Pump Motor Protection

[lottolearn]

I am designing a pumping station which will pump to a series of irrigation fields. Under normal operating conditions only two (2) or three (3) fields are irrigated at a time. The pump will operate at some 95psi which results in a 40 or 50 hp motor at the design flow. The pump should onlt operate on the first half of the curve under normal conditions. However, if the operator mistakingly opens more than three (3) irrigation zones, it is possible that the pump will operate `out' on the curve in excess of the rated horsepower. What is the best way to protect the pump from operating out of the normal range should the operator open more than the prescibed number of irrigation zones?

 

[captor]

One method would be to monitor total flow of water to the fields. If flow exceeds the amount for three fields, the flow monitor would alarm and/or shut-down the pump.
Calorimetric flow sensors or "flow-captors" are ideal for this application as they have no moving parts and are impervious to dirty water.

 

[MechanicalJoint]

Not sure what you mean by "in excess of the rated horsepower". Do you mean the pump will operate at a "bad" point of the pump curve or do you mean the motor that you are providing isn't adequate for that "bad" point on the curve?

Perhaps, if you provide more info about the system I could help you better.

 

[timb39]

Put in a pressure sustaining valve on each irrigation loop - that way if the operator adds a irrigation loop - the flow in each loop will cut back accordingly so that you will stil be in the desired portion of the curve.

Alternatively, you could consdeer variable speed pumps - If you do pay attention to the affinity laws

 

[civil21]

The simple and conservative approach is to correctly select the motor. If as you say the pump requires a 40 to 50 bhp at design flow, I suspect you have figured out that "out on the right of its curve" the pump requires a higher brake horsepower. See if a 60 or 75 horsepower motor will prevent "overload" out there.

Since this is for an irrigation system, the ability to put in complex controls is probably limited, but keep in mind if the operator is likely to open more valves than he or she should, he or she is just as likely to not open any, so make sure the discharge pipe material between the pipe and valves can handle the shutoff head (zero flow)of the pump and provide a high pressure and low pressure cutoff to the pump.