What are Valves?
A valve is a device for controlling the passage of fluid or air through a pipe, duct, etc, by opening, closing, or partially obstructing various passageways. The simplest valves are one-way-flap mechanisms that allow flow in one direction and shut with back-flow pressure to stop flow in the direction of origin.
Valves are technically fittings but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
The simplest, and very ancient, valve is simply a freely hinged flap that swings down to obstruct fluid (gas or liquid) flow in one direction but is pushed up by the flow itself when the flow is moving in the opposite direction. This is called a check valve, as it prevents or “checks” the flow in one direction.
Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems. Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home.
Even aerosol spray cans have tiny valves built-in. Valves are also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, valves are instead called dampers. In compressed air systems, however, valves are used with the most common type being ball valves.
What is the function of a valve?
A valve is a device or natural object that regulates, directs, or controls the flow of a fluid by opening, closing, or partially obstructing various passageways.
Valves provide several functions, including:
- Starting or stopping flow based on the valve state
- Regulating flow and pressure within a piping system
- Controlling the direction of flow within a piping system
- Throttling flow rates within a piping system
- Improving safety through relieving pressure or vacuum in a piping system
Parts of Valves
All valves have the following basic parts: the body, bonnet, trim (internal elements), actuator, and packing. The basic parts of valves are:
1. Valve Body
The valve body, sometimes called the shell, is the primary boundary of a pressure valve. He serves as the main element of a valve assembly because it is the framework that holds all the parts together.
The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints.
Valve bodies are cast or forged in a variety of forms and each component has a specific function and is constructed in a material suitable for that function.
2. Valve Bonnet
The cover for the opening in the body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are in many designs and models available.
A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is commonly connected to the body by a threaded, bolted, or welded joint. During the manufacture of the valve, the internal components, such as stem, disk, etc., are put into the body and then the bonnet is attached to hold all parts together inside.
In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts. Valve bonnets, although a necessity for most valves represent a cause for concern. Bonnets can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost, and are a source for potential leakage.
3. Valve Trim
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 do 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.
4. Disk and 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.
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 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 necessary 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.
5. Valve Stem
The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in the plug, Ball, and Butterfly valves, the disc is rotated to open or close the valve.
Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.
There are five types of valve stems:
- Rising Stem with Outside Screw and Yoke. The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are isolated from the flow medium by the stem packing. Two different styles of these designs are available; one with the handwheel attached to the stem, so they can rise together, and the other with a threaded sleeve that causes the stem to rise through the handwheel. This type of valve is indicated by “O. S. and Y.” is a common design for NPS 2 and larger valves.
- Rising Stem with Inside Screw. The threaded part of the stem is inside the valve body, and the stem packing along the smooth section that is exposed to the atmosphere outside. In this case, the stem threads are in contact with the flow medium. When rotated, the stem and the handwheel to rise together to open the valve.
- Non-Rising Stem with Inside Screw. The threaded part of the stem is inside the valve and does not rise. The valve disc travels along the stem, like a nut if the stem is rotated. Stem threads are exposed to the flow medium, and as such, are subjected to the impact. That is why this model is used when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem material.
- Sliding Stem. This valve stem does not rotate or turn. It slides in and out the valve to open or close the valve. This design is used in hand-operated lever rapid opening valves. It is also used in control valves are operated by hydraulic or pneumatic cylinders.
- Rotary Stem. This is a commonly used model in ball, plug, and Butterfly valves. A quarter-turn motion of the stem open or close the valve.
6. Valve Packing
Most valves use some form of packing to prevent leakage from the space between the stem and the bonnet.
Packing is commonly 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 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.
7. Valve Yoke and Yoke Nut
- Yoke. A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the Yoke holding a Yoke nut, stem nut, or Yoke bushing and the valve stem passes through it. A Yoke usually has openings to allow access to the stuffing box, actuator links, etc. Structurally, a Yoke must be strong enough to withstand forces, moments, and torque developed by the actuator.
- Yoke Nut. A Yoke nut is an internally threaded nut and is placed in the top of a Yoke by which the stem passes. In a Gate valve e.g., the Yoke nut is turned and the stem travels up or down. In the case of Globe valves, the nut is fixed and the stem is rotated through it.
8. Valve Actuator
Hand-operated valves are usually equipped with a handwheel attached to the valve’s stem or Yoke nut which is rotated clockwise or counterclockwise to close or open a valve. Globe and gate valves are opened and closed in this way.
Hand-operated, quarter-turn valves, such as Ball, Plug, or Butterfly, has a lever to actuate the valve. There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include:
- Large valves that must be operated against high hydrostatic pressure
- Valves they must be operated from a remote location
- When the time for opening, closing, throttle or manually controlling the valve is longer, than required by system-design criteria
These valves are usually equipped with an actuator. An actuator in the broadest definition is a device that produces linear and rotary motion of a source of power under the action of a source of control.
Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There a many different types of actuators, but the following are some of the commonly used valve actuators:
- Gear Actuators
- Electric Motor Actuators
- Pneumatic Actuators
- Hydraulic Actuators
- Solenoid Actuators
Valve Opening Methods Explained
While many valves accomplish similar goals, how they do so mechanically can vary. How a valve opens and closes will not only impact the overall performance but also determine how much control you have over the flow and how quickly the valve can operate.
Most valves fit into one of three categories:
- Multi-Turn valves: Think of these valves like a screw or piston. You crank the handle and the plug, plate, membrane, or other controlling obstruction moves into the path of the pipe blocking access. Depending on the valve, these can have higher or lower differentials allowing you to open or close them at various speeds.
- Quarter turn valves: Quarter-turn valves offer a full range of motion in a 90-degree turn of the handle. This makes them ideal for situations where precision isn’t as important as rapid action and easy opening or closing.
On top of the mechanical motion involved with a valve, also consider the method of actuation. In most cases, valves fall into one of three categories:
- Manual Valves: Typically adjusted by hand, these valves use handwheels, hand levels, gear wheels, or chains to actuate.
- Actuated Valves: Often connected to electric motors, air or pneumatic systems, hydraulic systems, or solenoids, these valves allow remote control and automation for high-precision or large-scale applications.
- Automatic Valves: Some valves activate when a specific flow condition is met. Examples include check valves closing during backflow or pressure release valves activating when an over-pressure condition is detected.
Types of Valves
Valves feature a range of characteristics, standards, and groupings the help to give you an idea of their intended applications and expected performance. Valve designs are one of the most basic ways to sort the huge range of valves available and find a good fit for a project or process.
Common types of valves include:
You might also see valves classified by function instead of design.
Common functional designations and their common design types include:
- Isolation Valves: Ball, butterfly, diaphragm, gate, pinch, piston, and plug valves
- Regulation Valves: Ball, butterfly, diaphragm, globe, needle, pinch, and plug valves
- Safety Relief Valves: Pressure release and vacuum relief valves
- Non-Return Valves: Swing check and lift check valves
- Special Purpose Valves: Multi-port, float, foot, knife gate, and line blind valves
Classification of Valves
The following are some of the commonly used valve classifications, based on mechanical motion:
- Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm, pinch, and lift Check Valves, moves in a straight line to allow, stop, or throttle the flow.
- Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, as in butterfly, ball, plug, eccentric- and Swing Check Valves, the valves are called rotary motion valves.
- Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through 90°, motion of the stem to go to fully open from a fully closed position or vice versa.
Classification of Valves based on Motion:
|Valve Types||Linear Motion||Rotary Motion||Quarter Turn|
The Most Common Control Valve Symbols
The control valve symbols on a P&ID differ depending on the type of valve specified for the application. Each P&ID has its own legend that identifies the symbols for the various equipment.
While there is some variation, examples of the standard symbols for control valves are shown below.
- Gate Valve Symbol
- Globe Valve Symbol
- Ball Valve Symbol
- Plug Valve Symbol
- Butterfly Valve Symbol
- Diaphragm Valve Symbol
- Check Valve Symbol
Valve Sizing Explained: Keeping Things Flowing
While valves might be a small part of your piping process or system in terms of space, they’re often a substantial portion of the design and build budget. They also have a significant impact on long-term costs and overall system performance.
Choosing proper valves size is essential to both optimizing costs and ensuring safe, accurate, and reliable operation. The first thing to consider is the overall size of the valve — both in terms of physical dimensions and in terms of internal size and flow rates (CV).
Choosing a valve that does not fit properly in the space required could result in added costs. Choosing valves that do not provide the ideal flow rate can lead to inaccurate flow control at the least and complete system failure at worst.
For example, if your valve is too small, it could cause reduced flow downstream while creating back-pressure upstream. If the valve is too large, you’ll find that flow control is drastically reduced the further you move from fully open or fully closed.
When choosing the proper size, be sure to consider both the connector diameter and the overall flow rate of the valves compared to your needs. Some valves offer excellent flow while others constrict flow and increase pressure.
This means sometimes you must install larger valves to adjust for flow than the adapter diameter alone might imply.
Valve End Connections: The Key to A Good Fit and Proper Operation
With sizing and design out of the way, it’s also important to consider valve end connections.
While the most obvious implication here is choosing an end connection compatible with your piping, there are also functional characteristics to common end types that might make one valve more suited to your needs than another.
Common valves connections and ends include:
- Screwed or Threaded: Often used in instrument connections or sample points
- Flanged: The most common ends for piping use
- Butt Welded: Typically used in high-pressure or high-temperature operations
- Socket Welded: Commonly used on small bore piping where threaded connections are not permitted
- Wafer and Lug: Often used for compact valves installed in systems with limited space
Application of Valve
Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dish and clothes washers and taps in the home.
Valves are found in virtually every industrial process, including water and sewage processing, mining, power generation, processing of oil, gas, and petroleum, food manufacturing, chemical, and plastic manufacturing, and many other fields.
People in developed nations use valves in their daily lives, including plumbing valves, such as taps for tap water, gas control valves on cookers, small valves fitted to washing machines and dishwashers, safety devices fitted to hot water systems, and poppet valves in car engines.
In nature, there are valves, for example, one-way valves in veins controlling the blood circulation, and heart valves controlling the flow of blood in the chambers of the heart and maintaining the correct pumping action.
Valves may be operated manually, either by a handle, lever, pedal, or wheel. Valves may also be automatic, driven by changes in pressure, temperature, or flow. These changes may act upon a diaphragm or a piston which in turn activates the valve, examples of this type of valve found commonly are safety valves fitted to hot water systems or boilers.