NPSHa Vertical Turbine Pumps
NPSHa Vertical Turbine Pumps
(OP)
Before a week ago I had no idea what NPSH was. We didn't cover it in Fluid Mechanics in school (Im a recent ME grad) and I had never worked with pumps before. So I am just now learning about pumps and NPSH. I am comfertable calculating NPSHa for centrifugal pumps but I am confused about calculating it for vertical turbine pumps.
Correct me if Im wrong, but NPSH is basically calculating how close the liquid is to boiling due to low pressure in the intake. With vertical turbine pumps the impellers are submerged so you would never experience low pressures in the intake. Why then is NPSH even calculated for vertical turbine systems? It seems like the major question is if the pump is powerful enough to push the liquid up the piping to the level at where it is discharged and be able to overcome any friction losses in the piping.
OK, so I guess after all that my main question is: is NPSH calculated for vertical turbine pumps and if so, how?
Thanks
Correct me if Im wrong, but NPSH is basically calculating how close the liquid is to boiling due to low pressure in the intake. With vertical turbine pumps the impellers are submerged so you would never experience low pressures in the intake. Why then is NPSH even calculated for vertical turbine systems? It seems like the major question is if the pump is powerful enough to push the liquid up the piping to the level at where it is discharged and be able to overcome any friction losses in the piping.
OK, so I guess after all that my main question is: is NPSH calculated for vertical turbine pumps and if so, how?
Thanks





RE: NPSHa Vertical Turbine Pumps
This is true whether or not the pump is horizontal or vertical.
Do an advanced search on this forum for NPSH. There have been many many threads devoted to the topic.
Don't feel intimidated by being new. If you do the search you can see that plenty of folks with lots more experience struggle with NPSH also. Ask more questins if you don't find the answers after the search.
rmw
RE: NPSHa Vertical Turbine Pumps
RE: NPSHa Vertical Turbine Pumps
RE: NPSHa Vertical Turbine Pumps
Assume you are at sea level - then the NPSHa is - atmospheric pressure at the water level plus the vertical distance from the water level to the impeller eye.
So roughly speaking if the impeller is positioned 10ft below the water level the NPSHa is approx 44ft - less any vapour pressure, inlet losses etc.
Or more precisely:
NPSHa = Ha - Hvpa (plus or minus Hst)- Hfs
RE: NPSHa Vertical Turbine Pumps
"The NPSHa calculation gives you the resulting head of liquid that must be standing above the inlet of a pump in order to maintain that liquid above its flash point in the low pressure area of the inlet to the impeller. The requiremenet is usually given by the pump manufacturer based on empirical data or actual testing"
Didn't you mean to say NPSHr?
RE: NPSHa Vertical Turbine Pumps
Thanks for your help! Your explaination is perfect. I also found some more information that supports what you said and it makes sense to me now. If you have an impeller that sticks 10 feet under the surface of the water, the higher pressure down there actually improves your cavitation situation, or makes it less likely to occur than if the pressure wasn't there. This is reflected by the higher NPSHa value.
Thanks guys. This site is great!
RE: NPSHa Vertical Turbine Pumps
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: NPSHa Vertical Turbine Pumps
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm
RE: NPSHa Vertical Turbine Pumps
RE: NPSHa Vertical Turbine Pumps
RE: NPSHa Vertical Turbine Pumps
On very large high flow mixed and axial flow pumps NPSHr can be quite high right of BEP where pumps can run at times of upset in a plant, therefore NPSHa is a critical component of correct pump / sump design.
However, will agree the point with you that having sufficient submergence is usually ok to ensure cavitation free operation.
RE: NPSHa Vertical Turbine Pumps
1. Pumps that cavitated because they had inadequate NPSH available but did meet the OEM requirement for submergence.
2. Pumps that cavitated because they had inadequate submergence but did meet the OEM requirement for NPSH available.
3. Pumps that cavitated despite the fact that they had adequate NPSH available and adequate submergence but had suction recirculation cavitation resulting from pre-rotation of the fluid because of a poorly designed sump or a poorly designed impeller.
All of these are important. Submergence might be the most common cause of problems, but it is not the only cause.
Johnny Pellin
RE: NPSHa Vertical Turbine Pumps
Johnny is correct. Be sure your sump is designed properly. How the water comes into the sump can be very important. A water fall effect can cause air entrainment in your pumpage causing cavatation.
RE: NPSHa Vertical Turbine Pumps
As a matter of fact, introducing air to the inlet of a cavitating pump is one way to reduce the cavitation effect.
RE: NPSHa Vertical Turbine Pumps
RE: NPSHa Vertical Turbine Pumps
From NcNally Institute with some addition (xxx) by myself.
Air ingestion.
---------------- Both vaporization (NPSHa/r problem) and air ingestion (entrainment) have an adverse affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.
Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.
RE: NPSHa Vertical Turbine Pumps
The only adjustment I might make is to alter the manufacturer's recommended minimum submergence to consider any vapor pressure effects if the system was at elevated temperature. Thus, the vendor's recommended submergence based on 60F water should be increased if the water is actually at 160F. The water's vapor pressure in that case would have gone from 0.26 to 4.74 psia, and roughly (4.74-0.26)*2.31 or 10.35 ft.
OK - that's a lot. I hadn't anticipated much change at the start, so I'll retreat a bit and concede that for significant changes in conditions one should look at the NPSH effect.
RE: NPSHa Vertical Turbine Pumps
To make the situation even worse, this type of pump is often tested in an open sump in relatively cold water with no NPSH test possible. They might test for submergence, but NPSH requirements have to be taken on their word, untested in your specific pump.
Johnny Pellin