NPSHA measured in ft H20 or Slurry? 2

[sprintcar]

Hey Guys - I need a couple expert opinions!

During a discussion on horizontal slurry pumps, the question arose on measuring Available NPSH - ie the height of the working sump level above centerline of suction inlet - if this should be given in feet of clear water or in feet of slurry (of the average specific gravity) to avoid both cavitation and possible vortex effects.

If the slurry was uniform, non-settling with small solids, the SG could be considered in the height, but a settling slurry with larger solids could be primarily water during startup and at other points in the process. Thus, using only clear water in the NPSH calculations should reduce pump problems in real world operation.

Once the fluid approaches the impeller, SG is a definite factor, but the sump is another issue.

Thanks for your thoughts on this!

Keep the wheels on the ground
Bob
showshine@aol.com

 

[TD2K]

I'd calculate it both ways and put the start up NPSHA on the data sheet as a note.

If during startup, if the water is going to be clear (which sounds reasonable to me), then I don't see a problem taking credit for this if it's needed.

 

[sprintcar]

Thanks TD - the discussion also went into actual running conditions - larger solids result in very non-uniform SG, while water would remain constant. I think for best overall performance of the system, using NPSHR for water would result in a higher sump level, but help eliminate possible cavitation later on. Keep the wheels on the ground
Bob
showshine@aol.com

 

[abeltio]

To avoid future problems with the pump, the suction specific speed (SSS) needs to be added into the spec and vendors should confirm that the pump operates between the recommended SSS at all times.
See also thread407-34092 for a discussion on NPSH margins.
As usual... a good reference is:
Pump Handbook
by Igor Karassik, William C. Krutzsch, Warren H.Fraser and Joseph P. Messina (Editors) 2nd Edition
Mc-Graw Hill Book Co.
ISBN 0-07-033302-5
For slurries as well as for SSS (Ns).

Slurries are very tricky, because the viscosity changes so much the correction factor for head, flow and efficiency should be applied for normal running conditions.
This will give you a much bigger pump, which when running on water may cavitate because the flow will be so high that it will fall to the right of the safe working flow for cavitation.
This could be overcome using a VFD or throttling the discharge valve during start-up.
HTH
Saludos.
a.

 

[greg87]

Not sure why you're asking about whether to "give the NPSH in water or in slurry", because if you're expressing it in feet its the same, independent of specific gravity. Feet of mercury is same as feet of water. (When you convert it to psi, then the s.g. comes in the conversion) NPSHA is more than just the difference in elevations: You will certainly want to calculate/determine any pressure drop in the suction line based on the slurry. I assume the fluid is at ambient temperature - remember that the vapor pressure is a key part of the NPSHA ; it would be neglible if you're at 70 F.

 

[stanier]

Also what are you going to compare it to? The pump manufacture will only have data on the pump curve in terms of NPSHr for water.

The impeller is sure to wear and the NPSHr will be affected.

ANy slurry pump installation shouldhave short suction lines and preferably flooded suctions. If you are trying to pump out of a sump immerse the pump.

A good book on pump NPSH/suctions is by Grist. It consolidates 30 years of experience.

 

[vpl]

I'd like to confirm what greg87 is saying. "Feet of water" is a measurement term just like "inches of mercury" "pounds per square inch" or "atmosphere" all of which are units of pressure in the english/american (non-metric) system. You calculate the NPSH available looking not only at the elevation differences, but also the vapor pressure and the friction losses in the piping. If any of your units are in psia, you need to take the specific gravity of your slurry into account when converting units. (I'd calculate both clear water and slurry, and then use the one which resulted in the minimum NPSH).

Regardless of the NPSH, you also need a pump that can handle slurries. As abeltio said, slurries can be very difficult to process. A lot depends on the type of slurry and it's not something that can be solved by water-cooler conversation. You really need to know what you're processing, what the expectations are for settling, and a whole bunch of other factors. There are a lot of "fluids" out there that if they settle will never move again.

Patricia Lougheed

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