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Positive Displacement Pumps in Series

Positive Displacement Pumps in Series

(OP)
Hello,

I'm designing a filtration and oil transfer skid for gear oil. The reservoir is located too far from the main lube skid that it would reduce the pump performance due to the long suction line. To correct for this I will install a transfer pump adjacent to the reservoir which will pump the oil over to the main lube skid.

Therefore, I will have (2) positive displacement pumps in series. I'm wondering if I can select two identical pumps and run them in series? The transfer pump will operate at approximately 4-bar and the lube pump will operate at 8-bar.

The pump manufacturer's curves show that at 4-bar the pump will discharge 354 L/min and at 8 bar it will discharge 347 L/min due to loss of efficiency at higher discharge pressures. Will I run into any problems with an arrangement like this? The transfer pump will have an overflow relief back to tank but I don't suspect it will see much flow.

I've included a very basic schematic attached.

Or, should I select a larger displacement pump for the transfer portion so that I am guaranteed sufficient flow to the lube pump?

I cannot find any information online about recommendations or tips on this type of an arrangement.

Thank you.

RE: Positive Displacement Pumps in Series

Your first pump needs to be a slightly higher flow at all times. So 210 cc/rev.

Otherwise there is a risk of starving the second pump. No two pumps are ever truly identical.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Positive Displacement Pumps in Series

(OP)
LIttleInch,

Thank you - this is what I suspected and was looking for.

RE: Positive Displacement Pumps in Series

You cannot simply put two positive displacement pumps in series. You need a buffer tank and controls (or relief valve) between the two pumps to prevent over-pressuring or starving the inlet of the second pump. Or use a non-positive displacement booster pump.

Your drawing shows a relief valve at the discharge of your first pump. This makes your first pump functionally no longer a positive displacement pump. So the wording of your question does not match your drawing. As drawn, the first pump must have higher capacity than the second pump.

RE: Positive Displacement Pumps in Series

(OP)
Compositepro,

Thank you for the response. I wasn't familiar with the nomenclature for this type of an arrangement. The pump itself is still a positive displacement gear pump but will have an externally mounted relief valve to allow for the overflow.

Due to the available displacements from the pump manufacturer I will be selecting a 200 cc/rev booster pump and will be using a 180 cc/rev lube pump. This will result in a booster flow of approximately 354 L/min and a lube flow of approximately 298L/min with the excess flowing over the booster's relief. The lube flow is not critical as even with a smaller lube pump there will be flow over the lube skid's relief and back to tank.

I will look into a pressure compensated pump but those may be too expensive for this project.

Thank you.

Do anyone else - is there a rule of thumb for how much larger the booster pump's flow should be? For example should a booster be 5% more flow, 10%, or more?

Thank you.

RE: Positive Displacement Pumps in Series

Hi Asit859,
Is the second pump even necessary? Assuming you can use just one positive displacement pump closer to the reservoir, why would you not go this route? Are the friction losses too great on the discharge of the pump? Do you already have a pump here that you want to utilize?

I have not used two PD pumps in series. Centrifugal pumps are used in series to increase head on the line, such as booster pumps when the length of pipe is too long. For positive displacement pumps, they are constant flow pumps and will supply the proper pressure at a given flow rate, assuming the motor is large enough to handle the power requirements. Adding two in parallel could function the same way as described for the centrifugal arrangement, assuming the flow rates on each are constant. Like others have stated, you need to make sure the second pump is not starved of product.

Good luck.



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