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Irregular Pump Flow

Irregular Pump Flow

Irregular Pump Flow

(OP)
Can anyone explain the following behaviour?

Grundfos borehole pump with a flow of approx 2 m3/hr. Discharges to a tank open to atmosphere approx 300m away. The pump is fixed speed and the whole system operates manually. After between 15 and 90 minutes of pumping, the flow suddenly reduces to nearly zero for approx 1s, followed by a surge for approx 1s or 2s. Behaviour repeats at irregular intervals but with increasing frequency and severity. After operating in this manner for approx. half an hour the flow stops completely for approx 4s, followed by a surge. The pump was stopped at this point. Behaviour repeated over several tests with not enough data to discern a connection between pump rate and how quickly the problem presents.

Some possibilities:
Excessive drawdown in the borehole leading to cavitation or air entrainment
Irregular supply voltage
Trapped air in the pumping main

Any thoughts on the above would be appreciated, as would suggestions for anything else it could be.

Thanks

RE: Irregular Pump Flow

Definetely sounds like first option to me. If it was posisble to measure flow instantaneolsy or even over a period of say 15 seconds, my gues sis that you would find the supply rate gradually falling until ou suck the well dry, the pump doesn't like pumping air, but this delay allows the well to re-group a little until youreally have run it dry.

Plan B - run it at 1m3/hr by throttling a discharge valve somewhere along the way and see how long it lasts then.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Irregular Pump Flow

I think #1 as well. No pun intended.

Independent events are seldomly independent.

RE: Irregular Pump Flow

Probably No1 - although it's not cavitation or air-entrainment - simply insufficient inflow to the well. Try throttling the pump discharge (ie, less flow / time) as see what happens.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Irregular Pump Flow

(OP)
Thanks for the replies. No.1 was my best guess. I tried to test it by throttling the pump using a gate valve on the headworks, however even closing the valve 7 turns out of 8 did not seem to affect the flow. Once I closed the valve 7 1/8 turns, the flow suddenly dropped to about 15% what it was before. The pump was run like this for an hour then the valve opened again and the flow did not return but remained at 15%. Down at the headworks the pump could be heard knocking about in the borehole, so I wondered if it had been damaged by cavitation and sent out out balance.

RE: Irregular Pump Flow

Is this a new or existing installation ? - simply sounds like insufficient flow into the well. However without a lot more data it is all guesswork as to the problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Irregular Pump Flow

That's the problem using gate valves to throttle, but 15% of flow was probably a bit too much. You may have damaged the pump or motor running it that low for an hour as the water through the pump usually cools it as well. Sounds like you need to pull the pump and check it before trying another test, but this time include a globe valve in the system to allow you to throttle much more accurately and not so low - 25% of max flow should be your minimum and initially aim for 50%.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Irregular Pump Flow

(OP)
Artisi- it is an existing installation in Africa, hence new data is hard to come by now! I think low flow into the well is the best guess I have.

LittleInch- thanks for the advice. Replacing the valve is certainly necessary. I found out today that the pump has apparently been fine since the last test, so maybe I'll just put it down as an anomaly.

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