Every capture retrofit on an operating gas plant starts with one question: what is actually installed. The answer lives in a drawing archive that is partly vector, partly scanned, partly redlined, and partly wrong, and in a DCS database that knows what is wired but not what is piped. Until those two are reconciled into a current-state register, the FEED contractor cannot scope the tie-ins, the operations group cannot write the MOC, and the estimate has no basis.
This is a procedure for building that register, written for the owner's or the consultant's engineer who has been asked for it before the capture package is awarded.
Key takeaways
- Decide first which plant you have: a sour plant with CO2 already separated at the acid gas removal unit, or a sweet plant where the CO2 is in the flue gas. The scope of the register follows.
- Establish the governing revision per sheet from the drawing register before reading anything.
- The register is built from the drawings and the DCS database together, with the disagreements listed rather than resolved by guesswork.
- Every row carries its sheet and revision. A row without a reference cannot be defended in the MOC.
- The change report between the baseline and the retrofit IFC is the MOC package's content; the change report at as-built closes it.
Step 1. Which retrofit is this
| Plant | Where the CO2 is | What the retrofit touches | Register scope |
|---|---|---|---|
| Sour gas plant | Acid gas from the amine regenerator, already separated, mixed with H2S | Acid gas compression and dehydration, H2S handling or injection, the sulphur plant if there is one, the regenerator overhead | The acid gas removal unit, the regenerator overhead system, the sulphur or acid gas injection train, the utilities they draw on |
| Sweet gas plant, or fired equipment targeted | Flue gas from turbines, heaters and boilers | A post-combustion capture unit on the flue gas, new steam and cooling demand, flue gas take-offs | The fired equipment and its stacks, the steam and condensate system, cooling water, the electrical system the new load lands on |
| Both | The DCS and SIS, the flare and vent system, the plot plan and the hazardous-area drawings, in every case |
A sour plant retrofit is a compression project on a stream that exists. A sweet plant retrofit is a new process unit bolted onto the plant's stacks. The instrument counts differ by an order of magnitude and so does the archive that needs reading.
Step 2. Triage the archive
The drawing register, if the plant has one, is the map. If it does not, build one from the files: every P&ID, one-line, loop drawing, datasheet and cable schedule, with its number, title, latest revision and the form it exists in.
| Form | What it usually means | What to do with it |
|---|---|---|
| Native CAD or vector PDF | Issued by the last EPC or the owner's drafting group | Read directly; check the revision block against the register |
| Scanned PDF or TIFF | Older sheets, often the original plant | Read as a scan; expect redlines and stamps; record the revision from the block, not the filename |
| Redlined scan | A field markup that was never incorporated | Treat the redline as the governing state for what it marks, and record it as a discrepancy against the clean revision |
| Superseded revision | Kept for history | Exclude from the register; keep in the archive index |
| Missing sheet | Referenced by a match line or the register but not in the files | Record as missing; the walk-down covers it |
The output of this step is the governing revision per sheet. It is written down before any tag is read, because a register read from the wrong revision is worse than no register.
Step 3. Read the registers from the drawings
For each governing sheet, the instrument index, line list, equipment list and valve list are read as printed, in the plant's own tag convention, with the sheet number and revision on every row. On a plant built decades ago that convention is a house rule: area codes that mean something to the original EPC, letter codes that drift from ISA 5.1, loop numbers reused across units. The register records the tags as they are, with the sheet reference, and does not normalize them. A row like PT-NCG-1305 on the synthetic sheet stays PT-NCG-1305 even when the same transmitter appears in the DCS as a different string; the reconciliation step handles that, not the reading step.
Two things are worth pulling out of the read as it goes:
- Off-page connectors and match lines, because they are how the sheet set is checked for completeness. Every connector should land on a sheet in the register. One that does not is a missing sheet.
- Revision clouds and triangles on the governing revision, because they show what changed last, and the last change is the one most likely to be reflected in the DCS but not the datasheets, or the reverse.
Step 4. Reconcile against the DCS and the field
The DCS tag database export is the second source. It lists every wired point, its I/O type, controller and address, and it is authoritative for what is connected today. Reconciling it against the index produces four lists:
| List | Meaning | Action |
|---|---|---|
| On the drawing and in the DCS | Confirmed | The row is complete |
| On the drawing, not in the DCS | Never wired, decommissioned, local-only, or a drawing error | Walk-down decides; record the outcome on the row |
| In the DCS, not on the drawing | Added without a drawing update, or a soft tag | Walk-down decides; a real instrument gets a discrepancy record and, eventually, a drawing update |
| Tag string differs | The same instrument under two names | Record both, keyed on the sheet; do not rewrite either |
The walk-down is targeted from these lists rather than done blind. It covers the discrepancies, the missing sheets and the tie-in areas, and it produces the field notes that become the discrepancy register.
Step 5. Scope the tie-ins
With the register standing, the tie-in candidates can be listed against it. For a sour plant retrofit these are the regenerator overhead line, the acid gas header and its existing compression if any, the sulphur plant feed, the flare header, and the utilities. For a sweet plant retrofit they are the stacks and ducts of each fired unit, the LP steam header, condensate, cooling water, instrument air, the MCC and the DCS marshalling.
Each candidate gets a row: the host sheet, the host tag or line it modifies, the isolation available, the utility capacity at that point, and whether the connection can be made live or needs a shutdown. That row is the seed of the tie-in register the FEED contractor will finish. The current-state register is what makes it possible to write it without guessing.
Step 6. Hand the package over
What the FEED contractor needs, and will rebuild at the owner's cost if not given:
- the current-state instrument index, line list, equipment list, valve list and I/O list, with sheet and revision on every row;
- the drawing register with the governing revision per sheet and the missing-sheet list;
- the discrepancy register from the DCS reconciliation and the walk-down;
- the tie-in candidate list;
- DCS and SIS spare capacity by controller and I/O type;
- the utility balances at the tie-in points;
- the drawing files, in the form they exist.
Step 7. Use the change report twice
When the retrofit IFC set arrives, the comparison of the IFC registers against the current-state baseline, tag by tag, is the content of the MOC package: what is added, removed and modified, on which sheets. When the as-built set arrives, the same comparison against the IFC shows what was actually changed against what was approved, and closes the MOC. The as-built register then becomes the new baseline, and the next project starts from it rather than from the archive.
Tagsight reads the archive, scanned or vector, in the plant's own convention, into the current-state registers with every row referenced to its sheet, and compares revisions tag by tag so the change report comes with the register. The reconciliation decisions, the walk-down and the MOC remain the plant's engineering.