Advantages of scada systems in petrochemical
DCS (distributed control system) and scada (supervisory control and data acquisition) systems both emerged independently within the controls industry and by the early to mid 1970s they both had a strong following. DCS was a term coined by two major control vendors providing plant automation systems in the early 1970s while scada first appeared in print in 1973 as part of the Bonneville Power Administration Study into system automation started in the late 1960s. Although these systems have much in common, the goals of DCS and scada systems are quite different.
DCSs are process centric and have a direct connection to the data source. They are primarily focused on realtime states with past and present process variables the main focus. DCSs are most often directly connected to the hardware device I/O, and therefore require a very reliable network connection to equipment. As a process state driven system, a DCS is sequential in nature with alarms generated not when a point changes but when a process is run; trends primarily focused on data states past and present. In many instances DCSs are not at all aware of a change in state, but simply report the current realtime state at the instant the hardware is polled. In a DCS, events and alarms (both central concepts in scada) are secondary to process displays. Therefore, it is often necessary to set up alarms by hand on each point, and many DCSs have limited alarm filtering. Finally, DCSs are deployed only when the system has a very small geographical footprint, such as a factory or plant, due to the fact that they must have a continuous connection to the hardware.
Scada systems are data centric and have a database driven architecture. Scada systems have historically focused on control of remote equipment where telecommunications may be regularly interrupted. To overcome the unreliable telecommunications to remote field devices, scada systems are indirectly connected via a database to the field equipment and can continue operating even when telecommunications are temporarily lost. In addition, more complex polling capabilities are typically provided by scada systems to help address telecommunication restrictions. This allows scada systems to deal with controlling assets over a very large geographical area, making it suitable for electric grids, and oil and gas pipelines.
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