Check valves are built in a piping system to avoid the reversal of flow. In the pipeline, the flowing fluid activates these valves. The pressure of the fluid flowing through the device opens the valve, while the valve is closed by any reversal in flow. The weight of the check mechanism, backpressure, the spring, or a combination of these factors are responsible for the closure of the valve. Swing, tilting-disk, dual plate, non-slam and stop check are the general forms of check valves.
The classification of the check valve can be decided by the types of discs used; these are:
• Swing check valve
1. Top hinged disc.
2. Tilting disc.
• Lift check valve
1. Piston type
• Dual Plate check valve
• Non-Slam check valve
• Stop check valve
Swing Check Valve is the most common of all check valves, offering much less flow resistance when fully open. The closing disc swings open and shuts around a hinge outside the seat like a trap door. The disc seat is located at some angle to allow smoother opening at lower pressures, stronger sealing and less impact when closing at higher pressures. The loose pin hinge ensures that the disc closes when the flow decreases to zero.
As the size of the swing check valve increases, the weight and movement of the disc will eventually become unnecessary for the satisfactory operation of the valve. For this reason, swing check valves bigger than NPS 24 are mostly equipped as multi-disk or Dual plate check valve and have several swing discs placed on a multi-seater diaphragm around the flow through the valve. Dual plate Check Valve reduces the path length along which the centre of gravity of the disc travels and also reduces the weight of the disc by 50% compared to a swing check valve of the same size.
The swing check closes suddenly due to gravity which causes a rise in pressure, resulting in shock waves. These high-pressure waves impose extreme stress on the pipe system. This problem can be reduced by adding a Non slam check valve. This valve does not rely on gravity. As the upstream velocity of the fluid slows down, the spring help on the valve begins closing the disc.
Application of check valves.
In the case of pumps and compressors, check vales are used where back flow may harm the inside of the equipment and cause an unwanted system shutdown.
The most popular type of water pumps, i.e. centrifugal pumps, are not self-priming, so control valves are necessary for holding water in the pipes.
In lines feeding a secondary system where the pressure may rise above that of the primary system, check valves can also be used. Reverse flow can be caused by this pressure difference.
Check valves are mostly used when several gases or fluids are mixed into one stream. To avoid mixing of the fluid at the original source resulting contamination, a check valve is mounted on all of the different fluid streams.