The Parts and Functions of a Valve

A valve is a mechanical device that controls the flow of fluid and pressure within a system or process. A valve controls system or process fluid flow and pressure by performing any of the following functions:

• Stopping and starting the fluid flow
• Varying (throttling) the amount of fluid flow
• Regulating downstream system or process pressure
• Relieving component or piping over pressure
• Controlling the direction of fluid flow

Notwithstanding the type of the valve, all the valves have the following basic parts:

• The body
• Bonnet
• Trim (internal elements)
• Actuator
• Packing

Valve Body

The body, sometimes referred to as the shell, is the primary pressure boundary of a valve. It serves as the principal element of a valve assembly since it is the framework that holds everything together.

The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and piping through threaded, bolted, or welded joints.

Valve bodies are cast or forged into a variety of shapes. Though a sphere or a cylinder would in theoretical terms be the most economical shape to resist fluid pressure when a valve is open, there are many other considerations. For instance, many valves require a partition across the valve body to support the seat opening, which is the throttling orifice. With the valve closed, loading on the body is difficult to determine. The valve end connections also distort loads on a simple sphere and more complicated shapes. The ease of fabrication, assembly, and costs are additional key considerations. Thus, the basic form of a valve body typically is not spherical, but ranges from simple block shapes to highly complex shapes in which the bonnet, a removable piece to make assembly possible, forms part of the pressure-resisting body.

Narrowing of the fluid passage (venturi effect) is also a typical technique for reducing the overall size and cost of a valve. In other instances, large ends are added to the valve for connection into a larger line.

Related: Pressure Control Valve

Valve Bonnet

The cover for the opening in the valve body is the bonnet. In some designs, the body itself is split into two sections that bolt together. Like valve bodies, bonnets vary in design. Some bonnets operate as valve covers, while others support valve internals and accessories such as the stem, disk and actuator.

The bonnet is the second principal pressure boundary of a valve. It is cast or forged of the same material as the body and is connected to the body by a threaded, bolted or welded joint. In all circumstances, the attachment of the bonnet to the body is considered a pressure boundary. This implies that, the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts.

While valve bonnets are a necessity for most valves, they do represent a cause for concern. Bonnets can complicate the assembly of valves, increase valve size, represent a significant cost portion of valve cost and are a source for potential leakage.

Valve Trim

The internal elements of a valve are collectively termed to as a valve’s trim. The trim typically includes a disk, seat, stem and sleeves needed to guide the stem. A valve’s 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.

Disk & Seat

For a valve having a bonnet, the disk is the third primary principal pressure boundary. The disk provides the capability for permitting and prohibiting fluid flow. With the disk closed, full system pressure is applied across the disk if the outlet side is depressurized. Because of this, the disk is a pressure-retaining part. Disks are typically forged and in some designs, hard-surfaced to provide a good wear characteristics. A fine surface finish of the seating area of a disk is required for good sealing when the valve is closed. Most valves are named, in part, according to the design of their disks.

The seat or seal rings provide the seating surface for the disk. In some designs, the body is machined to serve as the seating surface and seal rings are not used. In other designs, forged seal rings are threaded or welded to the body to provide the seating surface. To improve the wear-resistance of the seal-rings, the surface is often hard-faced by welding and then machining the contact surface of the seal ring. A fine surface finish of the seating area is needed for good sealing when the valve is closed. Seal rings are not usually considered pressure boundary parts because the body has sufficient wall thickness to withstand design pressure without relying upon the thickness of the seal rings.

Stem

The stem, which connects the actuator and disk, is responsible for positioning the disk. Stems are typically forged and connected to the disk by threaded or welded joints. For valve designs requiring stem packing or sealing to prevent leakage, a fine surface finish of the stem in the area of the seal is required. Typically, a stem is not considered a pressure boundary part.

The connection of the disk to the stem can permit some rocking or rotation to ease the positioning of the disk on the seat. Alternately, the stem may be flexible enough to let the disk position itself against the seat. However, constant fluttering or rotation of a flexible or loosely connected disk can destroy the disk or its connection to the stem.

Two types of valve stems are rising stems and non-rising stems. The two types are demonstrated in the figures below:

For the rising steam valve, the stem will rise above the actuator as the valve is opened. This occurs because the stem is threaded and mated with the bushing threads of a yoke that is integral part of, or is mounted to the bonnet.

There is no upward stem movement from outside the valve for a non-rising stem design. For the non-rising stem design, the valve disk is threaded internally and mates with the stem threads.

Valve Actuator

The actuator operates the stem and disk assembly. An actuator may be a manually operated hand wheel, manual lever, motor operator, solenoid operator, pneumatic operator or hydraulic ram. In some designs, the actuator is supported by the bonnet. In other designs, a yoke mounted to the bonnet supports the actuator. Except for certain hydraulically controlled valves, actuators are outside the pressure boundary. Yokes when employed are always outside the pressure boundary.

Related: Electric Solenoid Valve

Valve Packing

Most valves employ some form of packing to prevent leakage from the space between the stem and the bonnet. Packing is typically a fibrous material (such as flax) or another compound (such as Teflon) that forms a seal between the internal parts of a valve and the outside where the stem extends through the body. Valve packing must be properly compressed to prevent fluid loss and damage to the valve’s stem. If valve’s packing is too loose, the valve will leak, which is a safety hazard. If the packing is too tight, it will impair the movement and possibly damage the stem

Also Read: Pneumatic Actuators