Control valves symbols

The symbol for the control valve in Fig. 8-47 is for a pneumatic positioning valve without a valve positioner. 

Data on spare and parallel equipment are often omitted. Valving is also generally omitted. A alve is shown only where its specification can aid in understanding intermittent or alternate flows. Instrumentation is indicated to show the location of variables being controlled and the location of the actuating device, usually a control valve. To help the reader better understand the process flow sheet, a list of commonly used symbols is presented in Fig. 5.9.1. 

This symbol is used to represent all types of control valve, and both pneumatic and electric actuators. 

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

On working flowsheets the detectors, transmitters, and controllers are identified individually by appropriate letters and serial numbers in circles. Control valves are identified by the letters CV- followed by a serial number. When the intent is to show only in general the kind of control system, no special symbol is used for detectors, but simply a point of contact of the signal line with the equipment or process line. Transmitters are devices that convert the measured variable into air pressure for pneumatic controllers or units appropriate for electrical controllers. Temperature, for instance, may be detected with thermocouples or electrical resistance or height of a liquid column or radiant flux, etc., but the controller can accept only pneumatic or electrical signals depending on its type. When the nature of the transmitter is clear, it may be represented by an encircled cross or left out entirely. For clarity, the flowsheet can include only the most essential information. In an actual design... 

An endothermic continuous stirred tank reactor (CSTR) is shown schematically in Figure 15.2. The symbols used in the control schematics are listed in Table 15.1 The controlled variable is the temperature of the product leaving the reactor, and the manipulated variable is the flow rate of steam to the heat exchanger, which adds heat to the recycle line. The final control element is the control valve and associated equipment on the steam hue. The sensor is a temperature sensor/trans-mitter that measures the temperature of the product stream leaving the reactor. The controller compares the measured value of the product temperature with its desired temperature (setpoint) and makes changes to the control valve on the steam to the heat exchanger. The process is the... 

This control loop is also indicated on the PFD of Figure 1.5. However, the details of all the instrumentation are condensed into a single symbol (LIC), which adequately describes the essential process control function being performed. The control action that takes place is not described e q)licidy in either drawing. However, it is a simple matter to infer that if there is an increase in the level of liquid in V-104, the control valve will open slighdy and the flow of benzene product will increase, tending to lower the level in V-104. For a decrease in the level of liquid, the valve will close slighdy. 

Because the level transmitter, I/P transducer, and control valve have neghgible dynamics, the corresponding transfer functions can be written as Gm(s) = Km, Gip s) = Kjp, and Gy(s) = Ky, The block diagram for the level control system is shown in Fig. 11.16, where the units of the steady-state gains are apparent. The symbol Hgp denotes the desired value of liquid level (in meters), and H p denotes the corresponding value (in %) that is used internally by the computer. Note that these two set-points are related by the level transmitter gain Km, as was discussed in Section 11.1. 

All valves, control and block valves, with an identification number. The type and size should be shown. The type may be shown by the symbol used for the valve or included in the code used for the valve number. 

The symbols used to show the equipment, valves, instruments and control loops will depend on the practice of the particular design office. The equipment symbols are usually more detailed than those used for the process flow-sheet. A typical example of a P and I diagram is shown in Figure 5.25. 

Standard symbols for instruments, controllers and valves are given in the British Standard BS 1646. 

A general knowledge of the symbols for flow, level, pressure, and temperature controllers, as given in Fig. 5.9.2, is needed to comprehend flow diagrams like the simple example presented in Fig. 5.9.3. In this vessel, with an inlet feed on top of the tank equipped with a flow controller, the level in the tank is maintained by a level controlling device. When the le el rises above the high level point, the level controller sends a signal to a valve actuator and the alve is opened to drop the le el. When the level approaches a specified value, the valve is closed. Complicated systems can be analyzed in the same manner used in this basically simple example. 

International standard symbols for instruments, controllers, and valves are given by the Instrumentation Systems and Automation Society design code ISA-5.1-1984 (R1992). Some companies use their own symbols though, and different standards are followed in some countries, such as BS 1646 in the UK and DIN 19227 and DIN 2429 in Germany (see references). 

In presence of certain substances, the phase transition temperature of thermo-sensitive hydrogels is altered. This phenomenon is used to design electrothermi-cally adjustable hydrodynamic microtransistors, which are also called chemostat microvalves (Richter et al. 2007a). The valve seat of the device (Fig. 14a) is tempered by a heater and an integrated temperature sensor is used for a closed-loop control. The volume phase transition temperature of PNIPAAm decreases with increasing alcohol content in water (Fig. 14b, solid symbols). Therefore, each critical alcohol concentration or volume phase transition correlates with one characteristic isotherm. Tempered at a particular isotherm the valve switches at a certain concentration (Fig. 14b, open symbols). 

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