Introduction
The control valve is a controlled device that regulates the flow of a liquid or gas in a system. This regulation is accomplished by the varying resistance that the valve introduces into the system as the valve is stroked. As the valve modulates to the closed position the system pressure drop shifts to the valve and reduces the flow in the system.
Principles of Operation
A control valve is comprised of an actuator mounted to a valve. The valve modulates flow through movement of a valve plug in relation to the port(s) located within the valve body. The valve plug is attached to a valve stem, which, in turn, is connected to the actuator. The actuator, which can be pneumatically or electrically operated, directs the movement of the stem as dictated by the external control device.
1.Pneumatic/Diaphragm Actuated
Pneumatic Actuators are direct acting and utilize an air signal from an external control device to create a modulating control action. The force of the air signal is received into the actuator through a top port and distributed across the full area of the actuator’s diaphragm. The diaphragm presses down on the diaphragm plate and spring return assembly, which then moves the valve stem and plug assembly downward to stroke the valve.
2.Electric Actuated
Electric Actuators are motor driven devices that utilize an electrical input signal to generate a motor shaft rotation. This rotation is, in turn, translated by the unit’s linkage into a linear motion, which drives the valve stem and plug assembly for flow modulation.
The valve is very important to the operation of the system. Without a properly sized valve the system will never operate at an efficient level.
Control valves contain four basic sections;
- Body
The Body contains the orifice and is the main housing through which the controlled fluid flows.
- Trim
The Trim is the part of the valve excluding the body that comes in contact with the fluid. It is composed of the valve seat, plug, disc and disc holder, and stem.
- Bonnet
The Bonnet is an assembly that provides a mounting for the actuator and a guide through which the stem must pass. It is composed of the centerpiece, packing, packing guide, and packing nut. The packing provides a seal between the stem and bonnet to prevent leakage.
(4) Actuator.
The Actuator consists of either pneumatic or electric means to provide the force to stroke the valve.
The control valve is the most widely used type of final element. Other types of final control elements are dampers or louvers, and variable pitch fan blades.
Different types of valves
- Globe Valves
- Gate Valves
- Butterfly Valves
- Ball Valves
- Cage Trim Valves
Valve characteristics
The cage of a valve can have different shapes of holes. The different shapes can control the flow in different characteristics. There are three main types of control characteristics. They are:
- Linear
- Equal percentage
- Quick opening
check this link to know more about Control valve characteristics/trim
Control valve flow coefficient (Cv)
Cv is the number of gallons per min of water passes through a control valve with a pressure drop of 1 psi
Control valve accessories
- positioner
- I/P converter
- solenoid
- volume booster
- air lock relay
- air filter
to know more about Control valve accessories check the links below Control valve accessories
Calibration of control valve positioner
I/P converter calibration
