Due to the sharp change of the flow velocity in the pressure pipeline, the hydraulic shock phenomenon causing the water flow pressure in the pipeline to rise or fall sharply is called a water hammer phenomenon. The water hammer of the low-speed centrifugal pump station has a starting water hammer, a valve closing water hammer and a pump-stop water hammer. As long as the water pump is started according to the normal operating procedure, it will not cause the starting water hammer to cause damage. In the case of an empty pipe, the change of the water flow pressure will be aggravated when the air in the pipe cannot be discharged in time to be compressed. The shut-off water hammer does not cause excessive water hammer pressure during normal operation. The water hammer of the pumping station caused by the sudden power failure or accidental operation of the pump is often large, generally up to 1.5 times or more of the normal pressure, which is highly destructive and often causes accidents. Therefore, the water hammer of the pumping station must be carefully analyzed and calculated to take the necessary protective measures.
A. Pumping water hammer analysis
There is no stopping valve on the pipeline to stop the pump water hammer, and there is no stopping valve in the pipeline to stop the pump water hammer process in three stages. Pump working conditions. When the power is suddenly interrupted, the unit still maintains a positive rotation due to the inertia of the unit and the water flow, and the water flow still flows in the normal flow direction. However, the forward rotation speed is gradually reduced, the flow rate and flow rate are gradually reduced, and the pressure is lowered. Under normal circumstances, when the unit inertia is large, when the water flow stops the flow due to the friction and gravity of the pipe wall, the unit still decelerates and rotates forward. At this stage, from the sudden interruption of power to the complete stop of the flow in the pipe (ie Q = 0).
Brake condition. The transient still water starts to flow backward under the action of gravity and static head. The reverse flow acts as a brake on the rotating rotor, which causes the unit speed to continue to decrease. When the energy of the forward rotor is exhausted, the pump stops rotating forward. Since the backflow water is subjected to the resistance of the forward-rotating impeller, the water pressure in the pump and the pipeline begins to rise. This phase starts from the backflow of the water (ie, Q=0) until the pump stops rotating (ie, the speed n=0). Turbine operating conditions. Under the action of the reverse flow, the pump begins to reverse and gradually accelerates, and the water pressure in the pump also rises. The reverse flow rate quickly reaches a maximum value, and the reverse rotation speed also rises. As the speed of the impeller rises, it is bound to bring water to rotate, preventing the water from flowing down. Instead, the backflow is reduced, the pressure in the pipeline is increased to a maximum, and the corresponding speed is also maximized. Subsequently, as the reverse flow continues to decrease, the reverse speed is slightly reduced and the unit is finally operated at a steady speed and flow. Since the unit at this time is subjected to reverse flow and idling without any load, this stable speed is called the flying speed. When there is no check valve, the pump outlet is in the process line of pressure, flow and speed change after the pump is suddenly stopped. The boosting water hammer value at the pump outlet can reach 10%?50% of the normal pressure, and the pressure drop water hammer value can reach normal. About 90% of the pressure. When the check valve is equipped with a check valve, when the check valve is installed at the pump outlet, the first stage of the water hammer process is the same as the case without the check valve. That is, the pipeline centrifugal pump is rotating positively, the pressure is reduced, and the maximum pressure reduction value is about 90% of the normal pressure. In the second phase, the check valve closes, causing a sudden increase in pressure, and the maximum boost value is about 90% of the normal pressure. The rotor of the unit has a slow downward rotation due to the effect of no backflow. After the pressure reaches the maximum value, it drops rapidly, then rises and falls again, with the static head 100% as the baseline, alternating up and down. Due to the frictional resistance of the pipe, the peak value of the water hammer wave gradually decreases, and finally stabilizes on the static head line. The distribution line diagram of the water hammer pressure along the pipeline has the maximum and minimum pressure distribution lines along the pipeline when the check valve and the check valve are not. The pressure drop near the pump outlet is greater, and the pressure drop is smaller near the outlet. If the negative pressure formed at a certain point in the pipeline (such as point C) is lower than the saturated steam pressure at the working temperature, the water will vaporize and cause water column separation. When the water column is closed, a huge partial pressure is generated, causing the pressurized water pipeline to be damaged. To destruction.
B. The danger of stopping the pump water hammer
When the pump station stops the pump water hammer accident, it will cause "running water", stop the water, and seriously cause the pump house to be flooded; some also cause the factory to be forced to stop production, destroy the railway, and some equipment is hammered by water. Pressure damage, and even cause casualties. For example, a pump station of a power plant transports water from a reservoir to a power plant. The pipeline has a total length of 13,200 m. The pipeline is laid along the topography and is gradually rising. Prestressed reinforced concrete pipes with working pressures of 1.OMPa and 0.8MPa were used. After the pipeline was put into operation, water hammer bursting accidents occurred several times. The power plant was forced to stop generating electricity, causing great economic losses. Pumping water hammer accidents easily occur under the following conditions. Single pipe supplies water to the concrete. When the height difference of the water supply terrain exceeds 20m, it is necessary to pay attention to the danger of stopping the pump water hammer. The total head (or working pressure) of the pump is large; the flow velocity in the water pipeline is too large; the water pipeline It is very long and the pipelines are fluctuating; the valves in the automated pumping station are closed too fast.
