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Key takeaways
- An isometric shows one line in 3D on flat paper. It is not to scale, but every dimension on it is exact.
- It carries everything needed to build and inspect the line: dimensions, elevations, welds, spool breaks, supports and the bill of materials.
- Most isometrics today are generated automatically from the 3D model, so errors in the model become errors in the shop.
What an isometric is
A piping isometric draws a single line (or part of one) in an isometric view: the three axes are drawn at 120° to each other, so vertical runs go straight up the page and horizontal runs go at 30°. It is a *not-to-scale* drawing, which is why every length is dimensioned.
Isometrics are produced from the 3D model by software such as Isogen, from systems like Smart 3D, E3D or PDMS, and AutoCAD Plant 3D.
What you will find on one
North arrow and coordinates. The north arrow orients the drawing. Coordinates (E, N) and elevations (EL) at key points tie the line to the plant grid.
Dimensions. Lengths are given between centrelines of fittings and to faces of flanges. Elevations are usually to the pipe centreline or bottom of pipe (BOP).
Welds and spool breaks. Shop welds are made in the fabrication shop; field welds are made on site. Field welds often have extra length (“field fit weld”) so the fitters can cut to suit. The drawing is divided into spools, the pieces that fit on a truck.
Supports. Support tags and types (rest, guide, anchor, spring) show where the line is held.
Bill of materials (BOM). The table lists every component with its size, rating, material and quantity. It is the source for the material take-off (MTO).
Header block. Line number, piping class, design and test pressure and temperature, insulation, painting, PWHT, NDE extent and revision.
Common mistakes to look for
- Missing high-point vents or low-point drains after rerouting.
- Supports shown on the model but not tagged on the iso.
- A field weld in a place nobody can reach to weld.
- BOM quantities that don’t match the drawing after a revision.
Onshore
Onshore spools are sized to fit a road truck, and site fit-up has plenty of room.
Offshore
Offshore spools are often fabricated with the module, but hook-up spools between modules are built with extra length and fitted after the modules are set, because steel and hull movement make exact dimensions impossible to predict.
What to take back to your desk
When you check an iso, trace the line with a pen from start to finish, ticking each fitting against the BOM. It’s slow, but it catches what software misses.
Educational content. Always design to your project specification.
