What is Valve Trim?

The trim, particularly the mating elements that regulate the stream to the controller’s demands, is at the heart of the control valve. Each process is distinct in its own right, necessitating particular flow control features. Valve trim includes the stem, plug, disc, seating surface, etc. It’s also the plug and seat arrangement’s actual shape. The flow properties of the valve are determined by the shape of the valve plug.

Type of Valves Valve Trims

• • Angle Valves

    • • Stem, Seat, Disc, Disc nut, back-seat bushing

• • Gate Valves

    • • Stem, Seat, Wedge, back-seat bushing

• • Globe Valves

    • • Stem, Disc, Seat, Disc nut, back-seat bushing

• • Disc Valves

    • • Disc, Disc nut, back-seat bushing

• • Lift check valves

    • • Seat ring, Disc, Disc guide

• • Swing check valves

    • • Seat ring, Disc, Disc nut, Disc nut pin, Disc holder, Disc holder pin, side plug, carrier pin

Valve Trim – Disc

The first valve trim part is a disc. The disc is the part that allows, throttles, or stops fluid flow depending on its position. Types of disks define the name of the valve such as gate, ball, plug, and needle valve’s disk are also of the same shape as the name.

A valve disc could be cast, forged, or fabricated. The valve disk is sometimes required hard facing to improve wear resistance. The disk needed a smooth machine surface to reduce friction with the seat. The valve disk is a pressure-retaining part.

That means the disk holds the pressure. When the valve is open, the disc does not perform pressure-retaining or -containing functions. However, when the valve is closed, the disc performs pressure-retaining functions.

A disc rests against the stationary valve seat when the valve is in the closed position. It can be moved away from the seat by the movement of the stem. However, in check and safety-relief valves, fluid flow and pressure move the disc away from the seat.

Valve Trim – Seat

The seat provides the seating surface for the disk. Here, you can see the gate valve seat in the above image. A valve may have multiple seats. In the case of a globe valve and swing check valve, there is one seat. A gate valve and ball valve have two seats; one on the upstream side and the other on the downstream side.

The valve leakage rate is directly proportional to the effectiveness of the seal between the valve disc and seat(s). Valve seats may be integral or replaceable rings. Valves are generally provided with a screwed, welded, or integrally cast or forged seat and hardened by heat treatment or by the hard facing of Stellite weld overlay.

A fine surface finish for the seating area is necessary for proper sealing. Some ball valves & plug valves used the non-metallic seat for non-critical services. 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 cannot be achieved only by metal seats.

Valve Trim – Back Seat

Another component of the valve that does not maintain pressure is the back seat. A stem shoulder serves as the foundation of the back seat. The stem creates a seal when it is fully extended. As a result, the valve shell’s leaking into the packaging chamber, and the environment is stopped.

Valve Trim – Stem

The stem connects the actuator and disk. It moves and positions the valve disk. The valve stem transports the required motion to the disc, plug, or ball for opening, closing, or positioning the valve. The stem connects the actuator, handwheel, or lever of the valve at one end and the disc on the other end.

In gate and globe valves, the linear motion of the disc opens or closes the valve, while in the plug, ball, and butterfly valves, the disc rotates to open or shut the valve. Stems are typically forged from stainless steel and connected to the disk by threaded or welded joints.

Valve Trim – Yoke

Yoke is another name for yoke arms. A yoke connects the valve’s actual mechanism to the valve body or bonnet. The yoke and the bonnet are built as a single piece to cover multiple valves. The yoke nut and yoke bushing are located on the top side of the yoke.

The Yoke is constructed tough enough to endure the forces, moments, and torque generated by the actuators.

Valve Trim – Yoke Brushings

It is the area where the valve stem enters through the top of the yoke and the internally threaded nut is held in place. Depending on which way the nut is rotated, the stem moves up and down in certain valves, such as gate and diaphragm valves. The yoke nut is positioned permanently over some valves, and the stem is rotated via it.

The yoke bushings are composed of softer material than the stem in order to need the least amount of effort to operate. On the valves that need a lot of effort to open or close, anti-freeze yoke sleeve bearings are provided. By doing this, the friction between the yoke bushing and the toughened stem will be reduced.

Valve Trim Chart

The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as Valve trim. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim.

A Valve’s trim performance is determined by the disk and seat interface and the relation of the disk position to the seat. Because of the trim, basic motions, and flow control are possible. In rotational motion trim designs, the disk slides closely past the seat to produce a change in flow opening. In linear motion trim designs, the disk lifts perpendicularly away from the seat so that an annular orifice appears.

Valve trim parts may be constructed of assorted materials because of the different properties needed to withstand different forces and conditions. Bushings and packing glands do not experience the same forces and conditions as the valve disc and seat(s).

Flow-medium properties, chemical composition, pressure, temperature, flow rate, velocity, and viscosity are some of the important considerations in selecting suitable trim materials. Trim materials may or may not be the same material as the valve body or bonnet.