Pressure relief versus Pump size
Pressure relief versus Pump size
(OP)
I am running tests that measure the pressure response beneath a pool drain after the drain is blocked suddenly and then the pump is turned off 1/2 to 1 second later. I am finding that a limit is reached in the effect of increasing pump size and flow rate on the time (dT) for pressure to equalize after the blockage. I surmise this is due to the pressure dropping towards a vacuum, but of course never exceeding it. However, it is difficult to explain why a larger pump could have dT less than a smaller one. Could it be that larger pumps have larger internal passageways and so relieve pressure faster than smaller pumps? The pumps are Starite Dura-Glas P2RA5EL(1 hp) and Pentair Challenger CHII-NI-3FE (3 hp).





RE: Pressure relief versus Pump size
First, I don't think its entirely reasonable to be comparing two completely different pumps (not only 300% power rating, but different mfgr's as well) and trying to extrapolate hard and fast generalizations based on their response to an effect as small as a 15 psi pressure difference.
Secondly, yes could be.
Thirdly, The smaller pump may be doing a better job at lowering the pressure. Did you have exactly the same pressure reading at the time each pump was shut off?
Lastly, The larger pump probably has a lot more reversing force on the impellers due to backpressure on vane areas that are significantly greater than the smaller pump, although the moment of inertia of the larger pump and motor may not be proportionally higher. I assume you do not have check valves in the discharges.
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RE: Pressure relief versus Pump size
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RE: Pressure relief versus Pump size
The pressure reading before the drain blockage is the water head (2 feet) minus the velcoity head, which does make the 3 hp pump a lower pressure than the 1 hp to start. The dT is measured when the pressure again reaches that flow pressure that existed before the drain blockage.
I see your last point as particularly enlightening--
That the balance of torques from the impellar/motor inertia(positive) and the vane drag (negative) favors the smaller pump. Also, the running discharge pressure is higher for the larger pump.
Thank you for your response.
RE: Pressure relief versus Pump size
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RE: Pressure relief versus Pump size
Your idea about the larger vanes/more drag has given me a plausible argument. Thanks again.
RE: Pressure relief versus Pump size
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