Distributed control system areas

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. 

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. 

With the advance of computer techniques, especially implementation of distributed control systems (DCS) to chemical processes, a large set of on-line measurements are available at every sampling period. The rational use of this large volume of data requires the application of suitable techniques to improve their accuracy. This goal has triggered the focus on research and development, during the last ten years, in the area of plant data reconciliation. Complete reviews on the subject can be found in the works of Mah (1990), Madron (1992), and Crowe (1996). 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

Where practical, the distributed control system hardware areas should be provided with an independent HVAC system. 

In 1989, the NDF Company opened a facility in Georgetown, South Carolina to produce low density polyethylene. Manufacturing of the polyethylene is done in two 50-ton reactors that are encased individually within their own 8-story-high process unit. The main raw materials for the manufacturing operations include ethylene, hexane, and hutene. The polymerization is completed in the presence of a catalyst. The hase chemicals for the catalyst are aluminum alkyl and isopentane. The reactor and catalyst preparation areas are on a distributed control system (DCS). A simplihed process flow diagram is attached. 

The challenge posed by establishing and maintaining communication between distributed controllers has also stimulated research in the area of networked process control (El-Farra et al. 2005, Mhaskar et al. 2007, Sun and El-Farra 2008, 2010). The central issue of maintaining closed-loop stability in the presence of bandwidth constraints and limitations in transmitter battery longevity is typically addressed by a judicious distribution of computation and communication burdens between local/distributed control systems and a centralized supervisory controller. 

In other areas of plant operations, the loss of personnel is compensated for by using increasingly powerful computer systems. For example, a Distributed Control System (DCS) can carry out many of the functions previously performed by several operators. Similarly, sophisticated design software lets one engineer carry out calculations that previously had to be done by a team. Yet there remain certain actions that have to be carried out by people the loss of skilled personnel in these situations represents a true loss. 

Distributed Control System (DCS) communicates with the PC which is connected to the Local Area Network (LAN).The software running on the PC stores process data to the process database on the data server. Data which is not available from DCS enters manually from PCs located in individual plants. There can be also other sources of data (for example Laboratory Information Systems) which are not shown in the Fig. 13-2. After transferring data to the server, data are available to all users connected to the LAN. 

Separate rooms should be provided for I/O wiring panels, the process distributed control computer hardware and related systems. Motor Control Centers (MCC), UPS, and battery rooms. A minimum 1-hour fire-rated partition wall should be provided to ensure isolation of I/O and MCC areas from control rooms. 

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