0% NPSH
0% NPSH
(OP)
Instead of tacking onto the discussion on NPSH below, I thought I start a new thread.
I know of the pump manufacturer's NPSHR test at 3% head loss. But is there a 0% NPSHR test? Of course anything is possible but is doing a 0% NPSHR test a norm or uncommon? Is is exactly as the name indicates? i.e, when the head starts to drop a little when you starve the inlet head then that's the 0% NPSHR.
I know of the pump manufacturer's NPSHR test at 3% head loss. But is there a 0% NPSHR test? Of course anything is possible but is doing a 0% NPSHR test a norm or uncommon? Is is exactly as the name indicates? i.e, when the head starts to drop a little when you starve the inlet head then that's the 0% NPSHR.





RE: 0% NPSH
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: 0% NPSH
RE: 0% NPSH
NPSHr increases in a non-linear manner in proportion to the difference between the actual flow rate and the BEP flow rate. The NPSHa increases for flow rates below the BEP flow rate thereby inherently at least mitigating some of the NPSHr burden from off-BEP operation. The potentially more serious situation usually involves operation at greater than BEP flows where NPSHa drops as NPSHr increases. Since the pump's power requirement usually increases quite rapidly as the flow rate exceeds the BEP flow rate, pump's don't usually operate very long is this mode anyway.
Sometimes, a pump's NPSHr is based on 1% rather than 3%, so it is usually a good idea to be sure what standard is used for a particular pump's ratings. Also, it is common for a pump's ratings to be based on operation at a constant shaft speed (often but not necessarily a synchronous speed such as 1800 rpm or 3600 rpm), Sometimes, smaller, direct-connected pumps may be rated based on operation as powered by their standard induction motor so that some modest variation in shaft speed may be included in the pump's rated performance curves. Usually, these small variations in the rated performance are not troublesome, but I have encountered situations where problems of apparent performance deficiencies have resulted from a misunderstanding of the manufacturer's rating system.
RE: 0% NPSH
RE: 0% NPSH
I agree that testing for the magic 0% point is a serious challenge. I have never been involved in such an effort (nor am I likely to ever be so involved), but I suspect that the most likely means may involve some type of sonic or ultrasonic instrumentation--probably very fancy and expensive.
RE: 0% NPSH
Johnny Pellin
RE: 0% NPSH
Higher temperature fluids will only have a 1% head drop applied.
0% in my interpretation would be the point of deviation from a straight line, no 3% head drop applied.
Since this is all dependant on the Test Engineer's definition of "deviation from a straight line," it's all subjective.
Did you know that 76.4% of all statistics are made up...
RE: 0% NPSH
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: 0% NPSH
You will see the 3% drop off point, and you can track the curve back to where you still like the shape (what you would consider a 0% value.) Measure the difference, and consider it as additional margin that you then specify.
If you want a 0% NPSH value, there is no test for it. Specify a 3% or 1% test and apply some margin to the results. You can get the information that you want, without specifying an impossible test for the pump vendor.
I've been reading for awhile, but had to join up just to post this!
RE: 0% NPSH
http://
Did you know that 76.4% of all statistics are made up...