A comprehensive format and guide for an Instrument Index document, commonly implemented using a spreadsheet (like Microsoft Excel) or a dedicated engineering database.
1. Document Header
Every page of the Instrument Index should have a standard header containing:
Project Name:
Project Number:
Document Title: Instrument Index
Document Number:
Revision Number: (e.g., 0, 1, 2 or A, B, C for preliminary issues; 0, 1, 2 for “Issued for Construction” and later)
Revision Date:
Status: (e.g., For Internal Review, For Approval, Issued for Construction, As-Built)
Page Number: Page X of Y
2. Main Body: The Index Table
The core of the document is a table with detailed columns. These columns are typically grouped by function (e.g., Identification, Process Data, Specification, Control System Interface).
Group A: Identification & Location
This is the most critical section for uniquely identifying an instrument and finding its documentation and physical location.
| Column Header | Description | Example |
| Tag Number | The unique identifier for the instrument, following ISA-5.1 standards. | PT-1001A |
| Service Description | A brief description of the instrument’s function. | REACTOR 1A OUTLET PRESSURE |
| P&ID Number | The Piping and Instrument Diagram number where the instrument is shown. | PID-10-PR-101 |
| Loop Number | The instrument loop number, grouping related instruments. | 1001 |
| Instrument Type | A short code or description of the physical instrument type. | Pressure Transmitter |
| System / Subsystem | The main plant system the instrument belongs to. | REACTION |
| Unit / Area | The plant unit or physical area where the instrument is located. | UNIT-10 / AREA-B |
| Location / Elevation | A more specific description of the physical placement. | Platform 2, EL +15.0m |
Group B: Process Information
This section links the instrument to the process conditions it will experience.
| Column Header | Description | Example |
| Line No. / Equip. No. | The pipeline number or equipment tag number the instrument is connected to. | 10"-HC-1023-A1 |
| Fluid / Phase | The process fluid and its state (liquid, gas, vapor). | HYDROCARBON / GAS |
| Operating Pressure | Normal operating pressure. Use units like $bar(g)$ or $psig$. | 15.5 bar(g) |
| Operating Temp. | Normal operating temperature. Use units like $^\circ C$ or $^\circ F$. | 120 ^\circ C |
| Design Pressure | Maximum design pressure of the line/vessel. | 25.0 bar(g) |
| Design Temp. | Maximum design temperature of the line/vessel. | 200 ^\circ C |
Group C: Instrument Specification & Procurement
This section details the instrument’s hardware and purchase information.
| Column Header | Description | Example |
| Range | The calibrated measurement range of the instrument. | 0 - 20 bar(g) |
| Setpoints (L/H/LL/HH) | Alarm or trip setpoints for switches or transmitters. | H: 18.0, HH: 19.5 |
| Manufacturer | The company that manufactured the instrument. | Rosemount |
| Model Number | The specific model number from the manufacturer. | 3051S |
| Datasheet Number | Reference to the detailed instrument specification sheet. | DS-PT-1001A |
| PO Number | The Purchase Order number used to procure the device. | PO-2024-INST-087 |
Group D: Control & Electrical Interface
This section describes how the instrument connects to the control and power systems.
| Column Header | Description | Example |
| System | The control system it connects to (DCS, SIS, PLC, F&G). | DCS (Distributed Control System) |
| I/O Type | The type of Input/Output signal. | AI (Analog Input) |
| Signal Type | The electrical signal format. | 4-20 mA + HART |
| Controller Tag | The associated software controller block tag in the DCS/PLC. | PIC-1001A |
| Cabinet / Marshalling | The cabinet where the signal is wired. | DCS-CAB-05 |
| I/O Card / Channel | The specific I/O card and channel number. | Slot 4, Channel 12 |
| Power Supply | The power source for the instrument. | 24 VDC (Loop Powered) |
| Junction Box (JB) No. | The field junction box the instrument is wired to. | JB-INST-115 |
Group E: Safety & Installation
This section is crucial for safety compliance and installation details.
| Column Header | Description | Example |
| Hazardous Area Class. | The hazardous area classification (ATEX, IECEx, NEC). | Zone 1, IIB, T3 |
| SIL Required | The Safety Integrity Level required by the safety analysis (LOPA). | SIL 2 |
| SIL Achieved | The SIL rating the selected instrument and loop can achieve. | SIL 2 |
| Hook-up Drawing No. | Reference to the installation detail drawing. | IHD-PT-001 |
| Loop Diagram No. | Reference to the detailed loop wiring diagram. | LD-1001A |
Group F: Status & Remarks
This section is for project management and additional notes.
| Column Header | Description | Example |
| Status | The current status in the project lifecycle. | Installed / Pre-commissioned |
| Remarks | Any additional comments, special requirements, or notes. | Requires special winterization. |
3. Example Snippet of an Instrument Index
Here is a small example of what the table might look like in practice (best viewed on a wide screen).
| Tag Number | Service Description | P&ID Number | Instrument Type | System | I/O Type | Range | Manufacturer | Model Number | Hazardous Area Class. | Status |
PT-1001A | REACTOR 1A OUTLET PRESSURE | PID-10-PR-101 | Pressure Transmitter | DCS | AI | 0-25 bar(g) | Rosemount | 3051S | Zone 1, IIB, T3 | Installed |
TE-2015 | HEATER E-201 TUBE SKIN TEMP | PID-10-HT-201 | Thermocouple | DCS | AI | 0-900 ^\circ C | Omega | TC-K-316 | Zone 2, IIC, T2 | Procured |
XV-1001A | REACTOR 1A OUTLET SHUTDOWN VALVE | PID-10-PR-101 | Solenoid Valve | SIS | DO | Open/Close | ASCO | 8320G174 | Zone 1, IIB, T4 | Installed |
LSHH-3005 | SEPARATOR V-301 HIGH-HIGH LEVEL | PID-20-SP-301 | Level Switch | SIS | DI | N/A | Magnetrol | Tuffy II | Zone 1, IIB, T3 | Commissioned |
Best Practices
Standardization: Strictly adhere to the project’s tagging philosophy, typically based on ISA-5.1. Consistency is key.
Revision Control: Use a formal revision control system. All changes must be tracked, dated, and initialed.
Single Source of Truth: The Instrument Index should be the definitive source. If there is a discrepancy between the P&ID and the index, it must be resolved. The index usually holds the most detailed and correct information.
Integration: The index is not a standalone document. It must be consistent with P&IDs, Instrument Datasheets, Loop Diagrams, Hook-up Drawings, and the Control System I/O database.
Management of Change (MOC): After commissioning, any modification to an instrument (e.g., range change, replacement) must go through a formal MOC process, and the Instrument Index must be updated accordingly. This is a critical safety and operational requirement.
Instrument Index
| Column Header | Description | Example |
| Tag Number | (Identification) The unique identifier for the instrument, following ISA-5.1 standards. | PT-1001A |
| Service Description | (Identification) A brief, clear description of the instrument’s function and location in the process. | REACTOR 1A OUTLET PRESSURE |
| P&ID Number | (Identification) The Piping and Instrument Diagram number where the instrument is shown. | PID-10-PR-101 |
| Loop Number | (Identification) The instrument loop number that groups related instruments (e.g., sensor, controller, valve). | 1001 |
| Instrument Type | (Identification) A short description of the physical instrument type. | Pressure Transmitter |
| System / Subsystem | (Location) The main plant system or unit the instrument belongs to. | REACTION / UNIT-10 |
| Line No. / Equip. No. | (Process) The pipeline number or equipment tag number to which the instrument is physically connected. | 10"-HC-1023-A1 |
| Fluid / Phase | (Process) The process fluid and its state (e.g., liquid, gas, vapor) that the instrument measures. | Hydrocarbon / Gas |
| Operating Pressure | (Process) The normal operating pressure. Use standard units like bar(g) or psig. | 15.5 bar(g) |
| Operating Temp. | (Process) The normal operating temperature. Use standard units like Deg C or Deg F. | 120 Deg C |
| Range | (Specification) The calibrated measurement range of the instrument. | 0 - 25 bar(g) |
| Setpoints (L/H/LL/HH) | (Specification) Alarm or trip setpoints for switches or transmitters. | H: 18.0, HH: 19.5 bar(g) |
| Manufacturer | (Procurement) The company that manufactured the instrument. | Rosemount |
| Model Number | (Procurement) The specific model number from the manufacturer. | 3051S |
| Datasheet Number | (Documentation) Reference to the detailed instrument specification sheet. | DS-PT-1001A |
| PO Number | (Procurement) The Purchase Order number used to procure the device. | PO-2024-INST-087 |
| System | (Control System) The control system it connects to (e.g., DCS, SIS, PLC, F&G). | DCS |
| I/O Type | (Control System) The type of Input/Output signal (e.g., AI, AO, DI, DO). | AI (Analog Input) |
| Signal Type | (Control System) The electrical signal format. | 4-20 mA + HART |
| Cabinet / Marshalling | (Control System) The cabinet where the signal is wired. | DCS-CAB-05 |
| I/O Card / Channel | (Control System) The specific I/O card and channel number. | Slot 4, Channel 12 |
| Power Supply | (Electrical) The power source for the instrument (e.g., Loop Powered, 230VAC, 110VAC). | 24 VDC (Loop Powered) |
| Junction Box (JB) No. | (Electrical) The field junction box the instrument is wired to. | JB-INST-115 |
| Hazardous Area Class. | (Safety) The hazardous area classification (e.g., ATEX, IECEx, NEC). | Zone 1, IIB, T3 |
| SIL Required | (Safety) The Safety Integrity Level required by safety analysis. | SIL 2 |
| Hook-up Drawing No. | (Installation) Reference to the installation detail drawing (process and mechanical connection). | IHD-PT-001 |
| Loop Diagram No. | (Installation) Reference to the detailed loop wiring diagram from sensor to control system. | LD-1001A |
| Status | (Project) The current status in the project lifecycle. | Installed |
| Remarks | (Miscellaneous) Any additional comments, special requirements, or notes. | Requires special winterization. |