8.6 Spool Fabrication Workflow, Shop Versus Field Welds & Advanced Pipe Fabrication

Spool Fabrication Workflow, Shop Versus Field Welds & Advanced Pipe Fabrication

Core Trade Concept: Module 08402, Advanced Pipe Fabrication, is a 50-hour module — the longest in the Boilermaking curriculum. It exists because getting a complex piping system built is not primarily a welding problem; it is a sequencing, documentation and dimensional-control problem. A spool that arrives at site with the wrong heat number, no PWHT chart, or 6 in. of missing tie-in allowance stops a crew cold no matter how good the welds are.

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1. From Isometric to Cut List

   PIPING ISOMETRIC (Section 6.2)
            |
            v
   SPOOL SHEET  -- the iso broken into shippable pieces at
            |      shop/field weld boundaries
            v
   CUT LIST + BILL OF MATERIAL
            |          (each line: NPS, schedule, material spec,
            |           cut length, quantity, spool number)
            v
   MATERIAL RELEASE  -- QC releases stock against the MTR
            |
            v
   FABRICATION TRAVELER (router) -- the step-by-step record
                                    with AI/QC hold points

Where a spool is broken is an engineering decision driven by three constraints: shipping envelope (a spool must fit on a truck and through the plant), erection access (what a crane can land in one lift), and welding position (a weld the shop can turn on rollers is far cheaper and more reliable than an overhead field weld).

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2. Weld Numbering and the Shop/Field Distinction

Every weld on a spool sheet carries a unique number, and each is classified:

Field fit welds are the system's tolerance budget. Real structures are not where the model says they are: nozzles are off elevation, steel is out by fractions, and two spools that were both fabricated to tolerance can still fail to meet. The FFW carries a deliberate over-length — often several inches — that the crew trims and bevels to close the gap.

Never trim an FFW to the drawing dimension in the shop. Doing so eliminates the tolerance absorption and guarantees a gap in the field that has to be closed with a pup piece and an extra weld.

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3. The Fabrication Traveler and Hold Points

The traveler (also called a router or ITP record) travels physically with the spool and records each step in order:

1. Material receipt and identification — MTR reviewed, heat number recorded, PMI on alloy
2. Cut and heat-number transfer — the number is transferred to the offcut before the cut
3. Bevel and fit-up — hi-lo verified, root opening gauged
4. Weld — welder ID stamped or recorded against the weld number, filler metal lot recorded
5. Visual and NDE — VT, then RT/UT/PT/MT per the drawing and code
6. PWHT — chart attached, thermocouple locations recorded (Chapter 10)
7. Post-PWHT NDE and hardness — required on CSEF materials such as P91
8. Hydrostatic test — if the spool is tested at shop level
9. Cleaning, blasting, coating
10. Final dimensional check and release

Authorized Inspector hold points stop work. A step marked as a hold point cannot be passed until the AI has physically inspected and signed (Section 13.2). Working past an unsigned hold point can void code acceptance of everything downstream of it.

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4. Handling, Storage and Protection

• Alloy is segregated. Chrome-moly and stainless are stored physically apart from carbon steel, with colour coding and PMI verification on issue. A single misplaced length in an alloy rack becomes a latent failure years later.
• Stainless is protected from carbon steel contamination. Separate grinding wheels, wire brushes, slings and lay-down areas. Iron embedded from a carbon steel tool rusts and initiates pitting on a stainless surface (Section 2.2).
• Bores are capped. Open spool ends collect rain, rodents, welding rod stubs and gloves. Every field crew has a story about what came out of a line during the steam blow.
• Spool marking is permanent and visible: spool number, line number, heat number, material spec — applied with a marker approved for the material.
• Lifting points are planned. A long, thin spool with a flange on one end has an off-centre CG (Section 11.2), and a spool lifted from a single point sags and yields.

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5. Advanced Fabrication Situations

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6. Realistic Trade Scenario: A Tie-In That Arrived Short

A refinery turnaround has a 12 in. Schedule 80 alloy tie-in scheduled for the first 24 hours of the outage. Two spools were prefabricated eight weeks earlier from a laser scan of the existing steel.

At fit-up the two spools are 1-1/4 in. short of meeting.

What went wrong and what happens next:

1. The FFW allowance was trimmed in the shop. The shop, working to a clean drawing dimension, cut the field-fit end to the nominal length rather than leaving the specified 6 in. of trim allowance.
2. Accumulated tolerance did the rest. Both spools were individually within PFI ES-3 linear tolerance, and the existing header had moved slightly since the scan; the errors happened to stack in the same direction.
3. The recovery is a fabricated pup piece: a 12 in. Schedule 80 alloy section cut to 1-1/4 in. plus two root openings, machined-bevelled on both ends, with two additional code welds where there should have been one — each needing preheat, PWHT and radiography on the outage critical path.
4. Cost: roughly eighteen hours added to a 24-hour critical-path activity, and a permanent extra weld in a high-energy line.

The lesson is the one this whole chapter is built around: dimensional discipline and the field fit allowance are worth more on an outage than welding speed. A crew leader who checks that FFW ends arrive long has prevented the most expensive failure mode in pipe fabrication.