Suction Lift

[impeller1]

Good day,

A pretty baic question here..sometimes the basics get a little confusing I have a pump operating on a suction lift. Im trying to find the differential head across the pump, I have a discharge pressure gauge so that would effectively account for all losses and fittings on the discharge end downstream of the gauge and would give me the total discharge head. Suction is a suction lift condition. However how do i find the total suction lift? I know it's the distance from the impeller centreline to the water level in the sump plus all suction line losses. But since it's a open suction, how does atmospheric pressure play into the total suction lift? (In my system velocity head is negligible)

 

[photoengineer]

You would attached a pressure gauge to a location close to the pump inlet and measure the pressure there. The total pressure head developed by the pump is equal to the difference between the two gauges.

In a suction lift condition, the pressure reading will be less than atmospheric pressure (14.7 psi thereabouts).

 

[impeller1]

Sorry forget to mention I don't have a gauge readily available..

Regards

 

[BigInch]

Suction gage pressure is the same as the pressure equivalent of the atmospheric head about 33.9 ft - height from the fluid level to pump centerline - probably a bit of intake loss - pipe and fitting friction loss to suction flange.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

That's absolute pressure, so subtract 14.7 psi for gage pressure.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

If you can't measure the inlet condition by gauge - simply measure the height from the water level to the pump centreline add "something" to account for losses in inlet pipe and any fittings on the inlet line - call it total inlet head which you can add to your discharge head. Depending on accuracy needed the "something" you add will be as accurate a the time you are prepared to send calculating the losses in the line and any fitings in the line.

 

[impeller1]

Thanks BigInch and Artisi..so what you're both basically saying is that atmospheric pressure cancels itself out in this calculation in order to get gauge pressure..

 

[photoengineer]

If the location where you are pumping from is open to the atmosphere and the location where you are pumping to is open to the atmosphere, you can ignore the atmospheric pressure. However, if either of the locations is not open to the atmosphere (pressurized tank), then atmospheric pressure should be figured into your equations.

 

[JJPellin]

But, that affect would show up in the gauge pressure at the suction and discharge. Atmospheric pressure is not relevant otherwise. Since you do not have a pressure gauge at the suction, you need to calculate the gauge pressure at that location using the suction lift, fluid properties and flow loss. It should be a negative number. Once you subtract this negative number from the measured gauge pressure at the discharge, you will have the differential pressure which you can then convert to differential head. Atmospheric pressure does not come up anywhere in these equations.

Johnny Pellin

 

[BigInch]

If you always include atmospheric pressure, you don't have to worry about when you should and when you shouldn't. Vapor pressures are given in absolute pressures. NPSH equivalents are absolute pressures. Three less possible sources of errors, if you include the atmospheric pressure term. IMO it helps to keep the procedure straight in the mind and on a spreadsheet. No surprize that I've noticed a lot less confusion when explaining pump suction requirements to lesser experienced engineers too.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[impeller1]

Thanks everyone..I think I'm clear now.