Page 78 - Valve Selection Handbook

Manual Valves 65

Applications

Duty:
Controlling flow
Stopping and starting flow
Frequent valve operation
Service:
Gases essentially free of solids
Liquids essentially free of solids
Vacuum
Cryogenic

PISTON VALVES

Piston valves are closing-down valves in which a piston-shaped clo-
sure member intrudes into or withdraws from the seat bore, as in the
valves shown in Figure 3-20 through Figure 3-24.
In these valves, the seat seal is achieved between the lateral faces of
the piston and the seat bore. When the valve is being opened, flow cannot
start until the piston has been completely withdrawn from the seat bore.
Any erosive damage occurs, therefore, away from the seating surfaces.
When the valve is being closed, the piston tends to wipe away any solids,
which might have deposited themselves on the seat. Piston valves may
thus handle fluids that carry solids in suspension. When some damage
occurs to the seatings, the piston and the seat can be replaced in situ, and
the valve is like new without any machining.
Like globe valves, piston valves permit good flow control. If sensitive
flow adjustment is required, the piston may be fitted with a needle
shaped extension. Piston valves are also used for stopping and starting
flow when flow resistance due to the tortuous flow passage is accepted.
Construction

The seatings of piston valves are formed by the lateral faces of the
valve bore and the piston. A fluid-tight contact between these faces is
achieved by a packing that either forms part of the valve bore, as in the
valves shown in Figure 3-20 through Figure 3-22, or part of the piston, as
in the valves shown in Figure 3-23 and Figure 3-24. Packings commonly