A check valve
is a valve that allows flow in only one direction: from upstream
(where these labels do not get reversed if the flow reverses direction). There are a number of different physical arrangements to make this happen:
- Lift Check: In this kind of valve, the upstream pressure lifts up a piston until the fluid passes through to the other side of the valve. The disadvantage of this is that a high upstream pressure is required to get any flow. However, by the addition of a spring behind piston, you get essentially a pressure relief valve.
- Swing Check: In this kind of valve, the upstream pressure pushes a disc which swings open to allow flow through the valve. A variation on this is a wafer check which is mounted between flanges. Another variation allows counterweights to be added to the shaft that the disc turns on to adjust exactly how much pressure is required to start the flow through the valve.
- Ball Check: This configuration is used for the smaller pipe sizes and is similar to a lift check except that the upstream pressure pushes against a ball and the fluid flows around the ball.
The valve orientation
is important. Situations where you have a vertical pipe
with a downwards flow do not suit the use of check valves generally. You have to change the piping to have a horizontal section or then put the check valve into an existing horizontal section.
The ISA symbol for a check valve is:
Direction of flow --->
Stops flow <---