Check valves are automatic valves that rely on the differential pressure of the flow to open and close. Check valves are unique because they can permit the flow only in one direction. They do not work like ball valves or gate valves that have bi-directional flow. Besides, check valve manufacturers indicate the flow direction with an arrow on the valve's body. This unique characteristic enables the check valve to prevent the reverse flow or backflow of media, thus playing a great role in the safety of the maze of pipelines.
Technically, there are two ways to install check valves:
The valve installed in front of the water pump and at the end of the vertical suction pipe is called a bottom valve. This kind of installation works ideally when the pump is installed higher than the level of water or any liquid. It is also called the negative pressure method. The main objective of installing the valve in front of the pump is to fill the pump without allowing it to run idle without any water.
The second method of installing the check valve behind the pump is ideal when the liquid level is higher than the pump. This facilitates the easy closing and opening of the water pump without closing the pump outlet valve.
The working mechanism in check valves is similar in all the models. As far as check valves are concerned, the rule of thumb is to allow the flow only in one direction and to prevent any reverse flow. To operate the right way, check valves rely on minimum upstream pressure to allow them to open. This pressure is called the cracking pressure. The cracking pressure differs with the size and valve design.
Here's a breakdown of what happens when the valve is in operation:
The liquid pressure forces the control mechanism or disc to open and facilitate the flow from the inlet to the outlet.
If the inlet pressure is lower than that of the outlet pressure, the disc will automatically close the channel and prevent reverse flow.
In swing check valves, it is in the shape of a disc, held by a hinge on the upper end. A swing check valve is one of the most common types of valves used everywhere, from simple domestic applications to complex industrial purposes. It works well for large diametre pipelines with medium or low-pressure force. Swing check valves are not suitable for applications with high viscosity fluids and solid particles, but they can work perfectly in both horizontal and vertically inclined pipelines.
Ball check valves
The control mechanism in ball check valves is a free-floating ball or a spring-loaded ball that rests on a sealing seat close to the inlet. When the fluid pressure is more than the cracking pressure, the ball floats from its seat and allows the flow.
Lift check valves
True to its name, the lift check valve works with a guided disc that literally lifts off the valve seat to allow media flow. Gravity plays an important role in the lift check valve's working mechanism. To close, the cracking pressure has to overcome gravity so that the disc can reseat with correct alignment. Lift valves also have spring to help them close. But some valve models without springs rely only on gravity to swing shut.
Water hammer is one important issue to keep in mind concerning check valve working mechanism. If the valve closes too fast, it may lead to a water hammer which can be very noisy and even damage the valve. To prevent the occurrence of a water hammer, it is important to buy valves with a buffer function to extend the closing time.
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