Positionality

Various accessories can be suppHed along with the control valves for special situations. Positioners ensure that the valve stem is accurately positioned following small or slowly changing control signals or where unbalanced valve forces exist. Boosters, which are actually pneumatic amplifiers, can increase the speed of response or provide adequate force in high pressure appHcations. Limit switches are sometimes included to provide remote verification that the valve stem has actually moved to a particular position. 

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. 

The key parts of the positioner/actuator system, shown in Fig. 8-74 7, are (1) an input-conversion network, (2) a stem-position feedback network, (3) a summing junction, (4) an amplifier network, and (5) an actuator. 

FIG. 8-74 Positioner/actiiators (a) generic block diagram (h ) an example of a pneumatic positioner/actuator. 

Figure 8-74b is an example of a pneumatic positioner/actuator. The input signal is a pneumatic pressure that (1) moves the summing beam, w ch (2) operates the spool valve amplifier, which (3) provides flow to and from the piston actuator, which (4) causes the ac tuator to move and continue moving until (5) the feedback force returns the beam to its original position and stops valve travel at a new position. Typical positioner operation is thereby achieved. 

Static performance measurements related to positioner/ac tuator operation are conformity, measured accuracy, hysteresis, dead baud, repeatability, and locked stem-pressure gain. Definitions and standardized test procedures for determining these measurements can be found in ISA-S75.13-1989, Method of Evaluating the Performance of Positioners with Analog Input Signals and Pneumatic Output . 

The stiffness characteristic of the positioner/actuator varies with frequency. Figure 8-75Z indicates the stiffness of the positioner/actu-ator is high at low frequencies and is directly related to the locked-stem pressure gain provided by the positioner. As frequency increases, a dip in the stiffness curve results Trom dynamic gain attenuation in the pneumatic amplifiers in the positioner. The value at the bottom of the dip is the sum of the mechanical stiffness of the spring in the actu-... 

FIG. 8-75 Frequency response curves for a pneumatic positioner/actuator (a) input signal to stem travel for a 69-inch spring and diaphragm actuator with a 1.5-inch total travel and. 3-15 psig input pressure (h ) dynamic stiffness for the same positioner/actuator. 

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. 

Some tvpical examples of where the dvnarnics of the positioner are siifficientlv fast to improve process control are the following ... 

In a dhfribiifed control system (DCS) process loop with an eJeetronie transmitter. The DCS controller and the electronic transmitter have time constants that are dominant over the positioner response. Positioner operation is therefore beneficial in reducing valve-related nonlinearitv,... 

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,... 

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

Select a positioner with a faster response characteristic. 

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. 

Rotaiy stem-valve designs are normally offered only in their naturally occurring characteristic, since it is difficult to appreciably alter this. If additional characterization is required, the positioner or controller may be charac terized. However, these approaches are less direct, since it is possible for device nonlinearity and dynamics to distort the compensation. 

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. 

Positioners should be used to aehieve aeeuraey and repeatability of the eontrol-loop response. This is partieularly important with the open-loop eontrol method diseussed above. 

In the multistage compressor, the vanes are rectangular and located in a radial position ahead of the first impeller, with a linkage connecting the vanes to a power positioner. From that point, the control is affected in the same manner as the single-stage. 

One interesting advantage of the one-component waterborne adhesives is that many are re-positionable. For example, if, for some reason, the two substrates are poorly aligned when the bonding occurs, this problem can be corrected by heating the bonded substrates above the bond activation temperature. The two substrates... 

Diffuser vanes are used to decelerate a high velocity flow to create a pressure rise. They are usually at the periphery of each impeller. The variable diffuser vane system may be controlled manually by a handwheel or automatically by a hydraulic or air-operated positioner. See Figure 12-44B. 

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