MCB Positive Displacement pumps will not work.

[BB3242]

The problem starts with a MCB process. The pumps are internal rotary gear, and pump from a 30 foot high tank. They are about 20 feet from the tank, the centerline of the pumps no higher or lower than the outlet from the tank. Some of the givens,
Temp-240 Deg.
Product Visc.- 450 cST
Spec. Gravity- 1.15
Pump Perf.

1050 gpm
160 ft. head discharge.

I have ruled out the possibility of the NPSHa being a problem. The atmospheric pressure puts the NPSHa over the top. The problem is that the pumps have a positive return that when there is too much pressure within the pumps they pop the return and the discharge goes to the suction. The pumps cavitate wildly before this happens and continue to cavitate after. The seal ends up being one of the failure modes on the pump.
The problem is not the discharge either. I have walked down the line and found no problems. There is a mobil centrifugal working at same flow and same conditions and is pumping when necessary. Now the main problem is that this is all information I have to work with without having been able to see the pumps in action. I can only see the centrifugal that is working there now. The centrifugal connects in at about 5 feet from the outlet of the tank, and then about 10 feet from the discharge of the two pumps. So it essentially bypasses about 30 feet of line in or about the pumps. Any trouble shooting ideas?

Any help is much appreciated.

 

[25362]

Is the pumped fluid a mixture of chlorobenzenes ?

 

[BB3242]

Main Column Bottoms of the the FCC so it is sort of a mixture of asphalt similar substances. I do not believe that there are and Chloro benzenes.

 

[25362]

FCC bottoms are slurries containing catalyst fines and are sent to settlers, which might not be 100% efficient. What would happen to a internal gear rotary pump handling a residue with some solid particles ? I leave the pondering to you.

 

[katmar]

What size is the suction line? Could there be a jammed valve or other blockage between the branch to the centrifugal and the suction of the gear pumps? It sure sounds like an NPSHa problem to me.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BB3242]

The suction line is 8 inch.
Yeah I would imagine the same thing with the catalyst heal and how that would screw up the pump.

 

[GSTeng]

I am no pump expert by any means but I once had similar problem with a pump when I was an intern. Everyone blamed the intern and said my pump was undersized. It turned out that the pump was wired up wrong by the electrician, instead of three phase delta configuration it was a three phase Y configuration. They switched it and it worked great.

Try to think out of the box and see if there may be something simple, that is not mechanical, that may be causing the problems.