Pneumatic signal

Pneumatic signals are represented by solid hnes that are cross-hatched intermittently. The signal from the I/P transducer to the pneumatic positioning valve is pneumatic. 

The variable-pitch arrangement at constant motor speed changes the pitch of the olades through a pneumatic signal from the leaving-water temperature. As the thermal load and/or the ambient wet-bulb temperature decreases, the blade pitch reduces air flow and less fan energy is required. 

Key or Principal Process Lines Utility. Service. Auxiliary Process Lir.es Existing Lines In a System Flow Arrow. Indicates Flow Directior Pneumatic Signal Electric Signal... 

Pneumatic signal t, or undefined signal for process flow diagrams... 

The death knell for pneumatic control equipment has been predicted for at least the past 15 years. So far this has not happened, but it is still predicted. The major reason why pneumatic equipment is so popular is that the pneumatic control valve is cheap and requires little maintenance. The pneumatic system also has the advantage of posing no problems in the presence of flammable substances. (Extreme care must be exercised if electrical signals are used in such environments.) One major problem with pneumatic systems is the delay encountered in sending a pneumatic signal over 300 ft (90 m). However, this can usually be avoided by mounting the controller next to the unit instead of in the control room. This does not affect the monitoring of the process, which can still be done in a remote location. 

Analog controllers use continuous electronic or pneumatic signals. The controllers see transmitter signals continuously, and control valves are changed continuously. 

In alternative forms of these systems, the applications program is stored in the processor the analyzer microprocessor digitizes only the detector signal and transmits the digitized values to the data processor. Applications program event commands are received in real lime from the data processor and convened at the analyzer to electrical and pneumatic signals for sample valve actuation, column switching, sample conditioner conirul. and so on. 

The distance over which pneumatic signals can be transmitted is limited by the volume of the tubing and the resistance to flow. The dynamics of pneumatic systems can generally be approximated by a first order lag plus a dead time (Sections 7.S and 7.6). Tubing may be made of copper, aluminium or plastic, and is normally of S mm ID. Pneumatic receivers can be in the form of indicators, recording devices and/or controllers. 

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. 

If the output from the controller is electric (e.g. 4-20 mA), then this can be converted to a pneumatic signal (with pressures up to 600 kN/m2) using an electropneumatic converter (Section 6.12.4). The resulting pneumatic signal then... 

Electronic (4- to 20-mA) signals are represented by dashed lines. In Fig. 8-49, these include the signal from the transmitter to the controller and the signal from the controller to the I/P transducer. Pneumatic signals are represented by solid lines with double crosshatching at regular intervals. The signal from the I/P transducer to the valve actuator is pneumatic. 

Dynamics of Positioner-Based Control Valve Assemblies Control valve assemblies are complete, functional units that include the valve body, actuator, positioner, if so equipped, associated linkages, and any auxiliary equipment such as current to pneumatic signal transducers and air supply pressure regulators. Although performance information such as frequency response, sensitivity, and repeatability data may be available for a number of these components individually, it is the performance of the entire assembly that will ultimately determine how well the demand signal from the controller output is transferred through the control valve to the process. The valve body, actuator, and positioner combination is typically responsible for the majority of the control valve assembly s dynamic behavior. On larger actuators, the air supply pressure re lator capacity or other airflow restrictions may hmit the control valve assembly s speed of response. 

BESET BY I I PNEUMATIC SIGNAL I I EL ECTRONIC SIGNAL MANUAL... 

Most control valves (final control elements) are actuated by pneumatic signals using instrument air. Pneumatic actuators are preferred in situations in which electronic... 

Instrument supply or connection to process Undefined signal Pneumatic signal... 

RESET BY I [pneumatic SIGNAL I IeL ECTRONIC SIGNAL I I MANUAL... 

Electric motors may be variable-speed, reversible motors or simple solenoids (for on-off control). Because the former are expensive, it is a common practice to transduce the electric signals to pneumatic signals and use inexpensive diaphragm valves. 

The valve positioner, which is usually contained in its own box and mounted on the side of the valve actuator, is designed to control the valve stem position at a prescribed position in spite of packing friction and other forces on the stem. The valve positioner itself is a feedback controller that compares the measured with the specified stem position and makes adjustments to the instrument air pressure to provide the proper stem position. In this case, the setpoint for the valve positioner can be a pneumatic signal coming from an l/P converter or the 4-20 mA analog signal coming directly from the controller. A valve with a deadband of 25% can provide flow rate precision... 

Booster relays are designed to provide extra flow capacity for the instrument air system, which decreases the dynamic response time of the control valve (i.e., the time for most of a change to occur). Booster relays are used on valve actuators for large valves that require a large volume of instrument air to move the valve stem. Booster relays use the pneumatic signal as input and adjust the pressure of a high flow rate capacity instrument air system that provides pressure directly to the diaphragm of the valve actuator. 

Transducers Many measurements cannot be used for control until they are converted to physical quantities (such as electric voltage or current, or a pneumatic signal, i.e., compressed air or liquid) which can be transmitted easily. Transducers are used for that purpose. For example, strain gauges are metallic conductors whose electric resistance changes when they are subjected to mechanical strain. Thus they can be used to convert a pressure signal to an electric one. 

The output signal of a feedback controller depends on its construction and may be a pneumatic signal (compressed air) for pneumatic controllers or an electrical one for electronic controllers. 

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