Check valves, or non-return valves, are installed in pipeline systems to allow flow in one direction only. They are operated entirely by reaction to the line fluid and therefore do not require any external actuation. In this text, the expected, or desired direction of flow is termed 'forward flow’, flow in the opposite direction is 'reverse flow'.

Reasons for using check valves, which include: • Protection of any item of equipment that can be

affected by reverse flow, such as flow meters,

strainers and control valves. • To check the pressure surges associated with

hydraulic forces, for example, water hammer.

These hydraulic forces can cause a wave of

pressure to run up and down pipe work until the

energy is dissipated. • Prevention of flooding.

• Prevention of reverse flow on system shutdown.

• Prevention of flow under gravity.

• Relief of vacuum conditions.