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Suction/discharge pressures

Suction/discharge pressures

Suction/discharge pressures

(OP)
More help please!
If I have a pump connected to the outflow of some storage tanks (which have a height of 10m) does this head pressure relate to the suction pressure for pump selection, or is this going to be zero as it is not forced?
Also, how do you calculate discharge pressure -
I hope you can help
thanks
M

RE: Suction/discharge pressures

You should add it to suction pressure and this, infact, also reduces your required discharge head. To calculate discharge pressure you should add the static discharge head, frictional losses in pipe lines and pressure drop across equipment if any.

Regards,

Eng-Tips.com : Solving your problems before you get them.

RE: Suction/discharge pressures

MetalGuru,
You mention that the height of the storage tank is 10m, but if the pump will be draining the tank fluid level..ie..fluid level not constant, this should be taken into account when specifying the service pump. I believe NPSH is one of the critical criteria for pump selection.
Regards, ktm5

RE: Suction/discharge pressures

Ignoring possible cavitation problems due to low NPSH,
Pumps produce a pressure differential (dP).
Any increase/decrease on suction side results in a corresponding increase/decrease in discharge side.

PUMPDESIGNER

RE: Suction/discharge pressures

MetalGuru:

I think this is an easy problem; however, I'm going to have to assume what you mean because your description can be interpreted at least two different ways.  I'm assuming you are connecting the suction side of a centrifugal pump to the outlet nozzle on a tank.  This suction nozzle is located 10 meters below the normal operting tank liquid level.

The suction pressure the "eye" of the centrifugal pump sees is:

Suction pressure, absolute = tank vapor space pressure + 10 meter head of fluid above the pump - the fluid's vapor pressure (at the flowing temperature) - (the losses in the tank outlet, outlet nozzle, any fittings & valves and entrance to the pump impeller eye).

I don't understand what you mean by "forced", unless you mean that you don't have a gas blanket pressure on top of the liquid in the tank's vapor space.  You calculate the discharge pressue according to the performance curve pertinent to that centrifugal pump (not just any pump) and the TDH (total developed discharge head) characteristic to your system.

I strongly recommend you visit the following site and spend some time going over all the relative features that address your specific questions and problems.  All the answers are there:
http://www.mcnallyinstitute.com/index.html

I hope this has been of help.

Art Montemayor
Spring, TX

RE: Suction/discharge pressures

MetalGuru,
I agree with Montemayor regarding the McNally website. I have Bill McNally's Reference Manual for Centrifugal Pumps and Mechanical Seals on CD-ROM and it is a GREAT reference for pump users and specifiers. Almost anything you want to know about the subject is there.
Regards,
ktm5
(also of Spring, Texas)

RE: Suction/discharge pressures

(OP)
Thanks for that link Montemayor, it has been very useful, and I haven't got to read all of the advice yet!
One thing that is apparent though - is that I should be using a positive displacement pump such as a three screw design for transferring oil between tanks.
There is less information about sizing this type of pump, although it seems less complicated than a centrifugal pump. Could someone tell me what are the main elements in sizing a PD pump?
Thanks

M

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