WATER CAD FILE TYPE PLUS
These include an I to P converter, plus valve position and condition monitoring in an integral unit mounted on the valve body. 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. A positioner also allows the use of higher air pressures for valve actuation, which is particularly useful with cylinder type valve actuators. As a mechanical linkage is used for this, the positioner has to be mounted on the valve body. 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". This is a dedicated controller with a pneumatic nozzle and flapper arrangement and a mechanical linkage to sense the valve actuator movement. This signal is then fed to the valve actuator or more commonly, a pneumatic positioner. These are usually located locally to the control valve and convert 4-20 mA to 3-15 psi (or (0.2 - 1.0 bar).
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To allow the construction of hybrid systems, where the 4-20 mA is generated by the controller, but enables the use of pneumatic valves, a range of current to pressure (I to P) converters are available from manufacturers. 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.Īs the pneumatic operation of valves, compared to motorised operation, has cost and reliability advantages, pneumatic actuation is still an industry standard. The modes of failure operation are requirements of the failure to safety process control specification of the plant.
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Valve actuator - which moves the valve's modulating element, such as ball or butterfly.The HART, Fieldbus Foundation, and Profibus are the most common protocols.Īn automatic control valve consists of three main parts in which each part exist in several types and designs:
WATER CAD FILE TYPE VERIFICATION
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. 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. Normally with 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.Īir-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 opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators.
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4.1 List of common types of control valve.