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General problem
In case of a pump shut down or after a power failure, the pump will stop immediately, but the water column remains the same due to its physical mass (inertia). This creates a vacuum behind the pump discharge outlet. After a while, the water column stops and starts the reverse flow, highly accelerated by the considerable forces of the vacuum. This results in a fast back-flow which is suddenly stopped by the free swinging non-return valve and creates violent and dangerous water hammer which can destroy the pump and the valves, and also cause dangerous pipe bursts.
To prevent this problem, the consulting engineers have developed different possibilities of preventing this serious problem by planning and designing relatively expensive anti-water-hammer facilities such as surge vessels, equilibrating tanks, water relief valves, suction lines etc.. All these solutions make sense, but they are expensive.
Conventional methods
There are a couple of conventional anti-water-hammer solutions, e.g. surge vessels, equilibrating tanks, water relief valves and suction lines around the pump.
Surge vessels
In the pump discharge line, behind the free swinging non-return valve, a surge vessel is connected to the main pipeline. Surge vessels are partly filled with water and air, where the air is constantly maintained under a defined pressure. In case of a vacuum occurring in the main pipeline, the water from the tank is pressed into the main pipe, thus balancing the vacuum and closing the non-return valve.
Suction line
With this method a bypass around the pump is connected with the suction side of the pump. A nonreturn valve prevents the backflow during normal operation. In case of a power failure, the vacuum draws off the bypass water from the suction side of the pump to equalise the dangerous negative pressure.
There are some more methods practiced all over the world to prevent water hammer effects, but all of them have major disadvantages: They are very expensive, require considerable planning and time expenditure and need frequent maintenance.
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