What is the relationship between the selection of pump motor power, the process, and the head?
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According to the information provided, the power P of the water pump motor is calculated as P = solution density * flow rate * head * gravitational acceleration / (pump efficiency * motor efficiency). Are the flow rate and head in this formula referring to those under rated operating conditions? I purchased a centrifugal pump with a flow rate of 300 m3/h and a head of 60 m. The manufacturer specified that the shaft power required for this pump is 64.3 KW, with an efficiency of 76.2%. I chose a motor with an output power of 75 KW. Selection is made in accordance with a motor margin of 1.15; it seems appropriate to use a 75KW motor for a pump with an axis power of 64.3KW. However, during the equipment acceptance testing, it was found that the pump’s power actually reached 85KW, resulting in an over-power issue with the water pump. The manufacturer’s explanation is that the motor power is determined based on the rated flow rate and head specified in the project, rather than based on the maximum flow rate; therefore, there is no issue with the design selection of the pump. According to the pump performance curve provided by the manufacturer, the flow-rate head curve is a line that slopes downward from the upper left corner to the lower right corner, with a relatively gentle slope. As a result, in the latter part of the curve, the product of flow rate and head is much greater than the rated value of flow rate times head; in such cases, theory dictates that a motor with higher power should be used. I just want to ask whether the motor power of a water pump is indeed determined based on the rated flow rate and head; does such an overpower condition indicate that the motor power was selected improperly? Are there any relevant standards that specify how to select the power of a motor? If so, what are the standard numbers? I hope the experts here can give me some advice!
As long as the power consumption does not exceed 75 kW at 300 m3/h x 60 m, the manufacturer is correct. If you use it at full capacity, you need to make this clear to the manufacturer, who will then select a larger motor for you. I haven’t heard of any standards for selecting power levels; it’s all determined based on actual needs.
If the parameters you provided to the manufacturer included the rated flow rate and the maximum flow rate, you can negotiate with them; if only the rated flow rate was given, then the manufacturer is in the right.
This post was last edited by Ah on 2016-1-8 at 11:15. Pump motors are primarily selected based on the shaft power of the pump at its rated flow rate and head. Based on what the poster said, I understand that the flow rate during the acceptance test is much higher than the pump’s rated flow rate. Why did the poster use such a high flow rate during the acceptance test? If the process truly requires it, sometimes motors are selected based on the principle of covering the end of the power curve; however, this must be specified, otherwise the manufacturer’s selection will not be considered incorrect.
What was said upstairs is correct: during the acceptance tests, the flow rate is much higher than the pump’s rated flow rate. This flow rate and head can be used in the formula you mentioned, and it’s clear that the motor power will be exceeded. The motor selected by the manufacturer based on the power required under rated operating conditions has sufficient headroom, so there is no problem. It’s unnecessary to choose a motor that is too large; it’s also a waste.