Switching between power-frequency pumps and variable-frequency pumps

Regarding such issues, I recall that discussions were held on the forum before: (Here I will only outline the steps for switching from an inverter-driven pump to a conventional frequency-driven pump; you can infer the reverse process by using these steps.) 1. Initially, the operator is required to set the inverter to manual mode and set the output to 100%. 2. The pump switching shall be carried out normally at the outdoor operation site. 3. Next, the operator switches the frequency converter to automatic mode, sets the appropriate values, and gradually opens the relevant control valves to their full extent. 4. Finally, gradually open the outlet valve of the variable-frequency pump from outside until it is fully open. The key to this operation is to first treat the variable-frequency pump as a conventional frequency pump, and then activate the variable-frequency system to make it function.

When switching from a power-frequency pump to a frequency-modulated pump, it is necessary to ( ) before starting the frequency-modulated pump. A: Open the bypass valve of the air-controlled regulating valve fully. B: Open the drain valve. C: Close the outlet valve of the power-frequency pump. D: Reduce the flow rate at the outlet valve of the power-frequency pump

When switching from a power-frequency pump to a frequency-regulated pump, the bypass valve of the air-open control valve should be closed () before starting the pump. A: Fully open B: Fully closed C: 50% open D: Keep as is

I’m stuck – what are the key points related to these two questions? We have never used variable-frequency pumps. Why is it necessary to fully open the bypass line of the control valve when switching a variable-frequency pump? Will this cause the flow rate to increase instantly? ； Why should the outlet valve of the power frequency pump be closed? Will this cause the flow rate to decrease instantly? Please provide an explanation. Thank you

The steps mentioned above are completely correct. The key point is that when switching pumps, the frequency conversion should be set to operate at the normal frequency. One thing to note though is that even if the frequency conversion is set to 100, the outlet pressure of the pump still won’t be as high as that of a pump operating at the normal frequency; in practice, this may result in pressure buildup at the outlet of the frequency-conversion pump, preventing it from delivering the required flow rate. In my opinion, the key to handling these two options is to find ways to reduce the pressure in the downstream system or to prevent pressure buildup there, so as to ensure a smooth pump switchover. This is just my personal view; please correct me if I’m wrong.

There are certain differences between some practical practices* and theory