P&ID ISO Valves - Check Valve Symbol

What is a Check Valve?

A check valve (also known as a retention, non-return or one-way valve) is a device designed to prevent backflow in a piping system. It is essentially a one-way gate that utilises the changes in water pressure within the pipes to open/close. As the fluid flows through the intended direction, the valve remains open and allows it to pass through. If the fluid flow direction reverses, the change in pressure will cause the valve to close, creating a seal to restrict backflow.

Purposes of a Check Valve

As mentioned above, a check valve is designed to prevent backflow in piping. Other purposes include:

• Protecting equipment: Backflow can damage pumps, compressors, and other mechanical components. Check valves are a safeguard against these damages.
• Preventing contamination: In systems where different fluids are involved, backflow can inadvertently lead to cross-contamination of these liquids. Check valves are used to prevent this sort of mixing.
• Maintaining system pressure: Check valves can help maintain water pressure by preventing loss of fluid due to backflow. This is important for applications where consistent pressure is required such as irrigation systems.
• Preventing water hammer: Water hammer or hydraulic shock results from a pressure surge in the pipes, often occurring after an abrupt direction change or a burst from a previously clogged pipe. Check valves help mitigate the effects of water hammer by dispersing the kinetic energy of the moving liquid, preventing damage to the system.

Types of Check Valves

Here are some common types of check valves:

• Swing check valve: The most commonly used type, operating with a disc which swings on a hinge or a shaft. As the inlet pressure increases, the disc swings off the valve seat, allowing fluid to flow through.
• Life check valve: This type uses a disc which lifts to allow the flow of fluid upon an increase in inlet pressure, making it easy to install horizontally, vertically and angularly.
• Dual plate check valve: Named for its double metal plates, this type is commonly used in industrial operations and refineries. Its compact yet lightweight design can handle huge fluid backflow and is easy to install.
• Non-slam check valve: This type is designed to handle water hammer that results from accumulated pressure in the valve. It utilises a spring to push the disc forward to the valve seat as water pressure decreases, softly closing it to prevent shock.

What is a Check Valve Symbol?

A check valve symbol is a graphical representation of this device in a P&ID schematic. It is comprised of a triangle with two horizontal lines extending from two opposite sides of the triangle with a vertical line inserted into the meeting point between the triangle’s point and the horizontal line.

The horizontal lines represent the check valve’s inlet and outlet parts, while the triangle and vertical line indicate that the flow is allowed upstream but backflow is restricted.

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