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The presence of the spring enables the disc check valve to be installed in any direction. 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. 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. Where a ‘zero leakage’ seal is required, a soft seat can be included. The body is designed to act as an integral centring collar that facilitates installation. 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 disc check valve consists of four main components: the body, a disc, a spring and a spring retainer. This broad definition covers a variety of different designs, including disc check valves and wafer versions of swing or split disc check valves. By definition wafer check valves are those that are designed to fit between a set of flanges. To overcome this, wafer check valves have been developed. Wafer check valvesīoth lift and swing check valves tend to be bulky which limits their size and makes them costly. This can be overcome by fitting a damping mechanism to the disc and by using metal seats to limit the amount of seat wear. With abrupt changes in flow, the disc can slam against the valve seat, which can cause significant wear of the seat, and generate waterhammer along the pipe system. This means that there is typically a larger pressure drop across a swing check valve than across other types. In addition, they create turbulence, because the flap ‘floats’ on the fluid stream. Swing check valves produce relatively high resistance to flow in the open position, due to the weight of the disc. As can be seen from Figure 12.3.2, the whole mechanism is enclosed within a body, which allows the flap to retract out of the flow path. 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. Reverse flow will cause the disc to shut against the seat and stop the fluid going back down the pipe. With flow in the forwards direction, the pressure of the fluid forces the disc to hinge upwards, allowing flow through the valve. Swing check valvesĪ swing check valve consists of a flap or disc of the same diameter as the pipe bore, which hangs down in the flow path.
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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). It incorporates a piston shaped plug instead of the cone, and a dashpot is applied to this mechanism.
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The piston-type lift check valve is a modification of the standard lift check valve. 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 DN80, above which, the valve would become too bulky. In addition, the use of a metal seat limits the amount of seat wear. 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. Furthermore, the design of a lift check valve generally limits its use to water applications, subsequently, they are commonly used to prevent reverse flow of condensate in steam traps and on the outlets of cyclic condensate pumps. If a metal seat is used, the lift check valve is only suitable for applications where a small amount of leakage, under reverse flow conditions, is acceptable. With reverse flow, the cone returns to its seat and is held in place by the reverse flow pressure. When the flow into the valve is in the forward direction, the pressure of the fluid lifts the cone off its seat, opening the valve. The inlet and outlet ports are separated by a cone shaped plug that rests on a seat typically metal in some valves, the plug may be held on its seat using a spring. Lift check valves are similar in configuration to globe valves, except that the disc or plug is automatically operated. The different types of check valve and their applications are discussed in this module, along with the correct sizing method. Although check valves can effectively shut off reverse flow, they should never be used in place of an isolation valve to contain live steam, in a section of pipe.Īs with isolation valves, there are a number of different check valve designs, each suited to specific applications.