Page 115 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.65

FIGURE 2.36 Diaphragm valves. (a) Straight-through valve in open position; (b) weir-type valve
in closed position.

Another design has an extended body for connections to grooved end piping. In this
valve, the sealing member is the disk itself, which is fully encapsulated with a resilient
material selected for the service conditions at hand.
Butterfly valves have continued to grow in popularity, generally at the expense of gate
valves, because they are lightweight, easy to install, low in cost, easy to actuate, and easy
to insulate and also because they feature one-quarter turn operation, tight shutoff (due to
resilient seal), and a variety of construction materials.

DIAPHRAGM VALVES

A diaphragm valve uses a rubber, plastic, or elastomer diaphragm to seal the stem. The
diaphragm not only seals the stem but forms the closure element.
There are two styles of diaphragm valves, one having a body with a weir and the other
having a straight-through body. On the weir type, shown in Fig. 2.36, the stem is connected
to a finger arrangement, which in turn presses the diaphragm down onto a weir. This creates
an extremely tight seal that will seal even on some solids. This valve has been used exten-
sively in radwaste services and maintenance is extremely simple. On the straight-through
type (Fig. 2.35), the diaphragm presses onto the bottom of the valve body for seating.
Since the diaphragm is not metallic and forms the closure, the valve is severely limited
in pressure and temperature. A wide variety of diaphragm materials are available for use
with different fluids. To enable draining of horizontal pipelines, the weir valve must be
mounted 15° from the horizontal plane (because of the weir). This complicates installation,
especially with air actuators.

PINCH VALVES

A pinch valve (Fig. 2.37) uses a round elastomeric sleeve connected to the valve body from
inlet to outlet that completely isolates the liquid passing through the valve from all internal
valve components. Closure is made by a movable closure element outside the sleeve that
pinches the sleeve between the element and the valve body.
This type of valve is used for slurry and other liquids with highly corrosive properties.

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