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Npsh Available Of Pump


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#1 Zeeshan

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Posted 23 January 2008 - 10:03 PM

Dear Sirs,


I have a small query which has reference to the problem of Top PumpAorund (TPA) Pump, loosing suction (not motor tripping) in a Crude Distillation Unit. The pump lost suction when Atmospheric Column pressure was reduced from 2.2 Kg/cm2g to 1.8 Kg/cm2g. The design operating Atmospheric Column pressure is 1.6 Kg/cm2g, for particular Crude Mix.

My query is, does Colum pressure have any effect on NPSH available, for the pumps taking suction from Distillation Column?

Any comment/similar experience/troubleshooting of the referenced problem will also be helpful.

Pardon me if this type of problem has already been discussed...I just didn't find it...

Regards,

.......Zeeshan

#2 JoeWong

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Posted 24 January 2008 - 02:51 AM

QUOTE (Zeeshan @ Jan 23 2008, 11:03 PM) <{POST_SNAPBACK}>
My query is, does Colum pressure have any effect on NPSH available, for the pumps taking suction from Distillation Column?


As your feed operating pressure still same as its vapor pressure, don't see reduction in pressure has decreased the NPSHA.

#3 djack77494

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Posted 24 January 2008 - 02:12 PM

Joe, I don't quite agree with your assessment. If there is a sudden pressure reduction, then the liquid that is in the system before the reduction may be "superheated" and able to flash enroute to the pump's suction. I will concede that this would be a short term phenomenum, and the steady state condition would be restored before too long.
Doug

#4 JoeWong

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Posted 24 January 2008 - 02:48 PM

The pump may experience momentary "superheated" liquid which will provides additional benefits, however, level of "superheat" will decrease until it back to saturate liquid condition. Personally i don't really have idea how fast "superheated' liquid can "stay", i would guess it would recover fast.

Maybe the pump has very low margin between the NPSHA and NPSHR. With pressure reduction in column, more light end flash off from same tray and the liquid viscosity increase with more heavy end in it. NPSHA drop (due to higher frictional drop) may lead to higher tendencies of cavitation.

However, i believe there is pressure compensented temperature control at the reboiler may tends to reduce heating as overall operating pressure drop, lesser light end flash at lower temperature at pump-around tray. This counter the effect as discuss earlier.

With small reduction of pressure (0.2 kg.cm2), i think above the system may not see above effect.

#5 Zauberberg

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Posted 26 January 2008 - 11:51 AM

Ordinarily, column (or suction vessel) pressure reduction should not affect your pump NPSHA - assuming there is enough time for new vapor-liquid equilibrium to take place. However, as Doug pointed out, if the residence time of liquid in the accumulator (or partial draw-off) tray is insufficient, cavitation will occur. This phenomenon is opposite of sudden increase in vessel operating pressure, accompanied by apparent "subcooling" of liquid at the bottom of the vessel and pump suction line - because time is needed for new equilibrium conditions to be established. This is the way how you can start some troublesome pumps lacking of NPSHA.

From operational point of view, this pressure reduction is followed by increased demand for TPA flow. Increase in actual flowrate (I guess you might have TRC-FRC control loop) causes NPSHR to increase according to the NPSHR vs Q pump curve, and this could be one possible explanation of sudden loss of positive suction head.

Personally, I didn't have experience with this kind of problem - although I liked so much to change column operating pressures and watch for effects of increased/decreased pressure on other equipment in the system. Maybe is just that NPSHR/NPSHA margin is to tight, and that is easy to calculate - just take piping isometrics and fluid properties and see the output. Try to measure the time during this phenomenon (cavitation) and if still happens when you decrease system pressure at a rate - let's say - 0.05 bar per 20-30 minutes, I would say that something is definitely wrong with that pump.

Best of luck,

#6 Zeeshan

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Posted 28 January 2008 - 10:15 PM

Thanks to all of you, for response.

I also had same belief that suction vessel pressure is cancelled by vapour pressure in these cases, due to vapour-liquid equilibrium and I am still continuing it. However, time span in restablishment of equilibium is a good point to proceed further.

Doug,

The pressure reduction was not sudden (0.1 Kg/cm2 per 4 hrs).

Joe Wong,

Yes, there will be no significant change in vapour liquid traffic due to two counteracting phenomena..reduction in pressure and reduction in overall temperature profile.

Zauberberg,

Yes, there is a TRC-FRC control but the control is on reflux....not on TPA. However, TPA flow was also increased which led to increase in NPSHR but at higher flow rates also, NPSHR was lower than NPSHA. Yes, margin is little bit tight.

Overall, the time span in recovery of vapour liquid equilibrium is a good point to investigate further because other PumpAround pumps have started loosing suction too!!!!!!!!!!!!!!

Thanks once again to all of you.

#7 Zauberberg

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Posted 29 January 2008 - 09:36 AM

Total liquid inventory (which, divided by flow demand input from FRC, will give you "volumetric averaged" residence time) is something that can be calculated or at least approximated. By adding accumulator tray volume to the suction piping volume, it is easy to reach this averaged residence time. Especially for bigger towers (DIA>2.5m) the residence time for critical services - such are reflux or pumparound pumps - should not be less than 5 minutes, in order to provide you adequate liquid degassing time and safe margin in case of tower upsets.

Is your suction piping that is closest to the column (horizontal outlet nozzle and line segment) free of any pressure reducing devices (LCV, gate/globe valves, line reducers etc.)? Also, it can happen that your suction line is undersized in the case of operating the tower at lower pressures which causes TPA or reflux flow rate to increase. This is another issue that can be checked.

Regards,




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