In brief
A check valve is a self-acting valve that lets a medium pass in one direction only and prevents backflow by closing.
Contents
What is Check valve?
Check valves operate without external energy: the pressure of the flowing medium opens the closing element, and when flow stops or reverses, back pressure, a spring or the element’s own weight pushes it back onto the seat. They protect pumps against reverse rotation, prevent pipes from draining and separate circuits in hydraulics, pneumatics and process engineering.
Common designs are swing check valves, ball, disc and cone check valves, dual-plate wafer check valves and spring-loaded nozzle check valves, which close particularly quickly. In hydraulics, pilot-operated check valves and screw-in cartridges are also used. Important for selection are cracking pressure, nominal size or connection thread, nominal pressure, flow direction, permissible installation position, materials and type of seal.
Failures are frequently caused by dirt on the seat, broken or fatigued springs, wear from chattering at too low a flow rate and pressure surges when a slow-closing valve is slammed shut by the backflow. A leaking check valve is often only noticed indirectly, for example by pumps turning backwards or pressure loss at standstill. Regular checks when pumps stop help to detect such damage early.
In practice: maintenance and procurement
In a spare parts enquiry, the design, connection type and size, nominal pressure, cracking pressure, medium, temperature and installation position should be stated. If the design is changed during replacement, for example from a swing check valve to a spring-loaded valve, pressure loss and closing behaviour change.
Synonyms and abbreviations: non-return valve · NRV · swing check valve · backflow preventer
Frequently asked questions
How does a check valve work?
When the medium flows in the direction of flow, it pushes the closing element, such as a ball, disc or flap, away from the seat. When the flow decreases or reverses, the element closes by itself through spring force, its own weight or the back pressure. No controller or auxiliary energy is needed for this.
What is the cracking pressure of a check valve?
The cracking pressure is the pressure differential at which the closing element lifts off the seat and flow becomes possible. It depends mainly on the spring force. Too high a cracking pressure increases the pressure loss in the system, while too low a value can lead to leakage or chattering in certain installation positions. When replacing, the value should match.
Does the installation position of a check valve matter?
Yes. Gravity-operated designs such as some swing or ball check valves only work in certain positions, for example in horizontal pipes or with flow from bottom to top. Spring-loaded valves are usually independent of position. The permissible installation position is given in the manufacturer’s documentation and should be checked when replacing.
Why does a check valve slam when the pump is switched off?
Slamming occurs when the closing element reacts too slowly and the column of liquid already flowing back forces it abruptly onto the seat. This creates pressure surges that stress pipes and valves. The usual remedy is fast-closing, spring-loaded designs or damped swing check valves, whose suitability must be checked case by case.