4 replies to this topic

Hello Everyone!

i was just comparing between the centrifugal pump and postive displacement pump when i came to

a question that why a PSV is nessecary to have in the discharge line of the PD pumps , while in

the centrifugal pump we dont ?!!


Thank U

[Zauberberg]

If you understand the difference between dynamic and positive displacement machines, you can easily figure out why PSV is mandatory in reciprocating pumps. Have you done that part of the work?

[USR]

hi....the basic difference between a centrifugal pump and PD pump is that PD pumps are constant flow pumps i.e. they provide a constant flow irrespective of the discharge pressure...but not for a centrifugal pump for which the flow varies with discharge pressure.

Psv is needed for a PD pump as it is highly dangerous to run a PD pump with shut off head because even then there will be flow(whereas in case of centrifugal pumps flow is zero at shut off head)...as aresult the pump gets damaged...

[kkala]

A bit more explanations to above post, actually containing the answer.
Suppose you close a valve at discharge line. A PD (positive replacement pump) is like a piston delivering constant flow. Liquids are practically incompressible, so pressure in discharge line will be infinitively large.

On the contrary, a centrifugal pump will develop its shutoff head, shown by its curve. The resulting shut off pressure is often below the pressure limit acceptable by discharge pipe and pump (design pressure).

So a PRV is necessary at the discharge of PD pump, while it may not for a centrifugal pump.

Infinitively large shutoff pressure for PD pumps is theoretical, since there are leakages between piston and cylinder; but at any case pressure developed will be very high and rather unpredictable (causing pump damage, among others).

Edited by Art Montemayor, 24 May 2010 - 06:24 AM.

[sheiko]

On top of what have been said, i would add that:

The PD pump is limited in total developed pressure only by two factors:
(1) the physical strength of its metallic components, and
(2) the horsepower of its driver

The amount of positive head that a centrifugal pump can develop is limited by:
(1) the diameter of its impeller,
(2) the speed at which it rotates, and
(3) the horsepower applied to it.

All centrifugal pumps have a maximum total developed head that they can generate (shut-off head).
A PD pump, on the contrary, will not reach a maximum discharge pressure above which it cannot perform. It will, if allowed to, simply continue to raise its discharge pressure until the resistance found in its discharge system is overcome while displacing the same volumetric gas rate.

Edited by sheiko, 26 May 2010 - 05:44 AM.