Page 82 - Valve Selection Handbook

Manual Valves 69

ring that acts as a spring element and, as such, pre-stresses the packing.
When the piston moves into the seat, the friction ring comes to rest in the
seat bore, and any progression of the piston increases the packing stress.
National standards that apply specifically to piston valves do not exist.

Applications

Duty:
Controlling flow
Stopping and starting flow
Service:
Gases
Liquids
Fluids with solids in suspension
Vacuum

PARALLEL GATE VALVES

Parallel gate valves are slide valves with a parallel-faced gate-like clo-
sure member. This closure member may consist of a single disc or twin
discs with a spreading mechanism in between. Typical valves of this type
are shown in Figure 3-25 through Figure 3-32.
The force that presses the disc against the seat is controlled by the
fluid pressure acting on either a floating disc or a floating seat. In the
case of twin disc parallel gate valves, this force may be supplemented
with a mechanical force from a spreading mechanism between the discs.
One advantage of parallel gate valves is their low resistance to flow,
which in the case of full-bore valves approaches that of a short length of
straight pipe. Because the disc slides across the seat face, parallel gate
valves are also capable of handling fluids, which carry solids in suspen-
sion. This mode of valve operation also imposes some limitations on the
use of parallel gate valves:

• If fluid pressure is low, the seating force may be insufficient to produce
a satisfactory seal between metal-to-metal seatings.
• Frequent valve operation may lead to excessive wear of the seating
faces, depending on magnitude of fluid pressure, width of seating faces,
lubricity of the fluid to be sealed, and the wear resistance of the seating