PES vs. PLC vs. ESD

[dppalmer]

I read a good exchange on functionality and merits of DCS vs. PLC (see thread698-161284). Now that DCS functional often includes logical controls, instrumented process trips (i.e. interlocks) can be easily programmed into most DCS systems. However, as noted in referenced threads, critical trip systems are often implemented in dedicated PLC, particularly when high reliability is required (e.g. emergency shutdowns - ESD).
Is anyone aware of instrument symbology to differentiate operational interlocks that may be implemented in DCS from safety-instrumented interlocks (SIS) that must be implemented in ESD or PESS (programmable electronic safety system)? We have been using standard ISA symbology for all interlocks (using boxed hexagon)on P&IDs. However, creates ambiguity on whether trip system is SIS or an operational interlock or sequential control element. I have asked a couple of instrument engineers, but apparently there is no standard ISA symbol for differentiating a PESS from a PLC. Has anyone found a good way to do that short of labelling each symbol on the P&ID?

 

[Ashereng]

If you really want to be explcit, you can write under the symbol "DCS1", "PLC13", "SIS2", etc.

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[AGomaa]

Sometime the instrument tag can be used to distiSometime the instrument tag (tag or prefix/suffix) can be used to distinguish the control system that the instrument is connected tonguish

 

[AGomaa]

Sometime the instrument tag (tag or prefix/suffix) can be used to distinguish the control system that the instrument is connected to.
Sorry for the cut and paste error in my reply

 

[dppalmer]

I am hoping to use different symbols rather than additional text. Instrument tag prefixes need to follow ISA standards. Lengthening the instrument tag either in the prefix or suffix to designate control system makes it difficult to fit within symbols.

I have thought about using a boxed diamond (i.e. diamond within a square) for SIS rather than boxed hexagon which we use as our standard symbol to denote PLC I/O on P&IDs.

I can define symbols on legend sheet, but am interested to learn if there is a common practice in the absence of ISA standard for designating PESS input/outputs.

Thanks for feedback so far!

 

[controlnovice]

I don't know of a standard, but we started using a diamond within a square, like your example....

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