Problems caused by sudden power failure of multistage centri
Source:未知|Popularity:0|release date:2020-12-07 16:54:10
After the power is suddenly cut off, the centrifugal pump and 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 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 force the liquid to flow forward, while the decelerating pump acts like a throttle valve. The pressure in the discharge pipeline drops rapidly. In some cases, the pressure at the outlet of the pump or at a certain height along the pipeline may be lower than atmospheric pressure.
1. The transition process of the centrifugal pump power failure
The lowest pressure head that occurs during this movement phase is called the drop wave. At this time, since the liquid column is completely separated, the pressure can be low enough to cause vaporization. Therefore, during the separation period, the pipeline will be damaged due to external atmospheric pressure, which has happened many times. When the liquid column recombines after separation, its shock pressure may be enough 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 in accordance with the procedure. Before the reverse flow begins, it should be closed rarely even if it is closed.
2. Backflow control methods
Reverse flow can be controlled by a valve, or it can be controlled by putting air at or near 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 release, the pump will rotate like a hydraulic turbine at the runaway speed corresponding to the effective static head reduced due to system friction. However, when the reverse flow has been formed, the reverse rotation speed may exceed the runaway rotation speed in the steady state and reach a considerable value. The maximum reverse speed increases with the increase of pump efficiency and specific speed.
Calculations show that the maximum reverse speed can exceed 150% of the rated speed. This point should be considered when choosing a prime mover, especially when choosing a large electric motor. When there is a reverse flow, a pressure rise called a rise wave is generated in the discharge line. The maximum rising wave usually occurs shortly before the maximum reverse speed is reached, and the pressure increase it causes can exceed 60% or more of the rated outlet pressure of the centrifugal pump.