A control valve is a device capable of modulating flow at various degrees between minimum flow and full capacity in response to signals from external control devices. Control valves, often referred to as "final control elements," are an important part of each control loop, because they do physical work and are elements that directly affect the process.
What is a Control Valve?
Control valves automatically regulate pressure and/or flow rate, and are available for any pressure. If different plant systems operate up to, and at pressure/temperature combinations that require Class 300 valves, sometimes (where the design permits), all control valves chosen will be Class 300 for interchange-ability. However, if none of the systems exceeds the ratings for Class 150 valves, this is not necessary.
Control Valve Arrangement
The image below shows how a control valve can be used to control rate of flow in a line. The "controller" receives the pressure signals, compares them with pressure drop for the desired flow and if the actual flow is different, adjusts the control valve to increase or decrease the flow. Comparable arrangements can be devised to control any of numerous process variables. Temperature, pressure, level and flow rate are the most common controlled variables.
VALVE TYPES AND TYPICAL APPLICATIONS
Principles of Operation
The control valve consists of an actuator mounted on a valve. The valve modulates the flow through the movement of the valve plug in connection with the port located inside the valve body. The valve plug is attached to the valve stem, which, in turn, is connected to the actuator. Actuators, which can be operated pneumatically or electrically, direct rod movements as determined by external control devices. Pneumatic Actuary Trerice / Diaphragm Pneumatic actuators work directly and utilize air signals from external control devices to create modulation control measures. The strength of the air signal is received into the actuator through the upper port and distributed throughout the area of â€‹â€‹the actuator diaphragm. The diaphragm presses the diaphragm plate and spring back into the assembly, which then moves the valve stem and attaches the plug down to caress the valve. This actuator will move to the outgoing rod position in the event of an air signal failure. The choice of valve action (stem-In-To-Close or stem-In-To-Open) will determine the position of the signal failure.
1. Pneumatic / Diaphragm Moved
Pneumatic Actuator is direct action and utilizes air signals from external control devices to make modulation control actions. The strength of the air signal is received into the actuator through the upper port and distributed throughout the area of the actuator diaphragm. The diaphragm presses the diaphragm plate and spring back into the assembly, which then moves the valve stem and attaches the plug down to caress the valve.
2. Trerice Driven by Electricity
Electric actuators are motorized devices that utilize electrical input signals to produce rotation of the motor shaft. This rotation, in turn, is translated by the unit connection into a linear motion, which moves the valve stem and plug for flow modulation. In the case of an electrical signal failure, this actuator can be determined to fail at the position of the outgoing, inlet, or last bar.
Flow valve control coefficient (Cv)
Cv is the number of gallons per minute of water passing through the control valve with a pressure drop of 1 psi
Various types of valves
- • Globe Valves
- • Gate Valve
- • Butterfly valve
- • Ball valve
- • Cage Cut Valve
Valve cages can have various forms of holes. Different forms can control flow in different characteristics.
There are three main types of control characteristics. They:
- • Linear
- • The same percentage
- • Quick opening
Control valve accessories
- • position manager
- • I / P converter
- • solenoid
- • volume amplifier
- • air lock relays
- • air filter