performance and vibration are 24/7 monitored on these pumps, concept is to safely extend the service life of the units without major failure. Also, when a pump is removed and sent to shop for inspection/repair, normally all pump wear rings and berings are replaced.

Current balancing correction is intended to keep motor-pump vibration at low/acceptable levels but not sure what may be the hidden effects of trying to stabilize the motor-pump system from the upper end?

 

[raronen]

Tough question (not even BigInch took a crack at it).

It sounds like you (like almost everyone else) are monitoring the vibration of the pump from the readings above ground. Of course, as your OP mentions, the source of vibration is below ground and the readings above ground therefore have some limitations.

If I understand what you are doing correctly, it sounds like you are making a balance correction above ground for the below ground vibration. I suspect the downside might be that you are simply making your vibration readings look better when nothing down in the pump (where the wear is happening) has really changed.

Is it an acceptable practice? I don't know. When your pumps fail, how much damage is there? Does it look like you are going beyond normal wear (whatever that is) on your internal components before you pull the pump?

Alternatively, do you want to have a better means of monitoring these pumps, make them last longer, reduce the vibration, or are you happy with the current system and just want to know you are doing the right thing?

 

[xrmntech]

Thanks. Yes, so far it is working fine and economics are good BUT this is a short term conclusion. Idea is to safely extend unit service life by keeping "Upper or Surface" equipment/structure vibration below 0.1 ips. It is really compensating (masking?) pump shaft deflection caused by down pump components wear out. One of the factors in the equation is the high cost (begins with crane service) involved in any intrusive work on the pump and basically once it is pulled out, pump will go always to complete overhaul. Downhole pumps are in someway "Run to failure" equipment. Not catastrophic failure but you only pull them out once they failed (Performance or too high vibration).

There may be some papers covering this approach but haven't found any yet. With a spare unit available and a good moitoring system, economics drive the decision process.

 

[GPRnD]

I'd agree that it is just masking the true helth of your system by locally improving the vibration at one specific location (the one where you are measuring vibration).

Keep in mind that the below ground wear is still causing increased forces and loading on those components. At some point that wear and the resulting loads/movements will reach a critical level.

However since you have now "reset" the vibration baseline with your trim balance how are you going to know when the below ground wear becomes severe enough to warrant a shutdown ?

 

[electricpete]

I agree, tough question.

The link between above ground vibration and below ground vibration was tenuous to begin with. Primarily we look for change in observed behavior as indication something has changed in the machine. How severe was that change is much more difficult to quantify.

Balancing certainly will not hurt anything and will reduce stresses in certain parts of the machine near the top that now have reduced vibration. The fact that you are not addressing probable causes in less accessible areas is a calculated risk.

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[xrmntech]

That's the dilemma. Criteria now used is to pull out the pump only when there is a performance change (flow or head) or fast increase of vibration to a very high level, and a site inspection check confirming need for removal. All typical wear out components will be replaced (intended by design) but critical items like pump shaft and column (high cost) are of main concern. With accelerometer on motor upper and lower, eddy probe looking to pump shaft, system give us more confidence about stopping the unit if a major problem is present. We have also 24/7 monitoring on process parameters (pressures, flow, motor amps). We don't have accelerometers at lower section of pump as some installtion offered today in the market.

Removal, shipping, repair time, almost all details and cost will be similar if pump is removed before reaching the last end of the PF curve (Catastrophic). 6 Months to a year extension in unit service life may have a positive impact in production.

 

[electricpete]

I'm no pump expert. But just to try to talk thru some of the components and concerns for my own edification:

Wear rings clearance degradation will be detected by your hydraulci performance. That seems to be the main focus of your strategy.

Shaft concern - I don't think that change in 1x vibration creates much concern for the shaft. The reason is that it rotates with the shaft, so there is no fatigue occuring. If you have subsynchronous vibration, then shaft is flexing and I have more concern for the shaft. The reason for increase in 1x vibration is presumably...

Sleeve type bearing wear is likely cause of increasing vib over time. What happens as wear continues I'm not sure. At some point the shaft deflection becomes big enough to contact at another location. Then you'll hopefully see some distinct signs on vibration (?). Is it possible there is some seal/lub water flow path whose flow/dp can be monitored as indication of bearing condition?


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[GPRnD]

Have you ever run one of your units to failure or near failure?

If you did and got some vibration trend data, that might be the best way to make decisions in when to pull the unit.

0.1 in/s is a pretty low vibration level. Consider that the current API 610 allowable limit for vibration on vertical pumps is 0.2 in/s and for shaft movement is 4 mils peak to peak.

Personally (If I had no other past history or other site information), I'd set a maximum level before pull out at 2x this.

 

[xrmntech]

Yes. Two cases.

First case was subsynchronous vibration and 1st stage was found with broken wear rings and some parts got stuck (flow restriction). Pump head dropped 300 psig so it was a good call.

Second case vibration suddenly increased (jumped) and at same time pump head dropped 200 psig. Pump shaft broke at the third stage impeller key area. Area showed typical fatigue failure and crack initiation in the groove. Cycles, propagation and final break.

 

[GPRnD]

Interesting.

It sounds like monitoring the pump head is at least as good an indicator of the failure as vibration.

You didn't say what vibration levels you saw at and just before the time of failure, so its hard to say if these would be helpful in future trending and prediction of when to pull the pump.

However you bring up one good point. In the first failure you saw subsynchronous vibration. This is almost always a good indication that wear ring/bearing clearances are getting too high. This vibration should hopefully trend well against wear ring and bearing wear.

Something to consider anyway.


On your second failure, the fact that the fatigue failure was not near the final stages (where the shaft is most highly stressed in torsion), suggest that the failure may have at least as much due to a manufacturing defect, than to the increased vibration in the pump.

 

[xrmntech]

We use (standard): Alarm= 0,4 ips and Shutdown= 0,6 ips. There is 1 second delay in shutdown to avoid false trips ( instrumentation).

I'll report here any other finding. It may take some time until we confirmed it can be a good practice in the long term.

Thanks for all the inputs.

 

[poppeye]

A good baseline Vibration of pump and motor head upon installation coupled with dead head pressure matched to the manufacurers pump curve is a starting point. Monotoring these over time will give you degradation of the pumps performance. Impeller balancing is crucal in the repair of vertical turbines prior to assembly. Replacement of impeller and casing wear rings are standard when repairing all pumps. The increased clearances allow for turbulance and excessive axial and radial thrusting as fluid passes through these areas. There are materials available that can be used to tighten these clearances to 1/2 that of the manufacturers recommended specification. Greentweed offers several materials that can be used in the manufacture of bowl bearings, impeller and casing wear rings that cut the clearances in half, increase lifespan due their wear resistant capabilities, AR1 and WR300 are two that come to mind. Other areas of concern that should be looked into when having your Turbines repaired are, Impeller vane length. Impeller vane length can cause severe radial thrusting of an impeller if the vanes are not of equal length. This will cause premature wear of bowl bearings and wear rings. Bowl outlet difuser vanes and channels should be undamaged and smooth. From the Hp of your pumps these vanes are quite large. Polymer coating of these can decrease turbulence as the fluid passes from one impeller to another allowing for smooth laminer flow into the inlet of each stage. Also ensure that whoever does your repairs is not in the practice of shaft flipping. This is common place to save money in repairs especially in a competitive bid situation. Instead of replacing worn shafting, the shaft is flipped so the worn area is no longer at the corresponding bearing seating area. Upgrades to the bearings to wear resistant materials, balancing, and attention to detail during repairs can pay off with increased MTBR.

 

[dabluffrat]

by balanced do you mean trimming a balance disc or drum, or thrust balanced impellers?

Did you know that 76.4% of all statistics are made up...

 

[xrmntech]

One balance disc at upper motor end and one balance disc at couling (motor-pump) area.