Overview
In today’s industries, control valves are widely employing as a mechanical component. The fluids control these mechanisms in pneumatic or hydraulic systems. They direct fluid movement and control how much pressure it exerts in the fluid system. In the process control industry, the control valve is a final control element utilize to manipulate a flowing fluid, such as gas, steam, water, or chemical compounds. As close as possible to the setpoint, compensating for load disturbances and maintaining regulation process variables.
When people talk about valves, they are actually referring to a control valve assembly. As a rule, a valve consists of the valve body, internal trim parts, an actuator for driving the valve, and various valve accessories, including petitioners, transducers, pressure regulators, manual operators, and limit switches.
Control Valve
It is important to realize that the control valve is an integral part of the control loop. To say that it is the most important part of the loop isn’t accurate. A control loop can think of as an instrumentation chain. In any chain, the weakest link determines the strength of the whole.
Manufacturers are under extreme economic pressure in today’s dynamic business environment. So, they realize that raw material and scrap costs must be minimal. While productivity must Maximizing. Process control is identified as a method for improving financial returns and meeting global competitive pressures through the reduction of process variability in manufacturing processes.
In order to achieve business goals, process variability must be reduced. The control valve is one of many methods that can be used to achieve this. How well they can reduce process variability is dependent on many factors. It is important to consider more than one isolated parameter. The research within the industry has found that the particular design features of the final control element, such as the valve, actuator, and positioner, are very important for achieving good process control under dynamic conditions. It is essential that the control valve assembly is optimized or developed as a unit. The valve components are usually not designed as a complete assembly, which results in poor dynamic performance.
Special Control Valves
A wide range of control applications can be handled by standard control valves without a problem. Even for standard applications, corrosion and viscosity of the fluid, leakage rates, and other factors must be considered. As well as the standard ones, there are also many types of special ones that can certainly be applied to special applications.
One of the most important is a high-capacity control valve. A special valve is one with a diameter larger than 12 inches, a ball valve larger than 24 inches, and a high-performance butterfly valve larger than 48 inches. The static pressure load at shutoff increases geometrically as the valve size increases arithmetically. As a result, shaft strength, bearing loads, unbalanced forces, and actuator thrust all become more critical with increasing valve size.
To keep actuator and design requirements within reasonable limits, the maximum allowable pressure drop is normally reduced on large valves. Although some large-flow valves have lower working pressure ratings, their flow capacity remains excellent. For large-flow applications, long-stroke, double-acting pneumatic pistons are often specified. Installation and maintenance are complicated by the physical size and weight of the valve and actuator components. The valve body assembly needs to be installed into the pipeline and major trim parts need to be removed and replaced with heavy-duty hoists.
Applications
Other types of special valves include high-temperature control valves. Valve components that are intended for service at temperatures above 450°F (232°C) must be specified and designed with the temperature conditions in mind. In boiler feedwater systems and superheater bypass systems, elevated temperatures might cause standard materials of control valve construction to fail. In general, plastics and standard gaskets are unsuitable and must be replaced by stronger materials. Metal-to-metal seating materials should always be used. Flexible graphite packing materials made from semi-metallic or laminated layers are also commonly used.
In addition to the two types previously mentioned, low flow control valves also deserve our attention. There are a number of applications that require the control of extremely low flow rates. Because these compact and lightweight control valves are often used in laboratory environments where very light schedule piping/tubing is used, in addition to having very low flows, they are compact and lightweight. Control valves of this type are specially designed to accurately control very low flow liquids or gases.
Accessories for Control Valves
Control valve accessories are diverse, and some of them are very important to the overall performance of the machine. The main accessories are positioners. Positioners convert the input signal from a process controller into valve movement for pneumatically operated valves. A solenoid valve manifold is also an important control valve accessory. A solenoid valve must be selected depending on the actuator type and the desired failsafe operation. On double-acting pistons and single-acting diaphragm actuators, solenoids can be used.
Control valves also have a close relationship with limit switches. A limit switch operates discrete inputs to a distributed control system, signal lights, small solenoids, electric relays, or alarms. Two to four separate switches are operated by movement of the valve stem with the cam-operated type. The switches are housed in an assembly mounted on the actuator’s side. It can be used for either alternating current or direct current systems, as each switch adjusts individually. Valve-mounted limit switches are also available in other styles.
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Commonly known as air sets, these pressure regulators reduce the plant air supply to valve positioners and other control equipment. Reduced-air-supply pressures are typically 20, 35, and 60 psi. Integral to the positioner or nipple-mounted or bolted to the actuator is the regulator.
The pneumatic lock-up system is used with control valves to lock in existing actuator loading pressure in the event of a loss of supply pressure. Upon loss of pneumatic air supply, these devices can be used to move the valve to the fully open or closed position. The valve will return to its original position with the supply pressure restored. For control valves using diaphragm actuators, similar arrangements are available.
Additional Parts of Valve
The transducers receive direct current input signals and convert the electric signals to proportional pneumatic output signals using torque motors, nozzle flappers, and pneumatic relays. This nozzle pressure operates the relay and is piped to the torque motor feedback bellows to allow comparison between the input signal and the nozzle pressure. Transducers can be mounted directly on a valve and operated without the need for additional boosters or positioners, as shown. Positioners controlled by electro-pneumatics are used in electronic control loops to operate pneumatic diaphragm control valves.
The valve manufacturers often help to choose the correct and suitable control valve to best fit your applications. There are so many valve suppliers who offer pneumatic diaphragm-operated control valves and hydraulic control valves. There is also a motorized control valve available in the market. The most reliable control valve operating system is called pneumatic operation. It provides quick action and spark-free operation.
Conclusion
The control valve is not only used to control the fluid flow but is also very useful to control the fluid pressure in pipelines. There are many plants where control valves are used to maintain temperature as well. The valve positioner makes the control valve an unmatchable product.
