NPSHA calculation by gauge pressure

[moideen]

I have a fundamental question regarding NPSHA of condenser pumps (open circuit), in short, the formula, absolute pressure above liquid surface ± Suction Static - Friction Loss - Vapor Pressure. This is applicable prior to recognizing available NPSH before selecting the pump. My question comes after running the pump with suction gauge pressure. the way I understand to find the NPSHA after running the pump, gauge suction pressure +atmospheric pressure-vapor pressure. I found in some articles and YouTube videos that state adding the elevation pressure and friction head. Then formula defines like, absolute pressure + elevation +suction friction loss-vapor pressure. Is the suction pressure sum of the elevation and the friction loss? Then, why again add the elevation and friction head with suction pressure. It makes confusion. I think you can grasp my question?

 

[1503-44]

If your gage is immediately at pump suction, then the gage has already accounted for friction loss upstream and it is reading any liquid head above it. The only things it is not reading are atmospheric pressure, as that is the gage's reference point, vapor pressure and any remaining difference in elevation between the gage and the pump's centerline. It also does not read velocity head, which can be added to NPSHA, if the designer chooses to do so. Since velocity head is usually small, it is almost always ignored. If there are any fittings, especially bends, reducers etc. downstream, be sure to account for those friction losses too.

Fluid = water
Gage pressure = -2 psig or -0.87, say -0.9 ft head
Atm pressure 14.7 psia = 33.9 ft head
Vapor pressure = 1 psia = -0.4 ft
Velocity head (ignored)
Head at gage = 32.6 ft
Fitting loss downstream of gage = 0
Pipe friction 2ft long pipe downstream of gage = almost nothing, ignored
Elevation difference between gage and pump CL, Pump is 2ft higher than gage, so -2 ft head
NPSHA Head at pump CL = 30.6 ft

 

[moideen]

@1503-44: thanks, see your example calculation of gauge pressure -2 PSIG. Then, is it a vacuum? Then you convert to .87? how you get this value? The vacuum is no psig, it is inches of mercury(HG). So hg converting feet value 2hg *1.13=2.26ft. please correct me if I am wrong.
The YouTuber says diff stories who say that adding the absolute pressure plus elevation. Also, one article says the same story on running pumps.
NPSHA
Thanks

 

[1503-44]

There are gages that read vacuum, which you can use when the location is not suitable to use a mercury manometer.

OK I am So SORRY. I converted psig to feet wrong. Two times!

Gage pressure = -2 lbs/in2 gauge x 144 in2/ ft2 x 1 ft3/62.4 lbs = -4.6 ft
Vapor pressure = 1 psia x 144 in2/ ft2 x 1 ft3/62.4 lbs = -2.3 ft


Fluid = water
Gage pressure = -2 psig or -4.6 head
Atm pressure 14.7 psia = 33.9 ft head
Vapor pressure = 1 psia = -2.3 ft
Velocity head (ignored)
Head at gage = 27.0 ft
Fitting loss downstream of gage = 0
Pipe friction 2ft long pipe downstream of gage = almost nothing, ignored
Elevation difference between gage and pump CL, Pump is 2ft higher than gage, so -2 ft head
NPSHA Head at pump CL = 25.0 ft

In the Utube link Mr Griswold uses the same 5 variables as I do.
Note that his gage reads ABSOLUTE pressure.

Griswold's value 16.3 psia. Or 37.6 ft.
Using my value -2 psi_gage and converting that to absolute -2 + 14.7atm = 12.7 psia
.... which is 12.7 psia x 144/62.4 = 29.3 ft.
His vapor pressure gave 0.76ft. My vapor pressure gave 2.3 ft. My water is very cold.
His velocity head 5.2 ft. His velocity is high. My velocity is very very low. Ignored
His gage is 1 ft higher than the pump. +1. My gage is 2ft lower than the pump, so -2ft.
His pipe and fitting loss is -0.35 ft. Mine is very, very small. Ignored.

 

[TugboatEng]

I pump manufacturer I worked for in the past always added velocity head to their pressures. It looks better on paper. Velocity head was calculated, not measured.

 

[1503-44]

Mr Griswold in the Utube link has a velocity head of 5.2 ft. His flow is 400gpm through a 3" pipe, which gives a velocity of 18fps. That's about 3 to 4 times the velocity I usually see of 5fps, so my velocity heads are typically in the range of just 1 to 2 ft. Rather than calculate that small number, it is often ignored. That small of a safety factor for NPSHA calcs doesn't hurt and just might avoid cavitation when someone tries to increase the system flow rate in the future. At least in pipeline work, it is only a matter of time. Sooner or later the system will be pushed as far as possible.

It was good to see Mr Griswold included the velocity head in his NPSHA calculation. It is ignored in so many textbooks, without even a note they ignore it, that lots of engineers do not actually know that it can be included. Even still, I do not. At high velocities, where vel head can make a difference, I start getting water hammer problems and have to reduce velocity to get that under control.