Calculating NPSHa
Calculating NPSHa
(OP)
Hello all,
We recently installed a high volume (3000 GPM, water) closed flow loop to test multi-staged centrifugal pumps which includes a booster pump being fed by a tank. We are using a Labview program to plot the measured performance curve against the catalog curve for the multi-staged pump being tested, everything is kosher there. However with this booster pump I want to add fuctionality to the program by calculating and displaying the NPSHa at the booster pump on the program. My inputs to the DAQ card are a suction pressure transmitter (30inHg - 30psig), an RTD and a flowmeter. My question is this; I need a formula based on any or all of these inputs plus the calculated vapor pressure of the fluid at the specified temp. via the RTD to display the NPSHa in feet. Note: the pressure transmitter at the suction impeller is in psig. Any thoughts?
We recently installed a high volume (3000 GPM, water) closed flow loop to test multi-staged centrifugal pumps which includes a booster pump being fed by a tank. We are using a Labview program to plot the measured performance curve against the catalog curve for the multi-staged pump being tested, everything is kosher there. However with this booster pump I want to add fuctionality to the program by calculating and displaying the NPSHa at the booster pump on the program. My inputs to the DAQ card are a suction pressure transmitter (30inHg - 30psig), an RTD and a flowmeter. My question is this; I need a formula based on any or all of these inputs plus the calculated vapor pressure of the fluid at the specified temp. via the RTD to display the NPSHa in feet. Note: the pressure transmitter at the suction impeller is in psig. Any thoughts?





RE: Calculating NPSHa
NPSHa = (Ps + Pa - VP) * 144 / Water_Density_pcf + E
Where,
Ps = suction pressure at the gage, psig
Pa = atmospheric pressure, psia
VP = vapor pressure, psia, roughly 0.4 psia
Water Density = roughly 62.34 pcf
E = elevation difference between suction gage elevation and the pump centerline elevation, measured in feet. If the gage is above the pump centerline elevation, the sign of E is positive, so NPSHa at the pump will be increased, so add that elevation difference. If the gage is below the pump centerline elevation, the sign of E is negative and NPSHa at the pump is reduced by that elevation difference.
If there are any fittings, valves, or long pipe sections between the suction pressure gage and the pump, the NPSHa must be reduced due to frictional flow in each of those items, if they are significant.
If you need more accuracy, you'll need the temperature of the water to determine the exact water density and its vapor pressure.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Calculating NPSHa
Perfect approach. Just one thing. The OP has got a RTD to measure the temperature, and this parameter is sometime vital when dealing with NPSHa.
RE: Calculating NPSHa
The pressure transmitter is actually mounted on the suction flange of the booster pump and it is on the pump centerline, that is as close to the impeller as we can get. And yes we have an RTD to measure the temp. of the water. Having that said, can I assume the following:
NPSHa (in feet) = (Ps (psig)+ Pa (14.7 psia) - VP (calculated by temp. from RTD))*144/Water density (62.34)
I am more or less looking at this as a reference number to warn the operator when conditions are close to cavitation. We will add a safety margin for warning levels and possibly shutdown when we get close to NPSHr
RE: Calculating NPSHa
This should work, but please be careful with this stuff, as cavitation can happen even at NPSHa values well above the NPSH required by your pump.
RE: Calculating NPSHa
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Calculating NPSHa
RE: Calculating NPSHa
What's the water temperature there?
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Calculating NPSHa
RE: Calculating NPSHa
VP will go from 0.4 to 6.8 psia. http://www
Density from 62.34 to 60.58 pcf
http
At 180 F, VP=6.8 psia and reduces NPSHa by 16.2 feet.
If you needed 22 ft NPSHr and NPSHa was 40-16.2 = 23.8 ft, I'd be tempted to set the alarm at least at 25 ft, so ya, that's 15% or so. Best would be to get hold of an ultrasonic monitor and check if there's any indications at that level, if you can. If you get cavitation noise, raise it to 20%.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Calculating NPSHa
RE: Calculating NPSHa
RE: Calculating NPSHa
RE: Calculating NPSHa
The alternative would be to set the alarm for NPSHa at the normal high temperature of 80F with a shutdown right above that.
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/
RE: Calculating NPSHa
Sure have an alarm set point for above the normal operating temp. but monitor the "cavitation" manually during any test period which is fairly normal proceedure when testing large pumps.
RE: Calculating NPSHa
RE: Calculating NPSHa
**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch
http://virtualpipeline.spaces.live.com/