ASME B31.3 / B31.4 / B31.8 Pressure Design
Typical: 0.0625" (1/16") for carbon steel, 0 for stainless
Standard: 12.5% per ASTM A530 / API 5L
B31.3 only. W = 1.0 below creep range; < 1.0 at elevated temperatures.
Understand pressure design formulas, code comparison, material selection, and temperature effects on allowable stress
B31.3 (Process Piping) uses t = PD/(2(SEW + PY)) with a Y coefficient for thick-wall correction. B31.4 (Liquid Transportation) uses t = PD/(2SEF) with a design factor F typically 0.72. B31.8 (Gas Transmission) uses t = PD/(2SFET) where F is the location class design factor (0.72 to 0.40) and T is a temperature derating factor. All codes add corrosion allowance and mill tolerance to the calculated minimum thickness.
After calculating the required nominal wall thickness (including corrosion allowance and mill tolerance), select the lightest standard pipe schedule whose wall thickness equals or exceeds the required nominal thickness. Standard schedules include Sch 5S, 10S, 10, 20, 30, STD, 40, 60, 80, 100, 120, 140, 160, and XXS. A pressure margin of at least 10% above design pressure is recommended for good engineering practice.
The standard manufacturing (mill) tolerance for seamless and welded pipe per ASTM A530 and API 5L is 12.5% undertolerance on wall thickness. This means the actual wall thickness can be up to 12.5% thinner than the nominal value. The required nominal thickness is calculated as t_nominal = t_required / (1 - 0.125) = t_required / 0.875.
The Y coefficient provides a thick-wall correction in the B31.3 formula t = PD/(2(SEW + PY)). Per ASME B31.3 Table 304.1.1, Y = 0.4 below the creep range. For ferritic steels Y increases to 0.5 at 950°F and 0.7 at 1000°F and above; for austenitic steels Y stays 0.4 through 1000°F, rises to 0.5 at 1100°F, and reaches 0.7 at 1150°F. The Y factor reduces the required wall thickness versus the simple Barlow formula by accounting for the strengthening effect of internal pressure.