Error 2
If a pump is operated with the discharge valve closed (Shut-Off), the motor draws maximum current (the motor is under maximum load).
Answer
Some pumps draw more power at Shut-Off, some pumps draw less power at Shut-Off, and some pumps exhibit little or no change in power from Shut-Off to maximum flow. The only way to know for sure how power relates to the flow rate is to have the manufacturer's published power curve for that specific pump. In general however the following roughly applies.
The Specific Speed ranges below are approximations, differences in pump design and construction affect behavior, therefore a specific pump may behave slightly differently than what is shown below.
Radial & Francis Vane Ns < 3500 - Power increases with increased flow.
High Francis Vane & Mixed Flow Ns = 4500 - 7000 - Power varies slightly with flow rate.
Mixed Flow Ns = 7000 - 10000 - Power varies slightly or decreases as flow increases.
Axial Flow Pumps Ns > 10,000 - Power decreases as flow increases
Most irrigation pumps have Specific Speed (Ns) values of Ns < 4500. This lower specific speed makes the pumps flexible, easy to control, reliable, and "well behaved" in systems with variable flow rates and less than ideal conditions. Sometimes a higher Ns pump is installed on an irrigation system by a person or company that may not know what the consequences are for misapplication of these high specific speed pumps. High Ns pumps (Ns > 8000), are difficult pumps to control and require a pump expert experienced in using those pumps. There are many bad consequences resulting from improper use of high Ns pumps, among them are burnt and overheated motors, vibration problems causing shaft and coupling failures, and electrical component failures. Irrigation designers and contractors, who are not pump experts, would be well advised to either find a pump expert or stay away from such pumps, because these pumps kick back hard and could make someone miserable and poor.
Related Subjects on this Website
Glossary - Best Efficiency Point (BEP)
Glossary - Power Characteristic
How Fluids Move Through Impellers
