KSB feed pump balancing disc damage
KSB feed pump balancing disc damage
(OP)
I am new to the forum and looking for some tips.
Application is on a 3x1 combined cycle project. Deltak HRSG's with GE 7 FA combustion turbines, D11 steam turbine.
HRSG's have duct burner.
GE 7 FA's have steam power augmentation used on a regular basis.
Have had 7 disc failures on KSB HGC-5 8 stage pumps.
Each HRSG has two 100% capacity pumps with 5KV drive motors.
Evidence on the pumps indicates "loss of water on the balancing disc". No evidence of cavitation on the impellers. The drive end bearing will indicate some wiping. Little or no bearing wear on the non-drive end bearing. Pump alignment and pipe stress has been checked and no mis-alignment has been found.
First indication any problem is a trip from the seismic vibration switch, (no continous analog X-Y vibration or axial monitoring available).
Recirculation flow in low flow conditions has been 10 to 20% above minimum required by manufacture, confirmed through DCS historian. (An analog control valve is used for recirculation control, not an arc valve.)
The NPSH has 20 feet of margin, on paper, for the worse case condition.
We plan to install temporary full time axial and X-Y vibration monitoring.
Has anyone else experienced this failure and what was the resultant findings?
Application is on a 3x1 combined cycle project. Deltak HRSG's with GE 7 FA combustion turbines, D11 steam turbine.
HRSG's have duct burner.
GE 7 FA's have steam power augmentation used on a regular basis.
Have had 7 disc failures on KSB HGC-5 8 stage pumps.
Each HRSG has two 100% capacity pumps with 5KV drive motors.
Evidence on the pumps indicates "loss of water on the balancing disc". No evidence of cavitation on the impellers. The drive end bearing will indicate some wiping. Little or no bearing wear on the non-drive end bearing. Pump alignment and pipe stress has been checked and no mis-alignment has been found.
First indication any problem is a trip from the seismic vibration switch, (no continous analog X-Y vibration or axial monitoring available).
Recirculation flow in low flow conditions has been 10 to 20% above minimum required by manufacture, confirmed through DCS historian. (An analog control valve is used for recirculation control, not an arc valve.)
The NPSH has 20 feet of margin, on paper, for the worse case condition.
We plan to install temporary full time axial and X-Y vibration monitoring.
Has anyone else experienced this failure and what was the resultant findings?





RE: KSB feed pump balancing disc damage
* A restriction in the balance line. Perhaps this one has a restriction orifice or venturi fitting to measure balance line flow. Could there be something lodged in this fitting?
* An assembly error. Improper set up could result in a mounting that allowed the balance disk to rub axially while the pump is running within its normal range of shaft positions. This could be an error in the mounting of the balance disk (split ring retainer, shimming) or the thrust bearing (if it has one).
* A design error. The specifications for the size and clearance of the balance disk could include an error that results in repeated failure. This would be hard to prove.
We don't have any pumps of this exact type or configuration, but I just finished overhauling a 9 stage Worthingon, stacked diffuser, high pressure barrel pump that probably has some similarities. I have never seen an axial rub in this pump at the balance disk face. A similar smaller pump in another unit used to rub the axial face of the balance disk/drum regularly. We found design problems and controls problems that were resulting in extreme variations in flow (much too high and much too low).
RE: KSB feed pump balancing disc damage
rmw
RE: KSB feed pump balancing disc damage
Thank you for your response.
These pumps utilize a "lift-off" assembly for initial start and do not have a true thrust bearing. The "lift-off" assembly, (two back to back roller bearings) has no indication of damage or wear.
These pumps have an oil bath "slinger ring" lubrication, not force lubed.
The balance system in a drum/disk assembly.
The balance line and fixed reducing multi-ring orifice have been checked and no debris or plugging was found. The reducing multi-ring orifice has been checked for correct part number but I do not know how to insure the reducing orifice is manufactured correctly.
After the pumps were rebuilt by the manufacture, the axial and radial clearances were checked and meet the manufactures clearance requirements.
There still could be a manufacturing error, but based on the thrust clearances it meets spec.
We are pulling more data from the DCS control system to further investigate "high flow" and "low flow" conditions.
rmw,
Thank you for your question.
At present we are looking into the extreme peaks and valleys the pump may be experiencing. The preliminary data does not indicate anything beyond pump guarantees. This data was a long term request, which averages the data. We are now pulling 1 second data from the areas in most obvious operation, which would be on start, shut down of the unit, and during power augmentation operation.
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
Where does your balance line discharge to? We sometimes use the flow from the balance line as part of our minimum flow so we will discharge it to the DA. We will also have a bypass for additional flow to meet our minimum required flow. I have seen where customers have tied the two together in a small diameter pipe. The pump has plenty of pressure to push its minimum flow through the line. While the pump is bypassing the differential across the balance line changes and the flow through the balance line drops off. And that is the problem. I guess the question would be where does your balance line discharge and where does your bypass discharge?
checman
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
We have had these pumps completely disassembled on several occasions and I have not seen any evidence of suction recirculation cavitation, cavitation on the impellers, pump casing, or pump nozzles. I am not brave enough to say that some flashing is not occurring, it is just not evident except that the balance disk is wiped. We plan to monitor both radial vibration and axial movement.
JJPellin and Checman,
The "spill back" or return flow from the balance disc returns to the suction nozzle, ~30 gpm. Total pump flow is ~2,000 gpm. The return line and cavities have been check for debris or plugging, no problems have been noticed.
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
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RE: KSB feed pump balancing disc damage
The odd thing with this is the total trashing of the balance disk assembly, minor wiping on the drive end bearing with no observable impeller damage. The photos returned from the job site had good pictures of the pump impellers and they looked brand new. If there is any evidence of pump cavitation, we have not found it.
In the interim we have another site going into outage later on in the month that also utilizes KSB boiler feed pumps. We will be inspecting those as well.
This is a warranty issue so we are quite naturally interested in getting to the bottom of the balance disc problem and were hoping someone else out there might have experienced similar failures with KSB Boiler feed pumps.
Thanks, Dan
RE: KSB feed pump balancing disc damage
What is your minimum bypass flow? Where does it discharge? jjpellin I have trouble shot several high pressure boiler pumps. After misalignment and pipe strain, heat from the balance line and bypass flow have been common problems. On some I have changed where the balance line discharges to and on others I have increased the bypass flow to dissipate the heat from the balance line in the pumps suction. I have some good stories on how pipes and buildings shook until changes were made. At the pump company I work for we have several pumps balance lines running back to the DA. When properly done it’s a good way to dissipate heat from the pump.
Always busy and outside the box. checman
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
The "spill back" or return flow from the balance disc returns to the suction nozzle, ~30 gpm. Total pump flow is ~2,000 gpm. The return line and cavities have been check for debris or plugging, no problems have been noticed.
We should know a little more tomorrow. As an aside, we are also going to install a pressure gage on the return line.
RE: KSB feed pump balancing disc damage
I am interested in the minimum flow through the analog recirculation control valve. How much and where does it go? Also is the valve slow open slow close? Thrust loads from valves that fly open and close can raise heck with balance disc or drums.
Regards checman
RE: KSB feed pump balancing disc damage
But maybe your term "valves" included all of the above.
Maybe I should have said "flow fluctuations" instead of "load fluctuations" above.
rmw
RE: KSB feed pump balancing disc damage
The Recirc Valve returns to the LP drum. I don't know the flow but I will ask.
Valve control is typically slow acting and I am unsure if this is the case at this plant. This is one of the items they will be looking at.
I think KSB shows up out there today so I should hear something tomorrow.
One other thing that we are currently unsure of is when is this damage occuring. Is it more likely to be occuring as a result of a pump start or stop and compounds with the number of starts and stops. Or is it a result of something happening during steady-state operations. Or something else.
It is an interesting problem.
Thanks for all the input.
Dan
RE: KSB feed pump balancing disc damage
RE: KSB feed pump balancing disc damage
The plant was originally built to operate as a base load unit but in reality it was being operated more as a peaking unit.
The owner was aware that valves and check valves on the suction and discharge of the bfp's leaked by but did not disclose the fact that no preventative maintenance was done on the equipment. The lack of maintenance was because the owner chose to ignore that over profits from peaking. They did not equate that with bfp problems.
There was enough leakage to where there was insufficient pressure in the balancing disk area for the disk to do its job during startup so for a short time the disk was not centered and there was metal to metal contact - gradually trashing the balancing disk.
This was proven out to the point that it is not a warranty issue anymore. It is a repair issue at the owner's expense.
Thanks again for your suggestions and input.
pdldavis
RE: KSB feed pump balancing disc damage
First let me say "thank you" for all your input and questions.
The final determination for the damage to the balance disks was the fact that the two HP discharge check valves and the level control valves to the HP drum were leaking. The plant cycles off each night and was attempting to bottle the unit to concerve temperature and pressures for a re-start the following morning, which is a standard practice. But because the HP checks and level control valves were leaking through, a vapor void was being created in the piping during the night. (I do not believe the vapor void ever made it all the way to the pump due to the head pressure of the LP drum, which supplies the suction pressure to the pump and there was no evidence of cavitation on any of the pumps during inspection by KSB.) The next morning when the pump was started, it was taking up to 9 seconds to develop pump pressure but the pump had 100% flow indication, which definately runs the pump off the curve. (It had to fill the void in the line to build pressure.) As pdldavis explained, during this condition, full flow low pressure, the fixed break down orifice could not supply enough pressure to overcome the thrust and therefore the balance disk was coming into contact with the fixed drum face.
The moral of this story is, equipment maintenance. No matter how many check valves or safeties you place in a process, if it is not maintained, failures will occur.
The majority of the confusion to this problem was due to attempting a solution from 1,200 miles from the site. Once I was on site and witnessed the operation and gathered historical data from the control system, the evidence was clear.
Once again, thanks to everyone for their input. I hope this helps someone else in the future.
RE: KSB feed pump balancing disc damage
rmw