Check Valve
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.
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 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.
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 valves
A 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 using a weighted lever. As can be seen from
below picture the whole mechanism is enclosed within a body, which allows the
flap to retract out of the flow path.
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 centering 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. This is shown in Figure above The presence of the spring enables the disc check valve to be installed in any direction.