KSB feed pump balancing disc damage 2

[rlee1234]

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?

 

[rlee1234]

To all,

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.

 

[rmw]

Now that is a "flow fluctuation" that I never would have imagined, but a "flow fluctuation" none the less.

rmw