In this article11 sections
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.
In short
- 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, boiler 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, since any detail exposed to freezing gets a tracing and insulation variant with its own bill of material.
Installation details by measured variable and service
The type table above groups details by mounting and phase. The table below goes one level further, to the measured variable and service a detail is actually drawn against, and the arrangement and accessories that follow from it.
| Measured variable / service | Mounting | Process connection | Typical impulse-line arrangement | Accessories in the bill of material |
|---|---|---|---|---|
| Differential-pressure flow (orifice taps) | Remote, on a stand near the orifice flange | Twin taps at the orifice flange pair | Two lines to a five-valve manifold, sloped continuously | Five-valve manifold, impulse tubing pair, equalizing and vent/drain valves |
| Gauge pressure | Direct mount at the tap, or remote where vibration or heat rules it out | Single tap into the line or vessel | Single short line, or none on close-coupled mounting | Root/isolation valve, gauge valve, pigtail or siphon on hot service |
| Level by differential pressure, no seal | Remote, on a stand at the reference-leg elevation | Two nozzles, upper and lower | Wet or dry reference leg, sloped and vented like any DP hook-up | Five-valve manifold, tubing for both legs, drain pot on a wet leg |
| Level by differential pressure, with diaphragm seals | Direct, flanged onto the vessel nozzles | Flanged nozzle connection, rated to match | Filled capillary from each seal to the transmitter, no field-fillable line | Seal flanges (upper and lower), capillary of the specified length, shipping bracket |
| Temperature with thermowell | Direct, thermowell into the line or nozzle, head on or remote from the well | Thermowell into the process, immersion length set by service | None; the thermowell is the process barrier | Thermowell, extension nipple, head-mount or remote transmitter, cable gland |
| Pneumatic control valve air supply and positioner | Positioner on the valve yoke or actuator | Compressed-air connection at the actuator, not a process tap | Air-supply tubing from a local filter regulator, output tubing to the actuator | Filter regulator, supply tubing, mounting bracket, actuator tubing |
| Analyser sample take-off | Remote, at a probe on the process line | Sample probe or connection sized for the required flow | Sample line to the analyser, with a return or vent line | Sample probe, isolation valve, sample tubing, fast-loop return piping |
| Pressure gauge with siphon or gauge valve | Direct, siphon coiled below the gauge | Single tap through the gauge valve | Short run through a siphon coil on steam, or a snubber on pulsating service | Gauge valve, siphon or snubber, gauge, drain plug |
Two rows carry no impulse line to a transmitter in the differential-pressure sense. The pneumatic hook-up is a compressed-air circuit, and the analyser take-off is a sample circuit; the accessories column reflects that rather than a liquid-filled measurement leg.
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, which are measured variable, mounting arrangement and 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.
From bill of material to material take-off
The hook-up bill of material is written once, per detail. The bulk material take-off for the project is that bill of material multiplied by the number of tags assigned to it: fifty PT tags on detail HU-PT-DM-01 order fifty manifolds, fifty pairs of isolation valves and fifty runs of impulse tubing, not the one set shown on the drawing.
The multiplication only holds if the instrument index carries a hook-up detail number against every tag. A tag with no detail assigned drops out of the take-off silently, no manifold gets ordered for it, and the gap surfaces on site rather than in procurement. The same problem runs in reverse: a detail revised after the take-off was cut changes the bill of material for every tag pointing at it, and a take-off run against the superseded revision under-orders or over-orders across the whole set.
One item does not multiply by tag count at all. Diaphragm-seal capillary length, and any seal fill-fluid volume, is set per tag, driven by the actual mounting distance between seal and transmitter head, so it is pulled from the instrument datasheet or the location plan rather than averaged from the detail.
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.
The other documents that travel with a hook-up package
A hook-up detail assumes three other documents already exist and agree with it: where the tag physically sits, how its tubing and cable get there, and what it bonds to.
- Instrument location plan (or layout) places every tag on the unit or plot drawing, with an elevation and, where it matters, the area classification zone it falls in.
- Tubing routing drawing shows the physical path from the process tap or air supply back to the instrument, distinct from the hook-up's own arrangement view.
- Cable routing drawing shows the physical path from the field device to the junction box and onward to the marshalling cabinet.
- Mounting-stand and support detail covers the local structure a remote-mounted instrument sits on, positioned by the location plan.
- Junction box location and detail fixes where each JB sits, its size, cable entries and internal terminal layout.
- Instrument earthing detail shows how each device, JB and cabinet bonds to the plant's earthing system.
None of these is issued in isolation. Each is checked against a register that also has to agree with it.
| Document | What it fixes | Must agree with |
|---|---|---|
| Instrument location plan | Physical position, elevation and area classification zone of every tag | Instrument index, plot / equipment layout |
| Tubing routing drawing | Physical route of impulse or pneumatic tubing from tap to instrument | Hook-up detail, location plan |
| Cable routing drawing | Physical route of instrument cable from device to junction box and cabinet | Cable schedule, loop diagram |
| Mounting-stand and support detail | Dimensions and material of the local support structure | Location plan, hook-up detail |
| Junction box location and detail | JB position, size, cable entries, terminal arrangement | Junction box schedule, cable schedule |
| Instrument earthing detail | Bonding of device, JB and cabinet to the earthing system | Cable schedule, area classification drawing |
Checking a hook-up package before it is issued
A package ready to issue clears a short, mechanical check before it does:
- Every detail carries a unique number and a current revision.
- Every tag on the instrument index points at exactly one detail number, with none left blank.
- Every bill-of-material item description resolves to an entry in the project material specification, not a free-text description invented on the sheet.
- Slope direction and the vent/drain locations are stated on the sheet itself, not left to be inferred from the arrangement view.
- The winterisation scope on the detail matches the tracing schedule for every tag assigned to it.
Each of these is a cross-check against another document. No detail is checked in isolation, because nothing on a hook-up package is used in isolation either.
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.