Understanding the Basic Parts of Valve

Valves have some basic parts in common irrespective of the type of valve.These basic parts of valves serve different functions during the operation of valve.Understanding of basic parts of a valve practically helps plant operators and maintainers to perform their duties in a better way.

Basic Parts of Valve

Following are the basic parts of a valve:

• Body of valve            
• Bonnet of valve
• trim (internal elements) e.g Stem and disc ,seat and sleeve of valve stem
• actuator of valve
• packing of valve

1. Valve Body

Valve body, sometimes called the shell, is the primary pressure boundary of a valve. It serves as the principal element of a valve assembly because it is the framework that holds everything of valve together.

Body of valve has to withstand many forces inside the valve.It is the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. Body of valve also experiences many forces due to end connections of a valve. It receives inlet and outlet piping through threaded, bolted, or welded joints.

The basic form of a valve body ranges from simple block shapes to highly complex shapes in which the bonnet, a removable piece to make assembly possible, form part of the pressure resisting body. 

2. Valve Bonnet

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

The bonnet of valve  is 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 cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the welded joint or bolts that connect the bonnet to the body are pressure-retaining parts.

Bonnets of valves can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost.Bonnets of valves are a source for potential leakage.

3. Valve Trim

The internal elements of a valve are collectively called as 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. Contact of disk with seat matters most when evaluating the performance of valves.Disk to seat contact determines the presence of  leakage from a valve.

Because of 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 design, the disk lifts perpendicularly away from the seat so that an annular orifice appears.

4. Valve Disk and Valve Seat

For a valve having a bonnet, the valve disk is the third primary principal pressure boundary. The valve 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. For this reason, the disk is a pressure-retaining part. Disks are typically forged and, in some designs, hard-surfaced to provide good wear characteristics. A fine surface finish of the seating area of a disk is necessary for good sealing when the valve is closed. Most valves are named, in part, according to the design.

The seat or seal rings provide the setting 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 rings. A fine surface finish of the seating area is necessary for good sealing when the valve is closed.

5. Valve Stem

Valve stem connects the actuator and disk for positioning the disk in accordance with requirement. 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 necessary. Typically, a stem is not considered a pressure boundary part.

Two types of valve stems are rising stems and non-rising stems. These two types of stems are easily distinguished by observation. 

 For a rising stem 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 an 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.

6. Valve Actuator

Valve actuator operates the stem and disk assembly. An actuator may be manually operated hand wheel, manual lever, motor operated, solenoid operated, pneumatic operated, or hydraulic operated.Valve actuator basically directs the disc to open ,close or throttle depending upon the type of valve.

7. Valve packing

Valve packing prevent leakage from the space between the stem and the bonnet. Valve Packing is commonly graphite ring, 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 a 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.