In this article8 sections
An instrument location layout is the scaled plan drawing that shows where each field instrument and junction box physically sits on a plant, with the tag and the elevation an installer needs to find it. Projects also call it the instrument location plan, the instrument location drawing or the instrument layout, and the names describe the same document. It is drawn over the plot plan or the piping general arrangement for each area, unit or deck level, and it answers the question a P&ID leaves open. The P&ID records that an instrument exists and what it is connected to in the process. The location layout records where a fitter will mount it, how an operator will reach it, and where its cable, tubing and air supply start.
The drawing is used at three stages. Detailed design takes it as the start point for cable, tray, tubing and junction box routing. Construction uses it to put each device in the right place at the right height. Operations and maintenance use it to find an instrument that has a tag on a screen and no obvious home in the field, and that last use decides whether the drawing is maintained after handover.
What the location layout shows
The drawing shows each field instrument as a symbol and a tag at its plan position, with its elevation, against enough of the surrounding plant to find it on site. A question from ISA's Certified Control Systems Technician program states the core in one line: an instrument location plan shows the location, elevation and tag number of each instrument. Everything else on the sheet exists to make those three facts usable.
| Element | What the drawing carries | Why it is there |
|---|---|---|
| Background | Equipment outlines, structures, platforms, pipe racks and major lines, drawn light | An installer finds an instrument by the equipment beside it |
| Orientation and grid | North arrow, plant grid lines, a key plan, match lines to neighbouring sheets | Positions can be stated against the grid rather than scaled off the sheet |
| Symbol and tag | One symbol per field instrument, tagged exactly as on the instrument index | The tag is the join to every other document |
| Position | Where the instrument body sits, and the process tap where the two differ | A remote-mounted transmitter can sit metres from the point it measures |
| Elevation | Tap elevation and instrument elevation, stated separately | Grade, a platform or a height on a line changes the access, the installation and sometimes the calibration |
| Mounting | Close-coupled, on a stand or on a bracket, by reference to the installation detail | Fixes where the body ends up and what the fitter brings to it |
| Junction boxes | Each box with its tag, position and elevation | The field cable from a wired instrument usually runs to one of these boxes |
| Instrument air | Header take-offs or manifolds, where the project shows them | Air tubing for positioners and actuated valves starts here |
| Area classification | Zone boundaries, where the project overlays them | A device or box on the wrong side of a boundary needs a different certification |
| References | The P&IDs, installation details and background drawings used, with revisions | Ties the sheet to the inputs it depends on |
The same quiz question puts specification numbers on the instrument list and the installation details, and wiring on the conduit and wiring schedules or the cabling diagrams. Some published formats print more: instrument type, measured variable, range or setpoint, manufacturer and model. Those values already live in the instrument index and the datasheet. A figure printed on a layout has two owners and two revision cycles, and the copy nobody maintains ends up wrong, so keep the tag on the drawing and the values on the register the tag points to.
The location layout against the P&ID and the installation detail
The three drawings describe the same instrument and answer different questions: what it is connected to, where it is, and how it is mounted.
| P&ID | Instrument location layout | Installation detail | |
|---|---|---|---|
| Answers | What the instrument is connected to | Where the instrument is on the plant | How a class of instrument is mounted |
| Drawn to scale | No | Yes, in plan | No, a typical arrangement |
| Issued per | Process system or section | Area, unit or deck level | Installation type |
| Elevation | Only as a process note | Tap and instrument, both stated | Relative arrangement only |
| Tags | Every instrument in the process | Every field instrument in the area | None; the index assigns tags to each detail |
| Read by | Process, controls and safety reviewers | Installers, routing designers, maintenance | Pipefitters and instrument fitters |
The installation detail and the location layout depend on each other. The detail decides whether a transmitter is close-coupled or remote-mounted, and the layout decides where the remote one stands. A layout that shows a stand where the assigned detail is close-coupled, or the reverse, describes two different installations of the same tag. Instrument installation details covers the detail side.
How it is built from the P&ID and the plot plan
A location layout is built by combining the documents that say which instruments exist, the drawings that say where the plant is, and the details that say how each instrument is mounted.
| Input | What it supplies | What a revision to it changes on the layout |
|---|---|---|
| Instrument index | The field tag population, including local gauges and instruments with no wiring | A tag added, deleted or renumbered |
| P&ID | What each instrument is on, and where it sits relative to valves and branches | The line or vessel a tap belongs to |
| Plot plan, equipment layout or piping general arrangement | The background: equipment, structures, platforms, pipe racks | Instruments left beside equipment that has moved |
| Piping isometrics or the 3D model | Where each tap lands, with its orientation and elevation | Tap positions and elevations |
| Installation details | Close-coupled, stand or bracket mounting for each tag | Where the instrument body sits relative to its tap |
| Vendor and equipment drawings | Nozzle positions on vessels and packaged equipment | Level, analyser and package instrument positions |
| Area classification drawing | Zone boundaries | Certification of devices and boxes near a boundary |
The usual sequence on a new unit runs in this order.
- Take the field-mounted tags from the instrument index, so each tag is placed once and none is missed because it sits on a crowded P&ID sheet.
- Find what each tag is on from the P&ID, then confirm the tap's position and elevation on the isometric or in the model.
- Place the instrument. A close-coupled detail puts it at the tap. A remote detail puts it on a stand or bracket as close to the tap as practicable, where an operator can read and maintain it.
- Check the access route for each position: grade, a platform or a ladder, and the side of the equipment the instrument faces.
- Group nearby wired instruments to junction boxes, keep intrinsically safe circuits apart from the rest, and place each box where cable can reach it from the tray or trench.
- Annotate the tag, both elevations, the installation detail and the junction box, and record the P&ID and background revisions the sheet was drawn against.
On a project with a 3D model, the positions are commonly produced as plan views of the model, and the model becomes the reference for tap positions. The sheet still has to carry tags and elevations in a form an installer can read on site.
How it feeds cable routing, junction boxes and the cable schedule
The location layout is the start point for each document that routes something to or from a field instrument: cable, tray, tubing, instrument air and heat tracing.
| Document | What it takes from the location layout | The join that has to hold |
|---|---|---|
| Junction box layout and junction box schedule | Box positions, and the instruments grouped to each box | Each wired tag on the layout lands in exactly one box on the schedule |
| Cable routing and tray layout | Start and end points for each run, instrument to box and box to equipment room | Routes start where the instruments and boxes are drawn |
| Cable schedule | From and to references, and the estimated lengths used for procurement before routing is final | The same box tag on both; lengths taken again when a box moves |
| Tubing and impulse line routing | Tap and instrument positions, and the height between them | The rise or fall on the layout matches the installation detail |
| Instrument air distribution | Take-off and manifold positions | Each air-operated device within reach of a take-off |
| Heat tracing layout | Positions of traced instruments and their impulse lines | Each winterised installation detail has a traced position |
| Instrument index | The location drawing number on each row | Each field tag appears on exactly one layout, and the index names it |
| Loop diagram | The junction box the loop passes through | The box on the loop diagram is the box on the layout |
The junction box positions are the decision on this drawing with the widest reach. A box moved after the cable schedule is issued changes the length of each cable that lands in it, the trunk route back to the equipment room, and the certification the box needs if the move crosses a zone boundary. The change reaches procurement only if the cable schedule is revised with the layout. Showing the boxes in the piping model early, so piping routes around them, costs less than moving them after a clash. For the box itself and the wiring path beyond it, see marshalling cabinet vs junction box.
A worked area around separator V-201 in Unit 34
The area below is separator V-201 in Unit 34, classified Zone 1 IIB T4, with five field instruments and one intrinsically safe junction box, JB-A-03, on a stand at grid B3 beside the vessel skirt, +1.2 m. The liquid outlet 6"-CS-102-A1A leaves the bottom of the vessel, runs past TT-104 and XV-301 at grade and rises into the pipe rack. The P&ID is PID-034, elevations are stated above grade, and all five tags sit on one sheet of the Unit 34 location layout, which the instrument index names against each of them.
| Tag | Grid | Process connection | Instrument | Reached from | Cable to |
|---|---|---|---|---|---|
PT-101 | C3 | V-201 top nozzle, +6.0 m | Close-coupled above the nozzle, +6.3 m | Top platform, +5.4 m | JB-A-03 |
LT-103 | C3 | V-201 nozzles, +1.5 m and +4.0 m | Stand at grade, +1.0 m | Grade | JB-A-03 |
TT-104 | C4 | Thermowell in 6"-CS-102-A1A, +1.2 m | Head-mounted on the well, +1.2 m | Grade | JB-A-03 |
FT-102 | C5 | Orifice taps in 6"-CS-102-A1A, pipe rack, +5.5 m | Stand at grade below the taps, +1.0 m | Grade | JB-A-03 |
XV-301 | C4 | In line on 6"-CS-102-A1A, +1.0 m | Actuator and solenoid, +1.6 m | Grade | Non-IS box beside JB-A-03 |
Read across, the rows say four things the P&ID and the index cannot say on their own.
Two instruments sit away from their taps, and both distances go somewhere. FT-102 measures at +5.5 m and is read at +1.0 m, so each of its two impulse lines falls 4.5 m, and the installation detail, the tubing take-off and the support steel all have to provide that run. LT-103 sits 0.5 m below its lower nozzle. Where its legs or capillaries are filled, that 0.5 m enters the level calibration, so the layout and the datasheet have to carry the same figure.
One instrument depends on a structure the P&ID does not show. PT-101 is reachable only from the top platform. If a structural revision shortens or deletes that platform, the transmitter loses its access and nothing on PID-034 changes to say so. The location layout is where the loss becomes visible.
One box fixes four cable lengths. Four of the five devices land in JB-A-03, so its position sets four field cable runs and the start of the trunk back to the equipment room. The cable schedule's estimated lengths for those four cables are taken off this sheet.
Segregation shows up as geometry. The solenoid circuit on XV-301 is not intrinsically safe on this project, so it cannot share JB-A-03 with the transmitter loops. The second box beside it is the physical trace of a segregation rule, and it changes the junction box schedule, the cable schedule and the trunk count together.
Checking a location layout
Most location layout errors are disagreements with another document, so the check is a set of cross-references against the registers and drawings the sheet depends on.
| Check | Against | What it catches |
|---|---|---|
| Each field tag appears on exactly one layout | Instrument index | Missing tags, tags drawn twice on overlapping sheets, retags not carried through |
| Background revision | Current plot plan and piping layout | Instruments drawn beside equipment that has since moved |
| Tap positions and elevations | Isometrics or the 3D model | Taps moved by piping after the layout was drawn |
| Mounting and instrument elevation | The installation detail assigned on the index | A close-coupled detail on an instrument drawn on a stand, or the reverse |
| Access | Platforms and structural drawings | Instruments above grade with no platform, ladder or reach |
| Junction box assignment | Junction box schedule | A tag in one box on the layout and another on the schedule |
| Segregation and area | Area classification drawing, cable segregation groups | IS and non-IS circuits drawn into one box, or a box on the wrong side of a zone boundary |
| Package instruments | Vendor drawings | Skid instruments placed by neither the project nor the vendor |
| Input revisions | The P&ID and background revisions named on the sheet | A sheet drawn against superseded inputs |
Four failures recur often enough to name.
One elevation per instrument. A layout that carries a single elevation records either the tap or the body, and the reader cannot tell which. FT-102 above is the case where the two differ by 4.5 m.
A background that froze. Equipment positions move while the piping is laid out, and a layout drawn on an early plot plan keeps the old positions under current tags. Naming the background revision on the sheet makes the drift detectable.
Package instruments nobody placed. Skid instruments are often detailed on the vendor's drawings, with the skid shown on the project layout as an outline. That works when the outline carries a note pointing at the vendor drawing and the index rows carry the same reference. It fails when each side assumes the other placed them.
Changes recorded on the P&ID only. An instrument added under management of change gets a tag, a P&ID cloud and an index row. The location layout is easy to leave off the change's impact list, because nothing on the P&ID points at it.
Where practice varies
The drawing's job is constant across projects. Its name, scope and format are set by the project drawing list and the owner's specification.
One drawing or two. Some projects show junction boxes on the instrument location layout. Others issue a separate junction box location layout and carry the boxes on the instrument layout as background. Either works when one of the two is named as the reference for box positions.
What goes on the sheet. Instrument air manifolds, heat tracing and hazardous area boundaries sit on the location layout on some projects and on their own drawings on others.
How positions are stated. Some owners want grid coordinates, some want dimensions from gridlines or column lines, and some accept a scaled position with elevations.
Brownfield sites. Where no current layout exists, the positions come from a field walkdown against the instrument index, and the sheet is issued as-found before a change is designed against it.
Handover. The drawing is classed against the owner's document types. CFIHOS, the handover specification owner-operators write into their contracts, defines a layout diagram as a diagram showing the precise location of assets at a plant, process unit or area basis relative to surrounding structures and equipment, which is the location layout's job in one sentence.
Keeping the layout current with the registers
The location layout stays useful only while it agrees with the instrument index and the current P&ID revision, and it drifts as soon as it is revised on its own schedule.
Three habits keep it in step. Carry the location drawing number on each field row of the instrument index, so a tag can be found from the register and a sheet can be checked against the tags that point at it. Put the layout on the impact list of each change that adds, deletes, retags or moves an instrument, next to the index, the junction box schedule and the cable schedule. And issue it as-built from a field check rather than from the last design revision, because instruments move during construction for access and clashes, and the as-built sheet is the one maintenance uses for the life of the plant.
Tagsight gives an engineering team its instrument index built from the P&ID set and kept current with each revision, so the tag population a location layout is checked against is the current one.
For the mounting each position assumes, see instrument installation details. For keeping drawings and registers aligned through change, see MOC and P&ID documentation. The location layout is one sheet in a set that has to move together: the P&ID, the instrument index, the junction box and cable schedules and the layout, issued against the same revision and changed in the same change.