Calculation of positive displacement pump

[EngMecBra]

Hi

I'm doing a calculation of a positive displacement pump and I have a question.

At suction flange the pressure is 1 kgf/cm2 g (considering all suction losses).
Considering all downstream pressures losses, the required discharge pressure is 6 kgf/cm2 g.

Should I calculate required design pressure for this pump like centrifugal pumps (required discharge pressure less suction flange the pressure) resulting 5 kgfc/cm2 g, or should I consider 6 kgf/cm2 g as pump discharge pressure?

Tks

 

[LittleInch]

For pump power purposes you can use the differential head / pressure (5 in your example) and flowrate, but otherwise you need to state flowrate requirement as discharge pressure will be whatever the downstream system dictates and could be higher or lower, but flow for a fixed speed pd pump will be more or less the same (+/- 10%).

My motto: Learn something new every day

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

 

[micalbrch]

I would always specify as the discharge pressure whatever I see on the discharge side pressure gauge. You should mention towards possible vendors the differential pressure but the discharge pressure is 6 kgf/cm²g (under the assumption that your calculation is correct).

 

[EngMecBra]

LittleInch and micalbrch, tks for your answers.

The design pump of selected pump from pump manufacturer is 7.5 barg, then in this case it's ok no matter considered calculation. But I would like to know if the design pressure were 5,5 kgf/cm2g, for example, it'd be ok or I'd have problems.

Take care

 

[LittleInch]

Yes you would have problems. As I said the 5 bar differential presusre is for the power calcualtion, but the discharge pressure is what it is - 6 bar in your case so 5.5 would be too low.

Please tell me you have a full flow pump recircualtion line set somewhere between 6 and 7.5 barg. In reality your gap between 6 and 7.5 is probably too small and your re-circ valve will either leak or not protect your pump or your downstream pipeline. What is your maximum inlet pressure? the pump will just pump the flow so could add 5 barg to whatever your inlet pressure is.

My motto: Learn something new every day

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

 

[micalbrch]

With 5.5 bar design pressure a pump is not suitable for 6 bar discharge pressure. I would even specify or ask the manufacturer how they understand "design pressure". Is it the pressure the casing can withstand? Or is it the maximum pressure the pump can work against with the supplied configuration (motor etc. - see LittleInch' advise). API specifies "design pressure" but who know how you or your supplier understands it.

 

[dcasto]

you need the discharge pressure set so that the mechanical design of the pump is not exceeded. All the forces must be know.