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Pressure relief versus Pump size

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mrelet

Mechanical
Nov 1, 2001
49
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).
 
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I take it you refer to the pressure stabilizing at 0 psig.

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.



Going the Big Inch! [worm]
 
And... the reverse transient pressure wave created by the large pump quickly running out flow and the much higher flowrate in the discharge pipe, may be many times higher than what the smaller pump can do with its tiny flowrate.

Going the Big Inch! [worm]
 
There is no check valve in the system.

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.
 
Without a check valve, it may be somewhat easier for the larger pump to get a faster reverse rotation, due either to the higher discharge pressure x vane area, or a reverse flow transient. Put a check valve in and see if you get the same effects.

Going the Big Inch! [worm]
 
Thanks Big, but a check valve would stop pressure equalization altogether. In the run up to my question about pumps, I blocked the drain-- all suction inflow to the pump. A check blocks the discharge when pressure reverses.

Your idea about the larger vanes/more drag has given me a plausible argument. Thanks again.
 
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