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Valves are necessary in water supply system, in order to regulate the flow of water and to minimize the wastage of water. Valves either start/stop the flow of water or control (throttling) the speed and capacity of water or direct the flow in one direction only.
Also read: What are Different Types of Valves?
Reflux valve is commonly known as Check valve and Non Return Valve (NRV). Water tends to flow from higher pressure to lower pressure. Mains pressure is kept higher to suffice water distribution in a house. Sometimes mains pressure is reduced due to need of water in fire fighting. When this happen water from house distribution tends to flow towards the mains. To stop this return flow towards the mains check valve is installed.
Check valves are installed at the connection of home sewer line to the city sewer line. When city sewer lines are filled to capacity, water from city sewer line tends to flow towards home sewer lines. Check valves functions here to stop this backflow of city sewer to home sewer lines.
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Check valves directs/regulates the flow strictly in one direction only i.e. never in opposite direction even during pressure drop. The valve stops/check the flow in reverse direction.
It is operated by pressure alone, having no external means of control. The flow of water is controlled by means of a disc rising and falling onto a seat with the pressure (lift check). The return of the disc may sometimes be aided by a spring (spring-loaded check). The flow of water may also be controlled by means of a flap swinging up and down with pressure onto a seat (swing check). In order to avoid the pressure loss in the line, generally larger size of valve is selected.
Also Read: About Sluice Valve or Gate Valve
Mostly check valve is provided on downstream side of pumps. So that check valves would not prevent water hammer in the pipeline. They just prevent return flow through the pump and avoid water hammer pressure reaching the pumps. Normally a check valve installed will not reduce water hammer pressure, although it may limit the lateral extent of the shock.