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The meaning of bypass is to bypass or bridge. In centrifugal pump technology, it refers to the pipeline that plays a critical role in closed-loop control or serves as a balancing device. Under closed-loop control, the operating flow rate of a centrifugal pump may be higher than the available flow rate in its pipeline. Because some of the bypass flow is diverted out.

The bypass flow can be directly returned from the pump outlet to the pump inlet through a narrow loop, or it can be recombined with the suction side flow through different devices (such as coolers, deaerators, etc.) (after a delay) before returning to the pump inlet.

There is usually only one type of bypass in centrifugal pumps, namely the minimum recirculation line (also known as the minimum flow bypass), which is used to ensure that the pump always operates above its minimum continuous stable flow rate to avoid cavitation phenomenon. There is also a bypass in centrifugal pumps, which serves as a balancing device and is mainly used for centrifugal pumps with high outlet pressure, such as non self balancing BB4 and BB5 boiler feed pumps. As a balancing device, it is used to compensate for the axial thrust of the pump; Another example is the two-stage BB2 pump, which serves as a balancing device to reduce the pressure in the sealing chamber on the outlet side.

Reason for setting bypass

Centrifugal pumps rely on fluids to maintain their cooling and lubrication during operation, and they also rely on the fact that the discharged fluid has a place to go. But what if the process requires only a small portion of the minimum flow required by the pump? What if the downstream valve is closed? What happens when the basic hydraulic requirements of the pump are no longer met?

This is the best time to apply the minimum flow bypass. When there is a problem of meeting the minimum flow requirement and/or to prevent the pump from operating at the dead center for a long time, the most commonly used method is bypass. Therefore, the bypass is designed for:

1) Operational behavior. Prevent further operation of the pump within the low flow range.

2) Closed loop control. Suitable for pumps with downward sloping pump input power curves at high flow rates (such as spiral slurry pumps, vortex pumps).

3) Thermodynamics. Prevent the pumped fluid from heating up in the low flow range. The bypass flow is branched through an automatic recirculation valve installed on the outlet pipe of the pump, usually high-pressure and overpressure pumps (such as boiler feedwater pumps).