Let’s discuss the opening degree of the centrifugal pump outlet valve
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There is a centrifugal pump on site, and the outlet valve is closed completely from the moment it starts operating until it reaches 50% open. During this process, the outlet pressure gradually decreases, which is something everyone is aware of. However, when the valve goes from 50% open to fully open, the pressure remains unchanged, which indicates that the flow rate does not change during this period. Does this mean that the size of the outlet pipeline is designed to be too large? Second question: When testing all the centrifugal pumps on site, at the pressure level required, the opening degree of the outlet valve is only 20% in some cases, and even 10% in others. The process operators insist that the outlet valve should be fully open, claiming that this has always been the practice. However, with some pumps, the motor gets overloaded when the flow rate increases, and in other cases the outlet pressure does not meet the requirements, meaning the desired pressure level cannot be achieved. Does this also indicate that the size of the process outlet piping is designed to be too large?
The first issue is that there’s no problem with the pipeline size design; the flow rate specified for the pipeline must be greater than the pump’s maximum rated process flow rate； The second issue is that you need to take into account the back pressure of the pump. In other words, the pressure behind the pump’s outlet valve during testing is different from the process pressure under normal operation; it is generally much lower than the process pressure. Assuming the pressure exerted by the pump remains unchanged, then at the same valve position, the flow rate will be higher due to the lower back pressure. Therefore, in order to achieve the rated head, the outlet valve needs to be set at 10–20% of its full opening. As for what the operator said – that the outlet valve is usually kept fully open – that is also correct. The outlet valve of a centrifugal pump is generally a gate valve, which serves only to shut off flow; its position is either fully open or fully closed, as otherwise it could damage the valve’s sealing surface. During normal operation, the pump’s outlet valve is kept fully open, and there is also a flow control valve on the outlet pipeline
The pump’s outlet valve is used for shutting off, not for regulation. There are generally only two states: either fully off or fully on. First question: Gate valves are generally used; once they are opened to 50%, the proportion of local resistance in the total pipeline resistance becomes very small, so opening them further has little effect. Second question: The control performance of the gate valve is extremely poor; it is unable to regulate flow rate. Also, control valves are generally used to adjust flow rate; I’ve never seen valves used to adjust head pressure.