Complete and accurate documentation is critical to the commissioning and ongoing maintenance and operation of a PLC or DCS system and should be a high priority of design contract administration.
PLC or DCS system documentation provided by the system designer and/or system integrator should include the following:
- One-line diagrams
- Piping and instrumentation diagrams (P&IDs)
- Sequences of operation
- Instrument data sheets
- Points list
- Loop diagrams or I/O wiring diagrams
- Binary logic diagrams
- Control schematics
- PLC / DCS program listing
- HMI description (screenprints and database)
- Software configuration management documentation
- Facility physical and information security policies
All PLC or DCS documentation should be furnished in both hard copy and appropriate electronic format. The electronic format should be capable of revision; i.e. scanned documents or portable document format (pdf) files are not acceptable, although pdfs may be submitted as archive copies in addition to revisable files.
All system documentation should be treated as sensitive and this applies to PLC or DCS system diagrams and documentation as well. At a minimum, document security measures should include:
Marking all documents
Controlling access to documents
Properly destroying outdated or unused documents
Symbols and identification
Individual control loops should be identified by loop number following the convention established by ISA S5.1.
Each instrument, actuator and control point should have a unique tag number incorporating the loop number and identification letters per table 1 of ISA S5.1. All equipment and instruments should be consistently identified by their complete tag number on all drawings and points lists.
Piping and Instrumentation Diagram (P&ID)
P&IDs should be provided for all mechanical systems. P&IDs should depict all components of mechanical systems including vessels, pumps, compressors, chillers, heat exchangers, piping, valves and instruments and the control relationships between them. Symbols used should comply with ISA S5.1 – Instrumentation Symbols and Identification and ISA 5.3 – Graphic Symbols for Distributed Control/Shared Display Instrumentation, Logic and Computer Systems.
Sequences of operation
Written sequences of operation should be provided for all control loops.
Sequences of operation should include the following minimum information:
- Manual and automatic control modes;
- Normal system conditions and modes of operation;
- Contingency conditions and modes of operation;
- Effect of all operator controls;
- Description of all interlocks and permissives;
- Identification of failure state of all outputs.
- Instrument data sheets
- Instrument data sheets should be provided per ISA S20
Check the following instrumentation checklists:
- Transmitter Loop Checklist
- Process Switches Checklist
- PID Controller Checklist
- PLC Digital Input Checklist
- Instrument Installation Checklist
Points list
A points list should be provided that includes all real and virtual input, output and control points.
Provide the following information, where applicable, for each point:
- System
- Equipment
- Loop number
- Tag number
- Controller, rack, and slot
- Point type
- Field location
- Range
- Failure state
Example of Point List
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