Centrifugal Pump - API Plan 11 1

[OilBoiler]

Hi all,

I have a centrifugal pump that runs naphtha and its lubrication system is patterned after API Plan 11. The pump is rated for only 260#, but pressure gauges at the discharge line reads well above 300#. We have already ensured that pressure gauges are working properly. How is it possible we are getting higher discharge pressures? Is it because the seal mechanism?

Thank you in advance.

 

[MortenA]

Is the "rated pressure" at the expected operating point or the shut-in pressure?

Best regards

Morten

 

[phex]

as far as i understand this, you are saying that the pump should be capable of a maximum discharge pressure of 260#, but actually does 300#?

is the throughput like in design calcs? is the discharge line clean, i.e. is the line pressure drop as expected? have you checked power supply? rpm? is there any variable in your design calcs wrong?

tell us what you checked already, it should be more efficient then just guessing

 

[sshep]

I think that your seal plan has nothing to do with the higher than expected discharge pressure. I would check for things like flow (is it at rated point?), specific gravity (very likely not same as design), impeller diameter (easy to trim down if needed), speed, and/or suction head.

Good luck, sshep

 

[Nicksun]

Hi,

I would like to suppose the rated pressure 260Psig is designed as H2O. So there should be a conversion factor as for your medium which is the density of naphtha which is less than 1.
So 300Pisg by 0.6(????)should be less than 260pisg.

Am i right?
thank you.

Nicksun

 

[OilBoiler]

(I'm a newbie Process Engr, so please bear with me!)

Thanks!

The shut-in pressure is 276#. I was wondering if the high discharge pressure was due to the seal plan, because some changes were already being made to that in another project.

Well, it seems that this pump is rated for 230-250 gpm. There is a downstream CV from this pump that seems to be open only between 12-15%. The flow transmitter for this valve shows a fairly constant flow of only around 112 gpm coming out from this pump. The inlet pressure for this pump is fairly constant because it is the pressure head from an upstream drum, whose level is kept at 40%. This level is controlled by the same CV downstream of the pump in question.

The spec sheet for the pump says that the specific gravity of the fluid is 0.68, does that still mean that the pump was designed for water? I thought that the performance curves were based on the fluid whose service it was designed for.

Thanks again for your help!

 

[SeanB]

The shut in pressure you quote, is that from the datasheet or did you calculate it yourself? If you did not calculate it you should do so as a check. You say that the seal plan was modified on a previous project. Is there any chance that the impeller was changed as well?

 

[TD2K]

Your seal plan won't be causing your high discharge pressure.

Discharge pressure is the sum of the differential head produced by the pump plus the suction pressure. Differential head is read off the pump curve as so many feet of the liquid you are pumping (a centrifugal pump at a fixed flow will produce the same head whether you are pumping a light or heavy fluid). Convert the differential head from your pump curve to a differential dP (Head = 2.31 * dP / SG) Where SG is the specific gravity of the fluid you are pumping. Check to see if this matches your measured discharge pressure.