Operation of the centrifugal pump return line
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Hello, experts. I would like to have everyone compare the advantages and disadvantages of the two backflow operation methods for centrifugal pumps. I appreciate your guidance: The first method is to start the centrifugal pump, then adjust the backflow to 30% of the pump’s rated flow rate, set the control valve to automatic mode, and then feed material into the system until the rated flow rate is reached. The second method is to start the centrifugal pump, adjust the backflow to the pump’s rated flow rate, set the control valve to automatic mode, and then feed material into the system until the rated flow rate is achieved
Recirculation is only necessary when the discharge flow rate outside the pump is less than 30%; otherwise, it can be omitted.
There is a type of valve called a self-controlled backflow valve, which can control the backflow volume on its own, adjusting it based on the flow rate.
Before starting up, the return valve must be opened. It is primarily used to remove non-condensable gases from the pipes and the pump itself. Additionally, it allows for adjusting the outlet pressure of the pump; once the pressure is stable and higher than the system pressure, the outlet valve can be opened, and then the flow rate required by the process equipment can be adjusted as needed.
Bypass line, installed to protect the equipment
For centrifugal pumps, neither of these two operations is appropriate. Firstly, the outlet valve of the centrifugal pump should be closed at the time of startup. Once the pressure at the pump’s outlet reaches the pump’s rated pressure, the outlet valve can be slowly opened until it is fully open, thereby achieving the flow rate required for production. Of course, this flow rate is regulated by the control valve on the outlet pipeline, rather than by the pump’s outlet valve. If cavitation of the medium inside the pump occurs when the outlet valve is fully open and the flow rate is adjusted using a control valve, it indicates that the pump’s flow rate design is too high. Under normal conditions, a centrifugal pump does not require a return line, as it does not experience overpressure issues. Setting up a backflow in a centrifugal pump is essentially a waste of energy.
Pumps with reflux, as you mentioned, are used in hydrogenation and hydrocracking processes, and they are generally high-pressure pumps. It is standard practice to enable a certain amount of reflux first; otherwise, the pump may get overloaded. So following that approach will not work