7 replies to this topic

[ayan_dg]

Does cavitation takes place in positive displacement pumps? Do they require priming ? Does NPSHA & NPSHR has the same significance & importance when dealing with positive displacement pump ?

[JoeWong]

<!--quoteo(post=15384:date=Jan 9 2008, 03:03 AM:name=ayan_dg)--><div class='quotetop'>QUOTE (ayan_dg @ Jan 9 2008, 03:03 AM) <a href="index.php?act=findpost&pid=15384"><{POST_SNAPBACK}></a></div><div class='quotemain'><!--quotec-->Does cavitation takes place in positive displacement pumps?<!--QuoteEnd--></div><!--QuoteEEnd-->
Yes.
<!--quoteo--><div class='quotetop'>QUOTE </div><div class='quotemain'><!--quotec-->Do they require priming ?<!--QuoteEnd--></div><!--QuoteEEnd-->
Yes for those positive displacement pumps that i have seen.
<!--quoteo--><div class='quotetop'>QUOTE </div><div class='quotemain'><!--quotec-->Does NPSHA & NPSHR has the same significance & importance when dealing with positive displacement pump ?<!--QuoteEnd--></div><!--QuoteEEnd-->
Yes. In addition, there is acceleration head in positive displacement pumps suction and further reduced the NPSHA.

[djack77494]

ayan_dg,
Please search the topics on this website for more detailed answers to your queries. I know that there was a pretty extensive thread on this topic here recently.
Doug

[Austro]

"Yes. In addition, there is acceleration head in positive displacement pumps suction and further reduced the NPSHA."

Could you explain exactly what acceleration head is? I searched on this forum and elsewhere for an explanation of it and my best attempt at explaining it is it is the mechanical energy lost from the stop and go fluctuations of the pump. Much like it takes more gas to perform stop and go driving as opposed to driving at a continuous speed for the same distance. That seems more like NPSHR though. I'm not sure.

[djack77494]

Could you explain exactly what acceleration head is?

To describe acceleration head, picture a length of pipe or tubing filled with a liquid at rest. Now induce a flow to get that slug of liquid to move into a cavity; e.g. the cavity of a PD Pump. It takes energy to induce that fluid movement or, in other words, to accelerate the liquid from zero velocity to some positive velocity. The energy comes from the fluid itself and is refered to as a head loss. In this case since we're talking about accelerating the liquid, it is called the acceleration head. Once the cavity is filled, the liquid is brought to rest again, but no useful energy is recovered when the liquid's kinetic energy is brought to zero.
HTH,
Doug

[Austro]

Could you explain exactly what acceleration head is?

To describe acceleration head, picture a length of pipe or tubing filled with a liquid at rest. Now induce a flow to get that slug of liquid to move into a cavity; e.g. the cavity of a PD Pump. It takes energy to induce that fluid movement or, in other words, to accelerate the liquid from zero velocity to some positive velocity. The energy comes from the fluid itself and is refered to as a head loss. In this case since we're talking about accelerating the liquid, it is called the acceleration head. Once the cavity is filled, the liquid is brought to rest again, but no useful energy is recovered when the liquid's kinetic energy is brought to zero.
HTH,
Doug

Perfect. Still, kinda seems like it was from the pump but yeah, since the water entering the pump has lower head, obviously that means lower NPSHA rather than higher NPSHR. Kind of trivial anyway.

[djack77494]

[/quote]Perfect. Still, kinda seems like it was from the pump[/quote]

I think it important to understand that the energy does NOT come from the pump. Until the fluid enters the pump and has work done on it, it does not receive any energy from the pump. This is important to understand when doing NPSH calculations. Starting with your liquid at suction vessel conditions, all the energy to get the fluid moving, overcome frictional losses, and get into the pump come from the fluid itself. The energy can be viewed as "pressure energy" (really internal energy), and is consumed in the process. At the point where you get the fluid into the pump, the pressure must not have dropped below the vapor pressure of the liquid OR you will get vaporization/cavitation.
Doug

[deltaChe]

Does cavitation takes place in positive displacement pumps? Do they require priming ? Does NPSHA & NPSHR has the same significance & importance when dealing with positive displacement pump ?

They are two types of PD pumps (1) Reciprocating (2) rotary.
The reciprocating pump is generally used for low viscosity, low flow and high head duites.
Acceleration head is caused by flow in the suction and disicharge piping is not constant, and it must accelerate
and decelerate a number of times for each revolution of the crankshaft.

ha = LVNC/kg
ha= acceleration head of liuqid of being pumped
L=actual length of suction line
V=liquid velocity in suciton line
N=speed of pump crankshaft rpm
C=constant
k=constant (depend on fluid compressibilty)
g= gravitiational constant