Electropneumatic Valves

A gas chromatograph is used for the primary separation of the components in gas oil. An automatic unit feeds the chromatograph with samples and a back-flushing unit has heen added in order to remove heavy hydrocarbons, which might otherwise choke the column. TTie carrier gas (nitrogen) is controlled by three electropneumatic valves, as shown in Fig. 4.4. In the normal mode with valve A open and valves B and C closed, carrier gas flows through columns 1 and 2 to the detector. 

Fig. 4.4 Schematic layout of gas and liquid flow lines. A, B and C are electropneumatic 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. 

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

Electro-pneumatic equipment/circuit information (e.g., control valves and other electropneumatic valve circuits) is usually shown on the electrical loop diagram... 

With the exception of the dead-band booster, the application of booster relays has diminished somewhat by the increased use of current-to-pressure transducers, electropneumatic positioners, and electronic control systems. Transducers and valve positioners serve much the same functionality as the booster relay in addition to interfacing with the electronic process controller. 

The circulatory fluid is ejected by an electropneumatically driven ventricular pump. Downstream of the pump, an aortic valve assembly is located two different models have been built in order to offer lateral or frontal view of the prosthesis movements. Suitable stent adapters allow to test prostheses of different type and size. The aorta is a variable compliance rubber tube. Through a rigid conduit the fluid is conveyed to the laminar flow assembly which controls peripheral resistances. Aortic compliance and peripheral resistances are hydropneumatically controlled. The fluid, passing through a venous reservoir open to atmospheric pressure, reaches the left atrium. This is a rigid wall chamber in which a hydropneumatic system relates cardiac output to venous return, reproducing Frank--Starling s Law. Between atrium and ventricle there is another valve test assembly which allows to test mitral valves. 

Electropneumatic interface and control of hydraulic panel valves... 

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