Pump NPSHa
Pump NPSHa
(OP)
One of the centrifugal pumps(30gpm/100ft head) in my facility is facing many seal failures. This pump pumps 200 deg F water from a OPEN TANK with 4' static head.
Is Surface pressure equals vapor pressure here??
The suction pressure reads 20 - 40 inch water( fluctuates a lot) and discharge pressure reads 40 psi. If surface pressure equals vapor pressure( 11.5 psi at 200 F), then
NPSHa = 11.5 - 11.5 + 1.7(4' static head) - 1psi(Hfs) = 0.7psi which is equal to 20 inch water. Are my calculations right? Thanks for any help.
Is Surface pressure equals vapor pressure here??
The suction pressure reads 20 - 40 inch water( fluctuates a lot) and discharge pressure reads 40 psi. If surface pressure equals vapor pressure( 11.5 psi at 200 F), then
NPSHa = 11.5 - 11.5 + 1.7(4' static head) - 1psi(Hfs) = 0.7psi which is equal to 20 inch water. Are my calculations right? Thanks for any help.





RE: Pump NPSHa
RE: Pump NPSHa
Some thoughts:
• When estimating the NPSHA, the external pressure is the atmospheric pressure, not the VP of water. Are atmospheric conditions changing a lot ?
• VP of water ~ 11.5 psia at 200 deg F. Is the temperature fixed or variable ? VP changes at the rate of ~0.25 psi/oF near 200 deg F.
• 4 ft of water static head at 200 deg F ~ 1.6 psi. Consider water when converting ft to psi.
• friction drop in straight suction pipe < 0.15 psi
• an open tank may capture dirt of various sorts, is the strainer cleaned regularly ?
RE: Pump NPSHa
Chances are there is NOT enough NPSHa. Do you know exactly what value of NPSH IS required by the pump?
http://virtualpipeline.spaces.msn.com
RE: Pump NPSHa
NPSHa = 14.7(atm pressure) - 11.5(vp at 200 F) + 1.7(4' static head) - 1psi(Hfs) = 3.9psi which is equal to 9 ft.
I cannot understand y the suction pressure guage reading is 20 inch water( 0.7 psi). NPSHr for this pump is 4' but the pump is still able to pump the fluid with frequent seal failures.
RE: Pump NPSHa
Other items to check besides NPSH:
-Seal flush plan- with hot water the seal flush will need cooled, and the seal chamber will need to be pressurized to avoid water flashing across the hot seal faces.
-Pumps with very low NPSH requirements are very sensitive to off BEP operation causing vibration or recirculation which can shorten seal life.
RE: Pump NPSHa
Chem2603. Please add information. For example,
→ Is the suction pressure gage calibrated ?
→ What reading does it show when there is no flow ?
→ Where is it located with reference to the pump's center line ?
→ Is the water heated with a bottom steam coil ?
→ How is the "almost" constant water temperature measured and controlled ? For example, if it is 208 deg F, the VP = 13.6 psia, 2 psi higher than at 200 deg F. At 210 deg F this difference is 2.6 psi.
→ What kind of water is it? For example, salts in hard water would tend to plug the strainer.
→ How can you be sure the strainer contributes 0.7 psi to friction drop ?
RE: Pump NPSHa
See also thread407-171080: Insufficient NPSH and Airbound.
RE: Pump NPSHa
1. MATERIALS OF SEAL
2. TEMPERATURE OF SEAL BOX
3. STUFFING BOX TYPE
4. DOES THERE NEED TO BE A FLUSH LINE
5. IS THE PUMP CAVITATING
6. HOW FAST IS THE PUMP TURNING
7. IS THE PUMP OPERATING AT BEP OR OFF THE CURVE
LET ME KNOW IF YOU WOULD LIKE I WILL RUN ALL OF THE NUMBERS FOR YOU
RE: Pump NPSHa
I assume the last term which you called Hfs? is the friction loss due to piping & the strainer. 0.7 psi seems a bit high for this service, but may be fine. When you are dealing with NPSH calculations, it is preferable to work with feet or meters of head rather than pressure units.
There may be a concern with vortexing if you don't have a vortex breaker and if your piping is poorly configured. If the tank's drawoff nozzle is submerged by 4 feet then that is not likely, but if your tank is elevated above the pump, then the nozzle could be too close to the liquid surface.