PIPING SPOOLS
When piping is fabricated in a workshop, the initial isometric drawings are expanded into spool drawings. A spool is a pre-assembled section of piping consisting of pipes, fittings, and flanges that can be prefabricated. It does not include elements such as bolts, gaskets, valves, or instruments.
A spool sheet is an orthographic drawing that represents one specific spool, derived either from a piping General Arrangement (GA) drawing or an isometric drawing. Each spool sheet typically includes the following details:
- Instructions for the Welder: The spool sheet guides the welder on how to fabricate the spool.
- Bill of Materials: It lists the specific components such as cut lengths of pipe, fittings, and flanges required to assemble the spool.
- Material Specifications: Information about the material of construction is provided, including any special treatments or coatings needed for the piping.
- Quantity of Spools: The sheet indicates how many identical spools are required.
Each spool is assigned a unique number to simplify identification. The corresponding isometric drawings are identified by the line number they represent, and both the spool and spool sheet are labeled using this isometric sheet number as a prefix.
Spools typically exclude straight pipe sections longer than 6 meters, as such lengths are usually welded in the field during installation. The size of a spool is also limited by transportation constraints, as larger spools may be more difficult to transport to the site.
In general, carbon steel piping with a nominal bore (NB) of 40 mm or smaller is usually fabricated on-site. For larger bore sizes, specifically carbon steel and alloy steel piping with a nominal bore of 50 mm or more, prefabrication in a workshop is the standard practice. Larger diameter pipes, due to their handling complexity, are also more economically produced in a workshop environment.