Distribution control

A central location where instmment leads are short is preferred. In modem faciHties with distributed control systems, all units are controUed from a central control room with few operators. Only a few roving operators are available to spot trouble. It is desirable to deep process equipment a minimum of 8 m away from the control room. Any equipment and hydrocarbon-containing equipment should be separated by at least 15 m if possible. Most control rooms are designed with blastproof constmction and have emergency backup power and air conditioning. The room is pressuri2ed to prevent infusion of outside air that may have hydrocarbon content in the explosive range. 

Fig. 2. Distributed control system stmcture, where PIU = process interface unit LCUj = local control unit, model 1 LCU2 = local control unit, model 2 ...

Microprocessor technology permitted these technical issues to be addressed in a cost-effec tive manner. In the mid-1970s, a process control architecture referred to as a distributed control system (DCS) was introduced and almost instantly became a commercial success. A DCS consists of some number of microprocessor-based nodes that are interconnec ted by a digital communications network, often called a data highway. The key features of this architecture are as follows ... 

Figure 8-62 depicts a hypothetical distributed control system. A number of different unit configurations are illustrated. This system consists of many commonly used DCS components, including multiplexers (MUXs), single/multiple-loop controllers, programmable logic controllers (PLCs), and smart devices. A typical system includes the following elements as well ... 

The component controllers used in the controller subsystem portion of the DCS can be of various types and include multiloop controllers, programmable logic controllers, personal computer controllers, singleloop controllers, and fieldbus controllers. The type of elec tronic con-troUer utihzed depends on the size and func tional characteristic of the process apphcation being controlled. See the earlier section on distributed control systems. 

Currently, the trend in process control is away from centrahzed process control and toward an increased number of small distributed-control or PLC systems. This trend will put emphasis on the evolution of the fieldbus controller and continued growth of the PC-based controller. Also, as hardware and software improves, the functionality of the controller will increase, and the supporting hardware will be physically smaller. Hence, the traditional lines between the DCS and the PLC will become less distinct as systems will be capable of supporting either function set. 

Distribution of benzodiazepines in I-octanol - water system was investigated by a direct shake flask method at the presence of the compounds used in HPLC mobile phases the phosphate buffer with pH 6,87 (substances (I) - (II)), acetic and phosphate buffer, perchloric acid at pH 3 (substances (III) - (VI)). Concentrations of substances in an aqueous phase after distribution controlled by HPLC (chromatograph Hewlett Packard, column Nucleosil 100-5 C, mobile phase acetonitrile - phosphate buffer solution with pH 2,5, 30 70 (v/v)). 

Snyder, D. R. 1989. Understanding Distributed Control. Chemical Engineering, 96(5), 87-79. 

Distributed Control System (DCS) A system that divides process control functions into specific areas interconnected by communications (normally data highways) to form a single entity. It is characterized by digital controllers, typically administered by central operation interfaces and intermittent scanning of the data highway. 

A distributed control system (DCS) normally uses input and output modules which contain eight, sixteen, or more inputs or outputs. Failure of the module will simultaneously disable a large number of control loops. Attention to the assignment of input/output points to the modules makes the plant more tolerant of a failure of an input or output module (CCPS, 1993a). For a more detailed discussion of process control systems, see the process control part of Section 4.4, and Sections 6.4 and 6.5. 

In one site, the project team installing a DCS (Distributed Control System) carefully developed and tested techniques to make the displays clear for red-green color blind personnel (see discussion in 6.4). The displays were effective and were applauded by the operators. However, in subsequent DCS installation projects at the same site, different project teams made no provision to make displays visible to red-green color blind personnel. This inherently less safe condition was found during a design review at one unit and at the pre-start-up safety review for another unit. 

Rousseau, R.W. and Howell, T.R., 1982. Comparison of simulated crystal size distribution control systems based on nuclei density and super-saturation. Industrial and Engineering Chemistry Process Design and Development, 21, 606. 

Unfortunately the address of the gateway in the control computer used for the data transfer was the same as that used to connect to the distributed control system (dcs). As a result data flowed from the simulator through the control computer to the dcs and replaced the current input data by historic data. Some conditions on the plant started to change, but fortunately this was soon noticed by alert operators, and the plant was brought back under control. 

Improved boiler design/operation (cogeneration), and distributed control systems... 

Modified ball mill configuration, particle size distribution control, improved grinding media and linings, high-pressure roller press for clinker pre-grinding, high-efficiency classifiers, roller mills... 

Some of the same physical properties of the liquid and gas phases as well as temperature and pressure and the amount of entrained liquids (or solids if present) and the expected particle size and iLs distribution control the design and performance of these units also. 

This chapter presents the entire procedure for performing heat and weight balances. The last section of the chapter discusses the use of the distributed control system and computer in automating the process... 

If the unit does not have a distributed control system (DCS), a debottlenecking project is the right time to justify it. If it does have a DCS, advanced control projects should be justified. A DCS ... 

Distributive Control System (DCS) is a digital control system that has a distributive architecture where different control functions are implemented in specialized controllers. 

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