Pipefitting
Pipefitting: layout, fabrication, and installation
This domain tests the pipefitter's ability to turn drawings into installed, code-compliant piping: reading isometrics and spool sheets, calculating cut lengths and offsets, selecting fittings and supports, and executing safe, sound joints. Questions favor field math, fitting take-outs, and recognition of correct practice over memorized trivia.
Core rules and concepts
- Work from a current drawing and tag every spool; verify line number, size, schedule, service, and specification before cutting.
- Long-radius 90° elbows have a center-to-face (take-out) of 1.5 × NPS. Short-radius elbows are 1.0 × NPS.
- For 45° offsets, travel = offset × 1.414 and offset = travel × 0.707; for 22.5° offsets use 2.613 and 0.383.
- Butt-weld fit-up requires the specified root gap, proper bevel, alignment, and no internal misalignment beyond code; tack welds are part of the joint and must be sound.
- Threaded joints use tapered NPT threads; excessive engagement or damaged threads cause leaks. Socket welds require a 1/16 in. gap before welding.
- Supports and hangers carry weight, control movement, and protect equipment; spacing and type follow the specification, not guesswork.
Worked example
A 6 in. NPS Schedule 40 line runs between two long-radius 90° elbows, center-to-center 60 in. Take-out = 1.5 × 6 = 9 in. per elbow. Face-to-face pipe length = 60 − (2 × 9) = 42 in. If the specification requires a 1/8 in. root gap at each joint, cut the pipe to 42 − (2 × 1/8) = 41¾ in. The reasoning: subtract fitting take-outs to convert centerlines to faces, then subtract gaps so the assembled joint meets the drawing.
Traps that cost marks
- Using center-to-face as the cut length; forgetting two take-outs, root gaps, or valve face-to-face dimensions.
- Mixing 45° and 22.5° constants, or applying an offset constant to a rolling offset without the true vertical and horizontal components.
- Assuming all 90° elbows are long radius; short-radius and reducing elbows change take-out.
- Ignoring specification breaks: different schedules, materials, or services may require different fittings, supports, or joint types.