static head

[asifraza0]

I'm sizing an centrifugal pump for a water service in a close system. The system starts from the surge tank, pump, few heat exchangers and complex piping network. Eventually the pump discharge comes back and joins the pump suction. The surge tank is 30 ft elevated and is the highest point in the system. The second highest point in the system is the bank of heat exchangers where the pump discharges.

1) I've calculated the discharge pressure by taking a static head of 15 ft. 15 ft is the height from the grade to the highest inlet nozzle of the heat exchangers.
2) Then I've added dynamic losses, control valve losses, filter pressure drop etc to arrive at a total discharge pressure.

A counter argument is: Since the pump discharge comes back and joins the suction in a closed loop system. The static head is irrelevant and should not be included in the sizing calculations. Risk of not adding the static head means the pump will be undersized.

Please commnent and share your thougths.

Thanks in advance

John


Asif Raza

 

[BigInch]

Add static head on one side and subtract it on the other. Doesn't cost anything and when you don't have a closed loop, you can still use the spreadsheet.

I hate Windowz 8!!!!

 

[asifraza0]

Thanks BigInch:
What about the start-up conditions?

Thanks

John

Asif Raza

 

[JohnGP]

Your surge tank is at the highest point in the system, so the complete pipework can be liquid filled (provided you can vent the air) to start. Then you have the suction and discharge static heads in balance. Similarly when starting anytime with the system full, you have that benefit. It's not like when you have to pump over a hill from a low point, where you have an initial high static discharge head to overcome.
Cheers,
John

 

[Artisi]

JonhGP's comment:
that's provided the system can be fully charged from another source, if you need to use the system's pump to fill the loop, then the pump must be capable of generating sufficient head to reach the high point, this can be at a much reduced flow rate but it must be capable of pumping to the static head.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JohnGP]

Artisi, yes I agree, but a diagram of the system wasn't provided, so I was guessing somewhat as to how it was set up. Systems I have seen return flow to the pump via the surge tank - asifraza0 just said that the pump discharge eventually comes back to the pump suction. In any event, I think the system would need to be initially filled from an external source.

 

[asifraza0]

Thanks JohnGP and Artisi: You agree that if the system is not filled by an external source then the static head need to be accounted for while sizing. During first start, the pump has to work against the static head. As I mentioned that the surge tank is the highest point in the system, the surge tank can be used for filling the whole system since the surge tank will provide enough static head to fill the whole system. My colleague also mentioned that there is a filling nozzle in the suction line where a small portable pump can be used to fill the entire circuit from the truck. But regardless, how the system is filled that is one thing. I want to make sure that we agree, that if the system is pre-filled then the pump does not have to work against the static head. Then the pump will only sized for all dynamic losses such as line friction loss, H-E's and control valves if any.

Thanks

John

Asif Raza

 

[Artisi]

Quote. I want to make sure that we agree, that if the system is pre-filled then the pump does not have to work against the static head. Then the pump will only sized for all dynamic losses such as line friction loss, H-E's and control valves if any. Unquote

Correct.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[asifraza0]

Thanks Artisi for the clarification.

Good discussion..

Asif Raza