loss of head

[robtgarn]

what would cause one of our simi shrouded impeller centrifugal pumps to loose head as we go out on the flow? The test stand has been double checked and everything else with the pump seems normal.

 

[BigInch]

Its a centrifugal pump. They produce lower heads as flow increases at constant speed.

Do you mean it is losing more head than the curve says it should?

You're going to have to supply more information about exactly what symptoms its exhibiting, or flow, head, power consumption data to get any kind of a meaningful answer.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[robtgarn]

Yes, the head diverges from the published curve as we go out in flow for one of our model pumps. At shutoff we match-up to the published pretty good but it drops off as we go out in flow. Other model pumps of ours match the published curve well using the same test stand. Velocity head data was double checked.

 

[BigInch]

Doesn't sound like a test stand problem, unless you are not furnishing sufficient NPSH at higher flowrates.

Is this model significantly different than other models?
Are these flowrates higher than you've tested before?

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[robtgarn]

I think you hit on something. The pumps that are fine are much smaller. We are going to check the turbulence from the discharge and how it may effect the inlet.

Thanks, I will let you know.

 

[Artisi]

What will turbulence from the discharge tell you in relation to insufficient NPSHa, you need to measure or calculate the NSPHa in relation to the NPSHr shown on the pump curve.

What do you mean by semi shrouded, is the impeller front clearance adjustable against the casing or a wearplate?

 

[robtgarn]

I will clairify, the pump is being run on a test rig, I believe the discharge pipe is too close to the inlet and may be introducing turbulance/air into the inlet. There is no NPSH issue as it is an open tank with 6' of tubing before the pump and flooded.

 

[quark]

(Presuming the pump is running in correct direction)Do you see any vortexing in the tank?

 

[Artisi]

sounds like typical air entrainment - this will reduce pump performance.
Ensure the discharge is placed below the water surface level in the holding tank so that air is not driven into the tank.

 

[BigInch]

Had another thought.

How are you measuring the head? With inlet and outlet Pressure gages and using a flow meter, yes? How are you calculating the pump's differential head exactly?

(Discharge Pressure - Suction Pressure)psig * 144/62.4

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[robtgarn]

Yes, we do see a vortex too close to the inlet for comfort.
We are subtracting the inlet conditions and taking into account the velocity head. We have inlet pressure gage, discharge pressure gage and flow meter. Hope to rerun the test Monday with the discharge moved further away.

Thanks for all the help, will let you know what happens.

 

[BigInch]

Thought maybe you wern't considering or trying to measure V incorrectly, wrong meter for the velocity. OK, you're too smart for me. Must be one of those other things asso with a high velocity phenomenon, turbulence, elbow too close, vortexing air, etc

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization