Pump Sizing

[RyanMcCauley]

This may seem like a simple questions, but is there a rule of thumb for sizing pumps in regards to a factor of safety for the pressure losses throughout the system?

I'm working on a project that needs a new pump spec'd out. I've already ran pressure loss calculations throughout the entire system, which came to about a loss of 20 psi. When I spec the pump, how much of a factor of safety should I incorporate above the calculated pressure loss?

Hopefully my question makes sense to everyone else.

Thanks,
Ryan

 

[SNORGY]

If for a centrifugal, a rule of thumb I have seen used is 25% rise to shut-off head from BEP (Best Efficiency Point), and BEP is intended to be where your target flow rate is specified. Whatever factor of safety on differential head that can be applied falls out from that.

The trouble is, lots and lots of pumps have flat characteristics left of BEP and, therefore, you can't meet the 25% rise to shut-off.

With that in mind, maybe the best "rule of thumb" that I have come across is to apply the contingency to the *dynamic* head losses that would be encountered in the piping system. Cameron Hydraulic Data Book suggests that this should be 15% - 20% higher than the tabulated values of pressure drop per unit length, which they base on new, clean commercial pipe. Other references speak of an "age factor" (Crane TP 410-M page 1-7) similar in magnitude. So, my rule of thumb would be "20% on dynamic head loss or the difference in head loss across any control valve over its operating range, whichever is greater".

In practice, it would usually boil down to multiplying the calculated Darcy Friction Factor by 1.2.



Regards,

SNORGY.

 

[Artisi]

If you are talking about centrifugal pumps and you are confident in your head loss and static head (total head) and flow calculations a good method for initial sizing of pumps is to select a pump that will operate at or near to BEP with a 85-90% impeller trim.

 

[RyanMcCauley]

Thank you both for you quick, insightful responses. That gives me a great starting point. Thanks.

Ryan