Why must one or more non-return valves be used in installations of submersible electric pumps?

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Why must one or more non-return valves be used in installations of submersible electric pumps?

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Non-return valves are used to maintain the pressure in the system when the pump stops. They also prevent possible problems of water hammering, counter-thrust and counter-rotation: Water hammer : If the valve nearest the pump is placed at more than 10 metres from the static level of the water, or the lower valve leaks whereas the upper one holds, a pressure drop could be created inside the piping. At the next pump start, the water displacement at high speed fills the vacuum and beats against the upper valve and the water in the delivery pipe causing a water shock. Counter-thrust : Without a non-return valve, or with a faulty non-return valve, in the first electric pump start phase operation could occur without water column conditions in the delivery pipe). This causes an axial thrust on the impellers and pump shaft towards the delivery. This axial thrust is transmitted, through the electric motor/hydraulic part coupling, directly to the motor creating a counter-thrust operating conditio