NPSH requirement in PD pumps 4

With most PD pumps, it is referred to as NIPR (Net Inlet Pressure Required),and is expressed in pressure units, as opposed to head units.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

I think its a question of terminolgy:

Cavitation may occur in the feed line as pr. the tread that BigInch refers to. But cavitation as in a centrifugal pump where the wheel is damaged due to cavitation volume that occurs at one location in the pump due to low pressure and collapses again near the wheel as the pressure increases in the fluid i dont think that happen. Wether the cavitation then may occur inside the pumphouse and damage something there i dont know.

The keay seem to be pulsation dampning or maybe something like a progressive cavity pump, a gear pump or similar.

Mvh Morten

Thank you all for replying.

So does it mean that the suction piping for PD pumps also needs to follow the principles which are followed for centrifugal pump?

Karassik et al in their "The Pump Handbook" have an excellent discussion of this very thing. There is even an equation developed therein, from which you can calculate the rotational speed of a plunger type PD pump that will give rise to vapour development and collapse at the plunger faces (plunger separation phenomenon).

In very general terms, the NPSHA/R calculations are the same for PD pumps as for centrifugals, except that for PD pumps there is the added consideration of acceleration head.

My own short answer to your question is that "cavitation" (if defined as formation and collapse of vapours inside the pump due to changes in pressure along the fluid path during operation) does indeed occur. My own personal experience is that the effects in a plunger pump can give rise to spectacular failures and seeing pieces of pump casings, plungers and crankshafts strewn all around the pump skid. I haven't seen that in centrifugals.



Regards,

SNORGY.

A severe PD cavitation recovery can actually generate quite high pressures as the vapor pockets can be larger and collapse quickly as pressure is increased rapidly as that extra acceleration head finally catches up and starts feeding liquid again... actually a waterhammer generator.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

Cavitation is there because fluids boil and our atmospheric pressure is constant and finite.

If the absolute atmospheric pressure is 14.7 psia (32.3 ft), and the vapor pressure of the pumped liquid is 1 psia (2.3 ft), and you need to draw the liquid up more than 30 feet (32.2-2.3), mother nature won't let you -- no matter how good your pump is or what type it is. The lift height drops with velocity head as the through velocity gets higher.

With positive displacement pumps, cavitation likely does occur. Maybe just not so much.

Atmospheric pressure "constant and finite". Not by any stretch of the imagination. Try drawing water at an elevation of 10,000 ft and see how far you can lift it.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

with PD pumps, the NIPR is a function of, the suction valves and the geometry of the pump. I could make a pump that would require 10,000 feet of water hsuction head,given a strong enough spring and some complex geometry.

You have enough head pressure to overcome the spring tension on the suction valve without going below the bubble point. Most PD pumps I work with require 20 feet of NIPR and we set the feed system up in the air 25 feet at least.

I should restate my earlier point:

Cavitation is there because fluid boils and our atmospheric pressure is releatively constant and finite at on a given piece of ground (that's not underwater).

If the absolute atmospheric pressure is 10.1 (23.3 feet)psia at an altitude of 10000 feet, and the vapor pressure of the pumped liquid is 1 psia (2.3 ft), and you need to draw the liquid up more than 21 feet (23.3-2.3), mother nature won't let you -- no matter how good your pump is or what type it is. The lift height drops with velocity head as the through velocity gets higher.

If your pump is really good, you can better approach this theoretical maximum. You won't better it. With positive displacement pumps, cavitation likely does occur. Maybe just not so much.

The point is, the best you can do as far as suction lift for a reservoir exposed to atmosphere is related to the local atmospheric pressure, and the vapor pressure of the fluid.

You can always make a pump that has a higher NPSHR. But not one that has lower.

and... the fluid's density.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

The accelration of the fluid has to be taken into account in establishing the NPSHa for a PD pump. referThe Reciprocating Pump Theory, Design and USE by Miller.

Just a clarification: Acceleration head is only a significant factor if the positive displacement pump has pulsating output flows.

Gear pumps and screw pumps probably wouldn't have a problem. Piston Pumps and diaphragm pumps could.