Valve positioners

The symbol for the control valve in Fig. 8-47 is for a pneumatic positioning valve without a valve positioner. 

Transducers The ciirrent-to-pressiire transducer (I/P transducer) is a conversion interface that accepts a standard 4-20 rnA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range (normally (),2-L0 bar [3-15 psig] or, less frequently, 0,4-2,0 bar [6-30 p.sig]). The output pressure generated by the transducer is connected directly to the pressure connection on a spring-opposed diaphragm actuator or to the input of a pneumatic valve positioner. 

Valve Positioners The valve positioner, when combined with an appropriate actuator, forms a complete closed-loop valve-position control system. This system makes the valve stem conform to the input signal coming from the process controller in spite of force loads that the actuator may encounter while moving the control valve. Usually, the valve positioner is contained in its own enclosure and is mounted on the control valve. 

Additional situations vvEere valve positioners are used are as follovv s ... 

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. 

Digital Field Communications An increasing number of valve-mounted devices are available that support digital communications in addition to, or in place of, the traditional 4—20 mA current signal. These control-valve devices have increased functionality, resulting in reduced setup time, improved control, combined functionality of traditionally separate devices, and control-valve diagnostic capabihty. Digital communications also allow the control system to become completely distributed where, for example, the process PID controller could reside in the valve positioner or in the process transmitter. 

Speed-Control Systems The most common sensing element is mechanical some systems are hydraulic or electronic. For valve positioner they all have a hydrauhc servo as first choice, with an occasional choice of pneumatic for lighter loads. 

Valve positioners are little feedback controllers that sense the actual position of the stem, compare it with the desired position as given by the signal from the controller and adjust the air pressure on the diaphragm to drive the stem to its correct position. Valve positioners can also be used to make valves open and close over various ranges (split-range valves). 

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. 

Figure 7.120 is a simplified view of a spring diaphragm actuator. The actuator receives a pneumatic signal from the controller via a booster flow enlarger or a valve positioner and can be adapted in the form of an air-to-open or an air-to-close mechanism. 

Fig. 7.119. Pneumatically operated control valve (a) double-spring actuator with single-ported globe valve (b) exterior view of double-ported control valve with valve-positioner fitted on the side...

The use of a valve positioner is generally beneficial with relatively slow control loops. Fitting a positioner within a fast control loop will decrease the stability of the loop(<6). 

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. 

FIG. 8-80 Valve and actuator with valve positioner attached. (Courtesy Fisher Controls International LLC.)... 

A particular type of booster relay, called a dead-band booster, is shown in Fig. 8-88. This booster is designed to be used exclusively between the output of a valve positioner and the input to a pneumatic actuator. It is designed to provide extra flow capacity to stroke the actuator faster than with the positioner alone. The dead-band booster is designed intentionally with a large dead band (approximately 5 percent of the input span), elastomer seats for tight shutoff, and an adjustable bypass valve connected between the input and output of the booster. The bypass valve is tuned to provide the best compromise between increased actuator stroking speed and positioner/actuator stability. 

Adding a cascade slave to a fast loop can destabilize the primary if most of the process dynamics (time lags) are within the secondary loop. The most common example of this is using a valve positioner in a flow-control loop. The... 

Currently HART transmitters and valve positioners are state of the art. They do not differ outwardly from their pure analogue predecessors. A wide range of varying functions and measuring principles is available with a HART interface. Numerous products are deliverable with a certificate for intrinsic safety, too. The same holds for safety barriers, multiplexers and remote 1/O systems which are suitable for HART communication as well. 

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