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NSPH and PUMP PERFORMANCE DOUBTS

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Pucp

Materials
Aug 6, 2001
32
Hi, I will like more about water pumps, because now I have to deal with them, I have read a lot but francly I have a lot of doubts. So I will appreciate a lot if you answer some of my questions:

1)How can I measure the NPSH - R in a new pump? Does I have to introduce friction losses and wait until the onset of cavitation and then measure the head at this point at the suction...the NPSH - R will be the difference between this head and the vapor pressure of the water?
2)How can I change the NPSH - A? can I introduce friction losses closing valves?
4)What happen if I put a valve before the suction of the water and after I start the pump with the valve complete open, I start to close it. Does it makes the friction losses increase? Does the flow changes?
3)What happen with both the NPSH - A and the NPSH - R if I increase the flow rate? Does the NPSH - A go down because of the friction losses increase? and does the NPSH - R goes up because of the same reason?
3)How can I change the head with the same flow rate? Can I introduce losses without changing the flow rate?
4)In what part of the system can I change the flow rate, at the final? with a valve?
5)Can you recommend me a book where I can find how to calculate the hydraulic forces in the impeller?

Again, I will appreciate your reply, I know that some of them will be so easy to you but for me are all the way around. Thanks in advance. Best Regards
 
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There seems to be some confusion in you numbering. I have used the one you have proposed ;-)

1) Sounds about right
2)For testing? Under normal operation you wouldnt want lower NPSH. A valve would do fine anyway
4) Well a centrifugal pump delivers head and since the dP downstream the pump also is head then one must assume that the flow would actually remain constant - as long as you dont go below NPSH-r
3) NPSH-r does not change - its a theretical value thats measure at pump inlet. If you close a valve then NPSH-a drops but flow cf. 4) should remain constant.
3) I dont really follow you
4) A valve on the discharge side will reduce the flow (you move up the pump curve)
5) Sorry

Best Regards

Morten
 
Pucp,

1. You really can't measure it unless you have a test lab with a closed loop or suction throttling device. Here is how NPSHR is measured:

a. Do a perormance test with AMPLE (nebulous term, especially away from BEP) NPSHA. This will give your characteristic curve.
b. Select about 10 flow points. For EACH flow you are going to set your discharge head/throttling valve to hold a specific flow. Throttle the suction and measure the head. You may need to tweak to discharge head/throttling to keep the flow the same. Near BEP, the head will hold very steady as you drop the NPSH by throttling the suction. It will rise a little, then as you decrease NPSHA more, it will suddenly drop FAST. The NPSHA which produces a 3% head drop at your flow is defined as the NPSHR at that flow. Do this whole process over again for other flows.

Very costly. Typically you can rely on mfgs standard curves unless you have a critical application like a high vapor pressure fluid (low SG or hot, like a condensate extraction pump....)

2. As MortenA noted, you want to maximize NPSHA. Don't forget that if you are pumping from an open system you have 34' minus losses plus static elevation. You will see many pump specs written that specify max NPSHR of 10', and they are pumping out of a water tank vented to atmosphere and they have 40' NPSHA!

4?. Dont do that! It serves no purpose!

3?. NPSHA will generally drop with higher flow due to increased losses. But if your piping system is reasonable well designed, and you do not have a long run of pipe, the increase should not be large. NPSHR increases. Honestly you do not need to know why. If you are really interested, read Stepanov and Karassic.... on the simple level, NPSHR increases come straight out of Bernoulli's equation...more velocity = higher local pressure drop which precipitates cavitation (hence oversized impeller eyes for low NPSH applications). It is much more complicated though. Velocity/Inlet vector vs impeller vane orientation can have a large effect on cavitation onset. Generally post grad level stuff that you only need to know if you are a hydraulic desinger or high level consultant working on projects where an NPSH problem can cost big $$$.

3??. Get a different pump, or use a VFD.

4. Use multiple pumps in parallel or use a VFD (within limits, as you may run up the curve to fast.)

5. So you do want to be a hydraulic designer! Tough field! Karassic (sp) and Stepanov are the classics.

 
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