A valve is one device that directs, regulates or controls the flow of the fluid by opening, closing, or partially obstructing various passage-ways. Valves are technically kinds of fittings, but, are usually discussed as a separate category. In an open valve, fluids flow in a direction from higher pressure to lower.
Valves have many uses, which includes controlling water for irrigation, industrial uses of controlling processes, residential uses such as on/off and pressure control to clothe and dish washers and taps at homes. Even, aerosol has a tiny valve built in it. Valves are also used in military and transportation 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 ball valves.
Valves vary widely in application and form. Their sizes are typically in the range from 0.1mm -60cm. Also, special valves can have a diameter exceeding 5 meters or more.
The most common use of the term valve refers to the poppet valves which found in the vast majority of the modern internal combustion engines, such as, those in most fossil fuel powered vehicles.
Basic Types of Valves by Operating Principles
• Ball valve, on/off flow control without pressure drop
• Butterfly valve, on/off control in large diameter pipes
• Choke valve, raises or lowers a solid cylinder placed around / inside a second cylinder with holes or slots. Used for high pressure drops found in oil and gas well – heads
• Diaphragm, controls flow by movement of a diaphragm
• Gate valve, mainly for on/off control with low pressure drop
• Globe valves, good for regulating flow. Uses a cylinder movement over a seat
• Knife valve, similar to gate valves, but more compact. Used for slurries or powders on/off control
• Needle valve, for accurate lineer motion control
• Pinch valve, for slurry flow regulation and control
• Piston valve, for regulating fluids that carry solids in suspension
• Plug valve, slim valve for on/off control but with pressure drop
• Solenoid valve, an electrically actuated valve for hydraulic or pneumatic fluid control
• Spool Valve, pretty similar to the choke valve
Butterfly valve is a kind of valve that regulates or isolates the flow of a fluid. A disk is the closing mechanism that rotates.
Principle of operation
Operation is similar to a ball valve, which allows for a quick shut off. Butterfly valves are usually favored because they cost less than other type of valve, and they are light weight so they need less support. The disc in butterfy valves positioned the disc in the center of a pipe. A rod passes through this disc to an actuator on the outside of the butterfly valve. Rotating the actuator turns the disc either parallel or horizontal to the flow. Unlike a ball valve, the disc is always present within the flow, so it causes to a pressure drop, even when it is open.
A butterfly valve is from the family of valves called quarter turn valves. In operation, the valve is fully open or shutted when the disc is rotated a quarter turn. The butterfly is a metal disc mounted on a rod. When the valve is shutted, the disc is turned vertically so that it completely blocks off the passage – way. When the actuator is fully open, the disc is rotated a quarter turn so it allows a barely unrestricted passage to the fluid.
There are many kinds of butterfly valves, each of them are adapted for different pressures and different usage. The zero – offset butterfly valve, which uses flexibility of rubber, has the lowest pressure rating. The high performance butterfly valves have double offset valve, used in slightly higher pressure systems, is offset from the centre line of the disc seat and valve body seal, and the center line of the bore. The valve best suited for high – pressure systems is the triple offset butterfly valve. In this valve, the disc seat contact axis is offset, which acts virtually eliminates sliding contact between disc and seat. In the case of triple offset valve the seat is made of metal so that it can be machined such for a bubble tight shut – off when in contact with the disc.
1. Concentric butterfly valves: this resilient seated butterfly valves have resilient rubber seats with a metal disc in resilient seated valves.
2. Doubly – eccentric butterfly valves: different type of materials are used for seat and disc.
3. Triply – eccentric butterfly valves: the seats are either laminated or solid metal seats.
Wafer – style butterfly valve
Wafer style butterfly valves are the valve designns to maintain a seal against the bi – directional pressure differential to prevent it from any backflow in systems designed for undirectional flow. It accomplishes that with a tightly fitting seal. Gasket, o-ring, precision machined, and a flat valve face on the upstream and downstream sides of the valve.
Lug – style butterfly valve
Lug style valve has threaded inserts at both sides of the valve’s body. That allows them to be installed into a system using two sets of bolts with no nuts. The valve is installed between two pipe flanges by using a separate set of bolts for each flange. That setup permits any side of the piping system to be disconnected without disturbing other side.
A lug style butterfly valve is used in dead-end service generally has a reduced pressure ratings. For example, a lug style butterfly valve which is mounted between two pipe flanges has a 1MPa pressure rating. The very same valve mounted with one flange, in dead-end service, has a 520 kPa rating. Lugged valves are highly resistant to chemicals and solvents. Also, they can handle temperatures up to 200 °C.
Rotary valve constitutes a derivation of the general butterfly valve and is usually used in powder processing industries. Instead of being flat, the rotary butterfly is equipped with pockets. When closed position, it acts just like a butterfly valve and it is tight. But when it is open, the pockets allow dropping a defined amount of solids, which makes this valve suitable for dosing bulk product by gravity. This kind of valves are usually small sized, pneumatically activated and usually rotate 180 degrees back and forth.
Use in industry
In the chemical, pharmaceutical, and food industries, butterfly valves are used to interrupt product flows in the process. The valves used in these industries are manufactured according to cGMP guidelines. Butterfly valves usually replaced ball valves in many industries, petroleum, due to lower cost and easy installation, but, pipelines containing butterfly valves cannot be pigged to clean.
Many valves are controlled by hand with a handle attached to the its stem. If the handle is turned 90 degrees, the valve is called a quarter turn valve. Butterfly, ball valves, and plug valves are often quarter turn valves. If the handle is a full circul with the stem as the axis of rotation in the cente, then the handle is called a hand – wheel. Valves can also be controlled by actuators. They can be electro – mechanical actuators such as an electric motors or solenoids, pneumatic actuators, or hydraulic actuators. Actuators can be used for the purposes of automatic controls such as in washing – machine cycles, remote controls such as the use of a centralised control room, or because of manual control is too difficult for example when the valve is very large. 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.