Positioner application

Positioner Application Positioners are widelv used on pneumatic valve actuators, VIore often than not, thev provide improved process-loop control because thev reduce valve-related nonlinearitv, Dvnarnicallv, positioners maintain their abilitv to improve control-valve performance for sinusoidal input frequencies up to about one half of the positioner bandwidth. At input frequencies greater than this, the attenuation in the positioner amplifier netvv ork gets large, and valve nonlinearitv begins to affect final control-element performance more significantlv. Because of this, the most successful use of the positioner occurs when the positioner-response bandwidth is greater than twice that of the most dominant time lag in the process loop. 

In a process loop with a pneumatic controller and a large process time constant. Here the process time constant is dominant, and the positioner will improve the linearitv of the final control element, Some common processes with large time constants that benefit from positioner application are liquid level, temperature, large volume gas pressure, and mixing,... 

With the exception of the dead-band booster, the application of booster relavs has diminished sornewEat bv the increased use of ciir-rent-to-pressiire transducers, electropneiirnatic positioners, and electronic control svsterns. Transducers and valve positioners serve much the same fiinctionalitv as the booster relav in addition to interfacing with the electronic process controller. 

One important application of pneumatic transmission is in the operation of diaphragm actuators. These are the elements generally employed to drive the spindles of control valves (Section 7.22.3) and, if hard-wired transmission systems are employed, require devices which convert electric current into air pressure or air flowrate, i.e. electropneumatic (E/P) converters. The basic construction of a typical E/P converter is illustrated in Fig. 6.77. A coil is suspended in a magnetic field in such a way that when a current is passed through the coil it rotates. This rotation is sensed by a flapper/nozzle system (Section 7.22.1). The nozzle is supplied with air via a restrictor and its back pressure actuates a pneumatic relay. The output from the latter is applied to the feedback bellows and also acts as output from the E/P converter. Electropneumatic valve positioners employ the same principle of operation. 

Devices mounted on the control valve that interface various forms of input signals, monitor and transmit valve position, or modify valve response are valve control devices. In some applications, several auxiliary devices are used together on the same control valve. For example, mounted on the control valve, one may find a current-to-pressure transducer, a valve positioner, a volume booster relay, a solenoid valve, a trip valve, a limit switch, a process controller, and/or a stem position transmitter. Figure 8-80 shows a valve positioner mounted on tne yoke leg of a spring and diaphragm actuator. 

Requiring tight shutoff in large valves is particularly expensive in applications where spring and diaphragm actuators are preferred or where positioners normally would not be required. This is due to the necessity of changing to piston actuators in order to get the necessary thrust. 

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