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At present we are operating a diesel column bottom pump (saturated liquid at around 300 deg C). Pump runs for 24-30 hours and suddenly discharge pressure drops beyond permissible value and pump needs to be stopped. All other operating parameters are well with in the range. We donot understand the phenomena why discharge pressure needs to be dropped suddenly when all other parameters are same. One of the idea is internal recirculation, pump is developing the vapour some how internally then vapour accumulates and after considerable time pump needs to be stopped. Please advice.
 
Answers
02/02/2014 A: Bruce Carr, Suncor Inc, matukaze1@hotmail.com
Where abouts on the pump curve are you running? Discharge recirculation will start to occur at ~20% below BEP.... Suction Recirculation ~30% below BEP... Either will cause a build up of heat, if the liquid is already at saturation it will surely start to boil... If the flow rate is constanly running at these rates consider changing the size of the impeller or installing a recycle line back to the tower and running within +5% -10% of BEP. How are your seals handling this?
28/01/2014 A: Satish Angadi, Haldor topsoe, satish.angadi@gmail.com
I also agree to previous answers, of less NPSH.
Also one more information is missing, is the pump operating at lower than its normal operating point? If yes, then is the minimum flow recirculation line open? Excessive heat of churning can lead to more than acceptable vapor generation in the pump.
I can suggest to increase the operating level in column bottom, to increase suction head and see if this problem can be mitigated.
27/01/2014 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Mr. Bowers gave the answer while I was working on another question. I agree. You say that all other parameters are met. Does that include the available NPSH? That could be just barely adequate and occasionally inadequate.
27/01/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
It sounds like inadequate NPSH at times.