5 replies to this topic

[ARC]

Dears

I have been working on the capcity expansion project. I am stuck at the charge pump itself. There are two charge pumps. The rated capacity of the pump is 1086 gpm. Now the capcity is increased to 1318 gpm. Hence two pumps are used with 659 gpm flow rate through each pump. At the reduced flow rate now the pump is giving higher discarge pressure (1251 psig) than the discharge line design pressure (1239 psig). Also the line PSV is set at 1239 psig.


Is there any method/procedure with which the pump discharge pressure can be reduced to safe levels with thw same reduced capacity (659 gpm)?

One of the options found in the literature was throttling the discharge gate valves. But i am not very much sure about this.

[blanchy]

Hello ARC,

Pls dont say pressure when you talk about pumps lets call it as head.

First I have to ask, what is your pump type, centrifuge? If your pump's type are centrifuge, your pump discharge head (pressure) is related only your pump's impeller diameter and your pump's or motor's rpm.
HEAD= D^2 (in inches and if your rpm is 1800)

Waiting for your detailed description...

Bye

[pleckner]

You should have a basic understanding of the pump curve so I would suggest reading up on this. You can find many references to pump fundamentals by doing a search on this forum. You can also go directly to the hydraulic institute's website ( www.pumps.org).

For a centrifugal pump, pump head varies inversely with flow rate (flow goes down and head goes up). The first thing you have to ask is how did you set the flow of each pump in the first place? This was probably done by throttling a control valve downstream in the charge line (but you don't give details). You might consider moving the control valve closer to the pumps and upstream of the PSV then consider changing out the control valve trim to introduce more pressure drop to get the same flow. But my first recommendation would be to investigate putting in a reduced sized impeller into the pumps. This will allow the same flow but at a lower head because the pump follows a different curve, one which is lower than the original curve.

[Profe]

Hi Arc

If I understand well, you pretend use the two pumps in paralel. What about spare pump?

For a better understanding of your problem, I suggest you that attach a schematic draw of your feed system, and additionally the pressures at suction points, and if you attached the pumps curves I think will be better.

Mmmmmmmmmh, by the way, review the affinity laws for pumps.

Good luck.

[ASH25320]

Hi Arc

If I understand well, you pretend use the two pumps in paralel. What about spare pump?

For a better understanding of your problem, I suggest you that attach a schematic draw of your feed system, and additionally the pressures at suction points, and if you attached the pumps curves I think will be better.

Mmmmmmmmmh, by the way, review the affinity laws for pumps.

Good luck.

Dear ARC,
If you are referring to centrifugal pump set up then your system is an ideal candidate for Variable Frequency drive(VFDs), Don't waste energy by throttling the discharge valve, but an efficient control method like VFD for controlling pump capacity.

[Padmakar Katre]

Dear,
I agree with Mr Pleckner what he suggested but regarding reducing the size of an impeller which is normal pracice either to increase or decrease the impeller size in case of revamp and to use them when the friction losses becomes significant in the later stage or greater capacities are required (this is in case if you are replacing an impeller by a lower size impeller) if you are concern about the line design pressure one option I will suggest you to put one pressure control valve in common minimum circulation line which will control the pressure by regulating the minimum circulation flow.If you find any other option more economical than what we have discussed here please share with us.
One thing to ask you more what is your sparing philosophy in this case you have three pumps i.e. 2 running and one pump standby which is still not clear.Waiting for your reply.