Valve

A valve in mechanical engineering is a device that regulates the flow of fluids, fluidised solids, slurries or liquid, by opening, closing, or partially obstructing various passageways. In electronic engineering the valve refers to an electronic devise that regulates the flow of current integrated with other devices. Here this article refers to mechanical valve only.

Cross-sectional diagram of an open globe valve.
1. body
2. ports
3. seat
4. stem
5. disc when valve is open
6. handle or handwheel when valve is open
7. bonnet
8. packing
9. gland nut
10. fluid flow when valve is open
11. position of disc if valve were shut
12. position of handle or handwheel if valve were shut

Application

A large variety of valves are used in a vast variety of applications. Sizes range from tiny to huge. The cost of valves ranges from very cheap simple disposable valves in some items to very expensive valves for specialized applications, such as HF acid valves. Often not realized by some, small valves are even inside some common household items including liquid or gel mini-pump dispenser spigots, spray devices, some rubber bulbs for pumping air, etc., manual air pumps and some other pumps, and laundry washers. Valves are almost as ubiquitous as electrical switches. Often a valve is part of some object, the valve body and the object made in one piece; for example, a separatory funnel. Faucets, taps, and spigots are all variations of valves. Many fluid systems such as water and natural gas lines in houses and other buildings have valves. Fluid systems in chemical and power plants and other facilities have numerous valves to control fluid flow.

Valve parts

Body

The majority of the valve consists of the valve body, including most of the exterior. The valve body is the vessel or casing that holds the fluid going through inside the valve. Valve bodies are most commonly made of various metals or plastics, although valve bodies fused with glass laboratory items in one piece are also made of glass.

Ports

The body has two or more openings, often called the ports, through which the fluid can enter or leave the valve. Valves with two ports are the most common. Valves with three ports are the next most common. Less often used are valves with four or more ports. Rheodyne makes rather small valves with six ports and Valco makes similar small valves with six or ten ports. Extra ports which are not needed can be closed off. In some cases, a valve port is immediately connected to a spray nozzle or container.

Connections to ports

Although valves are sometimes made connected or integral with some other object like a vessel, many valves are manufactured initially as a separate item to be connected afterwards to some other object such as a pipe, tube, or pump head on at least one of the valve ports either by a manufacturer or the user. In valves which are essentially faucets, taps, spigots, or sampling valves, one (or more) of the ports is often left unconnected or just has a short length of piping. Commonly valves are made with some means of connection at the ports. Means of connections at valve ports can include treads, compression fittings, gluing or cementing (especially for plastic), or welding (for metals). In fluid systems, valves are often connected to piping or tubing. A certain type of valve may come in various sizes. Valve sizes are commonly given by the outer diameter of tubing or pipng the ports connect to; for example, a 1-inch valve is sized to connect to 1-inch outer diameter tubing.

Discs and rotors

Inside the valve body, flow through the valve may be partly or fully blocked by an object called a disc. 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 can be called a rotor. Ball valves are valves which use spherical rotors, except for the interior fluid passageways. Plug valves use cylindrically-shaped or conically-tapered rotors called plugs. Other round shapes for rotors are possible too in rotor valves, as as long as the rotor can be turned inside the valve body. However not all round or spherical dics 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 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, particularly when the valve is shut (closed). 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.

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 the these. A valve with a rotor would be controlled by turning the stem. The valve and stem can be treaded such that the stem can 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 even though though they have two ports.

Valves in which the 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. Valves in which the 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. These terms do not apply to valves with no stem nor to valves using rotors.

Bonnet

A bonnet basically 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 were put into the body and then the bonnet was 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,

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, with maybe qualitative descriptions in between. Some valves are specially designed to regulate varying amounts of flow. Such valves have been called by various names like 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 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 desgnated 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.

3-way valves

3-way valves have three ports. 3-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.

In valves having more than 3 ports, even more flow-directing schemes are possible. For examples, see Selector Valves on Specialty Mfg. Co.. Such valves are often rotor valves or ball valves. Slider valves have been used also.

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 huge. 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 commode 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-tight. 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.

Different types

Main classifications can be:

Used by itself

1. Gate valve

2. Check or non return valve

3. Butterfly valve

4. Globe valve

5. Diaphragm valve(Shrader)

6. Pressure reducing valve

7. Safety valve

8. Foot valve

9. Vent cock or pet cock

10.Plug valve

Used as part of machinery

1. In fuel fired engines

2. In fluid compressors

3. Automobile tyre valve(Shrader)

4. Diaphragm valve(Saunders)

In houseold appliances

Pressure cookers, water heaters etc.

See also

Category:Plumbing valves, for modified valve types and details. [1]

Construction

Gate valve [2]

The gate valve has a gate with seats on both sides of the gate. These seats butt against the seats on the body tight to prevent the leak of fluids. A spindle operates the gate up and down by turning a wheel attached to the spindle. The gate is fixed flexibly to the spindle so that they align correctly and sit tight on the seats.

Check valve [3]

The check valve has an hinged swinging flap with one side seating to close against the seat in the body. The flow of fluid in one direction keeps the check valve swing open and any reverse flow immediately closes the flap preventing reverse flow of fluid.

Butterfly valve [4]

The butterfly valve has a circular flap covering the full bore of the pipe. This is rotated centrally across the diameter by a spindle, one end of which extends outside. It may have a visual indication outside. When the flap is across the pipe the valve is in closed position and when it is in line to the pipe length it is in open position.

Globe valve [5]

The globe valve has a vertical threaded spindle with a hand wheel on the top. At the inside end it has a floating valve which sits tight against a seat in the body preventing the fluid flow.

Diaphragm valve(Shrader)

This diaphragm valve is a special type of valve meant only for automobile or similar mobile machinery wheels.

Pressure reducing valve

The pressure reducing valve is for maintaining a set pressure at the outlet of valve. The spindle on the top has a rubber diaphragm in a closed chamber instead of a hand wheel. The top or bottom of the chamber is conneted to the outlet of valve to sense the outlet pressure to regulate the pressure at the outlet of valve. At the inside end of the spindle it has a valve which varies the opening between the valve and the valve seat in the body to maintain a constant pressure at the outlet of valve.

Safety valve.

See [6]

Foot valve

The foot valve is similar to a globe valve but has no spindle coming out of the body. This is generally mounted in vertical pipe line. It has inside spring loaded or is heavy enough to drop back closing the seat in the body. When the pump starts its suction created makes this valve open and allow fluid to flow. With pump stoppage the water head on the valve is sufficient to close it back to prevent the entrapped fluid.

Vent cock or pet cock.

The vent cock or pet cock is similar to a plug valve but much smaller in size. It is used on top of pump bodies and also in such equipment where the fluid has to be vented to see that no other fluid except the one to be handled occupies all the volume inside. This is generally made of brass.

Diaphragm valve(Saunders)

The diaphragm valve has vertical threaded spindle with a wheel on the top but with a rubber or similar flexible diaphragm attached at the other end, inside. The diaphragm is a flanged one and is embedded in between the suitable valve body flanges. This diaphragm being flexible, sits tight against the body seat when closed, to prevent the fluid flow. This is also used in pipe lines carrying corrosive liquids.

Used as part of machinery

In fuel fired engines

Generally tappet type valves are used in engines using Otto cycle. The are controlled by a cam shaft.

In fluid compressors

Generally reed type or channel type or puppet type valves are used. They are generally controlled by springs.

In automobiles

Generally automobile tyre valve(Shrader) is used for tubes and tyres. These are of special design to suit those places only.

In Water treatment plant units

Diaphragm valve(Saunders),in Water treatment plant units, in Thermal power stations.

In Household appliances

Pressure cookers have dead wieght safety valves. Water heaters have spring loaded safety valves. Similar other pressure appliances have safety valves of suitable design.

Please see safety valve [7]

Uses

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

In industries

The requirement of valve type at each location varies depending on the duty called for. Generally the use of valves are at locations indicated below.

Valves are used at the pump suction and discharge. Normally two or more pumps are connected to work in parallel.

The Gate valve is used at the outlet of contrifugal pump to minimise the starting load on its driving machine and also for isolating the discharge side from other pumps connected to work in parallel.

The check valve is also used at the outlet of centrifugal pump for quick auto closing in case of sudden pump stoppage to avoid reverse flow when the pumps are working in parallel.

The foot valve is used at the suction end of pipe line with negative suction locations.

Diaphgragm type valve is used for control of corrosive fluids.

Vent cock is used on top of pump bodies and such places where the fluid has to be vented to see that no other fluid except the one to be handled occupies all the volume inside.

Safety valves are used on top of equipment to open at a certain set pressure to see that the pressure in the equipment does not exceed the design limit.

In fuel fired engines

Generally puppet valve is used in engines using Otto cycle. The are controlled by a cam shaft.

In fluid compressors

Generally reed type or channel type or puppet type valve is used. These types are self actuating.

Design

In each category and location mentioned above the design and construction of each type of valve varies depending on the specific requirement at the place of use.

Design aspects

This involves the type of valve to be used and the material of its construction. These are:

The gate valves are normally in big sizes, ranging from 3 inches to 20 inches, for water pump outlets. These are made of Cast iron with valve seating made from brass and inserted.

The check valves are normally in big sizes, ranging from 3 inches to 20 inches, for water pump outlets. These are made of Cast iron with valve seating made from brass and inserted.

The globe valves are generally of small sizes up to say one inch dia. and made of bronze with hand wheel for manual operation. The needle valve can also be considered as a small size (say about half an inch) globe valve. The needle valve is generally used at the inlet to pressure gauges or control equipment to isolate the same from air supply for maintenance etc.

The diaphragm valves are mainly used in pipe lines handling corrosive fluids such as in demineralising plants, and corrosive liquid storage tank pipes. The diaphragm coming into contact with corrosive fluid is made of rubber or similar non corrosive flexible material.

The pressure reducing valve is used wherever the fluid pressure reduction is required such as in compressed air lines. The body parts are generally made from gun metal or bronze to avoid corrosion and to avoid corrosive products to go along with the fluid handled.

The safety valve-(to be included)

The foot valve is used at all pump suctions having negative suction. The foot valves are made of brass for smaller sizes say up to two inches and for bigger sizes it is made of cast iron. The seating surfaces for bigger sizes are generally brass.

Vent cock is used on top of pump bodies and on top of air cylinder to vent out any fluid from the top not forming the working fluid.

The plug valve is rarely used in industries. However they are used mostly by water supply agencies to control the supply to individual consumer. They are generally made up of high density PVC.