Final controlling equipment

DIN standard final controlling equipment ( Stelleinrichtung ) and final controlling element ( Stellglied ). It is worth noting that the term actuator used in Fig. 6.2 conflicts with the actuator definition presented above which shall serve as the basis for this chapter. 

Selective and Override Control When there are more controlled variables than manipulated variables, a common solution to this problem is to use a selector to choose the appropriate process variable from among a number of available measurements. Selec tors can be based on either multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. 

A boiler house enclosed eight large boilers attended by two men. Failure of the combustion air supply shut down one of the boilers. This boiler shutdown created conditions beyond the control of just two men and lack of proper combustion control equipment finally caused seven of the eight boilers to shut down. Amid the confusion caused by low instrument air... 

Equipment Selection Criteria and Guidelines A number of factors should be considered to determine when to select which type ofvapor/gas/solid-liquid separator (blowdown drum or cyclone separator) to handle a multiphase stream from a relief device, and which final control or destruction equipment (emergency flare, emergency scrubber, or quench pool/catch tank) should be selected. These factors include the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

The selection of vapor/gas/solid-liquid separators and final control and destruction equipment can be helped by considering the advantages and disadvantages of each, as given below. 

May be used as final step after other control equipment, e.g., emergency scrubber or quench pool. 

The sensor, transmitter, and control valve are physically located on the process equipment ( in the field ). The controller is usually located on a panel or in a computer in a control room that is some distance from the process equipment. Wires connect the two locations, carrying current signals from transmitters to the controller and from the controller to the final control element. 

Medical uses for Udel resin include surgical trays, nebulizers, flow controllers for blood, and respiration regulators. Transportation applications center around automotive fuse housings, electrical connectors, and switches. Electrical and electronic end uses include coil bobbins, housings, connectors, bushings, capacitor film, and business machine parts. Finally, water, heater dip tubes, milking machine parts, pollution control equipment, and some filtration membranes are made. 

Risks linked with chemical processes are diverse. As already discussed, product risks include toxicity, flammability, explosion, corrosion, etc. but also include additional risks due to chemical reactivity. A process often uses conditions (temperature, pressure) that by themselves may present a risk and may lead to deviations that can generate critical effects. The plant equipment, including its control equipment, may also fail. Finally, since fine chemical processes are work-intensive, they may be subject to human error. All of these elements, that is, chemistry, energy, equipment, and operators and their interactions, constitute what we call process safety. 

First, we need to define generally what we are talking about A pressure relief device is any device that can purge a system from an overpressure condition. More particularly, an SRV is a pressure relief device that is self-actuated, and whose primary purpose is the protection of life and equipment. Through a controlled discharge of a required (rated) amount of fluid at a predetermined pressure, an SRV must prevent overpressure in pressurized vessels and systems, and it operates within limits which are determined by international codes. An SRV is often the final control device in the prevention of accidents or explosions caused by overpressure... 

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... 

The actuator system for a process control system in the CPI is typically composed of the control valve, the valve actuator, the I/P transmitter, and the instrument air system. The actuator system is known industrially as the final control element. In addition, a variety of optional equipment is designed to enhance the performance of the actuator system, such as valve stem positioners and instrument air boosters. 

Other final control elements include relays to start or stop various equipment, variable-drive motors for fans or pumps, heavy load elec-trohydraulic actuators, and so on. 

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