Page 81 - Valve Selection Handbook

68 Valve Selection Handbook

Figure 3-23. Piston Valve Adapted for Draining Vessels, Seat Packing Mounted on
Piston; the "Ram-Seal" Principle. (Courtesy of Fetterolf Corporation.)

Figure 3-24. Piston Valve, Standard Pattern, Seat Packing
Mounted on Piston. (Courtesy of Rich. KlingerAG.)

The piston valve shown in Figure 3-23 carries the seat packing on the
end of the piston instead of in the valve bore. The packing is supported
thereby on its underside by a loose compression ring. When the piston
moves into the final closing position, the compression ring comes to rest
on a shoulder in the seat bore so that any further progression of the piston
causes the compression ring to tighten the packing. Because the packing
establishes interference with the seat in the last closing stages only, the
operating effort of the valve is lower over a portion of the piston travel
than that of the foregoing valves.
The piston valve shown in Figure 3-24 also carries the seat packing on
the piston. However, the loose compression ring is replaced by a friction