Prom the most general analysis, there can be established the following general characteristics /and given some convenient particular observations in each case according to our
objective/ related to the components used in any industrial process, namely;
- Each component /i.e. measuring instruments, electro
mechanical devices, etc./ is physically located in only
142 -one place, invariant with time#
Since all the components have precise and fixed physical locations within the process, a given number of inter
related components can be grouped into different Control Sections# Each Section would have its own symbolic diagram.
Superpositions between two or among several Control Sec
tions could be avoided if one of the superposed parts is considered as a new independent Control Section. In this form, the possibility that one or more of the components belongs to more than one drawing does not occurr# This is illustrated in Pig. 3.2 with an example.
THREE SECTIONS WITH SUPERPOSITIONS THE SAME SECTIONS TRANSFORMED AND IN THE SHADED AREAS CONSIDERED AS SIX DIFFERENT SECTIONS
Fig. 3.2
When more detailed information is required from some part of a given picture shown on a display screen, in case of sophisticated graphical displays it is commonly extracted vising windowing facilities. In our conception, when this
is the case, the more detailed part must be considered as a new Section with a new independently programmed symbolic diagram. However, specific components could occasionally appear in more than one symbolic diagram. In these cases, it is necessary to specify some priority level among
several different drawings containing a given common com
ponent /or components/, in order to select automatically that drawing with the higher priority level when an emergency
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-situation takes place with the common component to all them. More than two different priority levels are not required, because in alarm situations it is sufficient to show only one diagram of all those containing a common damaged component. Nevertheless, updating of common components must be done by the computer in all the di
agrams containing any number of them.
Division of the whole process area in Control Sections depends on the density of components desired within each one of them, that is, on the level of detail desired for each drawing. It does not depend on the physical dimension or area occupied by the components considered, because we deal always with the symbolic representation of the
corresponding Control Sections considered in the plant, but of course, it depends on the total area of the
available useful surface on the display screen.
Every component realizes, either continuously, periodically, intermittently or even occasionally, precise and predefined tasks all of them inherent in its particular design and purpose.
Components with only one task and those with more than one task, operating regime, stable states, etc., must be
independently considered from the control point of view.
The action or effects caused by the operation of each component, can influence only one Section, more than one Section, or the whole plant.
However, each Control Section can be always considered from particular points of view as a whole, in spite of the possible direct influences arising from other Sec
tions, because in each Control Section all the variables related to it should be generally measured independently of /although influenced by/ the actual state of the
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-remainder of the plant. A given hierarchy order must be established in these cases when the whole system is designed.
Components are commonly of different nature, that is hydraulic, pneumatic, electric, electromechanical,
electronic, etc.
Due to this and because the information required from them is sensed by different types of sensors, it may be requi
red that their outputs be opportunely transformed to electrical signals by the corresponding transducer equip
ment.
Some types of components supply only analog values about their particular state /i.e. measuring instruments/, or inversely they adopt different stable states for analog values of a variable /i.e. valves with continuous setting possibility, etc./
This means that in some step of the information system, the conversion of analog signals to digital signals and viceversa will be required.
Some components require control and others do not require it.
In common industrial plants, control operations are per
formed automatically in the component itself by means of sensors, stabilizer circuits, regulators of different types, etc., in general only based on specific operating conditions of the particular component being controlled.
When a highly automated system is implemented and a control is required depending on logical or other decisions given by the computing system or by the operators /to close a valve, to start a pump, etc./, then the control must be performed with the help of a given command sent
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-opportunely from the Supervision Center. In the first case the control is local and depends mainly on the current
component’s state; in the second case the control is remote and depends mainly on the conditions of the remainder of the plant, on operator decisions, or on previously
conceived /and programmed/ computer decisions. Both types of controls can be used together on the same component, because the local control performs mainly regulation tasks over a given variable previously set for the component, while the second one performs the change in the component
of the value of other particular variables related to it.
As a general rule, not every component would require this remote control.
Each type of component is periodically watched b y the
operators from particular points of view and with different objectives at a given frequency, or what is the same, it is supervised at different time intervals /each second, each two minutes, etc./
The corresponding time interval must be defined previously depending on the type and characteristics of the component, its dynamic stability, etc., but it will be advantageous if the possibility is present of requests by means of commands given by the operators located in the Supervision Center.
The total number of components of different types required by a complex plant, is high.
This implies special design of the system in connection with the scanning, information processing, searching, and
so on.
The reliability and durability of the components and
auxiliary means must be, and is always expected to be, as great as possible.
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When new means with given specific purposes are added to a process, as a general rule they must have higher
reliability and longer durability than the existing parts, - When a new industrial plant is designed, it always uses
the most up-to-date components at that time.
At present, modularity and unification must be considered, among others, as basic considerations to be taken into account in new designs.
3.4 Basic information required about the process to be