An instrument installation detail, commonly called a hook-up drawing, is a typed standard drawing showing how one class of instrument is mounted, connected to the process, and supplied with air, power or heat tracing. It is issued per installation type rather than per tag, so one numbered detail governs every device installed the same way. Each detail carries an arrangement view, a bill of material with item numbers, and the slope, vent and drain rules for its impulse lines.
This note is for the engineer issuing or checking a hook-up package: what each detail must carry, which types a project needs, and the rules that decide whether the reading is right.
Key takeaways
- A hook-up drawing shows how a class of instrument is installed. It is a type, not a per-tag document.
- One numbered detail serves many tags. The assignment lives on the instrument index.
- The bill of material is the working half of the sheet: item numbers, descriptions, quantities.
- Slope, vent and drain rules decide whether a differential pressure reading is true.
- Tracing, winterisation and air supply belong on the detail, installed by the same crew from the same sheet.
Hook-up drawing, loop diagram, datasheet
Three documents describe the same instrument and answer different questions.
| Document | Answers | Scope |
|---|---|---|
| Installation detail, hook-up | How the device is mounted and connected to process, air and tracing | One per installation type |
| Loop diagram | How the signal travels from device to I/O card terminal | One per loop |
| Instrument datasheet | What the device is: range, materials, connections, performance | One per tag |
The standard hook-up types
The set splits by mounting, by the phase in the impulse line, and by whether process fluid enters that line at all.
| Hook-up type | Typical service | Why |
|---|---|---|
| Direct mount, close coupled | Clean gas or liquid gauge pressure, local gauges | Shortest path, fewest leak points, no impulse line |
| Remote mount on stand or bracket | Vibrating lines, hot service, poor access | Off the vibration and heat, within reach |
| Impulse lines, gas service | Flow and differential pressure on gas | Transmitter above the tap, lines rising, condensate drains back |
| Impulse lines, liquid service | Flow and differential pressure on liquid | Transmitter below the tap, lines falling, gas vents back |
| Sealed condensate legs | Steam flow, steam drum level | Equal filled legs give a known column on both sides |
| Diaphragm seal and capillary | Viscous, slurry, crystallising or corrosive service | No process fluid in the line; fill fluid and capillary length matter |
| Pneumatic supply hook-up | Positioners, actuators, air-operated valves | Filter regulator, tubing size, isolation at the header |
Winterisation cuts across the table: any detail exposed to freezing gets a tracing and insulation variant with its own bill of material.
What the bill of material carries
Each line carries an item number, a description keyed to the project material specification, and a quantity per hook-up.
| Item group | Typical entries | Quantity depends on |
|---|---|---|
| Process isolation | Root valve, block and bleed, needle valves | One set per tap; two taps on a differential hook-up |
| Manifold | Three valve or five valve | Five for differential pressure, three for gauge |
| Tubing and pipe | Impulse tubing size and material, nipples, unions | Run length plus slope allowance |
| Fittings | Compression fittings, tees, plugs, reducers | Per bend and per branch |
| Mounting and support | 2 inch stand, U bolts, bracket, sunshade | Remote mounted details only |
| Tracing and insulation | Tracer type, thermostat, insulation thickness | Winterised details only |
| Air supply | Filter regulator, tubing, isolation valve | Pneumatic details only |
Numbering, and how one detail serves many tags
The number carries the three facts that define the type: measured variable, mounting arrangement, service. HU-PT-DM-01 reads as pressure transmitter, direct mount, first of that kind.
The assignment lives as a hook-up column on the instrument index, one detail number per tag. Every gauge pressure transmitter on a clean gas header, PT-101 among them, shares one detail. TT-104 on the same line takes another, because a thermowell is a different arrangement with a different bill of material.
Two rules keep the set from sprawling. Add a detail only when the bill of material changes. And when a detail is revised, every tag pointing at it inherits the change.
Slope, venting and draining
An impulse line is a liquid column the transmitter cannot see. Anything standing in it that the arrangement did not intend reads as a fixed offset, and calibration will not remove it.
On gas service the lines rise continuously to a transmitter above the tap, so condensate returns to the process instead of collecting in a low point. On liquid service they fall continuously to a transmitter below the tap, so entrained gas vents back into the line. On steam both legs carry sealed condensate to equal height, and unequal legs are a permanent zero error that reads as a process change.
Every high point takes a vent and every low point a drain, both valved. A detail without them cannot be filled, calibrated or maintained without breaking into the line.
Where the package starts
Every detail assignment starts from a tag on a P&ID, and a tag missing from the index is a hook-up nobody counted. Tagsight gives your engineers an instrument index built from the P&ID set, ready for their review, so the hook-up column starts from a checked tag list. The wiring side of the same tags carries on into the cable schedule.