Effect of Pump Discharge Static Head

[Sawsan311]

Dear All,

I am currently evaluating scenarios scenarios for existing pumping system,appreciate your feedback on the below:

A-Scenario A: pump has a shut off head as per pump curve, discharge system was elevated initially with a static head of 5 m.Then it was decided later to increase the static head to 15 meters, do you believe no change will occur to pump shut off as it is always a zero flow scenario. Can we say that pipeline pressure rating would not be impacted as long as it was designed for the pump's shut off pressure? In other words, during normal operation,flow delivered by pump would be less compared to original case as the system curve is now raised and intersects at lower flow with the pump curve which is independent of the static height.

B. Scenario -B if the receiving system on discharge is located at -20 meters elevation with respect to pump's eye. The design pressure of this system should take into account pump's shut off pressure (shut off head at maximum density) + the static height of 20 meters with maximum density?

C. Scenario-C : I was discussing with one of the engineers on the adequacy check of en existing system pressure rating post upgrading pump's impeller. Bigger pump impeller yield higher shut off head. Hence with the use of the maximum density shut off pressure is estimated. I believe there is additional need to add the maximum system's suction pressure (design pressure of upstream suction) and the suction maximum static height to the estimated shut off pressure from pump curve...The engineer was stating that the blocked in discharge will lead the upstream suction side to reach its design pressure kindly please advise. In all cases,it shall be assured the shut off head on pump curve is with a rise of at least 10% from the rated head.

Thank you very much,

 

[mk3223]

The pump calculation should be similar for the scenario A and B. Regardless the static head in the pump discharge, the pump should be selected to have sufficient flow and delivery pressure per design requirements. The pump may need to be upsized as the discharge static head increased too much which may reduce the flow becoming under the design requirement.

The pump suction head is needed to provide the NPSHa, which is required to be higher than the pump NPSHr. So, the suction head may required to be increased for the larger pump, as specified by the pump manufacturer.

 

[Sawsan311]

Thank you mk3223 for your feedback,
In point A, I was mainly focusing on the effect of the discharge static height on the existing pump's shut off pressure and which I believe is independent as the discharge static head contribution of the system doesn't affect pump shut off head even if the static height increases.Do you agree?

Thanks

 

[LittleInch]

scenario A - no impact on pump shut off pressure because you've shut the pump off from the downstream system!
If the shut off was in the downstream system it will see a lower pressure due to the elevation change from 5 to 15 m but the pressure at pump discharge won't change
If the pipeline is now at a fixed altitude of +15m then it's pressure rating could be decreased by 10m compared to the 5m elevation, but the pipeline would need to never fall bleow 15m elevation relative to the pump centre line.

Scenario B - correct

scenario C - a pump is a differential pressure machine, therefore discharge head/pressure is inlet head/pressure plus the differential it creates.
Not sure what your engineer is saying, but in certain circumstances the downstream pressure could be seen by the upstream side of the pump. Use of check valves and relief valves may be required to not overpressure the upstream side of the pump.


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

 

[Sawsan311]

Thank you very much LittleInch for your response,I totally agree with your comments,


Additionally, for scenario C and specially in parallel pump operation having one of the pumps standby offline with a closed inlet, in case of a common blocked discharge on the downstream the suction side of the standby pump along with its suction side would be pressured up to the limit of the shut off of the other pumps. Therefore, we usually fully rate the pumps starting from their suction isolation valves.

However, I was mainly pointing out to the case of revamped upgraded pump impeller which mandates re-checking the design pressure of the system based on the revised shut off head from pump curve with maximum static height and suction PSV set pressure of the suction vessel.

Thank

 

[aliansari]

Scenario A. Pipeline pressure will not be impacted because the piping system is designed for shut off pressure. Highest pressure in the system would be at pump discharge flange and at the back end, it would be pump discharge pressure minus the pressure equivalent of 15 meter head in the non flowing shut off conditions.

Yes discharge flow during normal operation would be less if the system head is increased from 5 meters to 15 meters. Please refer to the pump curve, where reduction in flow can be estimated against the revised and increased system head

Scenario B. Yes, if the back end of the piping system is located 20 m below (-20 m as you mentioned)the pump eye, the pressure at that end would be 2 bar higher than the pump eye in a non flow shut off condition. You need to design the system at pump shut off plus 20 meter equivalent pressure using maximum density

Scenario C. If the pump casing accommodated a higher size of impeller, pump curve will be read with reference to the new impeller, where shut off head and rated head values will be higher, resulting in more flow rate at a given system head. Pump curve is always based on developed head. Discharge head is calculated by adding suction head into it and system design pressure is determined by actual discharge pressure at shut off (shut off head plus suction head converted into pressure). This is the reading shown by the pressure gauge at the discharge of the pump.

However suction side pressure will not increased to system design pressure, it will remain the same, irrespective of the discharge side pressure. Suction side pressure is needed only to calculated pump discharge pressure by adding it into the pressure developed by the pump. Suction side pressure is a function of suction side hydraulic configuration and has nothing to do with discharge side flow or pressure