In this article8 sections
An inter-discipline check (IDC), also called a squad check, is the internal review in which the discipline that owns a drawing circulates it to each discipline whose scope it touches, before the drawing is formally issued. On a P&ID, process engineering usually originates and circulates the sheet. Instrumentation, piping, mechanical, electrical, process safety and any package engineer check the items that belong to them, mark comments on the print, and sign the IDC cover sheet. The originator gives each comment a disposition, incorporates the accepted ones, and then sends the drawing to its checker and on to issue as IFR, IFA or IFC. An interface error found at IDC costs a redraft and a line in the comment register. Found after issue, the same error returns as a client comment, a vendor query or a site change.
This note covers the P&ID and the registers that travel with it: the instrument index, the I/O list, the line list and the instrument datasheets. Each contractor and owner writes its own IDC procedure, so the forms, return periods and colour codes differ between projects. The sequence below is the common shape, and the project procedure governs wherever it differs.
What an inter-discipline check is for
A P&ID is drawn by one discipline and built by all of them. Each symbol on the sheet commits another discipline's scope: an instrument bubble becomes an index row, a datasheet and, if wired, an I/O channel; a line number becomes a line list row and an isometric; a motor becomes a load list row and a starter. The originator cannot verify those commitments alone, because the data that proves them sits in other disciplines' registers. The IDC puts each commitment in front of the discipline that has to honour it while the drawing is still a working draft.
The IDC sits between the originator's own work and the formal check. A P&ID revision typically moves through draft, self-check, IDC, incorporation of the IDC comments, discipline check by the originating checker, approval by the discipline lead, and issue under a transmittal. The IDC repeats before each issue whose content changed, so a sheet that took client and HAZOP comments after IFR goes through IDC again before IFA, at least for the disciplines those changes touch.
An IDC signature records that the reviewing discipline checked its interfaces on that revision. Approval of the drawing stays with the originating discipline lead, and acceptance stays with the client.
Who circulates to whom
Most procedures drive circulation from an IDC matrix, a table naming each deliverable type and the disciplines that must review it. A typical matrix sends the P&ID to instrumentation, piping, mechanical, electrical, process safety and the package engineer, the instrument index to process, piping and electrical, and the line list to piping, instrumentation and materials. Limiting each circulation to the disciplines with a stake in the document keeps each review short enough to be done properly.
Two circulation methods are in use. In a parallel issue, each discipline receives its own print at the same time and returns it marked and signed. In a routed issue, one print travels from discipline to discipline with a routing slip, and each reviewer sees the comments made before theirs. Parallel is faster, and routed stops two disciplines marking contradictory changes on the same item.
The cover sheet or routing slip names the document number, the revision circulated, the date sent, the return date and the disciplines required, with a signature box for each that can be signed as "no comment".
What each discipline checks on the P&ID
The IDC is an interface review. Each discipline checks the items on the sheet that its own documents depend on, against its own register, and leaves drafting and internal consistency to the originator and the checker.
| Discipline | What it checks on the P&ID | Checked against |
|---|---|---|
| Process | Line sizes and design conditions against the heat and material balance, control philosophy, relief cases, notes and holds | Process datasheets, design basis |
| Instrumentation | Each instrument tagged once and matching the index, loop numbers, fail positions, trip and interlock references, SIS and BPCS symbols, instrument connection sizes | Instrument index, I/O list, cause and effect matrix, datasheets |
| Piping | Line numbers, sizes and specs, spec breaks at control valves and equipment, vents and drains, insulation and tracing codes, tie-in points | Line list, piping classes |
| Mechanical | Equipment numbers, nozzle marks, design conditions in callouts, package boundaries | Equipment list, equipment datasheets |
| Electrical | Motors, heaters, electric tracing, motor-operated valves, start, stop and run signals | Load list, motor list |
| Process safety | Relief valves with set pressure and discharge destination, car-sealed valves, HAZOP actions shown on the sheet | Relief valve list, HAZOP action register |
| Package engineer | Skid boundary, interface points, which instruments and valves the vendor supplies | Vendor drawings, supplier document requirements |
Instrumentation carries the largest share on most P&IDs, because the facts it checks have to agree in several documents at once. A control valve such as FCV-302 has a tag on the sheet, a row in the index, an AO channel on the I/O list, a datasheet whose line size must match the line list, and a fail position that the cause and effect matrix relies on where the valve takes a trip action. The instrumentation reviewer confirms that each of those facts agrees with the sheet as drawn.
How comments are marked and closed
The IDC print carries the colour legend from the procedure, and reviewers mark to it. Colour codes vary between contractors. One published scheme uses yellow for data checked and correct, red for data to be deleted, blue for data to be added, and green for remarks that are not to be drafted. Other procedures assign the colours differently, so the legend on the cover sheet settles it for each circulation.
Three habits make comments closable:
- Number each comment on the print and in the register, so its disposition can refer to it.
- Write the comment as the change required, located by tag, line number or grid zone. A question goes to the originator directly, and the answer comes back as a written comment.
- Keep to technical content within the reviewer's scope. Drafting style belongs to the checker.
The originator records a disposition against each comment. The usual set is incorporated, incorporated with modification, not incorporated with a stated reason, or on hold pending data such as a vendor drawing or a process calculation. Held items appear on the next issue as a hold cloud or hold note, so the sheet itself shows what is still open.
Closure takes two people. The originator records the disposition, and the reviewing discipline confirms it on the next revision by back-checking the incorporated sheet or accepting the stated reason. A comment the originator closes alone has a disposition nobody has checked.
The comment register carries the record.
| Column | What it holds |
|---|---|
| Comment number | Unique within the circulation, matching the number on the print |
| Document and revision reviewed | For example PID-001 Rev B, exactly as on the cover sheet |
| Location | Tag, line number, equipment number or grid zone |
| Reviewer | Discipline and name |
| Comment | The change required, written as an instruction |
| Documents affected | Other registers or drawings the comment changes, such as the instrument index or a datasheet |
| Disposition | Incorporated, modified, not incorporated with reason, or hold |
| Incorporated in | The revision that carries the change |
| Closed by and date | The reviewer's confirmation on back-check |
Many registers leave out the documents-affected column. It is the column that makes the register re-check below possible.
IDC against IFR review, HAZOP and model review
Four reviews touch a P&ID during design, and the same people attend several of them, which is why they get confused.
| Inter-discipline check | IFR review | HAZOP | Model review | |
|---|---|---|---|---|
| Reviewers | Other disciplines in the design team | Client or owner, sometimes the licensor | Study team with an independent chair and operations | Owner, operations, construction and the design disciplines |
| When | Before each formal issue | After issue for review | On a frozen P&ID set | At set model stages, commonly 30, 60 and 90 percent |
| Question asked | Does the sheet agree with each discipline's scope and registers | Does the design meet the owner's specifications and operating needs | Which deviations cause harm, and are the safeguards adequate | Can it be built, operated and maintained |
| Output | Marked prints and an IDC comment register | Comment resolution sheet with a review code | Worksheets and recommendations | Model review action list |
| Closed by | Originator, with reviewer back-check | Contractor response accepted by the client | Action owners, tracked to closure | Discipline leads |
The order matters for the P&ID. An IDC before IFR keeps interface errors out of the client's comment resolution sheet, where each one needs a formal response. Client comments and HAZOP recommendations then change the sheet, and those changes go through IDC before the next issue. A recommendation that adds a trip adds an initiator, a trip action, an I/O point and a cause and effect row, and each discipline on that chain has to see it.
Re-checking the registers against the revised sheet
IDC procedures are usually written around the drawing. The registers derived from it are separate documents with their own revisions, and they drift unless someone checks them against the sheet the IDC produced.
Take PID-001, circulated at Rev B, with three accepted comments:
- Process adds a high-high pressure switch, PSHH-401, on vessel V-201, tripping XV-301 closed.
- Piping moves FT-101 from line 4"-CS-101-A1A to 6"-CS-102-A1A.
- Instrumentation changes the fail position of FCV-302.
Rev C of the P&ID carries all three, and the registers change with it.
| Accepted comment | Instrument index | I/O list | Other documents |
|---|---|---|---|
| PSHH-401 added | New row | New DI, landing on the system the cause and effect matrix names | Cause and effect row closing XV-301, a setting on the alarm and trip setpoint list, a new datasheet |
| FT-101 moved | Line number updated | No change | Datasheet reissued for the larger line, which can change the meter size and process connection |
| FCV-302 fail position changed | Fail position updated | No change | Datasheet, and the cause and effect matrix where the valve takes a trip action |
The control is to key each register row to the P&ID number and revision it was last checked against, and to re-check the rows whose sheet revised. Three checks cover most of it: each tag on the revised sheet has a row, each row still points at a tag on the sheet, and the shared fields (line number, service, fail position, signal type) agree. A deleted tag keeps its row with a deleted status, because procurement or the vendor may already be working to it. The full comparison against a controlled P&ID set is set out in validating an I/O list against P&IDs.
Where IDCs fail
Most IDC failures are document-control failures around the review.
A stale revision circulated. The print sent for IDC lags the working file, often because drafting continued after printing. Reviewers mark a superseded sheet, and their comments duplicate changes already made or miss new ones. The control is to freeze the working file for the circulation and match the cover sheet revision to the master document register.
Comments closed without incorporation. The register says incorporated and the next issue does not show the change. Dispositions written from memory produce it, and so does a later redraft that overwrites an earlier correction. The reviewer's back-check on the issued revision is the step that catches it.
Registers left on the old revision. The P&ID moves to Rev C and the instrument index still describes Rev B. The drift surfaces at requisition, at vendor datasheet review or at loop check, each later and costlier to fix than the IDC. On an operating plant the same drift is what management of change documentation has to catch.
Silence read as agreement. A discipline misses the return date and the originator issues anyway. A "no comment" signature and a missing return mean different things, and the IDC record should show which one each discipline gave.
Comments raised on the wrong document. An interface problem that belongs on a datasheet, a vendor drawing or the cause and effect matrix is marked on the P&ID, incorporated there, and lost from the document that needed it. The documents-affected column in the register exists for this case.
Holds that reach construction. A hold cloud carried from IFR to IFC means a comment was parked and left unresolved. A hold needs an owner and a target date in the register, like any other open comment.
What a closed IDC leaves behind
A closed IDC leaves an issued revision that shows each accepted change, a comment register in which each comment carries a disposition and a reviewer's closure, and derived registers that cite the revision issued. Those are the records a client reviewer, a HAZOP team or a later change can trace a decision through. Tagsight returns the instrument index and I/O list built from the P&ID revision you upload, each row traceable to its sheet, so the register re-check starts from the current revision.