Check valves are installed in a pipeline and allow flow in one direction only (also known as a non-return valve). They work entirely by reaction to the contents of the pipe and hence do not require any external actuation.
Examples where you might use a Check valve;
Types of Check Valve
Lift Check Valve
The disk or plug lifts when the flow is from left to right. Limitations of this design are that they have to be installed horizontally.
The piston-type lift check valve is a modification of the standard lift check valve. It incorporates a piston shaped plug instead of the cone, and a dashpot is applied to this mechanism. The dashpot produces a damping effect during operation, thereby eliminating the damage caused by the frequent operation of the valve, for example, in pipeline systems, which are subject to surges in pressure, or frequent changes in flow direction (one example would be a boiler outlet).
With flow in the forwards direction, the pressure of the fluid forces the disc to hinge upwards. The weight of the flap is responsible for the closure of the valve; however, in some cases, closure may be assisted by the use of a weighted level. With abrupt changes in flow, the disk can slam against the valve seat and generate an affect called "water-hammer" along the pipe system. This can be overcome by fitting a damping mechanism.
Clamps directly between the flanges, usually available with or without a spring.
Split Disk / Dual Plate Check
Again clamps between flanges. Consists of two plates pivoted around a centre pin. Typically a lower pressure drop across the valve when compared to other check valve designs. You don’t get the slamming action which can occur with swing check types. This type can be made in very large sizes.
The pressure required to open the valve is determined by the spring. If a tight seal is required when closed a soft seat can be used. Because of the spring this valve can be installed in any direction. There is usually a greater pressure drop with this style because in the open position the contents of the pipe has to flow around the disk of the valve.
Ball Check Valve
Diaphragm Check Valve
Tilting Disk Check Valve
Tilting-Disc check (TDC) valves are designed for use in applications where rapid response (fat opening and closing) is required. The centre of gravity of the disc is very close to the axis of rotation thus the disc can open or close very quickly.
In most cases the size of a check valve is determined by the size of the pipework. An oversized check valve is indicated by continuous valve chatter.
Pressure Loss Chart
Even in the open position a check valve will have a pressure loss across the valve. This chart shows the pressure drop across a check valve of a given size at various flow rates for the particular medium stated (usually water).