In this article9 sections
An instrument material take-off (MTO) is the quantified list of bulk materials needed to install a project's instruments: impulse tubing, compression fittings, instrument valves and manifolds, instrument cable, cable glands, junction boxes, cable tray, and the stands and brackets that hold it all in place. It is built by counting installations and measuring routes. The instrument index says how many tags use each installation detail, the hook-up drawings say what one installation consumes, and the cable schedule and routing layouts supply the lengths. The instruments themselves, the transmitters, control valves and analyzers, are usually bought separately, each against its own datasheet on a material requisition. The take-off covers what connects those instruments to the process and to the control system.
This note describes the instrumentation bulk take-off on a process plant project. Piping and electrical take-offs follow the same logic from their own source documents, and the boundaries between the three are covered below.
What the take-off carries, and what it leaves to requisitions
| Item group | Typical lines | Counted or measured | Unit |
|---|---|---|---|
| Impulse tubing | Tube by outside diameter, wall thickness and material | Counted per hook-up, measured for remote runs | Metre |
| Compression fittings | Connectors, unions, elbows, tees, plugs, by size and material | Counted per hook-up | Each |
| Instrument valves and manifolds | Needle and ball valves, block and bleed valves, two, three and five valve manifolds | Counted per hook-up | Each |
| Instrument cable | Single and multi-pair, thermocouple extension, by type, core count and screening | Measured, cable by cable | Metre, then drums |
| Cable glands and accessories | Glands by entry thread, cable diameter and protection type, shrouds, earth tags | Counted per cable end | Each |
| Junction boxes | By entry count, terminal count, enclosure material and certification | Counted from the location layouts | Each |
| Cable tray, ladder and conduit | By width, material and finish, with bends, tees, reducers and covers | Measured from the routing layouts | Metre, each |
| Supports and mounting | Pipe stands, brackets, sunshades, U-bolts | Counted per hook-up and per junction box | Each |
| Identification | Tag plates, cable markers, core ferrules | Counted per tag, per cable, per core | Each |
The line between bulk and tagged items is drawn by the project, and a few items sit on it. A manifold can be bought fitted to its transmitter, in which case it arrives on the instrument requisition and leaves the take-off. A junction box can be bought pre-terminated. The tubing and fittings on a pneumatic control valve often arrive with the valve assembly. We treat every such item as belonging to exactly one list, with the other list carrying a note that it is excluded, because an item quietly present on both is ordered twice and an item quietly absent from both is discovered at installation.
Where each quantity comes from
| Source document | What it gives the take-off | A change there moves |
|---|---|---|
| Instrument index | The tag count per instrument type, and the hook-up detail number against every tag | Every line driven by a hook-up |
| Hook-up drawings | The bill of material for one installation | Every tag assigned to that detail |
| P&ID and line list | The process connection, and the line class that fixes material and rating at the root | Valve, fitting and tube material |
| Instrument datasheets | Process connection size and type, electrical entry, diaphragm seal and capillary details | Fittings, glands, anything set per tag |
| Instrument location layouts | Where each device and each junction box sits, and the routes between them | Remote tubing runs, stands, junction box counts |
| Cable schedule | Every cable, its type, its two ends and its routed length | Cable by type, glands per cable end |
| Cable routing and tray layouts | The tray network and its lengths | Tray, ladder, conduit and their supports |
| Junction box schedules | Entries, terminals and spare capacity per box | Glands, terminals, box size |
Two of these sources supply counts and the rest supply measurements, and the difference shows as soon as something changes. A counted line moves in steps: add one transmitter to an existing hook-up detail and one more set of valves and fittings goes on the take-off. A measured line moves continuously: relocate one junction box and every cable routed to it changes length, along with the tray that carries them.
Counting through the hook-up
A hook-up detail's bill of material is written once, for one installation, and multiplied by the number of tags the instrument index assigns to it, a mechanism covered in more depth in instrument installation details. The take-off is where those multiplications are added up across every detail on the project.
| Hook-up detail | Tags assigned in the index | Per installation | Take-off lines |
|---|---|---|---|
| Pressure, gas service, stand mounted (such as PT-101) | 14 | 1 two valve manifold, 3 m tube, 4 connectors, 1 stand | 14 manifolds, 42 m tube, 56 connectors, 14 stands |
| Differential pressure flow (such as FT-102) | 9 | 1 five valve manifold, 6 m tube, 8 connectors, 1 stand | 9 manifolds, 54 m tube, 72 connectors, 9 stands |
| Temperature, thermowell mounted (such as TT-104) | 22 | 1 cable gland at the head | 22 glands |
The figures are illustrative and the details belong to the project. What carries over to every project is that the index is the multiplier. A tag with no detail number drops out of the take-off and nothing is ordered for it. A tag on the wrong detail brings the wrong valves: a liquid-service transmitter taken off against a gas-service detail gets a mounting arrangement that traps gas in the impulse lines instead of letting it vent back to the process.
The first isolation valve at the process connection, the root valve, sits on the boundary with piping. On many projects it is piping scope, taken off with the piping MTO and shown on the hook-up as supplied by piping, with the instrument scope starting at its outlet. On others the instrument scope starts at the tapping. The hook-up drawing states the break and the take-off follows it.
Measuring cable, tray and routed tubing
Cable is taken off cable by cable from the cable schedule, using the routed length along the tray and not the straight-line distance between the two ends, with an allowance at each end for glanding and termination. The lengths are summed by cable type and then allocated to drums. The cable drum schedule decides how many drums of each type are ordered, which cables are cut from which drum to keep off-cuts short, and whether a run longer than a standard drum needs one of its own.
Tray, ladder and conduit are measured section by section from the routing layouts, with the fittings (bends, tees, reducers), the covers where the specification calls for them and the supports at the spacing the tray specification sets. Where the electrical discipline owns the tray network, instrumentation contributes its cable fill and the tray is taken off once, on the electrical side.
Tubing inside a hook-up is counted with the hook-up. Tubing outside it is measured: from a distant tapping to a transmitter grouped on a rack, to and from an analyzer shelter, and the instrument air distribution from the header to each consumer.
Allowances and holds
An allowance covers cutting losses, off-cuts too short to reuse, damage in handling and storage, and small field changes, none of which the net quantity can predict. The project sets it per commodity, usually higher for small fittings that are cheap and easily lost than for a junction box counted exactly, and the take-off shows it in its own column. The net quantity stays traceable to the index, the hook-ups, the cable schedule and the layouts; the gross quantity, net plus allowance, is what the requisition orders.
A hold marks a line whose quantity or specification is not yet confirmed: a detail waiting on vendor data, a junction box whose location has not been fixed, a tube material waiting on a line class decision. The line stays on the take-off with its hold flag and is left off the requisition until the hold is cleared, so the unknown is visible and nobody orders against a guess.
Issues and revisions
| Issue | Built from | Used for |
|---|---|---|
| Preliminary | Early instrument index, typical hook-ups, estimated routing | Estimating, and orders for long-lead bulk such as multi-pair cable and special alloy tubing |
| Intermediate | Approved hook-ups, fixed device and junction box locations, first cable schedule | The main bulk requisitions |
| Final | Design issued for construction, vendor data received, routed cable lengths | Top-up orders and site material control |
Every issue after the first carries its delta, listing the lines added, removed or changed since the last issue, by how much and why. Procurement acts on the delta, as a change to an order already placed, so a take-off reissued in full with no delta forces somebody to compare two spreadsheets by hand. Reductions matter as much as additions. Material already ordered becomes surplus unless the order can still be cut back, and the sooner a reduction is issued, the more of it can be.
The columns of an instrument MTO
| Column | What it holds |
|---|---|
| Line number | The take-off's own sequence |
| Commodity code | The project material code that ties the line to the material specification and to the requisition |
| Description | Item, size, material, rating and the standard it is made to |
| Unit | Each, metre or drum |
| Net quantity | The total from its sources |
| Allowance | The project's margin for this commodity |
| Gross quantity | Net plus allowance, the quantity requisitioned |
| Source | The hook-up detail, cable number or layout the quantity comes from |
| Revision and delta | The issue the line last changed in, and by how much |
| Hold | Whether the line is withheld pending information |
| Requisition | The requisition number that orders the line |
Where take-offs go wrong
The index trails the P&IDs. A transmitter added at a P&ID revision and not yet entered in the index has no hook-up detail and no materials. The take-off is internally consistent and short by one installation, and the shortage appears when the fitter reaches the device.
A tag carries the wrong detail. Service, orientation and mounting all live in the detail number. A wrong assignment orders plausible materials for the wrong installation, which is harder to spot than a missing line.
The material ignores the line class. Tube, fittings and valves at the process connection follow the line they connect to. A take-off that applies the default stainless steel to a line specified in a higher alloy orders material that cannot be installed.
Scope breaks are unstated. The root valve, the cable from a junction box to the marshalling cabinet, and the tray itself each belong to one discipline. Unless the project says which, they are taken off twice or not at all.
Vendor packages are taken off again. Instruments inside a skid are installed by the package vendor and their bulks belong to the package supply. The project take-off starts at the skid boundary, which the package drawings state.
Cable is measured as the crow flies. A straight-line distance between two ends under-orders every run that follows the tray, and the shortfall shows up when cables are pulled.
Who owns it
The instrument design team prepares the take-off, the lead instrument engineer approves each issue, and procurement and material control consume it: requisitions against the gross quantities during design, then warehouse issues against the same lines on site. At the end of construction a reconciliation compares what was issued with what was installed, and a take-off that kept its sources and its deltas is the one that can explain the difference.
Because most of its lines are counts of tags, the take-off is only as current as the instrument index behind it, and the index is only as current as the P&IDs it was built from. Keeping the index true at every P&ID revision is what keeps the next take-off issue a delta instead of a rebuild. For the documents the take-off draws on, see instrument index fields, instrument installation details and the cable schedule; for the tagged side of the same procurement, see the material requisition and technical bid evaluation.