What is Check Valve

Figure 1: One-piece check valve made from bar stock.

1. What it is

Check Valves are found in almost every industrial application acting as one-way or non-return valves. Check valves are simple devices, perform a vital function by preventing reverse flows, therefore preventing damage and ensuring efficient operations. For example, reverse flow can result in water hammer. This phenomenon can see repeated extreme pressure surges in connected pipework, valves and pumps that may fatally damage or rupture the system and its pipework. Even if a failure does not occur immediately the repeated impact of water hammer can promote fatigue that may also ultimately result in a loss of system integrity.

Figure 2: Check valves perform a vital function by preventing water hammer.

2. Technical Terminology

Cracking pressure – Refers to the minimum pressure differential needed between the inlet and outlet of the valve at which the first indication of flow occurs (steady stream of bubbles). Cracking pressure is also known as unseating head (pressure) or opening pressure.

Reseal pressure – Refers to the pressure differential between the inlet and outlet of the valve during the closing process of the check valve, at which there is no visible leak rate. Reseal pressure is also known as sealing pressure,seating head (pressure) or closing pressure.

Back pressure – a pressure higher at the outlet of a fitting than that at the inlet or a point upstream

3. How Does it Work

Featuring a single inlet and outlet, check valves are operated by a pressure differential. Above a certain upstream pressure, the valve will automatically open without requiring any other intervention. Known as the cracking pressure, this minimum operational pressure is one of the key characteristics specified in all check valves. Frequently check valves are placed in series, for instance in water systems to prevent backflow of contaminated water into clean water supply lines.

Figure 3: Structure of a check valve.

4. Application and Types of Check Valve

Chemicals, pulp and paper, food processing, water and wastewater treatment, industrial, marine and mining, pumps, pipelines, power generation and HVAC are among the many fluid flow and pumping applications that feature check valves.

With a huge range of potential applications, there are also multiple types of check valve which use many different materials such as metal, polymers and rubber. Common designs include swing or flap check valves in which a metal disc pivots on a hinge or trunnion to prevent reverse flow. Larger check valves are typically of the swing or flap type. As the name suggests, ball and spring ball check valves feature a ball that mounts in an appropriately profiled seat. Duck bill check valves rely on a flexible rubber diaphragm which creates a valve that is normally closed unless positive pressure is applied.

However, unlike their metal swing or flap check valve counterparts rubber duckbill check valves cannot rust, seize, or bind, typically increasing reliability and longevity. Similarly, rubber check valves do not suffer from mechanical wear.

Figure 4: Types of check valve.

5. Selection Criteria

Check valves have the following criteria to consider when selecting one for your application:

Material compatibility with the medium;
Line size for connection points;
Max pressure and cracking pressure requirement;
Installation orientation horizontal or vertical;
Envelope dimensions;
Accessibility needs for inspections and repairs;
Temperature (external and media).

6. Installation Orientation

As a one way valve only works in one direction, it is crucial to know the correct installation orientation. Often times, there is an arrow on the valve housing to signal the flow direction. Otherwise, you will need to examine the valve to ensure it is installed in the intended flow direction. If it is backwards, flow will not be able to move through the system and a build-up of pressure could cause damage.

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