Valves

Valve uses

Valves are a simple mechanical or simple electro-mechanical device that are used to control the flow of gases, liquids, powders, etc. through the pipes or the tubes from the tanks or other containers. Mostly, the valves depend on the design of mechanical barrier i.e., a ball, a plate, a diaphragm, a disc, that can be inserted and removed from the flow stream of the material that is passing by, or be rotated inside the flow steam. Some of the valves are designed into on-off models, while others are designed to allow proper control of the path of substances.

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 also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, valves are instead called dampers.

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.

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.

Valve functions

Valves play a critical role in increasing efficiency, improving quality control, and ensuring safety and reliability of a process. The primary consideration in valve selection is the desired function of the valve. Valves are incorporated into a piping system for one or more of the following reasons:

• Starting or stopping fluid flow
• Throttling flow rates within a piping system
• Regulation of pressure
• Regulation of fluid temperature in a piping system
• Redirecting the flow from one path to another
• Restricting the flow for system balance or to prevent over pressurization
• Prevent reverse flow of fluid
• Stopping the flow in case of a system failure
• Improving safety through relieving pressure or vacuum in a piping system

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.

Valve types

The most used types of valves are gate valves, globe valves, butterfly valves, diaphragm valves, ball valves, and check valves.

Gate valves are the most common type of valves found in process plants. Gate valves provide good shutoff and are considered suitable for almost all fluid services including air, fuel gas, feedwater, steam, slurries and viscous liquids. The valve design makes it difficult to control the flow. Furthermore, the fluid slapping against a partially open gate can damage the valve.

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. The design of these valves forces the fluid to change direction as it flows through; this creates high pressure loss and turbulence. They are not suitable for fluids containing particles, high viscosity fluids and other mediums that are prone to choking.

The “butterfly” refers to a rotatable disc mounted on a shaft. 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. 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.

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. Diaphragm valves are leak-proof, easy to maintain, clean, safe, and effective. They are suitable for moderate pressure & temperature applications. The working temperature and pressures of diaphragm valves depend upon the diaphragm material. The saddle in a diaphragm valve can prevent complete drainage of the pipe.  When used in continuous throttling applications, the diaphragm can wear out over time leading to valve failure.

A ball valve uses a hollow ball to stop or allow fluid flow. When the hole in the ball faces the flow inlet, the fluid flows through. A quarter-turn rotary motion of the valve shaft turns the ball by 90° blocking the flow. Ball valves offer ease of operation, high volume flow, high pressure and long service life. 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. Check valves are generally small, simple, and inexpensive. Industrial applications of check valves include feed water control systems, gas systems that mix multiple gases into a single gas stream, and fuel and oxidizer mixing systems.

Valve classification

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.

Valve function – Valves are incorporated in a process to perform different functions. They may be used to start/stop the flow or to regulate flow and pressure. They may be required to control the direction of flow, or for improving process safety. Based on flow control, valves are classified as either isolation valves or regulation valves. Isolation valves are designed to either completely restrict fluid flow or allow it without obstruction. These valves are not recommended for continuous use in the partially open state as they may provide poor flow control or suffer wear damage over time. 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 (diaphragm valves and butterfly valves) while others are recommended only for one. 

Mode of operation – Valves can be classified as manual, actuated or automatic based on how they are operated. Manual valves are typically operated by hand with the help of wheels, levers, pedals, knobs, or chains. These valves can be geared mechanically to change the direction of movement and/or to increase the operation speed or torque. In high-precision or large-scale applications, actuated valves are connected to electric motors, pneumatic systems, hydraulic systems, or solenoids, to enable remote operation and automation of valve function. Some valves are automatically activated when a specific flow condition is met. For examples, check valves close without any need for outside intervention during backflow. Similarly, pressure release valves are activated automatically when an over-pressure condition is detected.

End connections – Valves are connected to pipes through joints; these joints can be screwed, flanged or welded. Screwed end valves have threaded joints, to which the end of a pipe or another fitting is screwed. A flange is a plate or ring that forms a rim at the end of a pipe or valve; two flanges are bolted together to form a joint. Wafer type valves have thin valve body, which is placed between two pipe flanges, with the bolts surrounding the valve body. These valves are usually installed in systems with limited space. The butt weld valves have bevelled ends that match a similar bevel on the pipe. For socket welded valves the pipe is inserted into a socket and welded.

Mechanical motion – Based on the mechanical motion of the closing element, valves can be classified as Linear Motion Valves or Rotary Motion Valves. If the closing element in the valve moves in a straight line to allow or stop the flow, the valve is called a linear motion valve. When the closing element rotates or moves along a circular path the valve is called a rotary motion valve. Valves can also be classified as multi-turn or quarter turn depending on the extent of turning of the handle/wheel required to completely open or close the valve.

Construction material – The valve body can be made of stainless steel, alloy steel, brass, cast iron, or plastics depending on the desired temperature and pressure ratings, durability, and application. The gasket, diaphragm, packing, and valve seat are usually made of PTFE, POM, FKM, PA, EPDM, or NBR to achieve the desired level of seal and temperature/pressure ratings.

Pressure and temperature ratings – Valves may also be classified according to their pressure and temperature ratings. These ratings specify the maximum allowable pressure and temperature levels to which the valve may be safely subjected.

Valve selection

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, design for specific and specialised applications, maintenance needs, and the desired level of safety and reliability.