9 replies to this topic

[rychurek]

Hallo.


This is my first performance on this forum, so sorry in advance for any letter or substantive mistakes. My issue that makes me crazy is centrifugal pump discharge pressure. We have LPG centrifugal pump with control valve on discharge side that's maintain constant flow.As we increase flow, discharge pressure goes down (Bernouli energy conservation) but what's happen with discharge pressure when we would like maintain flow rate but increase pressure after control valve by for example increase head loss( extra heat exchanger). I know that dp on valve decrease so flow as well and we have to open valve more to reach preset value. In my mind i have pump curve and my first feeling is : if flow rate will be constant, so discharge pressure should be constant, and by energy balance: we add resistance by add heat exchanger but decrease resistance of control valve. Does it make sense?

[Art Montemayor]

Rychurek:

What you should furnish is a schematic flow diagram of what your centrifugal pump installation looks like so that we can all understand what you mean and refer to when you say “with control valve on discharge side that's maintain constant flow”. A centrifugal pump does not supply a discharge pressure; it simply follows its performance curve and will deliver the fluid flow rate that corresponds to the discharge head (pressure) that it has to overcome. It is that simple.

You do not increase the flow through a centrifugal pump. The pump will pump more fluid only when the total head it has to overcome is decreased (like opening a valve on the discharge of the pump). It is difficult to understand your writing, but I think you mean to say:

“In order to maintain a constant flow rate through a centrifugal pump, one must maintain a constant total head on the pump (which means that the suction and discharge pressures remain constant).” If that is what you mean, then you are right. HOWEVER, bear in mind that if you have a flow control valve on the pump’s discharge nozzle and you have process equipment downstream of the control valve (i.e., a heat exchanger), then the pump only sees the head (flow resistance) of the control valve as it is throttled. It does not sense the pressure drop within the heat exchanger. That is why a schematic flow diagram is needed to explain your specific query. That is also why you must relate to and understand the concept of a total head on a centrifugal pump. That is what the pump "sees" and responds to.

[rychurek]

Appreciate for very quick answer.


Sorry for mess in my description. I'm open for a constructive criticism with makes me valuable member of this forum
I described system includes my question . I know that's quite different compare to first query. I hope that is more clear to understand.


 Basic system drawing.xlsx   68.51KB   109 downloads

[Art Montemayor]

Yes, you are right. A detailed schematic is much better communications between engineers than a written process description. However, I think we still are having some language barriers or failures. Please refer to my comments on the attached Rev1 of your workbook.

I also took the liberty to tidy up your schematic so that it would read better and allow us to continue to make comments on it in the future.

I hope this helps to resolve your concerns.

Attached Files

•  Basic Pump System Drawing(Rev1).xls   304.5KB   125 downloads

[Anup Paul]

Dear rychurek,

There is some mismatch in your drawing with respect to your explanation.
Based on your explanation PV1 should be located in min circulation line. When FV2 opens to increase the flow, then there is decrease in head of the pump (as mention by Mr. Art. Pump will flow its curve) so PV1 opens to maintain the pressure (PV1 Opens more to send more flow via min circulation line).

[rychurek]

Art Montemayor:

Thank you for remarks on excel sheet. I tried to put some more details on it. Please relate to these changes
 Basic Pump System Drawing (Rev2).xlsx   71.52KB   74 downloads

[rychurek]

Dear Anup Paul
In almost 90% plant load pump discharge pressure is close to 4050Kpag and spare pump starts automaticly. So query content this topic. IS POSSIBLE INCREASE PUMP DISCHARGE PRESSURE TO PREVENT SWITCH PUMP BY DECREASE OF PC1 SETPOINT TO FOR e.g 3000Kpag. MY PRESUME IS: AS WE DECREASE PC1.SETPOINT PV1 START TO CLOSE---> FLOW RATE DECREASE(DECREASE PRESSURE DROP ACROSS FV2) SO FV2 OPEN TO MAINTAIN RSP. SO IN THESE NEW CONDITION WE HAVE FV2 MORE OPEN(LESS DP ACROSS VALVE) AND LESS OPEN PC1

[TS1979]

There are some mistakes if the system is designed as the EXCELL drawing indicated. There shouldn't be any presure control valve downstream of the pump. Pump is used to booster pressure while the pressure control valve to reduce pressure. That arrangement will waste energy. If you have to use a pressure control valve, it means that you have wrong pumps.

The correct control should be just using flow control valve located downstream or upstream R1, preferably the flow control valve located upstream of R1. Normally higher operating pressure will be better for R1 provided that the operating pressure is not over the MAWP.

You can have a pressure sensor downstream of the pump. When the pump P1A discharge pressure is low, you can automatically start the pump P1B.

[rychurek]

There are some mistakes if the system is designed as the EXCELL drawing indicated. There shouldn't be any presure control valve downstream of the pump. Pump is used to booster pressure while the pressure control valve to reduce pressure. That arrangement will waste energy. If you have to use a pressure control valve, it means that you have wrong pumps.

The correct control should be just using flow control valve located downstream or upstream R1, preferably the flow control valve located upstream of R1. Normally higher operating pressure will be better for R1 provided that the operating pressure is not over the MAWP.

You can have a pressure sensor downstream of the pump. When the pump P1A discharge pressure is low, you can automatically start the pump P1B.

Hi
I hadn't got opportunity to ask project engineer about this solution, but in two hydrogen plant(12 and 2 years old) prepared by different supplier is the same arrangement in desulphurization systems. The three feed LPG,Light Naptha and Natural gas can be used simultanously. LPG, LN are in liquid state where vaporized in shift gas-feed HX. Downstream of system is vapours so FV1 control Volume flow rate.In my opinion the thing is maintain constant pressure before FV1-plant load control valve,to more accurate control flow

[TS1979]

IMO it is not the normal configuration for the system control. I believe that one flow control valve will do the work. If you have a stable feed gas (or liquid) flow rate in the real operation, the pump should generate stable discharge pressure based on the flow. For my understanding, the higher operating pressure for the HDS unit, the better desulfurization result. Of course, the operating pressure must be below the MAWP for the vessel.

To get accurate steam flowrate to the reformer, you need pressure, temperature compensation for the process gas flow.