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?

Is this the version of the pump that has no thrust bearing?  If it has a thrust bearing, what is the configuration?  Has the thrust bearing been found to have failed at the time of the balance disk failure?  Is this a true balance disk or a combination drum/disk?  Without these details, I would suspect some of the following:
* 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).

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