Check valves are designed to prevent back flow in lines. The five principal types of check valves used are the tee-pattern lift check, the swing check, the tilting disk check, the wye-pattern lift check and the ball check. The swing check valve is the more commonly used.
Check (non-return) valves are installed in pipelines to allow flow in one direction only. It is operated entirely by reaction to the line fluid and therefore do not require any external actuation. Types including lift, disc, swing and wafer check valves.
The force gravity plays an important role in the functioning of a check valve and therefore the position of the valve must always be given consideration. Lift and ball-check valves must always be placed so that the direction of lift is vertical. Swing checks must be located to ensure that the disk will always be closed freely and positively by gravity.
The flow velocity of the fluid through the valve has a significant affect on the life of the check valve. The valve should be sized such that the fluid velocity under normal design conditions is sufficient to keep the disk fully open and pressure against the stop.
Types of Check Valves
Lift / Piston check valves
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 dash pot is applied to this mechanism. The dash pot 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.
The main advantage of the lift check valve lies in its simplicity, and as the cone is the only moving part; the valve is robust and requires little maintenance. In addition, the use of a metal seat limits the amount of seat wear.
The lift check valve has two major limitations firstly, it is designed only for installation in horizontal pipelines, and secondly, its size is typically limited to 3”, above which, the valve would become too bulky.
Swing check valve
Swing check valve consists of a flap or disc of the same diameter as the pipe bore, which hangs down in the flow path. With flow in the forwards direction, the pressure of the fluid forces the disc to hinge upwards, allowing flow through the valve. Reverse flow will cause the disc to shut against the seat and stop the fluid going back down the pipe.
In the absence of flow, 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 lever. Swing check valves produce relatively high resistance to flow in the open position, due to the weight of the disc.
In addition, they create turbulence, because the flap ‘floats’ on the fluid stream. This means that there is typically a larger pressure drop across a swing check valve than across other types.
Wafer check valves
Both lift and swing check valves tend to be bulky which limits their size and makes them costly. To overcome this, wafer check valves have been developed.
By definition wafer check valves are those that are designed to fit between a set of flanges. This broad definition covers a variety of different designs, including disc check valves and wafer versions of swing or split disc check valves.
Disc check valves
The disc check valve consists of four main components: the body, a disc, a spring and a spring retainer. The disc moves in a plane at right angles to the flow of the fluid, resisted by the spring that is held in place by the retainer. The body is designed to act as an integral cantering collar that facilitates installation. Where a ‘zero leakage’ seal is required, a soft seat can be included.
When the force exerted on the disc by the upstream pressure is greater than the force exerted by the spring, the weight of the disc and any downstream pressure, the disc is forced to lift off its seat, allowing flow through the valve. When the differential pressure across the valve is reduced, the spring forces the disc back onto its seat, closing the valve just before reverse flow occurs.