In this article9 sections
Equipment tag numbering gives every vessel, pump, compressor, exchanger, tank, package and motor on a plant an identifier made of an equipment-type letter and a sequence number, often with a unit or area prefix in front and a suffix letter for duplicated machines. P-302A and P-302B are two pumps in one service, E-401 is an exchanger, V-201 a vessel. The letters come from the project's numbering procedure, and the numbers are issued from the equipment list. Instrument tags are a separate series. Their letters follow ISA 5.1, or the plant coding system in force, and name a measured variable and a function. Their number is a loop number, and they are issued into the instrument index. The two series are kept apart by format, by register and by owner, and the index records the equipment each instrument serves in a column of its own, outside the tag.
The conventions described are those of process plants tagged to ISA practice; plants coded under KKS or IEC 81346 are noted where they differ.
In short
- An equipment number names an item by class and sequence; an instrument tag names a device by variable, function and loop. Shared digits mean nothing unless the procedure says so.
- No published standard fixes equipment letters industry-wide the way ISA 5.1 fixes instrument letters, so the project procedure governs.
- A suffix identifies one of several identical machines. Duty and standby are operating states and sit on the equipment list, outside the number.
- The two registers merge at handover, where an identical string in both series becomes two assets with one name.
What an equipment number encodes
An equipment number is the identity of one physical item for the life of the plant. The same string appears on the process flow diagram, the P&ID, the equipment list, the datasheet, the plot plan and the vendor documents, and after handover in the maintenance system. It has up to four fields, and the spare status travels beside it.
| Field | In 12-P-302B | What it carries | Where projects differ |
|---|---|---|---|
| Area or unit prefix | 12 | The unit the item belongs to, here Unit 12 | Omitted on single-unit plants, or folded into the sequence |
| Equipment-type letter | P | The class of item, here a pump | One or two letters, defined by the numbering procedure |
| Sequence number | 302 | The item's place in the series | Often banded, so the leading digit marks an area within the unit |
| Duplicate or train suffix | B | The second of two identical machines in one service | Some projects reserve it for trains and number duplicates in the sequence |
| Installed spare status | Outside the number | Duty or standby | Recorded on the equipment list and P&ID callout; some procedures reserve a spare letter |
Read left to right, 12-P-302B is the second of the two identical pumps numbered 302 in Unit 12, and on a single-unit plant the same machine is P-302B. The procedure chooses one form, and every document then uses it exactly, separators included.
Equipment-type letters are set by the project
ISA 5.1 gives instrument letters a published meaning, and a project that adopts it inherits the table. Equipment letters have no counterpart with that reach. They come from the owner's standard or the contractor's procedure, and vary between companies and sometimes between projects of one company.
| Letter | Usual meaning | Other meanings in use |
|---|---|---|
| P | Pump | The most consistent letter across procedures |
| V | Vessel | D for drums, where a project separates drums from other vessels |
| E | Heat exchanger | Separate codes for air coolers or plate exchangers on some projects |
| C | Compressor | Column, on projects that give compressors K |
| K | Compressor | Blower or fan on some projects, column in German-language practice |
| T | Tank | Tower or column, with tanks then taking TK |
| F | Furnace or fired heater | Filter on other projects |
| PK | Package | Numbered under the letter of its main machine on other projects |
An equipment number is therefore readable in full only with the procedure that issued it. Plant coding systems are the exception. KKS, used widely in power generation, and IEC 81346 reference designation classify equipment and instruments alike with codes the system defines. A project that adopts one inherits those codes the way an ISA project inherits instrument letters, and both kinds of item sit in one coded hierarchy.
Equipment tags and instrument tags side by side
| Equipment number | Instrument tag | |
|---|---|---|
| Example | P-302A | FT-102 |
| What the letters mean | The class of item, from the project procedure | Measured variable and function, under ISA 5.1 or the plant code |
| What the number means | A sequence within the unit or area | The loop, shared by every device in it |
| What a suffix means | A duplicate machine, or a train | A redundant device, or one measurement repeated on duplicated equipment |
| Who assigns it | Process or project engineering, from the first PFD | Instrumentation engineering, as loops reach the P&IDs |
| Register it is issued from | Equipment list | Instrument index |
| Drawings it appears on | PFD, P&ID, plot plan, general arrangement, equipment datasheet, vendor drawings | P&ID, loop diagram, instrument datasheet, hook-up detail, cable schedule |
| Where it lands | The maintenance system, and the electrical load list through its motor | The control system through the I/O list if wired, then the maintenance system |
The two series travel to different systems during design and arrive together at handover, often in one tag register built to a handover specification such as CFIHOS. Separate registers tolerate an equipment number and an instrument tag with identical characters; a merged register cannot hold both.
Where equipment numbers and loop tags collide on a P&ID
A P&ID carries both series on one sheet, and four situations cause most of the confusion.
Shared digits. Pump P-301, its motor M-301 and an on-off valve XV-301 can all carry 301. The motor shares the number by design where motors are numbered after their machines; the valve shares it by coincidence, as do vessel V-202 and pressure loop PIC-202. Some procedures derive loop numbers from the equipment served, so the first level loop on tank T-220 is loop 2201 (LT-2201), and there the shared digits mean something. Know which rule is in force, and key every lookup on the full identifier.
Shared letters. Some equipment codes are also valid instrument combinations. A strainer numbered ST-602 reads under ISA 5.1 as a speed transmitter on loop 602. The symbol settles it on the drawing; in a merged tag register nothing does. The procedure should exclude such codes or separate their number ranges.
Motor and driven machine. A motor is both a driver and an electrical load. Projects number it after the driven machine (M-301 for P-301), as a suffix to the machine's number, or in an independent electrical series. The equipment list records the driver rating on the pump's row, the load list and single-line diagram carry the motor under its own identifier, and its control signals reach the I/O list under one identifier or the other. If the two numbers are unrelated, the driven-equipment column on the load list is the only link.
Vendor packages. A package arrives with the vendor's own numbering inside its boundary, often a short series starting again at 1 that collides with the project series. The package takes a project number, commonly under the letter of its main machine, so a compressor package becomes K-501. Items inside the skid are renumbered into the project series or keep the vendor's numbers with a cross-reference, and the procedure has to say which, because vendor drawings, datasheets and spare parts lists are indexed by the vendor's numbers.
Duplicate, installed spare and train suffixes
Identical machines in one service share a sequence number and take a suffix letter each. P-302A and P-302B are two complete items with their own equipment list rows, motors and maintenance histories. The equipment list states the sparing arrangement, for example two machines each able to carry the full duty with one running, or three sized for half the duty with two running.
Duty and standby is an operating state, and plants that rotate duty to even out running hours change it at each changeover. The suffix therefore identifies the machine and says nothing about which one is running. A procedure that reserves a letter for the spare ends up with a machine called the spare that runs for months at a time.
Parallel process trains need a letter as well. If the suffix marks duplicates, the train goes into the prefix and both trains keep P-302A and P-302B under different prefixes. If it marks trains, duplicates are numbered in the sequence. Mixing the two conventions means P-302B is either the second pump or the train B pump, depending on who drew the sheet.
On instruments, PT-101A, PT-101B and PT-101C are three redundant transmitters voting on one measurement, while on a duplicated pump pair the same suffixes can mark one measurement repeated on each machine. The instrument index has to say which, because the two are designed, wired and tested differently.
One pump pair in the equipment list, P&ID, line list and instrument index
Follow P-302A/B, two feed pumps drawing from vessel V-201 and discharging through exchanger E-401.
| Document | How the pumps appear | What joins it to the rest |
|---|---|---|
| Equipment list | Two rows, P-302A and P-302B, with service, rating, driver, datasheet number and sparing arrangement | The equipment number, and the P&ID number on each row |
| P&ID, PID-002 | Both pumps drawn, with a callout giving number, service and rated duty | Equipment numbers on the symbols, line numbers on the pipe |
| Line list | Suction line 6"-CS-102-A1A from V-201, discharge line 4"-CS-101-A1A to E-401 | The from and to columns, which carry equipment numbers |
| Instrument index | FT-102 on the common discharge, with P-302A/B in its equipment column | The equipment column and the line number |
| Electrical load list | One motor row per pump | The driven-equipment column |
| Maintenance system | One functional location per pump after handover | The equipment number, or a location code mapped to it |
Each join runs through the equipment number. FT-102 relates to the pumps through the equipment column, and its loop number 102 has no relation to 302, so a renumbered pump is a change to the whole document set, however small the change on the P&ID.
Numbering failures at revision and handover
Renumbering after a revision. A unit boundary moves or a single pump gains a spare, and items are renumbered to keep the series tidy. The P&ID changes in one revision; the line list, instrument index, datasheets and vendor documents catch up over later ones, if at all. Keep a number wherever the item has not changed, give a new item a new number, and where renumbering is unavoidable, issue an old-to-new cross-reference and keep the superseded number on the equipment list row.
Reused numbers. An item deleted at revision C has its number issued to a different item at revision E. Everything that referred to the old item (HAZOP records, vendor quotations, superseded datasheets) now points at the wrong machine, and no document shows it. Retire deleted numbers and keep their rows on the equipment list with a deleted status.
Vendor numbers never mapped. The package is handed over with its vendor numbering intact, and the maintenance system receives two items called pump 1 from two vendors. The mapping belongs in the purchase order: the supplier document requirements oblige the vendor to use project numbers or submit a cross-reference for approval, and the equipment list carries both numbers.
Format drift. P-302B on the P&ID, P302B in the maintenance system and P 302 B in a vendor list are one pump and three strings, and a register matched by lookup fails on the difference. The procedure fixes the separators, and each register issue is checked by exact string comparison. For the handover end, see building the CMMS asset register.
The numbering procedure and who owns it
The rules above live in one document, called the equipment numbering procedure, the tagging specification or the plant identification philosophy. On an operating site it is usually a company standard; on a capital project the contractor issues a project procedure early in design that applies the owner's standard and fills its gaps. It should cover:
- the number format, with separators and permitted characters
- area and unit codes, and the sequence bands reserved for each
- the equipment-type letters, excluding codes that form instrument letter combinations
- suffix rules for duplicates, installed spares and trains
- the boundary between equipment numbers, instrument tags and piping special items, for strainers, filters and sight glasses
- motor numbering and the cross-reference to the load list
- package numbering and the mapping of vendor numbers
- retirement of deleted numbers
- any mapping to a plant coding system or maintenance functional locations
Ownership sits with one custodian per series. The lead process or project engineer issues equipment numbers from the equipment list, the lead instrument engineer issues tags from the instrument index, and document control issues the procedure under revision. A number taken from a drawing instead of its register is how a duplicate starts.
For the instrument side, see ISA 5.1 letter codes and serial and parallel loop numbering; for plants carrying a plant code beside ISA tags, see mixing ISA 5.1 and KKS.