A control valve consists of three main parts in which each part exist in several types and designs. Valve actuator as mentioned earlier is the operator which moves the valve's modulating element, such as ball, globe or disc depends on whether it is a ball, globe or a butterfly control valve respectively. The Valve positioner, ensures that the control valve has reached the desired degree of opening. This helps in overcoming the problems of friction and wear. The Valve body which houses the trim parts and internals of the valve along with the stem, sealing element, cage, seat rings etc.Basic guidelines about control valves, Fisher dvc6205, dvc6205
Basic Guidelines and Applications of Control Valves.
What is a Control Valve?
A control valve is a manual valve used to control fluid flow fitted with an actuator instead of a handwheel. The size of the flow passage can be varied as directed by a signal from a controller mounted on the actuator. In this article we have used the image of Fisher digital valve controller dvc 6200 series. An actuator can be pneumatic, electric or hydraulically operated. This enables the direct control of flow rate and other control parameters of process media such as pressure, temperature, and liquid level. To know more about the application of control valves please view the slideshare presentation below.
Control Valve Operation:
The opening or closing of control valves is usually done by electrical, hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed. Valve positioners are also used to ensure the valve attains the desired degree of opening. Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves required high pressure supply and return lines for the hydraulic fluid.
The pneumatic control signals are traditionally based on a pressure range of 3-15psi (0.2-1.0 bar), or more commonly now, an electrical signal of 4-20mA for industry, or 0-10V for HVAC systems. Electrical control now often includes a "Smart" communication signal superimposed on the 4-20mA control current, such that the health and verification of the valve position can be signalled back to the controller (fisher dvc 6200 in our case). The HART, Fieldbus Foundation, and Profibus are the most common protocols.To learn more on "Why digital valve controllers are preferred over pneumatic" [Click Here]
A control valve consists of three main parts in which each part exist in several types and designs.
Valve actuator as mentioned earlier is the operator which moves the valve's modulating element, such as ball, globe or disc depends on whether it is a ball, globe or a butterfly control valve respectively. The Valve positioner, ensures that the control valve has reached the desired degree of opening. This helps in overcoming the problems of friction and wear. The Valve body which houses the trim parts and internals of the valve along with the stem, sealing element, cage, seat rings etc.
Valve Control Action:
Control action determines the functionality of the control valve and its design with respect to the process application. Take the example of a pneumatic control valve, there are two outcomes of control actions possible."Air or current to open" - The flow restriction decreases with increased control signal value.
"Air or current to close" - The flow restriction increases with increased control signal value.
Control valves are also built with failure to safety modes, wherein based on failure of input plant air or electric input signal the valve will perform a predetermined action.
Air or control signal failure to close" - On failure of compressed air to the actuator, the valve closes under spring pressure or by backup power.
Air or control signal failure to open" - On failure of compressed air to actuator, the valve opens under spring pressure or by backup power.
The modes of failure operation are requirements of the failure to safety process control specification of the plant. In the case of a cooling water application it may be to fail open, however in the case of delivering a chemical in a process plant it may be to fail closed. So why do control valves fail?
[Click Here] to read my blog post on "What are the different causes of failures in Control Valves" To get a better understanding of how the control valve action works view the control loop diagram below.
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Valve Positioner:
As the pneumatic operation of valves, compared to motorised operation, has cost and reliability advantages, pneumatic actuation is still an industry standard. To allow the construction of hybrid systems, where the 4-20 mA is generated by the controller (fisher dvc 6200 in our case), but enables the use of pneumatic valves, a range of current to pressure (I to P) converters are available from manufacturers. These are usually located locally to the control valve and convert 4-20 mA to 3-15 psi (or (0.2 - 1.0 bar). This signal is then fed to the valve actuator or more commonly, a pneumatic positioner. This is a dedicated controller with a pneumatic nozzle and flapper arrangement and a mechanical linkage to sense the valve actuator movement. This ensures that problems of friction are overcome and the valve control element moves to the desired position, by comparing the actual valve regulating element position against the desired value; hence the name "positioner". To learn more on "How to read a piping and instrumentation drawing" [Click Here]
Types of Control Valves:
There are many different types of control valves in operation. However, they can be classified based on the two main forms of control action. the sliding stem and the rotary action. The most common and versatile types of control valves are sliding-stem globe, V-notch ball, butterfly and angle type control valves. Their popularity is derived from the rugged construction and the many options available that make them suitable for a variety of process applications. Control valve bodies may be categorized as Sliding Stem type consisting of Angle seat piston valve and Globe type control valve. The other being Rotary type consisting of Butterfly valve and Ball control valves.
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