Carbon Steel Pipe Spec Table

In-Depth Guide

Nominal pipe size (NPS / A) is neither the OD nor the ID — the most common dimension mistake

A pipe nominal size (NPS, or the "A" designation) is just a label, not the measured outer or inner diameter. For example, a 1/2" (15A) pipe has an actual OD of 21.3 mm, and its ID changes again with wall thickness (Sch). The key principle is that for a given nominal size the OD stays constant as the Sch changes, and only the ID varies — that is what lets same-size fittings and flanges fit regardless of wall thickness. Also, up to NPS 12" the actual OD is larger than the nominal inch, but from 14" up the OD equals the nominal inch. So when you need the OD, do not treat the nominal as millimeters — read it from the standard OD table.

SGP (KS D 3507) vs STPG (KS D 3562) — why you must not use SGP for pressure piping

They share the same OD system and look identical, but their guaranteed properties are completely different. SGP (SPP, ordinary piping carbon steel pipe) is for low-pressure general piping below about 1 MPa — water, air, low-pressure steam — with low guaranteed tensile and hydrostatic-test levels. Pressure and high-temperature lines must use STPG370 / STPG410 (pressure-piping carbon steel pipe), whose strength and testing are guaranteed. Using SGP on high-pressure steam, boilers or hydraulics just because it looks the same is a code violation and a rupture risk. Before ordering or installing, check the line pressure and temperature first and split SGP from STPG accordingly.

Black vs galvanized pipe, and zinc fume (toxic) and porosity when welding galvanized

Black pipe (uncoated) goes in indoor, painted or buried lines where corrosion is controlled, while galvanized (hot-dip zinc) pipe goes outdoors and in exposed or wet environments. The critical caution is welding galvanized pipe — welding straight over the zinc vaporizes it into a white smoke, which if inhaled causes metal fume fever (chills and fever), and the zinc gets into the weld and causes porosity and cracking. So on galvanized pipe, grind the coating off the weld prep first, ventilate well, and repair the damaged coating with a zinc-rich paint afterward. Where possible, joining galvanized pipe by threads, flanges or grooved couplings is safer than welding.

Threaded vs welded joints — size limits and leak traps

Small bore (roughly 50A and below) is usually joined with taper threads (PT/NPT) and sockets, but at 65A and up or higher pressure you move to butt weld (BW), socket weld (SW) or flanges. A threaded joint loses pressure strength because the pipe wall is cut away at the thread, and it tends to loosen and leak under vibration and temperature swings. Seal threads with PTFE tape or a liquid sealant — wrap the tape in the direction of thread engagement, note that over-wrapping can actually split the fitting, while a liquid sealant suits larger bores and higher temperatures. The larger the pressure and bore, the more welding or flanges, not threads, are the proper choice.

What the Schedule number is, STD/XS/XXS, and corrosion allowance

The Schedule (Sch) number is a thickness class based roughly on 1000 × P/S (P = working pressure, S = allowable stress), so higher pressure needs a higher Sch. In small bores Sch 40 ≈ STD and Sch 80 ≈ XS are nearly the same, but in large bores STD/XS and Sch diverge, so do not mix the two systems on big pipe. For the same Sch, the absolute wall thickness grows as the nominal size increases. In design, add a corrosion allowance (typically 1–3 mm) for the metal that corrosion and erosion will thin away, and let that push you up one Schedule — choosing on the table minimum thickness alone can leave the pipe short on pressure strength later in life.