A valve is a mechanical device used in fluid systems to control, direct, isolate, mix, or regulate the flow or pressure of a fluid, by modifying the passageway through the pipe. When a valve is in the open position, the passageway is unobstructed and the fluid flows in a direction from higher pressure to lower pressure. By partially or fully obstructing the passageway through the valve, the flow can be reduced or completely stopped.
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Technically, valves are pipe fittings but due to their critical function, wide variety of types, and operation options they are generally considered separately from other fittings. Valves are often the costliest components of the piping system in a plant; the cost of valves can be as much as 30% of the overall piping cost.
At the design stage of a manufacturing process or a piping system, engineers generally specify the type of valve suitable for the desired function on the Process and Instrumentation Diagram (P&ID). A P&ID is a schematic illustration of the functional relationship of piping, instrumentation and system components. Different types of valves commonly found on P&IDs are introduced and discusses in the following section.
Gate valves are inexpensive and easy to disassemble for maintenance. They offer low fluid resistance due to a straight path that allows the fluid to flow straight through without change in direction.
During use, gate valves are normally kept in either a fully open or a fully closed position as they are not recommended for throttling applications. The valve design makes it difficult to control the flow.
Furthermore, the fluid slapping against a partially open gate can damage the valve. The multi-turn design of these valves makes them unsuitable for applications where rapid operation is desired.
A globe valve generally has a spherical body that contains a movable plug element and a stationary ring seat. The two halves of the body are separated by an internal baffle. The plug element is connected to an operating hand wheel through a stem. As the wheel is turned, the screw action moves the plug towards or away from the ring seat. Globe valves can be used for stop/start function as well as to regulate fluid flow. They are useful in applications where good flow control is desired and leak tightness is important such as cooling water systems, feedwater and chemical feed systems, and fuel/lubricating oil systems.
Globe valves cost more than gate valves but provide better shut off and throttling control. The design of these valves forces the fluid to change direction as it flows through; this creates high pressure loss and turbulence. Globe valves are generally heavier than other valves. They are not suitable for fluids containing particles, high viscosity fluids and other mediums that are prone to choking.
Ball valves are smaller, lighter and cheaper when compared to gate valves of the same size and rating. They allow visual detection of the open or closed state of the valve. In an open state, the handle is aligned with the direction of flow; in a closed state the handle is perpendicular to the flow direction. In addition, ball valves offer ease of operation, high volume flow, high pressure, long service life and possibility of repair of seats and seals without removing the valve body from the line. On the other hand, ball valves are difficult to clean which can lead to contamination. They are not suitable for applications that require continuous throttling.
A check valve is used to prevent backflow in piping systems. The valve opens due to the pressure of the fluid passing through the pipeline. Reverse flow closes the valve and prevents the fluid from moving in the opposite direction. Check valves do not require external control. They work automatically and hence do not have an operating handle or stem. The simplest check valves are designed with a one-way flap mechanism.
Plug valves use a cylindrical or conically tapered plug to allow or restrict fluid flow. The plug has one or more hollow passageways that allow the fluid to pass through when the passage is aligned with the flow direction. A quarter-turn rotary motion is required to turn the plug and block the fluid flow. Plug valves perform well in slurry applications. The wiping action of the plug does not allow suspended particles to accumulate and form an obstruction. Typical applications of plug valves include gaseous, and vapor services, natural gas piping systems, oil piping systems, coal, mineral and sewage applications. They are also suitable for vacuum and high-pressure applications.
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Plug valves offer a long service life and high reliability which makes them suitable for corrosive, abrasive, and toxic materials. They are usually used as on-off stop valves to provide quick bubble-tight shutoff. The valves can be repaired and cleaned without removal of the body from the piping system. Plug valves are not recommended for throttling applications and applications that require high frequency switching. Compared to ball valves, they cost more and require greater force to actuate, due to higher friction.
The “butterfly” refers to a rotatable metal disc mounted on a rod. A quarter turn of the disc is required to open or shut-off the valve. In the closed position, the disk completely blocks off the passageway. In the open position, the face of the disc is parallel to the flow direction and allows nearly unrestricted fluid flow. Butterfly valves find applications in cooling water systems, compressed air or gas applications, fire protection, slurry services, vacuum service, and high-pressure / high-temperature water and steam services.
Butterfly valves have a short circular body which reduces space requirement and makes them light weight and easier to install. The compact design also makes them suitable for large valve applications. Butterfly valves have a high coefficient of flow. They offer good sealing even at low pressure. On the other hand, butterfly valves can be difficult to clean, and the throttling function is limited to low differential pressure and unguided disc movement can be affected by flow turbulence.
Needle valves are similar in design to globe valves. The plugging element in these valves is a long, tapered, needle-like plunger which fits into a matching seat to stop the flow. A stem is turned to insert or retracted the plunger, opening or restricting the flow path. The finely-threaded stem requires many turns to fully retract the plunger; this feature allows precise regulation of the flow rate. Needle valves are typically used in small diameter piping systems where very fine control of flow is desired. Needle valves are generally suitable for systems with low viscosity fluids flowing at low flow rates. The most common application is to regulate flow to fragile gauges in order to protect them from damage caused by an abrupt pressure surge. They are also used as bleed valves to relieve pressure or to drain off the fluid for analysis or for maintenance.
Pinch valves can be used to start, stop, and regulate fluid flow. Since these valves have fewer components, they are relatively cheaper and more leak-proof as compared to other valves. These valves are ideal for pneumatic conveying of solid materials and for handling of slurries. Pinch valves minimize turbulence and friction during flow. They are, however, not suitable for gases and for high-temperature or high-pressure applications. A high pressure-differential or vacuum in the system can cause the tubing to collapse or deform resulting in incomplete opening of the valve.
A diaphragm valve, also known as a membrane valve, consists of an elastomeric diaphragm, and a saddle/seat upon which the diaphragm closes. A linear compressor is used to push the thin, flexible diaphragm into contact with the seat/saddle to close the valve. Diaphragm valves also allow partial closure of the passageway making them suitable for throttling applications. They are suitable for viscous fluids and fluids containing solid materials as the solids are not trapped in the valve mechanism. Diaphragm valves are commonly used in water treatment facilities, pharmaceutical, food and chemical plants, vacuum services and corrosive applications.
There are several different ways in which valves can be classified. They can be grouped based on their function, type of operating mechanism, end connections, mechanical motion, construction material, pressure and temperature ratings and port size. When choosing a valve, it is important to consider each of these classifications to ensure that the valve is suitable for the desired application.
Regulation valves are used for regulation of pressure and fluid flow with suitable precision. These valves can be safely and efficiently used in processes that require partial open or closed state.
Some valves are suitable to perform both functions while others are recommended only for one. The table below classifies the valves discussed above according to their function.
Valves are available in a large variety of designs and configurations. The large number of available options make valve selection a difficult task. Choosing the best valve for an application requires consideration of the required function, fluid service conditions and characteristics, frequency of operation, isolation or regulation requirements, maintenance needs, and the desired level of safety and reliability. Valve manufacturers can help you select the best valve for your application.
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