OpenSpecEngineering

Dimensions for steel pipe and instrument tube, and a guide to telling pipe threads apart. Pick a pipe size and schedule to get its outside and inside diameters, wall, weight and NPT thread pitch. Piping designers, fitters and maintenance staff use it to order pipe, find the bore for a flow calculation and avoid mixing thread systems.

What the page covers

• Pipe Size, Schedule & NPT Threads: a picker for NPS 1/8″ to 24″ in every schedule ASME B36.10M [1] lists for that size (data from [5]). It shows the DN size, OD, wall, ID, plain-end steel weight and the NPT threads per inch from ASME B1.20.1 [3].
• Tube OD & Wall Gauge: common wall thicknesses for 1/8″ to 1″ tube and their BWG numbers, from the Swagelok tubing data [2].
• Thread Standard Comparison: NPT, NPTF, BSPP, BSPT, ISO 6149, SAE ORB and UN/UNF, with taper, sealing method, thread angle and typical use [3], [4].

Reading the sizes

NPS (nominal pipe size) is a label, not a measurement. Every schedule of one NPS has the same OD, and a higher schedule means a thicker wall and a smaller ID. STD, XS and XXS are the older weight classes; STD matches Sch 40 up to NPS 10. DN is the metric name for the same pipe. Tube, unlike pipe, is named by its real OD. A lower BWG number means a heavier wall. The plain-end weight is w = 10.69 (D − t) t lb/ft, with D the OD and t the wall in inches [1].

Limits and common mistakes

• The weight is for bare carbon steel. Add fluid, insulation and fittings for support loads.
• Use the ID, not the NPS, in a flow calculation. A 2″ Sch 80 bore is under 2″.
• NPT (60°) and BSPT (55°) look alike but don’t seal together. Count the threads: 1/4″ NPT has 18 TPI and 1/4″ BSPP has 19.
• NPTF seals dry only when both parts are NPTF. Mixed with NPT, use sealant.
• Straight threads (BSPP, ORB, ISO 6149) seal at a washer or O-ring, never on the thread.

Worked example

A compressed-air drop needs 2″ Sch 80 carbon steel pipe with threaded ends. You need the bore for the pressure-drop check, the weight for the hanger spacing and the thread pitch for the die.

1. Under Pipe Size, Schedule & NPT Threads, choose Nominal pipe size (NPS) 2″.
2. Choose Schedule Sch 80.

The picker shows, under NPS 2″ — Sch 80:

Check: 2.375 − 2 × 0.218 = 1.939 in, and 10.69 × (2.375 − 0.218) × 0.218 = 5.03 lb/ft. Sch 40 would give a 2.067 in bore with the same OD and thread. Take the 1.939 in bore to the Pipe Flow & Pressure Drop calculator.