The rate of fluid flow is regulated by a control valve as the position of the valve plug or disk can be changed by an actuator. Control Valves are used for maintaining a variable process as close as possible to the desired set point. Controller setpoints are mainly, pressure, flow rate, and temperature. The product parameters like concentration, density, level of the liquid, and others can also be controlled with the help of control valves. A control valve installation comprises an actuator, valve body, accessories, and position. The body comprises a bonnet assembly and trims parts. It has been designed to withstand the static pressure of the fluid and the differential pressure, allows the flow of fluid, and offers connecting ends of the pipe while supporting seating surfaces and a valve closure member.
Actuators are hydraulic, pneumatic, or electrically powered devices which offer the force for opening and closing the valve. The positioners can control and monitor the true actuator movement for maintaining a desired set-point. In the accessories section, there are pressure regulators, electro-pneumatic transducers, handwheels, limit switches, and position indicators. In the very basic terms, control valves can be defined as a power-operated device that can modulate the flow at a varied range of degrees among full capacity and minimal flow in response to a signal from the controlling system. A major role in daily life for increasing the profits of the process plants and conserving energy.
Control Valves mainly follow the instructions which it received from the controlling loop. Sensing the element sends a signal to the controller which is in turn conveyed to the positioned that is mounted on the valve. The positioned controls the valve position with the help of an actuator. For selecting the correctly fitted option, it is very important to follow the parameter of the media. These are state, flow rate; vapor pressure, inlet and outlet pressures, density, inlet temperature, molecular weight, critical temperature, specific heat ratio, viscosity, and critical pressure. When you are defining the flow rate, you must add certain safety margins. One must also consider the ambient conditions at system geometry, site, MOC, pipeline sizing, fail-safe positioning, and others.Selecting the right valve for your application –
Depending on the above, the suitability of the valve can be defined. There no single type of valve that can suit all the applications. Each of the valve types has a specific role. So for this, it is important to know which one you must choose for the task remaining on your hand. Here is a brief look at the Control Valves which are widely used.
- Pressure Reducing Control Valves – The diaphragm operated control valves for pressure reducing applications (PRV) can reduce a higher inlet pressure into a lower outlet pressure automatically. The pressure from the outlet is kept constant without any regard for changes in the rate of flow or the inlet pressure. The main control valve is pilot operated and is capable of precise setting with the easy adjustment of outlet pressure.
- Float Control Valves – Float Control Valves are automatically operated control valves which are intended for use in on-off or modulating service. These valves are either valve-mounted or remotely operated. The feature combinations include solenoid and pressure sustaining. They are applicable in places wherever it is required to maintain the water level