After the power supply is suddenly cut off, the centrifugal pump and motor will rotate freely, which will cause serious damage to the system.
Except in rare cases with flywheels, pumps and prime movers usually have only a small moment of inertia. Therefore, the pump will stop quickly and gently. At this time, unless the pipeline is very short, the inertia of the liquid will still make the liquid flow forward vigorously, while the decelerating pump acts like a throttle valve. The pressure in the discharge pipeline drops rapidly, and in some cases, the pressure at the pump outlet or at a certain height along the pipeline may be lower than the atmospheric pressure.
The transition process of centrifugal pump power failure accident is roughly as follows:
The lowest pressure head occurred in this movement stage is called dounsnrge. 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 pipeline will be damaged due to the external atmospheric pressure, which has happened many times. When the liquid column recombines after separation, its impact pressure may be enough to break 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, and even if it is closed, it should be closed very little before the backflow starts.
The reverse flow can be controlled by a valve, or by putting air at or near the outlet to empty the discharge pipe. If there is no resistance to reverse flow, the pump will stop and then accelerate in reverse. After discharge, the pump will rotate like a water turbine at a runaway speed corresponding to the effective static head reduced due to friction loss of the system. However, when reverse flow has been formed, the reverse speed may exceed the runaway speed in steady state and reach a considerable value. The maximum reverse rotation speed increases with the increase of pump efficiency and specific rotation speed.
The calculation shows that the maximum reverse speed can exceed 150% of the rated speed. This should be considered when selecting prime movers, especially when selecting large motors. When there is reverse flow, the pressure in the discharge pipeline rises as dounsnrge. The maximum rising wave usually occurs shortly before reaching the maximum reverse speed, and the pressure rise caused by it can exceed 60% or higher of the rated outlet pressure of centrifugal pump.
