Page 103 - Valve Selection Handbook

90 Valve Selection Handbook

Applications

Duty:
Stopping and starting flow
Infrequent operation
Service:
Gases
Liquids
Rubber-seated wedge gate valves without bottom cavity for fluids
carrying solids in suspension
Vacuum
Cryogenic

PLUG VALVES

Plug valves are rotary valves in which a plug-shaped closure member
is rotated through increments of 90° to engage or disengage a porthole or
holes in the plug with the ports in the valve body. The shape of the plug
may thereby be cylindrical, as in the valves shown in Figure 3-49
through Figure 3-53, or tapered, as in the valves shown in Figure 3-54
through Figure 3-58. Rotary valves with ball-shaped plugs are likewise
members of the plug valve family, but are conventionally referred to as
ball valves. These valves are discussed separately on page 101.
The shape of the port is commonly rectangular in parallel plugs, and
truncated triangular shapes in taper plugs. These shapes permit a slimmer
valve construction of reduced weight, but at the expense of some pres-
sure drop. Full area round-bore ports are normally used only if the
pipeline has to be scraped or the nature of the fluid demands a full area
round bore. However, some plug valves are made only with round-bore
because of the method of sealing employed.
Plug valves are best suited for stopping and starting flow and flow
diversion, though they are also used occasionally for moderate throttling,
depending on the nature of the service and the erosion-resistance of the
seatings. Because the seatings move against each other with a wiping
motion, and in the fully open position are also fully protected from con-
tact with the flowing fluid, plug valves are generally capable of handling
fluids with solids in suspension.