Definition:
Logic Diagrams graphically represent the logic implemented in the controls of a system. It can use one of a few different types of logic symbology per IEEE, ISA, or IEC. In a SIS, it would document what the logic solver is doing and the decisions it is making. Logic diagrams also facilitate review, change management, and troubleshooting for both engineers and operations personnel.
Logic diagrams are a step up from a main interlock list (MIL) as it can show more complexity than a MIL ever could. Often a SIS is fairly straightforward (it sees high pressure, then it closes a valve) that a MIL may be sufficient.
For a complete BPCS, which is substantially more complex that a SIS, logic diagrams are good engineering practice. Often done with a functional narrative (the description with words). It would be hard to fully document a complex BPCS, especially with complex speed control logic for a VFD, with only a MIL.
- Simple systems or a simple SIS –> often no logic diagram. Only a MIL and functional narrative
- Complex systems –> lean towards logic diagrams and a functional narrative.
Key Points:
- Common types include Ladder Logic, Function Block Diagram (FBD), and Boolean logic diagrams
- Used to document safety logic.
- Often included in the Safety Requirement Specifications (SRS)
- Some simple facilities do not use logic diagrams and use a combination of a MIL and a Functional Narrative.
Example:
A logic diagram showing that if any two out of three pressure transmitters detect high pressure, then the shutdown valve will close. This could be drawn using function blocks, relay symbols, or Boolean gates, depending on the engineering standard or application.
See also: Functional Narrative, MIL
Cited Source:
- IEC 61131-3.