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In pipelines operating under pressure, sudden changes in water velocity caused by external factors such as the abrupt closure of a safety valve or sudden stoppage of a water pump can lead to hydraulic phenomena known as water hammer.
This phenomenon occurs when water flows create hydraulic shock waves that collide back and forth like a hammer, potentially causing damage to valves and water pumps. When an electric water pump starts at full voltage, it can accelerate from stationary to rated speed in less than a second, and the flow in the pipeline can increase from zero to a rated flow. Because fluids have momentum and a certain degree of compressibility, the rapid change in flow can lead to pressure surges that cause high or low-pressure shocks within the pipeline, as well as cavitation. The pressure shocks can cause the pipe walls to vibrate and produce noise, similar to the hammering of pipes, known as the "water hammer effect." This effect is only related to the inertia of water and not related to water pumps.
Explanation of valve water hammer phenomenon
Water hammer occurs when sudden power outages or rapid valve closures produce inertia in pressure flow, generating water flow shock waves that resemble hammer blows, hence the name water hammer. The forces produced by the back-and-forth flow shock waves can be significant, potentially leading to the destruction of valves and water pumps.
Extend the opening and closing time of the valve.
Centrifugal pumps and coagulation pumps should be stopped when the valve is half-closed, 15%-30%, rather than fully closed.
In addition, air should be removed from the pipeline to fill it with water before starting the water pump. An automatic exhaust safety valve should be installed at high points in long-distance water supply pipelines. Water hammer caused by pump shutdown is mainly due to the quick closing of the backflow prevention valve in the water discharge pipe, and removing the backflow prevention valve can eliminate the danger of water hammer caused by pump shutdown, reduce head loss, and save electricity. According to experiments conducted in some large cities, the primary pumping station can be cancelled, but the secondary pumping station should not be cancelled. When removing the backflow prevention valve, water hammer pressure calculation should be carried out, and a micro-resistant slow-closing check valve should be installed on large-diameter pipes to reduce and eliminate water hammer. A buffering check valve and a micro-closed butterfly valve can be installed on the water pump discharge pipe to effectively eliminate water hammer, but because there is a certain amount of water reflux when the valve is in operation, the suction well must have an overflow pipe. A water hammer eliminator should be installed immediately after the check valve is installed.
The water hammer effect can be highly destructive: excessive pressure can cause the pipe to rupture, while low pressure can cause the pipe to collapse, and damage valves and fittings. When the power is cut off and the machine is stopped, the potential energy of the pump water system will overcome the inertia of the electric motor and cause the system to abruptly stop, leading to pressure shock and water hammer effects. To eliminate the serious consequences of water hammer effects, a series of buffering measures and equipment should be introduced into the pipeline.
A water hammer eliminator can effectively eliminate irregular hydraulic shock waves that may occur in transmission systems when fluid flows back and forth, wave surge, and other disturbances, without blocking fluid flow. The internal chamber of the water hammer eliminator is hermetically sealed, with a piston at its lower end. When a shock wave enters the water hammer eliminator, the water wave acts on the piston, causing it to move toward the gas chamber. The piston's motion is related to the gas pressure in the chamber and the size of the water shock wave. Under the dual effects of a certain pressure of gas and an irregular water shock wave, the piston moves up and down to achieve a dynamic equilibrium, effectively eliminating irregular water hammer shock waves.
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