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Anonymous Poster

Pump Puzzle Please

11/27/2010 8:47 AM

It is said that a centrifugal pump do not suck the water but that it is pushed up by barometric pressure, why do one then have to add the suction hight to the total head for pump selection and design ?
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Codemaster
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#1

Re: Pump Puzzle Please

11/27/2010 8:59 AM

Because unless you're delivering to a vessel under vacuum, there's atmospheric pressure on the pump delivery side as well.

Codey

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yesyen
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#2

Re: Pump Puzzle Please

11/27/2010 10:21 AM

Well said simply.

Further to elaborate:

In any pumping system, the pressure is the lowest around the eye of the impeller. In the shown illustration, arrangement C has the highest pressure at the suction (eye of the impeller) than arrangement A. And the suction pressure is the least in the arrangement B. If the suction level goes down further at some point, the liquid would start boiling at room temperature it self. This situation leads to cavitation. The chance of this happening is far less in arrangement C, wherein the suction level is high.

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JohnDG
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#3

Re: Pump Puzzle Please

11/27/2010 10:49 AM

"... why do one then have to add the suction hight to the total head for pump selection and design? ..."

Another way of thinking about this: if you're raising water from level A to level B, it doesn't matter where the pump is in terms of the work the pump has to do. It takes the same amount of work to raise a litre of water from A to B whether the pump is at (or even below) level A, or right up near B. So the total head is the same.

[As an aside - the quibble about the pump not "sucking" is just wordplay - if that definition of the action of a pump when above the lower water level is extended, the word "suction" may as well not exist in the English language].

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