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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 valveSwing Check shut off depends on
gravity (weight of disc) and reverse flow. The pivot point of the swing
check is outside the periphery of the disc and, the greater the head,
the greater the possibility that the fluid will flow back through the
valve before the disc can shut off.
To effect complete shut off, the disc of a swing check valve must travel
through a 90 angle arc to the valve seat, as shown in Fig. 1. Without
resistance to slow the disc's downward thrust, and encouraged by reverse
flow, the shut off results in slamming and damaging water hammer.
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