valve

valve

valve, device for controlling the flow of fluids (liquids and gases). Valves vary in construction and size depending upon their function. Some are classified according to their method of operation or design, e.g., butterfly, gate, globe, lift, needle, piston, and slide valves. Valves are also named for the functions they perform, e.g., check valve (which permits flow in one direction only) and cutoff, bypass, exhaust, intake, safety (see safety valve), and throttle valves. Valves are operated automatically, by hand, or by special mechanism. Valves are employed in the carburetor, diesel engine, internal-combustion engine, pump, and steam engine. In Great Britain an electron tube may be referred to as a valve. In anatomy and physiology the term valve includes the flaps of tissue that help to control the direction of the flow of blood in the heart.

A globe valve controls the flow of a fluid through a pipe, inlet, or outlet. To stop the flow elipsis

In mechanical engineering, a device for controlling the flow of fluids (liquids, gases, slurries) in a pipe or other enclosure. It exerts control by means of a movable element that opens, shuts, or partially blocks an opening in a passageway. Valves are of seven main types: globe, gate, needle, plug (cock), butterfly, poppet, and spool. Some valves operate automatically; check (or nonreturn) valves, for example, are self-acting valves that permit flow in only one direction. Safety valves open at a predetermined pressure; the movable element usually has a weighted lever or a spring strong enough to hold the valve closed until a particular pressure is reached.

Learn more about valve with a free trial on Britannica.com.

:For other uses, see Valve (disambiguation). For the electronic component, see Thermionic valve. For the game development company see Valve Corporation.

A valve is a device that regulates the flow of materials (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but usually are discussed separately.

Valves are used in a variety of applications including industrial, military, commercial, residential, and transportation.

Oil and gas, power generation, mining, water reticulation, sewerage and chemical manufacturing would constitute the majority of valves used by industry.

Plumbing valves are the most obvious in everyday life, taps for hot and cold water, gas control valves on cookers and barbecues, small valves fitted to washing machines and dishwashers, and safety devices fitted to hot water systems are all simple valves.

Most valves are operated manually, either by a hand wheel or lever. Some valves are 'self-operating' driven by changes in pressure, temperature or flow. These changes in pressure or temperature often act against springs or capsules filled with expanding substances. Examples of this type of valve found commonly are safety valves fitted to hot water systems or steam boilers.

More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an actuator. An actuator will stroke the valve depending on its input and set-up, allowing the valve to be positioned accurately, and allowing fine control over a variety of requirements.

Valves are also found in the Otto cycle (internal combustion) engines driven by a camshaft, lifters and or push rods where they play a major role in engine cycle control.

Applications

A huge variety of valves are available and have infinite applications with sizes ranging from 1/4" (6 mm) to 24" (600 mm). Special valves can be manufactured to be in excess of 200" (5000 mm) diameter.

Valves range from inexpensive, simple, disposable valves to items in exotic materials that in some instances cost thousands of dollars (US$) per inch (25 mm) of diameter. These items would only be used on specialized extreme applications.

Disposable valves are inside some common household items including liquid or gel mini-pump dispensers, aerosol spray devices, some rubber bulbs for pumping air etc.

Types/Designations

Valves can be categorized into the following design types and although there are hundreds of variations they all fit into these basic types:

  • Gate
  • Plug
  • Globe
  • Check
  • Butterfly
  • Diaphragm
  • Ball
  • SOLENOID
  • NEEDLE
  • HYDRAULIC

Also the valves can be classified as

  • Conventional Valve
  • Severe Service Valve

Valve parts

Body and Bonnet

The main part of the valve consists of the valve body and bonnet. These two parts form the vessel or casing that holds the fluid going through the valve. Valve bodies are usually metallic. Brass, bronze, cast or ductile iron, steel, alloy steels and stainless steels are very common. Plastic bodies are used for relatively low pressures and temperatures. PVC, PP, PVDF and glass-reinforced nylon are common plastics used for valve bodies

Ports

Integral to the valve body are the passages that allow flow into and out of the valve. These are called ports. These ports are obstructed or opened up by the valve member or disc to control the fluid flow. Valves with two or three ports are the most common, while valves with multiple ports (up to 20) are used in special applications. Nearly all valves are built with some means of connection at the ports. These include Threads (male or female); BSP or NPT are most common. Compression fittings, to suit tube s/s or copper. Glue or cement application (especially for plastic) almost always a socket type connection (not a butt) Flanges ANSI, BS, DIN, or JIS. (US, British, European, Japanese standards) Welding either Socket type or Butt type welds.

Disc / Rotor / Valve Member

Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc or valve member. Although valve discs of some kinds of valves are traditionally disc-shaped, discs can come in various shapes. Although the valve body remains stationary within the fluid system, the disc in the valve is movable so it can control flow. A round type of disc with fluid pathway(s) inside which can be rotated to direct flow between certain ports is usually called a ball. Ball valves are valves which use spherical rotors, except for the interior fluid passageways. Plug valves use cylindrical or conically tapered rotors called plugs. Other round shapes for rotors are possible too in rotor valves, as long as the rotor can be turned inside the valve body. However not all round or spherical discs are rotors; for example, a ball check valve uses the ball to block reverse flow, but is not a rotor because operating the valve does not involve rotation of the ball.

Seat

The valve seat is the interior surface in the body which contacts or could contact the disc to form a seal which should be leak-tight when the valve is shut. If the disc moves linearly as the valve is controlled, the disc comes into contact with the seat when the valve is shut. When the valve has a rotor, the seat is always in contact with the rotor, but the surface area of contact on the rotor changes as the rotor is turned. If the disc swings on a hinge, as in a swing check valve, it contacts the seat to shut the valve and stop flow. In all the above cases, the seat remains stationary while the disc or rotor moves. The body and the seat could both come in one piece of solid material, or the seat could be a separate piece attached or fixed to the inside of the valve body, depending on the valve design.

Seats can be integral to the valve body, that is "hard" metal or plastic. Nearly all metal seated valves leak, even though some leaks are extremely small.

"Soft" seats can be fitted to the valve body and made of materials such as PTFE or various elastomers such as NBR, EPDM, FKM. Each of these soft materials is limited by temperature (rough maximum temperatures are listed below)

NBR 80 °C

EPDM 120 °C

FKM 170 °C

PTFE 200 °C

The advantage of soft seats is that they are more likely to offer 100% tight shutoff when valve is closed.

There are advantages of Hard seated Valves as well in applications where there is heavy erosion due to the material flowing from the pipes then the metal seated valves are preferred over soft seated valves.

Metal seated Valves have longer life as well.

Gate Valves, Globe Valves, Check Valves are usually hard seated Valves and Butterfly Valves, Ball Valves, Plug Valves, Diapharagm Valves are Usually soft seated Valves.

Though there are some special cases where we do have hard seated Butterfly Valves and Hard seated Ball Valves as well.

Stem

The stem is a rod or similar piece spanning the inside and the outside of the valve, transmitting motion to control the internal disc or rotor from outside the valve. Inside the valve, the rod is joined to or contacts the disc/rotor. Outside the valve the stem is attached to a handle or another controlling device. Between inside and outside, the stem typically goes through a valve bonnet if there is one. In some cases, the stem and the disc can be combined in one piece, or the stem and the handle are combined in one piece.

The motion transmitted by the stem can be a linear push or pull motion, a rotating motion, or some combination of these. A valve with a rotor would be controlled by turning the stem. The valve and stem can be threaded such that the stem can be screwed into or out of the valve by turning it in one direction or the other, thus moving the disc back or forth inside the body. Packing is often used between the stem and the bonnet to seal fluid inside the valve in spite of turning of the stem. Some valves have no external control and do not need a stem; for example, most check valves. Check valves are valves which allow flow in one direction, but block flow in the opposite direction. Some refer to them as one-way valves.

Valves whose disc is between the seat and the stem and where the stem moves in a direction into the valve to shut it are normally-seated (also called 'front seated'). Valves whose seat is between the disc and the stem and where the stem moves in a direction out of the valve to shut it are reverse-seated (also called 'back seated'). These terms do not apply to valves with no stem nor to valves using rotors.

Bonnet

A bonnet acts as a cover on the valve body. It is commonly semi-permanently screwed into the valve body. During manufacture of the valve, the internal parts are put into the body and then the bonnet is attached to hold everything together inside. To access internal parts of a valve, a user would take off the bonnet, usually for maintenance. Many valves do not have bonnets; for example, plug valves usually do not have bonnets.

Spring

Many valves have a spring for spring-loading, to normally shift the disc into some position by default but allow control to reposition the disc. Relief valves commonly use a spring to keep the valve shut, but allow excessive pressure to force the valve open against the spring-loading. Typical spring materials include carbon steel (often cad plated), 304 Series stainless steels and for high temperature applications Inconel X750. Springs can be typical 'coil' types or 'bellville" washer stacks or even bimetallic elements which exert a spring force on temperature change.

Valve balls

A valve ball is also used for severe duty, high-pressure, high-tolerance applications. They are typically made of stainless steel, titanium, Stellite, Hastelloy, brass, or nickel. They can also be made of different types of plastic, such as ABS, PVC, PP or PVDF.

Valve operating positions

Valve positions are operating conditions determined by the position the disc or rotor in the valve. Some valves are made to be operated in a gradual change between two or more positions.

2-way valves

2-port valves are commonly called 2-way valves. Operating positions for such valves can be either shut (closed) so that no flow at all goes through, fully open for maximum flow, or sometimes partially open to any degree in between. Many valves are not designed to precisely control intermediate degree of flow; such valves are considered to be either open or shut, which maybe qualitative descriptions in between. Some valves are specially designed to regulate varying amounts of flow. Such valves have been called by various names such as regulating, throttling, metering, or needle valves. For example, needle valves have elongated conically-tapered discs and matching seats for fine flow control. For some valves, there may be a mechanism to indicate by how much the valve is open, but in many cases other indications of flow rate are used, such as separate flow meters.

In some plants with fluid systems, some 2-way valves can be designated as normally shut or normally open during regular operation. Examples of normally shut valves are sampling valves, which are only opened while a sample is taken. Examples of normally open valves are isolation valves, which are usually only shut when there is a problem with a unit or a section of a fluid system such as a leak. Then, isolation valve(s) are shut in order to isolate the problem from the rest of the system.

Although many 2-way valves are made in which the flow can go in either direction between the two ports, when a valve is placed into a certain application, flow is often expected to go from one certain port on the upstream side of the valve, to the other port on the downstream side. Pressure regulators are variations of valves in which flow is controlled to produce a certain downstream pressure, if possible. They are often used to control flow of gas from a gas cylinder. A back-pressure regulator is a variation of a valve in which flow is controlled to maintain a certain upstream pressure, if possible.

Three-way valves

Three-way valves have three ports. Three-way valves are commonly made such that flow coming in at one port can be directed to either the second port in one position or the third port in another position or in an intermediate position so all flow is stopped. Often such 3-way valves are ball or rotor valves. Many faucets are made so that incoming cold and hot water can be regulated in varying degrees to give outcoming water at a desired temperature. Other kinds of 3-port valves can be designed for other possible flow-directing schemes and positions; for example, see Ball valve.

The "motor valve" on a domestic heating system is an example of a 3-way valve. Depending on demand the motor head rotates the spindle to control the proportion of the flow that goes to the two outlet pipes: One to radiators, one to hot water system. In a conventional system the valve usually sits just after the pump and by the cylinder

In valves having more than 3 ports, even more flow-directing schemes are possible. Such valves are often rotor valves or ball valves.

Control

Many valves are controlled manually with a handle attached to the valve stem. If the handle is turned a quarter of a full turn (90°) between operating positions, the valve is called a quarter-turn valve. Butterfly valves, ball valves, and plug valves are often quarter-turn valves. Valves can also be controlled by devices called actuators attached to the stem. They can be electromechanical actuators such as an electric motor or solenoid, pneumatic actuators which are controlled by air pressure, or hydraulic actuators which are controlled by the pressure of a liquid such as oil or water. Actuators can be used for the purposes of automatic control such as in washing machine cycles, remote control such as the use of a centralized control room, or because manual control is too difficult; for example, the valve is large. Pneumatic actuators and hydraulic actuators need pressurized air or liquid lines to supply the actuator: an inlet line and an outlet line. Pilot valves are valves which are used to control other valves. Pilot valves in the actuator lines control the supply of air or liquid going to the actuators.

The fill valve in a toilet water tank is a liquid level-actuated valve. When a high water level is reached, a mechanism shuts the valve which fills the tank.

In some valve designs, the pressure of the flow fluid itself or pressure difference of the flow fluid between the ports automatically controls flow through the valve. In an open valve, fluid flows in a direction from higher pressure to lower pressure.

Other considerations

Valves are typically rated for maximum temperature and pressure by the manufacturer. The wetted materials in a valve are usually identified also. Some valves rated at very high pressures are available. When a designer, engineer, or user decides to use a valve for an application, he/she should ensure the rated maximum temperature and pressure are never exceeded and that the wetted materials are compatible with the fluid the valve interior is exposed to.

Some fluid system designs, especially in chemical or power plants, are schematically represented in piping and instrumentation diagrams. In such diagrams, different types of valves are represented by certain symbols.

Valves in good condition should be leak-free. However, valves may eventually wear out from use and develop a leak, either between the inside and outside of the valve or, when the valve is shut to stop flow, between the disc and the seat. A particle trapped between the seat and disc could also cause such leakage.

Types of valves

Generic Type Valves

  • Ball valve, for on/off control without pressure drop.
  • Butterfly valve, for flow regulation in large pipe diameters.
  • Choke valve: a valve that lifts up and down a solid cylinder which is placed around or inside another cylinder which has holes or slots. Used for high pressure drops found in oil and gas wellheads.
  • Check valve or Non-return valve, allows the fluid to pass in one direction only.
  • Diaphragm valve, some are sanitary predominantly used in the pharmaceutical and foodstuff industry.
  • Expansion valve, used pressure reduction of fluid in general. Type of restrictor valve to reduce refrigerant pressure and temperature in short period of time in adiabatic expansion.
  • Gate valve, mainly for on/off control, with low pressure drop.
  • Globe valve, good for regulating flow.
  • Knife valve, for slurries or powders on/off control.
  • Needle valve for accurate flow control.
  • Piston valve, for regulating fluids that carry solids in suspension.
  • Pinch valve, for slurry flow regulation.
  • Plug valve, slim valve for on/off control but with some pressure drop.

Specific Valve Types

Images

See also

External links

Search another word or see valveon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature