To clarify the issues raised in the definition of the control valve which can be gotten on Blackhawk supply for instance, we are going to list its proper functions.
1 – The control valve must retain the process fluids in which it is installed, withstand the severity of the operating conditions. The fluid passes through the control valve; it needs to have specific mechanical characteristics and chemical characteristics to withstand the process pressure, temperature, corrosion, erosion, dirt, and contaminants in the fluids.
2 – The control valve must respond to the controller actuation signal. This signal must be directed to the valve actuator, which is readily transformed into a force, which moves the stem, whose lower end is the obturator, which varies the fluid flow area through the valve.
3 – The control valve must manipulate the flow of the control medium by changing its opening to meet the needs of the process.
4 – The control valve must absorb the (variable) pressure drop in the line to compensate for the inlet and outlet pressure variations. The control valve is the only device that can generate or absorb a controllable pressure drop.
The control valve acts as a limiter in piping systems. By alternating its opening, it allows the flow resistance to vary, thus causing its flow to vary. The control valve works to adjust the flow continuously.
For the control valve to perform its functions effectively, it must contain the body, actuator, and bonnet. If you want or need to improve the valve’s performance, you can choose to add boosters, positioners, handwheels, switches, inversion relay, and transducers. These days, smart control valves are commonplace in the markets. The intelligent control valve is built based on microprocessors, incorporating the valve, controller, actuator, alarms, and digital ports in a single device.
The control valve body is the part that connects to the piping and has the variable fluid passage orifice. The control valve body is a pressure vessel where one or even two seats have been connected. The obturator (plug) is delineated, located at the end of the stem, activated by a pneumatic actuator. The relative position between the obturator and the seat, modulated by the signal coming from the controller, determines the value of the fluid flow that passes through the control valve body, having the pressure drop variation through the control valve itself.
In the body is:
- At headquarters
- Rod guide
The control valve parts that have direct contact with the process fluid are called TRIM. This appellation does not include the body, bonnet, flanges, and gaskets.
In a globe valve, the stem, plug, seat, guides, cage, and bushings are TRIM, and in rotary valves, the closing members, seat, stem, gaskets, and supports are included. You can see different collection on Blackhawk supply for instance.