Pd Rotery Screw Pump pressure vs flow

[crueltobekind]

Hello guys,

I've been struggling with the problem for a while now.hoping you guys can shed some light on the subject for me. so at the place where i work, there is a PD screw pump. now i am trying to figure out the flow rate for the fluid passing through this pump. i know the RPM, of the motor and i have this chart named image1. now if u look at the 250 series line, and 1750 RPM, you can see that the flow rate is 176 GPM, also, at the top of the table, it says these flow rates are flow rates at 500 PSI. i was talking to a staff member and i looked at the pressure gauge in the area, it says the pressure the pump is running at is about 75 PSI. so my question is. since the pump is running at 75 PSI, will it have greater flow rate than the chart in image 1 shows, or will it be the same. can PD pumps have the same flow rates while operating at different pressures? if so how is the discharge pressure controlled, while having the motor at the same RPM?

i have also attached a performance graph of the pump. some more problems here, first, the graph is made for @100 SSU, while our fluid is 300 SSU. and the graph only shows a section for flow rate and pressure head. it doesn't even show 75 PSI. does that mean the pump should not be operating at that pressure?

Sorry for the ignorance guys, im fairly new to the industry. i can use all your knowledge.


Thanks

 

[hydromech]

The actual flow is a function of the volumetric efficiency. A sensible pump manufacturer will quote the flow rates at the maximum working pressure with the stated volumetric efficiency.

As the pressure in the pump increases, oil leaks through the clearances between the screws and housing. Leaking from the outlet and back into the inlet.

Volumetric efficiencies are usually in the region of 95 - 98%

You can measure the output or work it out from the power consumption if you know the overall efficiency including the mechanical efficiency.

How accurate do you want to be?

Adrian

 

[LittleInch]

You might have had a better response in the pumps section, but you can't double post on this site, but if you try again use that forum.

PD pumps in theory pump the the same amount of liquid for each rotation of the shaft. In reality they don't do this with 100% efficiency as hydromech notes and as shown in your image 1 as there are small gaps between the moving parts which allow flow backwards during operation. The greater the pressure difference the greater the losses. You always need to look at pressure or head difference with a pump, but assuming your suction pressure is nearly 0 psi and the 75 is your discharge pressure the answers to your questions are:

Will it have greater flow at 75 psi than the nominal flow rate based on 500 psi - Yes. From the charts provided, my best guess is that at 75 psi differential you're doing about 195 to 200 gpm

Can PD pumps have the same flow rate whilst operating at the same pressure - No, but the difference is relatively small and varies depending on the type of pump

How is the discharge pressure controlled if operating at the same speed - by the resistance to flow of the downstream pipework. Whilst pump flow is marginally affected variable by differential presusre, PD pumps have a very wide pressure operating range. You always need to make sure that between the pump and the first isolating valve, there is a pressure relief or bypass valve (sometimes built into the pump) otherwise the pump can generate very large pressures on low or no flow until either the motor stalls or the pipe or flange leaks / ruptures. From 100 psi differenrc to 500 psi difference, your pump only varies between 195 to 173 gpm. You need to make sure that your donwstream pipework can always accept this flow otherwise the pressure will rise very quickly.

You only have 75 psi (I assume this is differential pressure), can you run it? I can't see why not. You are running it at very low power so essentially your motor is oversized and your overall efficiency is quite low (top curve in the pump curve sheet), but unless you are using the fluid as lubrication then it shouldn't really matter. A quick call to the pump vendor should set your mind at rest quite easily.

SSU is the viscosity of the fluid. This will affect the resistance to flow downstream, but within the pump will only really add some power requirement and may reduce some losses as it is more difficult for the fluid to leak past the small gaps in the pump, but at this level doesn't really make any difference. It's the losses in the downstream pipework that would change a lot from 100 to 300.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[crueltobekind]

Alright so i learned something new, might be basic knowledge for most. but was new for me.

Pd pumps are fixed volume, so the flow is directly related to the RPM, it is completely separate from pressure. PD pumps produce only enough pressure that is needed to overcome restrictions in the downstream pipeline.

 

[LittleInch]

More or less yes. As your pump curves show, flow rate is affected by differential pressure in practice, but not very much. They will only produce enough pressure to overcome downstream resistance. They do not work well if you restrict the flow rate or shut the pump pdf whilst it is still running.

Screw pumps are good at producing a smooth flow without pulses, but others can produce pulsed flow.

Glad you learnt something.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Crueltobekind,
That is the primary difference between a PD pump and a dynamic pump, PD pump capacity is a function of rpm (for a given pump configuration) and a dynamic pump capacity is a function of discharge pressure for a given pump configuration and rpm. In both cases pump power-required is a function of differential head, mass flow rate pumped, and pump efficiency.

Variations from this in PD pumps (the leakage mentioned above, compressibility of "incompressible" fluids, etc.) are all second-order effects and are rarely a significant factor in pump performance.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"