Air-actuated control valves each with a 4-20 mA “I to P” converter integral to a valve positioner. In this example each positioner is comparing the valve stem travel against control signal, and applying any correction. Normally control valve pdf free download a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are 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.
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. Valve actuator – which moves the valve’s modulating element, such as ball or butterfly. Valve positioner – Which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear. Valve body – in which the modulating element, a plug, globe, ball or butterfly, is contained. Showing the evolution of analogue control loop signalling from the pneumatic era to the electronic era.
Example of current loops used for sensing and control transmission. Specific example of a smart valve positioner used. 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. 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. In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed.
As the pneumatic operation of valves, compared to motorised operation, has cost and reliability advantages, pneumatic actuation is still an industry standard. This signal is then fed to the valve actuator or more commonly, a pneumatic positioner. As a mechanical linkage is used for this, the positioner has to be mounted on the valve body. A positioner also allows the use of higher air pressures for valve actuation, which is particularly useful with cylinder type valve actuators. With the development of cheap industrial micro-processors, “smart” valve positioners have become available since the mid-1980s and are very popular for new installations. These include an I to P converter, plus valve position and condition monitoring in an integral unit mounted on the valve body. This allows further verification that the valve is operating as desired.
A huge variety of valve types and control operation exist. The most common and versatile types of control valves are sliding-stem globe, V-notch ball, butterfly and angle types. Their popularity derives from rugged construction and the many options available that make them suitable for a variety of process applications. Instrument Society of America Standard S561.
Fisher control valve handbook” fourth edition 1977. Tutorials covering the sizing, capacity and characteristics of control valves. Article from a University of South Australia website. Samson AG Demo Valve Sizing Software. Basic Trial valve sizing programme.
Control Valve Sizing Calculator to determine Cv for a valve. This page was last edited on 9 November 2017, at 22:58. Example of current loops used for sensing and control. Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point.