Deadhead pressure Variable speed pumps

[Diborane]

Hi all,

When working with fixed speed its pretty easy to determine the dead-head pressure of the pump. Just look at the pump curve for your impeller size at 0 flow.


For a variable speed pumps I have been calculating the deadhead pressure at the maximum motor rpm. Is this too conservative for VSD pumps? At lower flow rates the variable speed drive should lower the motor rpm and thus the max TDH. What is the proper procedure for calculating the deadhead on these pumps.

 

[Artisi]

The "dead-head" as you call it - which I think is better described as the closed valve head or CV will change approximately as the square of the speed, this is usually sufficient for all general purpose needs.

Naresuan University
Phitsanulok
Thailand

 

[quark]

The link below shows you how to calculate the shutoff head for various speeds.

http://www.mcnallyinstitute.com/02-html/2-7.html

Regards,

 

[EGT01]

Diborane,

If the reason you are wanting to determine the dead-head pressure is to evaluate a potential overpressure condition, without knowing more details about your specific situation, I think I would continue to use your approach of considering the maximum motor speed.

I've always followed the general rule that no credit is taken for a typical process controller to reduce an overpressure event. If nothing else, an operator could place a process controller in a manual operating position such that it would not react to a change of input.

Now, if you have associated controls that would meet the requirements of a high integrity, safety instrumented system, that might be sufficient to take credit for the controls to react and reduce or prevent an overpressure event.

 

[Diborane]

Thank you quark that is a valuable reference.

Also thank you EGT01, your response pertains more to the safety concerns I had. The deadhead pressure was a specific concern for choosing flange/piping class. I had choosen the higher rated piping class and a comment was made regarding the existence of the VSD.