30 replies to this topic

[JoeWong]

Chiko,

I have the following opinion.

They are equivalent important and critical. Moreover, recip. demand higher NPSH due to pulsating effect.

What's makes you think "NPSH for recip. is not as much critical as that for centrifugal pumps." ?

JoeWong

[djack77494]

The NPSHr for a recip pump is often higher than that for a centrifugal pump. More importantly, the NPSHa for a recip pump is always less than that of a centrifugal for 2 reasons.

1) Every cycle (stroke) you accelerate the volume of fluid in both your suction and discharge piping from its minimum to its maximum velocity. That takes energy in the form of pressure losses called acceleration head.
2) Your flowrate is not a constant value (its average) like it would be for a centrifugal, but rather varies. Worst case hydraulic losses should be calculated based on the max flow and not the lower average flow.

The situation is fairly complex since you can have a variety of types of recip pumps - simplex, duplex, triplex, etc. - and single or double acting. Different types have different characteristics and need to be calculated differently. Picture the simplest type - a single acting simplex pump. Flow through the pump is like the upper half of a sine wave. Half the time the flow is zero. The rest of the time it varies from zero to a maximum and back to zero again.

The good news is that recip pumps tend to be tolerant of some entrained gases or vapors and are self priming. I think all the complacancy I'm detecting here is due to this forgiving nature of these pumps. Despite this, the pumps cannot violate the laws of physics. Ignore their inherent limitations at your own risk.

[jprocess]

Dear Doug,

You stated that the flowrate of reciprocating pumps is not a constant value. But if we look at the characteristic diagram of recip. pumps we find that it is approximately a constant value which gives pressure at pump shut off towards infinity. Could you please clarify this point?

Thanks in advance.

[Roger Susan]

NPSH for recip. is not as much critical as that for centrifugal pumps.
Why do you require NPSH for reciprocating pumps?

Not being an expert but having dealt a lot with design of pump systems, my understanding NPSH is important for any pump to avoid vaporisation in the pump it self when pumping a fluid.

I have a little query on this. Acceleration head has been indicated before for Recip pumps. We have a triplex pump and have designed the system appropriately. However this pump has added a suction pulsation dampner. From pump vendor this smooths the flow to only vary by 3%. So my question does acceleration head stay relevant in this case since there is no stroke maxmiums and minimums?

This does affect potential line size and suction vessel heights.
I would like to find a reference if possible about suction dampners affect on accelartion head term.

Thanks in advance.

[djack77494]

To jprocess,
The pump curve you refer to is, I believe, a curve of the AVERAGE pump flowrate vs. discharge pressure. It is overly simplistic to view recip pump curves in this way, since the instantaneous flowrates may vary widely.


To Roger Susan,
Suction and discharge pulsation dampeners, sometimes referred to as bottles or snubbers or accumulators, act to (nearly) eliminate a large part of the acceleration losses. To account for suction losses with a well designed suction pulsation dampener, you may calculate the suction losses in the "normal manner" using the average flowrate for the whole suction circuit, except for the (presumably very short) segment between the dampener and the pump itself. Likewise, if you have a dampener on the discharge side of the pump, you may treat the discharge hydraulic calculations in the same manner. If you google reciprocating pumps and pulsation dampeners (or really just the latter) you should find lots of good material on the subject.
HTH,
Doug

[jitendraprocess]

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