Low pump inlet pressure - calculating inlet size

[Hydromechdude]

Hi all,

I have a pump right now that at times is not priming properly on start up, which I theorize is happening because of low inlet pressure.

The line inlet is 3/8" diameter and the pump is an axial piston type producing about .6 GPM flow (or less). The reservoir is about one foot above the pump, with a three foot long suction line length. The suction line is not pressurized and it is believed that there is not enough fluid in the line in order for the pump to prime properly. This is because the reservoir seal friction causes the fluid going through/into the pump on start up to then pull a vacuum and prevent any fluid thereafter to enter the pump.

Being discussed as a possible solution is to increase the line diameter size that is running to the pump. This would create a much larger volume of fluid with which to prime the pump before the vacuum effects take over. At least, that's the theory.

What I'm wondering is how I would calculate the volume of fluid necessary in order to ensure the pump can prime itself. Also, what are your thoughts on adding a larger suction line to combat this problem?

I'll provide any more information that I can.

Thanks in advance.

 

[BigInch]

A larger diameter suction line would be a step in the right direction, if what you describe is happening, as well as probably a good thing to do in any other case too. Another would be to get some additional pressure in that supply tank. Moving the tank closer, or perhaps even supplying an intermediate "accumulator" tank closer to the pump, might also help.

Piston pumps need something called dynamic suction head, something in addition to the normal suction head required by centrifugal pumps. Lack of appropriate dynamic suction head may also be the cause of your trouble. As the piston in the pump does not cause a uniform motion of liquid from the suction side and a uniform discharge of liquid from the discharge, flow both in the suction line and discharge line can slow down and come to a halt before the piston begins to move and the fluid accelerates again. So, you not only have to provide static head, but the dynamic head needed to accomplish the acceleration that occurs on every cycle of the piston. If pressures actually go below the fluid's vapor pressure (for some liquids it may even be higher than vacuum range pressures), gas pockets will form too. What you must do is minimize these effects by using one, or all of the methods I mentioned above. Calculating "dynamic suction requirements for PD pumps" have been discussed in this forum a number of times, so try searching for that topic above. If you don't find it, come back and we'll fill you in on 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)

http://virtualpipeline.spaces.live.com/

 

[Hydromechdude]

Thanks BigInch. I'll take a look around and see what I can find.

Also, the reservoir is pressurized as is the suction line, under most conditions. However, what I'm trying to remedy is the occasion where the reservoir is no longer pressurized (~once every two weeks or so).

 

[Hydromechdude]

I wasn't able to find what I was looking for. The threads I was finding mostly dealt with requirements and the differences between PD vs. gear and centrifugal pumps.

Just a couple notes about the system.

- The reservoir is pressurized (and closed to atmosphere) under operating conditions. The concern that we are trying to deal with is for start up only.
- There is no evidence of cavitation for removed pumps. However, I would not expect there to be any, as the inlet line is pressurized under operating conditions. The exposure to this condition will be very small.
- Air in the inlet line could be contributing to the problem. We don't have an exact understanding of how much air is typically in the system.

The goal of the larger inlet line is to give the pump enough fluid to start producing pressure in order to pressurize the reservoir, before a vacuum in the reservoir is created.

I have a couple questions that I am curious about. The first is, would a larger suction line do what I'm hoping? The second is, how would I model/calculate the line size (and therefore fluid volume increase) needed in order to do this?

I believe I tried to be too vague in my initial post. My apologies for that. I hope this is clearer.

 

[BigInch]

I've got some docs attached here,


**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

and here,

**********************
"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)

http://virtualpipeline.spaces.live.com/