Negative pressure for a pump

[5303]

Hi there,
Could someone explain to me what a negative pressure means for a pump TDH? I am using a pump in a gravity feed slurry line, to increase the flow rate of the slurry. After taking into account all the head loss in the system, I ended up with a negative total delivery head for the pump. I quite don't understand what that means. I will appreciate any feed back.

 

[sprintcar]

Centifugal Slurry Pumps require a NPSH or net POSITIVE suction head to prevent cavitation. Typically, in a horizontal pump, this is measured from the center of the suction pipe at the pump to the top of the sump level. Since centrifugal pumps don't normally have suction lift capacity, there must be positive pressure at the impeller eye. If the sump level is 10' high, that's your Available head. Now you deduct all the piping lengths, restrictions, etc for the Net. Some folks factor slurry SG into this equation also. Check your Pump Curve from the mfr and see that you have adequate suction pressure to avoid cavitation. Also note that changes to NPSHA will have an effect on your discharge head as well. Keep the wheels on the ground
Bob
showshine@aol.com

 

[Kawartha]

5303,

I'm not quite sure what you are asking, but I think it is in regard to calculation of pump differential head.

To calculate the required pump differential head (in feet of liquid), add the following elements:
1) static head (feet of liquid),
2) surface pressure (at suction and discharge)
3) friction losses,
4) velocity head [discharge - suction] (velocity to 2nd power/2g)- for low specific speed pumps this can be ignored.

Head = Hs + Hp + Hfr + Hv

The best way to design your system is to use the above formula to calculate a system curve. Then superimpose a pump curve to select the pump.

If you are concerned with suction pressure (at pump suction), use the same formula as above to calculate the head available at the pump suction.

To calculate HPSHa, subtract the liquid vapour pressure from the head available at the pump suction.

The NPSHa must be greater than the pump required (NPSHr).

If I missed your question, please provide additional details.

Richard

 

[TD2K]

5303, if you've done the calculation right, a negative TDH means the suction pressure to the pump is higher than the discharge pressure.

Which raises the question, do you need a pump at all? If you are looking at increasing the flow rate through a piping system, that would imply you need to add head to the system to overcome additional line/equipment losses. I would suspect that you have a mistake somewhere in the calculations.

I suppose I could see 'some' operating cases falling into this example if you have significant changes in your source and final destination operating pressure but I've never come across a case like this.