Barlow's Formula (Pipe Pressure Rating)

Cut a pressurised pipe lengthwise in your imagination: the pressure acting on the projected area D per unit length must be resisted by two wall thicknesses in tension, which gives P = 2St/D directly. Peter Barlow published it in 1836 for cast-iron water mains and it has outlived nearly every rival. A 6.625 in OD line pipe with a 0.25 in wall in 35 000 psi material bursts, in theory, at 2 × 35 000 × 0.25/6.625 ≈ 2642 psi.

That theoretical number is never the rating. Codes such as ASME B31.3 and B31.8 multiply it by a design factor (0.72 for most gas transmission), a longitudinal-joint factor and a temperature derating, then subtract mill tolerance and a corrosion allowance from t, so the stamped working pressure is often less than half the Barlow result. Note too that D is the outside diameter here; some references write the same formula with mean or inside diameter and a correspondingly different answer, and being conservative means using OD.