Non-pressure Retaining Parts.

Valve seat(s), stem, yoke, packing, gland bolting, bushings, handwheel, and valve actuators are some of the major non- pressure retaining parts of a valve.

Valve Seat(s). A valve may have one or more seats. In the case of a globe or a swing-check valve, there is usually one seat, which forms a seal with the disc to stop the flow. In the case of a gate valve, there are two seats; one on the upstream side and the other on the downstream side. The gate-valve disc or wedge has two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow. Multiport plug and ball valves may have several seats, depending upon the number of ports in the plug or ball.

The valve leakage rate is directly proportional to the effectiveness of the seal between the valve disc and its seat(s). The valve standards MSS SP 61, API 598, and ASME B16.34 specify acceptable leak rates. A user may specify more or less restrictive leak rates to satisfy the application requirements.

Valve manufacturers have developed several designs of combination valve seats involving elastomer and metal seats that are effective in achieving the desired leak tightness, which is not readily accomplished with metal seats.

Valve seats may be integral, replaceable, or renewable seat rings. Small valves generally are provided with screwed-in, swaged-in, welded, or brazed-in valve seats. Large valves may have any of the seat designs listed for small valves, or have seats integrally cast or forged with the valve body and hardened by heat treatment or surfaced with hard material such as Stellite. Stellite is a trade name of the Deloro Company. There are other metals that can be used for hardening the seating surfaces.

Galling Prevention. In order to prevent or minimize galling of the valve disc and valve seats, it is common industry practice to maintain a hardness differential between the stationary seating surfaces of valve seats and the moving seating surfaces of the valve disc. The stationary valve seats are hardened slightly more than the disc-seating surfaces. When both the valve seats and the disc are hardened by use of Stellite, the valve is termed fully stellited. When only the seats are hardened by use of Stellite, the valve is called half stellited. Heat treatment is another method of hardening.

Valve Stem. The valve stem imparts the required motion to the disc, plug, or the ball for opening or closing the valve. It is connected to the valve handwheel, actuator, or the lever at one end and the valve disc on the other. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball, and butterfly valves, the valve disc is rotated to open or close the valve. With the exception of stop-check valves, check valves do not have valve stems.

Rising Stem with Outside Screw and Yoke. The outermost part of the stem is threaded, while the portion of stem inside the valve is smooth. The stem threads are isolated from the flow medium by the stem packing. Two different styles of this design are available: one having the handwheel fixed to the stem so that they rise together, and the other having a threaded sleeve that causes the stem to rise through the handwheel. The rising stem with outside screw and yoke (O. S. & Y.) is a common design for NPS 2 (DN 50) and larger valves. Some codes, such as ASME B31.1, Power Piping, require that an outside screw-and-yoke design be used for NPS 3 (DN 80) and larger valves in pressures above 600 psi (4140 kPa).