Reason for the decrease in outlet pressure after replacing the fixed-frequency pump with a variable-frequency pump
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After replacing the centrifugal pump’s fixed-frequency version with a variable-frequency pump, the pressure dropped from 9.5 to 7.5. I would like to ask the expert what caused this phenomenon There is another question: does the pressure behind the outlet valve of a centrifugal pump refer to the system pressure downstream of the valve? If it’s not that pressure, then how is pressure determined? I hope the teacher can provide an answer; thank you very much!
Personally, I think when a pump is equipped with variable frequency technology, its speed decreases. I believe that a lower speed results in less centrifugal force, and therefore lower pressure. The pressure generated by a pump is proportional to the centrifugal force; this is just my opinion for reference only, hehe
The pump is no longer rotating, and the pressure behind its outlet valve is the pressure of the system downstream. The pressure behind the pump outlet valve is equal to the pressure drop across a device located after the valve; for example, if the outlet is open to the atmosphere, then it is entirely due to the pressure drop. Downscaling reduces the pressure before the pump outlet valve, while the pressure behind the valve is determined by the system pressure and flow rate (the higher the flow rate, the greater the pressure drop). It’s not necessarily the pump outlet valve that controls the throttling at the pump outlet; there is also a control valve elsewhere, right?
If a new pump is installed, and the pressure drops at the maximum frequency of the variable-frequency pump, it is likely that the head capacity of that variable-frequency pump is lower. If the pump has not been replaced, or if the parameters of the variable-frequency pump remain the same, it should be checked whether it is operating at 50Hz.
During variable-frequency operation, the outlet valve is usually opened fully, which reduces the outlet resistance and results in a lower pressure. At this time, the pump’s outlet pressure should be relatively close to the system pressure; the key factor here is the resistance. During normal frequency operation, the outlet valve may not be fully open, leading to higher pipeline resistance, which is likely the main reason. Another possibility is that the frequency has not been set to 50HZ
The outlet pressure of the pump is increased by the centrifugal force generated by the rotation speed of the centrifugal pump; when the rotation speed is reduced due to variable frequency, both the flow rate and the pressure decrease. If the pressure in the downstream system is not constant, it will change in accordance with the outlet pressure of the pump. This situation needs to be taken into account.
Changing to variable frequency is done to save energy; by using variable frequency, the pump’s frequency is reduced, which in turn lowers its power consumption. With variable frequency control, whether flow rate or pressure is being regulated, the operation takes place at the set parameters rather than at the rated values; as a result, the outlet pressure of the pump decreases. If this pressure does not decrease, it indicates that the variable frequency conversion was not successful, there is no potential for energy savings, and the investment was a failure...
Firstly, there are two operating conditions: fixed-frequency and variable-frequency. In China, the fixed-frequency value is 50HZ; for variable-frequency operation, check whether it is also 50HZ – if it is, then it should be the same.
Was a new pump selected, or was only the original pump modified with variable frequency technology? Are all the export valves fully open this time? If only the pressure is reduced and the flow rate can be controlled, and your downstream system can accept it, then the modification can be considered successful~~