Regarding pump switching in multi-stage pumps
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Gentlemen, I would like to ask whether it is necessary to open the outlet return line in multi-stage centrifugal pumps equipped with such a line, during normal daily switching operations For pumps in operation, I think that when all the outlet valves are closed, if the pump is not stopped immediately, the return line should be opened slightly to ensure a minimum flow rate (it’s okay to open it more). I believe that if the pump is stopped right after the outlets are closed (without first considering whether the standby pump is working properly), there is no need at all to open this return line. As for the standby pump, is it necessary to open the valve on the return line to a certain degree before starting it? My understanding is that it shouldn’t be turned on. Given the characteristics of centrifugal pumps, the outlet valve should be opened only when outlet pressure builds up, in order to prevent vacuum formation. Additionally, the current is at its highest when the motor starts; opening the pump causes a sudden increase in current, so it’s usual to wait until the current levels down before opening the outlet valve. If a return line is used, there seems to be a risk of overcurrent. I wonder what everyone thinks?
If there is a minimum flow requirement, in my opinion it needs to be turned on!
The biggest fear when cutting a pump is the fluid flowing back, which can cause the impeller to loosen
Pumps shouldn’t be able to operate at a low flow rate for extended periods, right? It’s just a normal pump switching process – the outlet valve closes and the pump stops immediately. Is it really necessary to maintain this minimum flow rate? Can’t multi-stage pumps withstand some pressure buildup at all?
There is a flow-limiting orifice plate on the return line at the pump outlet, and a check valve is present at the outlet. When starting the pump, the outlet return valve is opened; once the pump is operating properly, the outlet return valve is closed. After the outlet check valve opens, the return line is quickly closed, and once the flow rate is normal, the pump continues to operate as usual
The outlet pressure of multi-stage pumps is very high; the outlet pressure of our 10-stage pump is 16 MPa. If the flow isn’t allowed to return, won’t it cause damage?
It’s best to ensure it is open; in our previous company, the multi-stage pumps required to be opened! The outlet pressure is 2.4 MPa. In our process, the pump outlet is divided into two pipelines; each pump has its own separate return line. As in a normal process, there is a gate valve for both the return line and the main flow path, with the valve in the main flow path serving as the outlet valve! Before starting, carry out all necessary preparations: open the inlet valve and the return valve, but do not open the outlet valve. Start the motor; once the pressure is normal, slowly open the outlet valve while gradually closing the return valve until it is fully closed. Throughout this process, the flow rate is adjusted manually using the control valve. The procedure for shutting down the pump is the same: start the standby pump and integrate it into the system, then gradually shut down the pump that was previously in use. Slowly close the outlet valve, open the return valve until the outlet is completely closed, fully open the return valve, and then stop the pump!
Stop the pump with the check valve fully open?
It doesn’t run for long; the pump stops as soon as the outlet valve is closed, or it starts running when the outlet valve is opened. If it were to run for a longer period of time, then the valve would definitely need to remain open
It depends on the pump’s configuration and the minimum flow rate required; the pump can be started at a lower speed. However, we have now switched to inverter control, and the return pipeline has been removed.
Yes, the backflow is the backflow of the pump, which prevents pressure buildup! When shutting down the pump, gradually close the outlet valve while gradually opening the return valve (ensuring constant outlet pressure throughout the process), until the outlet valve is fully closed and the return valve is fully open, at which point the pump is shut down!