Effect of second pump in the line on NPSHr
Effect of second pump in the line on NPSHr
(OP)
Hello,
I have a water system with a huge NPSHr, compared to the NPSHa. I can't do much about the NPSHa, so i need to somehow reduce my NPSHr. I'm wondering what the effect would be on the NPSHr of adding a second pump in the line, in serie with the first one, somewhere halfway the end.
Any thoughts?
Thanks,
M.
I have a water system with a huge NPSHr, compared to the NPSHa. I can't do much about the NPSHa, so i need to somehow reduce my NPSHr. I'm wondering what the effect would be on the NPSHr of adding a second pump in the line, in serie with the first one, somewhere halfway the end.
Any thoughts?
Thanks,
M.
Process - Piping
ing - EiT





RE: Effect of second pump in the line on NPSHr
If I were you I would add another pump, but make sure you run it at 1750rpm. Keep in mind: In series the flow stays the same, and the TDH stacks.
I would say you are better off replaceing what you have with a 1750rpm multistage pump, or a vertical can pump if you are really hurting for NPSHa.
RE: Effect of second pump in the line on NPSHr
A question for you, how can you have a water system with a huge NPSHr? - NPSHr is a pump requirement not a system requirement, do you mean that the pump NPSHr exceeds the NPSHa, if this is the case then you need to select a pump with a NPSHr lower than the NPSHa.
RE: Effect of second pump in the line on NPSHr
Best regards
Morten
RE: Effect of second pump in the line on NPSHr
by reducing flow, or speed or increasing head.
http://www.mcnallyinstitute.com/07-html/7-03.html
http://www.gouldspumps.com/cpf_0008.html
http://www.gouldspumps.com/cpf_0008.html
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"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
RE: Effect of second pump in the line on NPSHr
"Running pumps in parallel can solve NPSH issues in a particular system since you are utilizing half of the available value, more or less."
Its not clear how you would be using half of "the available value". What available value, NPSHa? Would you then be running 2 pumps at half flow?
Then why just not run one pump with a control valve to cut back on flow and increase the NPSHa in the suction line?
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"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
This sounds possible, but ineffecient, correct? If you were to replace the current pump, you might as well put in something that will do the job.
RE: Effect of second pump in the line on NPSHr
Of course replacing the pump with the proper model is best, but that option wasn't part of the question.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
I'm trying to model a system in Pipeflo. Ultra Pure Water system, closed loop. I have a source tank with water level at 10m. System requires a 80m3/h flow. Piping is PVDF, 225 mm. Larger pipes are not ok. Required head is about 15 m.
The program tells me the NSPHa is not sufficient. For a good NSPHa, i need to raise the level in the tank to 35 m. For obvious reasons, the level is limited to 10 m.
Fluid velocity: 1.5 m/s (which is too low actually)
At this point, i don't have a pump model. That's what i'm trying ot find out with this program, but it get stuck because of the NPSHa. Maybe a 2nd pump in serie would help, but Pipeflo doesn't like that option..
Process - Piping
ing - EiT
RE: Effect of second pump in the line on NPSHr
You should be able to do that easily with a 125-80-200 pump, needing approx 1.7m NPSH.
Perhaps you have too much suction piping/losses, and you run out of NPSH before the pump....where is your pump located in relation to the supply tank? What temperature is the water?
RE: Effect of second pump in the line on NPSHr
When i launch a calculation, i get an error message at the pump, that won't go away until i raise the level in the tank to 35m. Tank and pump are close together, so the solution is not there. Normal conditons for water.
How did you come up with 1.7m NPSH?
Process - Piping
ing - EiT
RE: Effect of second pump in the line on NPSHr
Reduce the specific speed & suction specific speed
by reducing flow, or speed or increasing head.
Stop using pipeflo for awhile and start reading the references I gave you.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
Do it the old school way.
RE: Effect of second pump in the line on NPSHr
I ran your 80m3/h at 15 m head through a pump sizing program, it came up with lots of options that require 1 to 2 m NPSH.
There's obviously something a bit wacky with the way the problem is set up, or the way the system is calculating.
RE: Effect of second pump in the line on NPSHr
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
RE: Effect of second pump in the line on NPSHr
However it sounds as if your Pipeflo program or the defaults or inputs are bad. Suggest you calculate by hand, with simplifications if necessary, this might help some insight into whats going wrong.
Cheers
Steve
RE: Effect of second pump in the line on NPSHr
I did a test with a very short line, and still had the same issue. So i figure something's wrong in the design settings, material specs, estimated pressure loss in equipment etc...
Biginch, i did check your references. Thanks. And yes, a booster pump is a possibility. I'm not familiar with these, tho, so i'll need to investigate that as well.
I realise this post is probably not in the right forum... sorry about that..
Process - Piping
ing - EiT
RE: Effect of second pump in the line on NPSHr
Process - Piping
ing - EiT
RE: Effect of second pump in the line on NPSHr
Bet you wouldn't have done that if you wern't trying to use a program first.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Effect of second pump in the line on NPSHr
RE: Effect of second pump in the line on NPSHr
Ted
RE: Effect of second pump in the line on NPSHr
Process - Piping
ing - EiT
RE: Effect of second pump in the line on NPSHr
**********************
"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/