ISA 5.1 Tagging Convention 2

DM2 · Jan 25, 2024

DM2 (Mechanical)(OP)25 Jan 24 00:09
I'm trying to understand ISA 5.1, Table 4.1 as it might apply to a Gas Detector.

Normally I see a gas detector tags as "ASH..." I've been told that the "S" is for "Safety" and the "H" is for High.
Table 4.1, "A" Column 1 is = Analysis (4-20mA Signal...right?), Blank for Column 2, Alarm for Column 3.
"S", Column 2 is "Safety" and "H", Column 5 is "High"

Does the table mean that the definition for a modifier listed in Column 2 or 5, has to be used as defined under the respective column?

I'm a little surprised I don't see "I", which under column 1 is "Current", and Column 3 as "Indicate". Section 4.2(2) tells me I can't modify the meaning for the 1st letter (Column 1), but I'd be using it in column 3 (output / Active Function) since it outputs a 4-20mA sign.

I'm wanting to assign a tag that allows me to distinguish the different types of Gas Sensors (CH4, CO, H2S, etc)

Since Column 2 is a "Modifier" and for "A" column 2 is Blank, would it make sense to assign a Combustible Gas detector "ACH...) (C for Combustible) or "ATH..." (T for Toxic)?
...also could I use "U" in place of the "H" considering the detector is used to record multiple levels of the gas?

Regards,
DM
"Real world Knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity."

GBTorpenhow · Jan 25, 2024

ASH doesn't seem correct because of the misuse of S for safety: 4.2(14) Variable Modifier safety is technically not a direct-measured variable but is used to identify self-actuated emergency protective primary and final control elements only when used in conjunction with Measured/Initiating Variables flow [F], pressure [P] or temperature [T]. And because of the critical nature of such devices, [FS, PS, and TS] shall be considered as Measured/ Initiating Variables in all Loop Identification Number construction schemes:

Unless its a 'switch' which doesn't make sense as you reference 4-20mA output.

I agree that A makes the most sense for primary variable as the detector is returning composition analysis, %LEL likely:
4.2 (3) Measured/Initiating Variable analysis [A] shall be used for all types of process stream composition and physical property analysis. The type of analyzer, and for stream component analyzers the components of interest, shall be defined outside the tagging bubble.

Annotate the bubble with COMB from table 5.2.2.

The function of the device is to transmit a 4-20mA signal to some other device, so T for output/active function.

So the device itself would be AT. The signal coming from the device would be AI. High alarm AAH, High High Alarm AAHH.

If the detector is part of a SIS then Z would be appropriate as a variable modifier for all the above: AZT, AZI, AZAH, AZAHH

DM2 · Jan 25, 2024

GBTorpenhow...thanks for your response.

ASH actually comes from API RP 14C, Table 1. Oddly enough API RP 14C, section 4.2 is their support of the use for "S" as "Safety". As you surmised, the gas detectors do return the %LEL or PPM as an analog value but gas detectors can be purchased with contacts for Low, High, & Fault to be monitored as a digital input to some systems (conventional fire panel as an example). Having said that, they're still processing the signal from the sensor and then internal firmware is driving the contacts. We use the 4-20mA which is the focus of my question.

4.2(14) Variable Modifier safety ~~is technically not a direct-measured variable but is used to identify self-actuated emergency protective primary and final control elements]~~

...well now I'm going down a rabbit hole...So would a Solenoid that activates a deluge valve (or something similar) have a tag of "SOV"?

We've been asked to us "AT" for gas detectors in the past.

In looking at Table 5.2.1, the "(*)" [bubble subscript?) would seem to provide the solution to "What is the target gas for the sensor?". Table 5.2.5, seem to suggest that a gas detector sensing H[sub]2[/sub]S would have a subscript of "H[sub]2[/sub]S" outside of the bubble?

Any recommendations on books, YouTube Video's, online classes, etc., that might help me to understand all this &*#?

Regards,
DM

"Real world Knowledge isn't dropped from a parachute in the sky but rather acquired in tiny increments from a variety of sources including panic and curiosity."

GBTorpenhow · Jan 25, 2024

SOV is an "english language abbreviation" style tag for solenoids that is common in the wild, but it isn't a valid ISA tag. S as a primary variable would be speed/frequency. A typical ISA compliant tag for a solenoid would be to tag it as an auxiliary device (Y) to the final control element that it trips/activates, using the same loop variable, so e.g. a control valve modulating flow (FV) might have a tripping solenoid (FY).

Because 'analysis' could mean many different things, yes, the bubble needs a label, H2S for hydrogen sulfide, GC for a gas chromatograph, O2 for oxygen, etc. per table 5.2.2.

ISA 5.1 is pretty well written and as far as learning "P&ID tagging strictly per ISA" goes all you should need is this standard. Of course, ISA 5.1 isn't law, and real world P&ID tagging often gets real messy real fast.

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ISA 5.1 Tagging Convention 2

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