Describe control valves
Control valves are used to control the flow and direction of fluid such as gas, water, steam or chemical compounds. They are controlled so that the load disturbance can be compensated. Control valves are used in gas centrifugal compressors to avoid damages in the compression machine. Control valves are used in the fluid power system. Automatic control of the modern plants is done with the help of control valves they can do an exchange of energy and reduction of pressure. The control valve is the final control element, it is the major part of the automatic control. Control valves will provide the required energy to the controllers.
What are the types of control valves
Control valves are classified into linear and rotary, linear motion control valves are comprised of globe, gate and diaphragm type closures. While the rotary motion control valves consist of ball, butterfly or plug closures. All of these valves have their own special features which could either be an advantage or disadvantage for the given application.
Features of linear valve
- The flow path is tortuous
- They have low recovery and can throttle small flow rates
- It offers a variety of special trim designs
- They are suited for high-pressure applications
- They are usually flanged or threaded
- Separable bonnet
Features of the rotary valve
- Streamlined flow path
- High recovery
- More capacity and less packing wear
- They can handle slurry and abrasives
- High rangeability and flangeless
- Integral bonnet
How to install a control valve and what are the factors that must be considered while installing a control valve
In case of installing a valve we must know it’s functions, we must know if it is able to prevent backflow, start the flow, stop the flow, regulate flow, or regulate pressure. Most of the details will be stamped on the valve body by the manufacturer, like the type of system, operating pressure direction of flow, etc. The operating characteristics of the valve must be known. We must aware of metal it is made of and the type of end connection it has. Operating characteristics and the material of the valve could affect the service a valve can provide. By checking the end connection we could know if the valve is suited or not for the installation in the system.
The installation of the valve should be done at accessible places and with a proper headroom for better operation. The stem position must be kept upwards whenever it is possible. It is ok if the stem position is horizontal or straight up but the inverted position or the stem pointing downward must be avoided. Before installation, the system should be checked so that we could know if the pressure is on the wrong side of the disk if so it could damage the valve.
How to do valve maintenance
Maintenance must be done periodically for valves so that the service life will be better. If a leak is seen then it must be taken care, sometimes it can be done by tightening a packing gland nut. Loose hangers could allow sections of a line to sag. Valves that are used for a long time and subjected to constant service must need gland tightening, replacing or a complete overhaul.
The valve can be checked visually to know if the seat and disk make good contact with each other and this known as spotting valves. Grinding valves grinding can be done manually to remove irregularities by grinding together the contact surfaces of the seat and disk.
What is a valve positioner
It is an air relay that is used between the controller output and the valve diaphragm. It will assure the exact position of the valve stem in accordance with controller output.
What are the types of actuators used in control valves
Spring and diaphragm are the most popular actuator type, the net output thrust is different between diaphragm force and opposing spring force. Output thrust is required and supply air pressure available dictate size. Diaphragm actuators are simple and dependable
This type is operated pneumatically with the help of high pressure, it provides high thrust and fast stroking speed. This actuator can give maximum force in both directions.
What are the actuation techniques in control valves
Manual actuation – Manual actuation is done by the operator and it is done according to the system requirements. Pushbuttons, pedals are needed for manual actuation. This type of actuation is commonly used when the system requirement is not critical
Mechanical actuation – Mechanical actuators include springs, plungers, rollers, cams, and mechanical linkages. Springs and plungers are most commonly used to provide force for valve operations.
Electrical actuation – Electrical actuators are solenoid actuators solenoid is an electromagnetic coil. So it is energized it will create a magnetic force and that will pull the armature, the valve is connected to the armature and it is actuated.
How can we obtain stability while choosing a valve
- It can be obtained by choosing the correct valve and proper Cv
- It can be obtained by removing nonlinearity: hysteresis, stiction
- It can be obtained by using the positioner correctly
- By choosing a strong actuator
- It can also be obtained with fast loops, reduce dead time, and time constant
How to obtain precision when choosing a valve
- By reducing stiction
- Strong actuator
- Supply pressure at the maximum pressure permitted
- Remove hysteresis by properly installing mechanical links
- Positioner must be properly adjusted and sized
How to obtain speed when choosing a valve
- Use strong actuator
- Pipe with large diameter or tube
- By using pressure regulator which has good flow characteristics
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