Distributive control system

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. 

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... 

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. 

The reactor pressure is reduced to 0 psig to flash off any remaining water after a desired temperature is reached. Simultaneous ramp up of the heat source to a new setpoint is also carried out. The duration spent at this second setpoint is monitored using CUSUM plots to ensure the batch reaches a desired final reactor temperature within the prescribed batch time. The heat source subsequently is removed and the material is allowed to continue reacting until the final desired temperature is reached. The last stage involves the removal of the finished polymer as evidenced by the rise in the reactor pressure. Each reactor is equipped with sensors that measure the relevant temperature, pressure, and the heat source variable values. These sensors are interfaced to a distributed control system that monitors and controls the processing steps. 

A detailed discussion of the application of digital computers and microprocessors in process control is beyond the scope of this volume. The use of computers and microprocessor based distributed control systems for the control of chemical process is covered by Kalani (1988). 

KALANI, G. (1988) Microprocessor Based Distributed Control Systems (Prentice Hall). 

The latest designs for onshore installations cater for a centralized control room, well distanced from the operating facility with sub control areas as part of a distributed control system (DCS). The sub-control areas are closer to the processes but contain fewer personnel and process control systems for the overall plant, so the overall risk level for the facility from a major incident is lowered. The outlying control buildings (sometimes referred to as PIBs or SIHs) still need to be sited against impacts from explosions and fires. 

Distributed Control System (DCS) - A generic, microprocessor based, regulatory system for managing a system, process or facility. 

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