Some booster pump
I realize that I may be a bit behind the times, but I thought I would add a little to this thread.NPSHr + 10% or 2-ft, whichever is greater, is the absolute minimum gap between NPSHr and NPSHa when used in water and wasterwater pumping applications. A more common standard is NPSHr + 35% or 5-ft (1.5m) whichever is greater. These standards work if you are pumping water/wastewater, the fluids have a temp between 50F and 90 F, and your pump is operating within 15% of BEC (flowrate at BEP).Pumping Station Design, 3rd edition suggests that if you are operating your pump outside of 15% of the BEC, then you might consider using NPSHr + 80%. This, of course doesn't mention the effect of pumping "hot" water which would also require a larger gap between NPSHr and NPSHa. This is where the 10% & 80% numbers come from, but both could be perfectly valid depending on your selected pump/turbine and the fluid properties.
For some booster pump, I used in the past to sum two NPSH margin, one that account for permanent cavitation phenomena (as per NPSH defined at 3% head losses, different impeller material will be impacted not in the same manner) then add a NPSH safety margin; The point is that working out of some API610 field like it is the case in water applications for example, the NPSH is estimated mainly relying on field experience of expertized people. That's a real difficulty.Not unusual to have up to 3 meters total NPSH margin to be on safe side.
The HI standard was withdrawn because of the disfunction nature of the HI committee dealing with the NPSH standard.There were too many competing points of view which resulted in a stalemate what was the correct approach in determining NPSH margin. Having sat in on many of those discussions, I can tell you it was like having your teeth pulled.I am not aware of any specific standard that fills the gap left by HI.Sadly some of the advice on this thread regarding margin is quite dangerous. NPSH margin for BFW service needs to be quite large. I have an Multi-stage pump internal methodology for determining it (as used by my pump company), which is decent. Depending on your criteria you end up with NPSHa/NPSHr margins of 1.5 to 3. The range is a result of the water temperature, Nss of the pump, operating point and desired impeller life. BFW service is v aggressive and needs large margins. Apply 10% margins is a sure fire way to end up with cavitation damaged impellers.
2011-08-16
