Pump NPSHa

[chem2603]

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.

 

[chem2603]

Suction line is 2"dia and 18ft long. 2 years ago they have installed a strainer. I dunno how much would be the friction loss thru a strainer so i added .7psi to 0.3psi due to pipe and fittings loss.

 

[25362]

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.

[&bull;] friction drop in straight suction pipe < 0.15 psi

[&bull;] an open tank may capture dirt of various sorts, is the strainer cleaned regularly ?

 

[BigInch]

25362, I read that the atmospheric pressure is such that it is also 11.5 psi. I think he must have an elevation of around 7000 feet. If its true that atmospheric pressure also equals 11.5 psia, the calculations should be roughly correct.

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

 

[chem2603]

The tank is 4' 11" tall with 4' head and water temp in the tank is almost constant. The suction line has 2 ball v/v's and 3 90's and a strainer. Total friction loss would be 1psi for a flow of 30gpm. Atmospheric condition deosnt change since this tank is in ISBL.Strainer will be cleaned once in 2 days. Given these conditions,

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.

 

[rzrbk]

.7 psig appears to be the correct suction pressure reading, as 3.2psi of your NPSH is supplied by atmospheric pressure which your gage will not pick up.

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.

 

[25362]

Chem2603. Please add information. For example,

[&rarr;] Is the suction pressure gage calibrated ?
[&rarr;] What reading does it show when there is no flow ?
[&rarr;] Where is it located with reference to the pump's center line ?
[&rarr;] Is the water heated with a bottom steam coil ?
[&rarr;] 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.
[&rarr;] What kind of water is it? For example, salts in hard water would tend to plug the strainer.
[&rarr;] How can you be sure the strainer contributes 0.7 psi to friction drop ?

 

[25362]

See also thread407-171080.

 

[pumpguykc]

HAVE YOU EVALUATED THE SEAL FACES AND ELASTOMERS ON THE PUMP. MANY TIMES PEOPLE RUN THEIR PUMPS ON THE RAGGED EDGE FORGETTING THAT THERE ARE THINGS THAT HAPPEN IN THE PUMP THAT CAUSE OTHER ISSUES. IF YOUR ARE RUNNING 200F-212F AND DO NOT HAVE GOOD COOLING TO THE SEAL FACES THIS COULD EASILY CAUSE FLASHING ON THE SEAL FACES ALLOWING THE PUMP TO RUN DRY. IS THERE A FLUSH FROM THE DISCHARGE OF THE PUMP TO THE STUFFING BOX ON THE SEAL? WHAT SPEED ARE YOU RUNNING YOUR PUMP? ALSO WHAT MATERIALS ARE YOUR SEAL FACES AND ELASTOMERS OF YOUR SEAL? THINGS THAT REALLY SHOULD BE CHECKED IF A PUMP SEAL IS CONTINUOUSLY FAILING:
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

 

[djack77494]

Double check your level, remembering that you must reference the tank's level relative to the pump's suction centerline. 4 feet of level in a 4'-11" tanks says that the pump centerline in only 11" above grade even if the tank operates totally full. Though I can't put my finger on it, things don't look right with your numbers. A 4'-11" high tank is a very small tank, but your flowrate in not comparably small. Nor is your suction line size. I'm also very suspicious about your strainer. What do you see when you clean it? Is there any way you could measure the dP across the strainer?

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.