5 replies to this topic

[stu]

dear all,

Have clarification on pump differential pressure & pump discharge pressure estimation.

Let us consider  two schemes of piping arrangement , one pump with 10 m rise & drop discharge line and another with straight length. Both pumps suction pressure, flowrate, discharge pipe total length, required pressure at user, discharge pipe friction loss(assumed same for ease understanding) are same. Please clarify whether both pump have same discharge pressure / differential head or pump with 10 m elevated discharge line have any difference even though static head gain or loss is zero i.e static head 10 m rise & drop balance.

Sketch for those scheme also attached for clarity.

 Thanks,

stu  

[breizh]

hi,

Let you consider the excel sheet attached (1rst page) with examples .

note : at start up the pumps need to be able to bring the material to the highest point of the discharge line.

Good luck

Breizh 

Edited by breizh, 29 May 2021 - 01:55 AM.

[stu]

Dear Breizh,

Thanks for the material. please suggest based on the sketch attached in my post, is it for scheme 1( 10 m rise & 10 drop) during startup only we require 10 m differential head higher than scheme 2 ( st.line), after that both require same diff. head?

For scheme 1 discharge side static head is -10 +10 = zero & for scheme 2 no rise & drop ,then static head here also zero, that means both pumps req. differential heads are same?

Thanks,

Stu 

[breizh]

Hi,

Yes for your understanding about static head .

Good luck 

Breizh 

[Pilesar]

By the criteria stated in the original question, the steady state pressure at the point of pump suction and the point of pump discharge will be identical for both schemes. Therefore the pump differential head will be identical for both schemes. You do not need more pump head for the vertical loop piping on startup as long as the discharge line is kept full of liquid.

   The original question is posted in the 'Student' section so I will answer as if the original poster were an engineering student. The problem is a classic fluid flow problem. Determine pressure drop in the discharge line for both schemes and compare the results. 

P(end)= P(start) - friction loss- fittings loss -component loss + elevation (start-end) + pump head

Solve the equation for P(start) at the point of pump outlet. P(end) is the point of the line at the final destination. Elevation (start-end) is zero for both schemes. Pump head is zero for both schemes since there is no pump in the discharge line. The attachment with the original question states the assumed friction loss in both schemes to be the same: "Discharge pipe friction loss = 0.2 bar (loss assumed same as straight length, for ease understanding, in real loss will be more than straight length because of bends)" This assumption is an artificial constraint and may not 'ease understanding' for readers since it conflicts with real world experience and application. If the discharge line is not kept full of liquid, the 'same pressure drop in the line' assumption is even more misleading.

[Bobby Strain]

A student for 12 years?

 

Bobby