Schematic drawings of process units showing equipment, instrumentation, piping, valves and other components and associated data are required for construction, operation and maintenance of process plants. These schematic drawings are initially prepared and used for the construction of the units and are frequently referred to as process and instrument diagrams (P&ID's).
The instrument symbols used in these drawings are generally based on Instrumentation, Systems, and Automation Society (ISA) Standard S5.1
P&ID's are also used for operation and maintenance of the unit after construction and are retained and used for many years.
P&ID's however have problems when being used for the Control systems information on them.
P&ID standards are very variable from company to company, what is shown and the numbering systems vary enormously
They are often very inaccurate, frequently missing details of control loops, contents of process packages etc.
P&ID's are not object oriented in any way. For example where are several identical units (ie one Unit Object) they still mostly draw one P&ID for each. If you are lucky. Sometimes you will find more than one unit (say 2 and a half) on one P&ID or one unit defined on 2 or more P&ID's!
The instrument database behind typical P&ID software is often inadequate for Control purposes. The text in them is often useless (we know FT means Flow Transmitter, we want to know what the flow is doing!)
P&ID's have a slow development cycle and are almost always out of date apart from the marked up paper one the process engineer has.
P&ID's are not designed for operational purposes, for example they are full of clutter such as nozzle sizes that are not relevant to the control, or even the plant operators.
They are usually a poor basis for control system graphics.
P&ID's totally fail to represent procedural functions
In fact for all but the simplest control loops, additional documentation such as Loop Diagrams is always needed
For more on P&ID's and PFD's see the University of Michigan Chemical Engineering Controls Wiki.
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