Flapper/nozzle system

A typical DP cell is shown in Fig. 6.19a<23). The associated flapper/nozzle system works on a force-balance principle (see Volume 1, Section 6.2.3). The output pressure P0 is linearly related to the difference in pressure (P, - P2), thus ... 

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. 

Thus, the pressure of the output to the valve P is the sum of Pi and the pressure produced by the wide-band proportional action contributed by the proportional bellows and the flapper-nozzle system (cf. equation 7.3). Note that for t > 0, P/ is no longer equal to P0 and thus equation 7.268 only strictly applies at t = 0. The value of 3 i (and consequently r/) depends upon the capacity C/B of the integral bellows and the the resistance to flow R/r through the integral restrictor. It is generally assumed that C/fi changes little and r/ is varied by adjusting R/r. 

Number of independent equations Number of degrees of freedom Number of independent variables Number of zeros of function Pressure upstream of nozzle in flapper/nozzle system Pressures applied to limbs of manometer tube or pressures downstream and upstream of orifice plate Distillation column pressure Pressure in feedback bellows of pneumatic controller Frictional drag per unit cross-sectional area of manometer tube... 

During the 1920s, a pneumatic amplification mechanism, the flapper-nozzle amplifier, that greatly increased the sensitivity of the pneumatic system, was developed. The principle of the flapper-nozzle based controller is shown in figure 7, and figure 8 shows a Foxboro controller circa 1922. 

The excessive sensitivity of the narrow-band mechanism leads to considerable instability in the control system. This sensitivity is reduced by introducing a feedback bellows as illustrated in Fig. 7.115. In this case, if P increases due to the movement of the flapper towards the nozzle, the bellows will expand. Thus, as pivot Y is moved to the left by a change in measured value or set point, X will move to... 

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