Piston valves are closing-down valves in which a piston-shaped closure 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 When the valve is being opened, flow cannot start until the piston has been completely withdrawn from the seat bore.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.

Figure 3-20: Piston Valve, Standard Pattern, Seat Packing Mounted in Valve Body, Piston Pressure Unbalanced. ( Courtesy of Rich. Klinger AG.)

Figure 3-21: Piston Valve, Standard Pattern, Seat Packing Mounted in Valve Body, Piston Pressure Balanced. ( Courtesy of Rich. Klinger AG.)

Figure 3-22: Piston Valve, Adapted for Draining Vessels, Seat Packing Mounted in Valve Body. ( Courtesy of Yarway Corporation.)

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. Klinger AG.)

Like globe valves, piston valves...