Basics of Valves
Basics of Valves
A valve is a mechanical device specially designed to control the fluid flow and pressure within a system or process. Valves are designed for either liquid or gaseous application. A valve carries out any of the following functions.
- Stopping and starting of flow of the fluid.
- Throttling the amount of fluid flow
- Controlling the fluid flow direction
- Regulating the downstream system or process pressure
- Relieving the over pressure in the system
- Regulating the temperature of process fluid
- Controlling the mixing of fluids
Since there is diversity of the types of systems, fluids, and environments in which valves are to operate, valves have many designs and types which satisfy one or more of the identified functions. By nature of their design, function, and application, valves comes in a wide variety of styles, sizes, and pressure classes. A multitude of valve types and designs safely accommodate a wide variety of valve applications. These days range of valves extends from simple water cocks to control valves equipped with microprocessors, which provide single loop control of the process. The most common types of valves in use today are gate, globe, plug, ball, needle, butterfly, check, pressure relief, diaphragm, pinch, and safety valves etc. Valves are manufactured from a number of materials which include carbon steel, alloy steel, stainless steel, iron, brass, bronze, aluminum, plastics and a number of special alloys. Valves can have threaded end connection, flanged end connection, welded connection, or other type of connection like union end connections in case of some plastic valves.
Regardless of type, all valves have the basic components which include valve body, bonnet, trim (internal elements), actuators, and packing. These are described below. A valve along with its basic components is shown in Fig 1.
Fig 1 A valve showing its components
- Valve body – It is sometimes called the shell and is the primary pressure boundary of a valve. It is the principal element of a valve assembly because it is the framework that holds everything together. It resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints. Valves bodies are usually forged, cast, or fabricated into a variety of shapes.
- Valve bonnet – The cover for the opening in the valve body is the bonnet. Bonnets vary in design. Some bonnets functions simply as valve covers, while other supports internals and accessories such as stem, disk, and actuator. The bonnet is the second pressure boundary of a valve. It is forged, cast or fabricated from the same material as the valve body and is connected to the body by a threaded, bolted, or welded joint.
- Valve trim – The internal elements of a valve are collectively referred to as a valve’s trim. The performance of a valve is determined by the disk, 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 and seat – These are part of the valve trim. For a valve, the disk is the third primary principal pressure boundary. The disk provides the capability for permitting and prohibiting fluid flow. The disk is a pressure retaining part. Disks are usually forged and in some designs they are hard surfaced to provide good wear characteristics. A fine surface finish of the seating area of a disk is necessary for good sealing. 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 improve the wear resistance of the seal rings, the surface is usually hard faced by welding and then machining the contact surface of the seal ring. Seal rings are not normally considered pressure boundary parts.
- Stem – The stem connects the actuator and the disk and is responsible for positioning of the disk. Stems are normally forged and connected to the disk by threaded or welded joints. The stems can be either rising stems or non rising stems.
- Valve actuator – The actuator operates the stem and disc assembly. An actuator may be a manually operated hand wheel, manual lever, motor operated, solenoid operated, pneumatic operated or hydraulic operated. In some designs the actuator is supported by the bonnet while in other designs a yoke mounted on the bonnet supports the actuators. Actuators are normally outside the pressure boundary of the valve.
- Valve packing – Most of the valves use some form of packing to prevent leakage from the space between the stem and the bonnet. Packing is commonly a fibrous material (e.g. flax) or a compound (e.g. Teflon) that form a seal between the internal parts of a valve and outside where the stem extends through the body. Valve packing is to be properly compressed to prevent fluid loss and damage to the valve’s stem.
Though all the valves have the same function of controlling the fluid flow and have the same basic components, the method of controlling the flow can differ. Normally the following four methods are used for controlling flow through the valve.
- Move a disk, or plug into or against an orifice as in globe or needle type of valves.
- Slide a flat, cylindrical, or spherical surface across an orifice as in a gate or plug valves.
- Rotate a disk or ellipse about a shaft extending across the diameter of an orifice as in a butterfly or ball valve.
- Make a flexible material into the flow passage as in diaphragm and pinch valves.
Classification of valves
- Classification as per function – By nature of their design and function in handling process fluids, valves can be classified into three categories namely (i) on-off valves which handle the function of blocking the flow or allowing it to pass, (ii) non return valves which only allow flow to travel in one direction, and (iii) throttling valves which allow for the regulation of the flow at any point between fully open to fully closed. In this type of classification of valves it is possible that specific valve body designs such as globe, gate, plug, and butterfly etc. can fit into one, two, or all three classifications.
- Classification as per application – Under this classification valves can be classified in three categories. These are (i) general service valves which have a versatile valve design that can be used in numerous applications without modification, (ii) special service valves which are specifically designed for a specific application, and (iii) severe service valves which are highly engineered to avoid the side effects of difficult applications.
- Classification as mechanical motion – Under this classification valves are classified in two categories. In the first category come linear motion valves which have a sliding stem design that pushes a closure element into an open or closed position. Gate, globe, pinch, diaphragm, split body, three way and angle valves fit into this classification. Linear valves are known for the simple design, easy maintenance, and versatility with more sizes, pressure class and design options. In the second category come rotary motion valves which use a closure element that rotates, through a quarter turn or 45 degree range to open or block the flow. Rotary valves are smaller in size.
- Classification as per port size – Under this classification valves are classified in two categories. Valves which are designed so that internal flow passageways are large enough to pass flow without a significant restriction are called full port valves. In these valves the internal flow is equal to the full area of the inlet port. Full port valves are used primarily with on-off services where the flow must be stopped or diverted. The second category valves are reduced port valves. In these valves closure elements restrict the flow. The restriction allows the valve to take a pressure drop as flow moves through the closure element, allowing a partial pressure recovery. Primary purpose of reduced port valves is to control the flow through throttling.