NPSHR for Parallel Pumps
NPSHR for Parallel Pumps
(OP)
Hello there! Been a lurker for many years while in school and finally took the plunge to register. I will try to contribute as much as I can (though I still have alot to learn).
I understand the very basics of what series and parallel pump configuration does to net head on flow capacity. However, for parallel configurations, I am a little unsure about NPSHR. I would assume that you would add each of the NPSHR for each pump and that would be that correct?
Also, if anyone knows of any good rotating machinary book I could use as a reference, I would appriciate it.
Thanks in advance!
I understand the very basics of what series and parallel pump configuration does to net head on flow capacity. However, for parallel configurations, I am a little unsure about NPSHR. I would assume that you would add each of the NPSHR for each pump and that would be that correct?
Also, if anyone knows of any good rotating machinary book I could use as a reference, I would appriciate it.
Thanks in advance!





RE: NPSHR for Parallel Pumps
An example may help to illustrate my points. I have two pumps piped in parallel. They both tie into a common suction manifold and a common discharge manifold. There are no control valves, orifices or other imposed pressure drops in the piping that connects the two pumps. Rather than running one pump at its rated flow of 1000 gpm, I run both pumps at 500 gpm each. The total flow is unchanged. The NPSH available to each pump increases very slightly. The flow through the common piping is still the same and so it has the same pressure drop and flow losses. But the flow through the individual suction line to each pump is running at a lower flow, lower velocity and so it has a lower pressure drop. For a typical API centrifugal pump, the NPSH required by each pump will drop as a result of the lower flow through that pump. So, I have a higher NPSHa and a lower NPSHr. In theory, I end up with a better NPSH margin and the pumps are less likely to cavitate.
If this is the case, why don't I prefer to run all of my pumps in parallel? First, running at 50% of rated flow, each pump will have a lower efficiency. Running further from the best efficiency point, each pump will have higher radial loads on the impeller. Each pump will be running further back on the curve. Depending on the shape of the curve, the pumps may not be sharing the flow equally. Especially if the curve is flat in this region, any slight differences in the condition of the pumps will drive one to carry a higher portion of the load and one to carry a lower portion. The pump at lower flow could drop below the minimum flow required to avoid suction recirculation cavitation or other destructive conditions.
It is much more common in the real world for pumps running in parallel to be less reliable than pumps running individually. But it really depends on where each individual pump is running relative to its best efficiency point and how much NPSH margin it has above required.
As a reference to the principals involved, I would suggest any of the following:
"Pump User's Handbook" by Bloch and Budris
"Centrifugal Pumps – Selection, Operation and Maintenance" by Karassik and Carter
"Cameron Hydraulic Data" edited by Heald
Johnny Pellin
RE: NPSHR for Parallel Pumps
I'm confused why you said that. What's up?
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHR for Parallel Pumps
I guess that it is refering to the fact that the efficiency tend to increase with flow increase. Is n it Johnny?
Wimple
RE: NPSHR for Parallel Pumps
RE: NPSHR for Parallel Pumps
Johnny Pellin
RE: NPSHR for Parallel Pumps
Johnny Pellin
RE: NPSHR for Parallel Pumps
Profit = Q * (TransportCost/Q - K * Q/eff)
but at a pipeline that is profit and that equation quarantees you'll never ever never even think about doing it. So, I quess what I'm saying is efficiency must be significant to you in order to be able to mention it in the first place. Know what I mean?
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHR for Parallel Pumps
We have a crude unit that runs a booster pump at 100,000 barrels per day. If they had an opportunity to sell more product from this unit, increasing the unit capacity to 101,000 BPD by running both booster pumps might pay out like a slot machine. The extra 1,000 BPD could generate a net profit of $3 per barrel. And that is truly net profit after accounting for raw materials costs and all processing costs. Over the course of a year, that 1,000 BPD would generate $1.1 million in additional profit. The energy cost for running two pumps at lower efficiency might be wasting 200 HP. At our current energy costs, that 200 HP would cost us about $70,000 per year. Who among us would not spend $70,000 to make $1.1 million? Even if I add in a penalty for reducing the pump reliability, I can't compete with the production incentives. The only thing that could trump this decision would be a significant impact on safety or environmental compliance
Johnny Pellin
RE: NPSHR for Parallel Pumps
RE: NPSHR for Parallel Pumps
You do know about McNally? www.mcnallyinstitute.com You could do some lurking around there too.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: NPSHR for Parallel Pumps
RE: NPSHR for Parallel Pumps
For transfer of crude oil from storage tanks to offshore facility about 5 km and the route is more or less flat.
My question is whether we need booster pumps before get in to shipping pumps to transfer oil?
or
we can instal directly the shipping / transfer pumps nearer to tank farm for transfer of oil?
what coudl problem will anticipate if we avoid booster pump.
the flow capcity required is about 45,000 bbl / hr at the rate of 325 psi.
Can any one suggest and comment on that.
Regards,
vijay
RE: NPSHR for Parallel Pumps
Basically no way to tell without knowing the entire configuraion affecting the hydraulics (headers, valves, viscosity and pipe diameter). You might only need transfer pumps. Supply the details in your new thread.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/