Pump as a flow restriction?

[TiCl4]

This is sadly not theoretical. I visited a site a while back where an ~30ft tall storage tank was being “pumped” to an ~6’ tall effluent pH control tank. Between the two tanks is a 2” pipe with a small pump. I don’t remember the exact characteristics of the pump, but it was something like a 0.75”x1”, 20 gpm @ 20’ head. Something around there, anyways. There is no flow control valve anywhere in the system.

The particulars of the pump aren’t greatly important, as this situation is rather unique. If the tank is full or even half-full, gravity flow would far exceed the capacity of the pump. This got me wondering.

If left off when gravity flowing, the pump would act as a flow restriction due to the restricted pipe diameter. What would happen if the pump were turned on and the tank is full and flowing? If gravity flow already exceeds (not a given - I haven’t measured it) the 0dP flow capacity of the pump, what would turning the pump on do to the flow rate? Naturally this would wreck the pump, but that made me interested about situations where gravity flow exceeds pump capacity.

 

[LittleInch]

A few options I think.

It might, just might, end up with a reverse DP and essentially acting as a poor efficiency generator as the impellor tris to go faster than the motor is set for and create less flow.

It might add a few feet DP and a few more GPM, but all depends on what the ultimate restriction is in the pump and whether you get to critical flow

The few DP feet it adds gets eaten by the extra resistance within the pump and no apparent increase in flow.

Try it and find out - it's only a small pump. Just let us know and take a few readings.

But if you have 30 ft max diff head (30-0) and you pump puts out 20 feet, then there might not be as much extra flow from gravity as you think.

once the head diff gets to <20 feet without it, then the pump will add more flow surely?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

In other words, your 2" pipe might start acting as if it was the "flow control valve".

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."