Diaphragm Actuation

FIG. 8-70 Spring and diaphragm actuator w ith an up fail-safe mode. Spring adjuster allow s slight alteration of bench set, (Cmiiiesij Fislwr-Biismnoiinl.)... 

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. 

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. 

The lever-operated valve may be opened intermittently, either manually or automatically, for 2 seconds via a timer-controlled diaphragm actuator. 

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. 7.120. Spring-diaphragm actuator (a) air-to-open (direct) mechanism (6) air-to-close (inverse) mechanism...

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. 

The amplifier network provides signal conversion and suitable static and dynamic compensation for good positioner performance. Control from this block usually reduces to a form of proportional or proportional plus derivative control. The output from this block in the case of a pneumatic positioner is a single connection to the spring and diaphragm actuator or two connections for push/pull operation of a... 

Compute the maximum flow for the diaphragm-actuated valve. For best results in service, pressure-reducing valves are selected so that they operate 60 to 70 percent open at normal load. To obtain a valve sized for this opening, divide the desired delivery, in lb/h, by 0.7 to obtain the maximum flow expected. For this valve, then, the maximum flow is 350/0.7 = 500 lb/h. 

Select the diaphragm-actuated valve size. Using a manufacturer s engineering data for an acceptable valve, enter the appropriate valve-capacity table at the valve inlet steam pressure, 225 psig, and project to a capacity of 500 lb/h, as in Table 6.16. Read the valve size as 3/4 in at the top of the capacity column. 

Accdg to Ref 3, pp 216—19 229, the original acoustic homing system was called Kranich and the later version Pudel. The Pudel fuze was not sufficiently developed to be used in combat, but the Kranich fuze was. The Kranich consisted of a light diaphragm-actuated mechanism which responded to the sound of airplane propellers at a range of 15 meters. It was constructed on the same principle as the Pudel fuze. It was planned to install the Kranich system on some Rheintochter missiles... 

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. 

The P and I diagram shows all the components that make up a control loop. For example. Figure 5.8 shows a field-located pressure transmitter connected to a shared display pressure indicator-controller with operator access to adjustments and high and low alarms. The pressure controller sends an electric signal to a fail-closed diaphragm-actuated pressure control valve. 

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