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Hello,
I have sized many centrifugal pumps, but never had chance size any positive displacement pumps. Today, I was asked to size a screw pump. Could anyone tell me what are the differences sizing centrifugal compared to positive displacemnt pumps? Do I also calculate just the total dynamic head? Thanks in advance for your help.
Flowie
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Positive Displacement
Started by Guest_Flowie_*, Feb 23 2005 09:42 PM
4 replies to this topic
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#2
Guest_Joker_*
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Posted 23 February 2005 - 10:07 PM
Hi Flowie,
In positive displacement pump you don't ne ed to calculate head. Instead you calculate the volumetric displacement. Assuming first that there is perfect tightness (100% efficiency) what you pump is the displacement (cc) x rpm x density of the fluid.
The pump WILL move that fluid unless there is an obstruction (for example a closed valve) in which case pressure will increase until something gives (burst pipe). Therefore a positive displacement pump is always protected by a safety valve which preferably discharges back to the pump suction.
The pump efficiency depends on the fluid (relatively high viscosity is better. However very high viscosity may lead to the fluid not filling the cavity and therefore you lose efficiency), on the discharge pressure (fluid slips back to the suction at high pressure).
Talk to a manufacturer for recommendations.
In positive displacement pump you don't ne ed to calculate head. Instead you calculate the volumetric displacement. Assuming first that there is perfect tightness (100% efficiency) what you pump is the displacement (cc) x rpm x density of the fluid.
The pump WILL move that fluid unless there is an obstruction (for example a closed valve) in which case pressure will increase until something gives (burst pipe). Therefore a positive displacement pump is always protected by a safety valve which preferably discharges back to the pump suction.
The pump efficiency depends on the fluid (relatively high viscosity is better. However very high viscosity may lead to the fluid not filling the cavity and therefore you lose efficiency), on the discharge pressure (fluid slips back to the suction at high pressure).
Talk to a manufacturer for recommendations.
#3
Guest_Guest_*
Guest_Guest_*
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Posted 23 February 2005 - 11:48 PM
Joker,
Thank you for your input. Now I understand that positive displacement pump's discharge pressure varies in order to maintain constant volumetric flow rate.
But what kind of specs or data points do I need to provide to the vendor to size a positive displacement?
Thank you very much for the help
Flowie
Thank you for your input. Now I understand that positive displacement pump's discharge pressure varies in order to maintain constant volumetric flow rate.
But what kind of specs or data points do I need to provide to the vendor to size a positive displacement?
Thank you very much for the help
Flowie
#4
ShaunHill
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Posted 24 February 2005 - 04:15 PM
You still need to provide the flow and head (diff pressure) for operation as both will be used to calculate the required horsepower of the pump.
You will also need to provide the basic fluid properties like density, viscosity etc that you would provide with a centrifugal pump.
Maximum discharge operating pressure should be included since the pump rating will need to be able to mechanically handle that pressure.
Also remember to include information on NPSHa, because PD pumps can also suffer damage from cavitation.
You will also need to provide the basic fluid properties like density, viscosity etc that you would provide with a centrifugal pump.
Maximum discharge operating pressure should be included since the pump rating will need to be able to mechanically handle that pressure.
Also remember to include information on NPSHa, because PD pumps can also suffer damage from cavitation.
#5
djack77494
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Posted 19 March 2005 - 02:19 AM
I concur with what Shaun says here, but take objection to the use of the term "head". Centrifugal pumps produce "head", which we tend to think of as pressure. Positive displacement pumps produce capacity, or volumetric flowrate. They will deliver that capacity against any opposing pressure unless/until it exceeds the design capabilities of the pump. Then, you will experience some sort of failure. Probably best would be an inability of the driver to keep the machine moving. Less good would be a containment failure.
Getting back to your original question, however, you need to specify the pump's design capacity and the differential pressure (to size the driver). If you have an elevated suction pressure, that would also need to be specified to ensure that the pump can mechanically "handle" the fluid. You might also specify the NPSHa, and, of course, include the fluid properties.
Doug
Getting back to your original question, however, you need to specify the pump's design capacity and the differential pressure (to size the driver). If you have an elevated suction pressure, that would also need to be specified to ensure that the pump can mechanically "handle" the fluid. You might also specify the NPSHa, and, of course, include the fluid properties.
Doug
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