Effect of the centrifugal pump outlet line bypass on pipeline resistance
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Dear colleagues, a centrifugal pump has an outlet flow rate of 200 cubic meters per unit time. The outlet valve is fully open, as is the outlet control valve, and the outlet pressure is 0.75 megapascals. At this point, the flow rate and pressure should represent the performance curve of this pump under the resistance present in the piping system. When the bypass valve next to the control valve is fully opened, the flow rate increases to 350 cubic meters per unit time, while the outlet pressure drops to 0.6 megapascals. I would like to ask why such a significant change in flow rate occurs when the bypass valve is opened. Does opening the bypass indeed reduce the resistance in the outlet pipeline, and can a small bypass section have such a big impact on the overall characteristics of the pipeline?
The installation location of control valves usually involves a change in diameter, which results in throttling compared to the overall pipeline diameter; it is then understandable why an increase in bypass flow occurs.
In such a situation, bypassing results in a change in the resistance of the straight pipe. Generally, when calculating the resistance of the straight pipe, the loss due to that resistance is very small; so why does the performance of the outlet pipeline increase by so much?
Personally, I think it’s likely that the control valve on the pump’s outlet pipeline accounts for a large portion of the pressure drop in that pipeline. When the bypass valve is opened, the bypass line and the control valve are connected in parallel, resulting in equal pressure drops; therefore, the flow rate in the bypass line is comparable to, or even greater than, the flow rate in the main line. As a result, the flow rate increases by almost half.