Problems caused by sudden power failure of multistage centrifugal pumps
After the power supply is cut off suddenly, the centrifugal pump and the prime mover will rotate freely, which will cause serious damage to the system.
Except in rare cases with flywheels, pumps and prime movers usually have very small moments of inertia. Therefore, the pump will stop quickly and gently. At this time, unless the pipeline is very short, the inertia of the liquid rest will still force the liquid rest to flow forward, while the decelerating pump acts like a throttle. The pressure in the discharge line drops rapidly. In some cases, the pressure at the pump outlet or at a certain height along the line may be lower than atmospheric pressure.
1. Transition process of power failure accident of centrifugal pump
The lowest pressure head that occurs during this phase of movement is called a descending wave. At this time, since the liquid column is completely separated, the pressure can be made low enough to cause vaporization. Therefore, during the separation period, the destruction of the pipeline due to the external atmospheric pressure has occurred many times. When the liquid column is recombined after separation, the impact pressure may be sufficient to rupture the pipeline or pump. Closing the valve on the discharge line can only make the situation worse. Therefore, the valve should be operated according to the procedure. Before the reverse flow starts, it should be closed very little even if it is closed.
2. Control method of reverse flow
The reverse flow can be controlled by a valve, or by using air at or close to the outlet to empty the discharge line. If there is no resistance to the reverse flow, it will stop the pump and then accelerate the rotation in the reverse direction. After being released, the pump will rotate like a turbine at the flyaway speed corresponding to the effective static head that is reduced due to system friction loss. However, when reverse flow has been formed, the reverse speed may exceed the steady-state flying speed and reach a considerable value. The maximum reverse speed increases as the pump efficiency and specific speed increase.
Calculations show that the maximum reverse speed can exceed 150% of the rated speed. This should be considered when choosing the prime mover, especially when choosing a large motor. When there is reverse flow, a pressure rise called a rising wave is generated in the discharge line. The maximum rising wave usually occurs shortly before reaching the maximum reverse speed. The pressure rise caused by it can exceed 60% or more of the rated outlet pressure of the centrifugal pump.