Wedge Design of Cast Iron Metal Seated Gate Valves

Wedge Gate & Metal-to-Metal Seating

The cast iron metal seated gate valve is usually furnished with a wedge-shaped gate, which is perpendicular to the process flow. The wedge-shaped gate valve uses two inclined seats and a slightly mismatched inclined gate (also called “wedge”) that allows for tight shutoff, even against higher pressures. The inclined wedge are designed 2.5° to 5° from the vertical plane, while the inclined seats are designed with a close, but not exact angle.  This inclination angle θ is called the “wedge half angle”. When the seat and wedge angles are slightly mismatched, either the seat or the wedge is designed with some free movement to allow the seating surfaces to conform with each other as the manual actuator force is applied.

The internal view of a non-rising stem (NRS) metal seated cast iron gate valve (left); The internal view of a rising stem (RS) metal seated cast iron gate valve (right).

The wedge of the cast iron metal seated gate valve is attached to the manual operator through the stem, which may be either fixed (rising stem) to the wedge or threaded (non-rising stem) to the wedge. As illustrated in the picture above, the stem is not integral to the wedge but rather uses a T-shaped collar that fits into a T-shaped slot in the wedge. The T-slot is usually parallel to the flow stream. A pair of copper alloy seat rings are embedded/ inserted into the seating areas of both the wedge and the valve body to form the metal-to-metal seal. This metal seated design offers enhanced durability, better resistance to higher temperatures and harsh environments, improved sealing performance, and longer service life compared to traditional soft-seated designs.

Different Wedge Designs

The drawing of three types of wedge design for cast iron metal seated gate valves. (a) flexible wedge; (b) solid wedge; (c) double disk (split wedge).

The wedge gate of commonly used cast iron metal-seated gate valves is divided into three types: solid wedge, flexible wedge, and double disk (split wedge).

• The flexible wedge (a) is a single-piece design, with two sealing surfaces supported on a central cantilever beam. There is a circular groove on its vertical midpoint face, which gives the wedge a certain flexibility. When the valve is closed, the slight deformation generated by the wedge compensates for the machining tolerances between the gate and the seat, ensuring complete alignment of the two sealing surfaces with the respective seats on both sides and forming a tight shutoff. The flexible wedge has a simple structure, and provides reliable sealing. It is less prone to wedging when the fluid temperature changes, and has relatively lower machining precision requirements for the wedge angle.
• The solid wedge (b) is a single-piece solid structure that cannot compensate for changes in seating dimensions due to pipeline loads or thermal fluctuations. High machining precision is required for the wedge half angle on both sealing surfaces, making manufacturing and maintenance relatively difficult. Wear between sealing pairs is prone during operation, and the gate may be wedged during temperature changes. Therefore, for sizes larger than DN50 (NPS 2), the use of solid wedge gate valves is generally not recommended for temperatures exceeding 121°C (250°F). Anyway, the solid wedge gate valves are considered the most economical, and almost all small-sized gate valves no greater than DN50 (NPS 2) adopt solid wedge designs, typically suitable for lower “pressure-temperature” conditions.
• The split wedge (double-disk) consists of two gate disks combined and hinged into a wedge-shaped gate with a spherical center pivot. This design allows for free adjustment of the wedge angle to achieve a good fit with the seats on both sides. The sealing wedge angle of the gate disk can be automatically adjusted by the centrally hinged spherical pivot, thus requiring lower machining precision. It is less prone to jamming or scratching when temperature changes occur. After wear on the wedge sealing surface, compensation can be made by adding shims at the pivot, facilitating repairs. However, the downside is that the structure is complex, with many parts, assembly is troublesome, and it is not suitable for viscous media.

Design Standards

Rising stem and non-rising stem metal-seated gate valves, made in accordance with DIN 3352 F4, supplied to Germany.

Cast iron metal seated wedge gate valves are generally furnished with flanged ends. They may be manufactured in accordance with a series of international standards: BS 5150, EN 1171, DIN 3352, BS 3464, MSS SP 70, BS 5163, GB/T 12232, JIS B2031, API 595, API 604, ASME B16.10, etc.