7 replies to this topic

[sruthildinesh]

can anyone please tell me how this" NPSH required" in pump curve is calculated by Manufactures??

[latexman]

Most likely pump manufacturers get it from actual test data.

[Art Montemayor]

This is a practical and insightful question.

I have read the procedure in the past, but I don't remember where I saw it and what I did with it. However, I do recall that basically, the manufacturer runs a specific lab test on the pump set-up, using water as the fluid. There is a pre-determined method on how to detect the exact moment the pump loses prime and fails to pump. This method is, I believe a "standardized" method used by most pump manufacturers. The pump does not "cavitate" - as some engineers erroneously assume - the moment the required NPSH is not met. The pump merely loses prime (gets liquid-starved) because the vapor pressure of the pumped liquid is reached and the liquid vaporizes - vapor binding the casing and not allowing further liquid to enter the impeller. Cavitation is another, totally different phenomena.

I consider this an important question because the correct answer gives process engineers a good, accurate handle on what is involved in determining the appropriate and successful suction conditions for their pump applications and installations. If I find the procedure, I will publish it here in the Forums.

Excellent point and question from a student; I wish all of the professional engineers that have worked with and under me in the past would have raised this question.

[pavanayi]

Obtained the following quote

"NPSHR (net positive suction head required) is defined as the NPSH at which the pump total head (first stage head in multi stage pumps) has decreased by three percent (3%) due to low suction head and resultant cavitation within the pump"

from the attached webpage (date accessed: 17 June 2012)
http://www.mcnallyin...html/11-12.html

And also see here
http://www.pumps.org...ail.aspx?id=714

[sruthildinesh]

can you tell me how NPSHR will change with flow???

[kkala]

A test curve example to specify NPSHr at a given constant flow rate can be seen in http://www.pump-zone.com/topics/pumps/centrifugal-pumps/performance-curves-and-npsh-tests, Fig 5. The NPSHr is considered as that NPSHa incurring 3% head loss, reported in post No 4 by pavanayi. During the mentioned test NPSHa is gradually reduced further, to make the point of NPSHr clearer. Of course to this specified NPSHr (concerning the operating point) a safety margin is added (e.g. 2 ft, depending on conditions) to result in the minimum requirement of NPSHa.
NPSH tests as above are performed in pump supplier shop using water as fluid. They are usually limited to pumps with critical NPSH requirements. Corresponding NPSHr for hydrocarbons (expressed as meters of liquid) is a bit lower compared to water. See Perry, 7th ed, pumping of liquids and gases, NPSH requirements for other liquids. However it is conservative to assume same NPSHr versus pump flow rate for all liquids, and this is applied at least here in the local work place.
Several NPSH tests were performed in 1992 for an alumina project (centrifugal pumps of constant RPM), but I can hardly recall the measuring equipment. Roughly, suction head was gradually decreased, while flow rate (hence total head too) remained constant. So there were globe valves at suction and discharge (for gradual throttling, in reverse direction); manometers at suction and discharge too, as well as a flow meter at the discharge line (*). This complies with my memory. Presented drop in head was rather abrupt, when it started declining.
For a general picture you can look at "Pump Handbook" by I. Karassik, W. Krutzsch, W. Fraser, McGraw-Hill (1976), Section 2.3 - Centrifugal pump performance, subsection "Cavitation" (p. 2-144 to 2-160), especially Fig 30, showing a NPSHr curve versus flow rate.

(*) editing note, Jun 20,2012: Also sensors to measure water temperature (to find its vapor pressure).

Edited by kkala, 20 June 2012 - 02:04 AM.

[fallah]

can you tell me how NPSHR will change with flow???

It increases with flow increasing!

[Shivshankar]

Hi,

Hope this helps.

http://www.irrigationcraft.com/n.htm
http://www.pump-zone...-and-npsh-tests

Regards
Shivshankar

Attached Files

•  M-3035.pdf   407.65KB   267 downloads
•  npsh.pdf   94.36KB   322 downloads

Edited by Shivshankar, 20 June 2012 - 08:15 AM.