Tighten every flange to the same bolt stress and the gasket stress comes out very differently from one size and class to the next. This chart shows how much, using ASME B16.5 and B16.47 flange bolting with B16.20 spiral wound or B16.21 non-metallic ring gaskets. Pick a flange standard, gasket and class, then move the slider. Values are for reference; confirm against the current edition of the governing standard. Send us your bolting list for a quote or call (281) 581-0176.
Related: Bolting List Builder · Gasket Stress Target Torque Calculator · Nut Factor (K) Guide · Torque Calculator (Beta) · All technical docs
Every size and class at a glance
Flange bolting from ASME B16.5 / B16.47. Spiral wound sealing element ID and OD from ASME B16.20; ring gasket ID from ASME B16.21, seated out to the raised face. Effective gasket width is reduced by 1/16 in. at the OD, the same as our Torque Calculator. Bolt load uses the thread root area (UNC to 1 in., 8-UN above).
Why gasket stress changes with flange size
The bolts squeeze the gasket, so gasket stress is simply the total bolt load spread over the gasket area:
where Sb is bolt stress, n the number of bolts, Ab the root area of one bolt and Ag the gasket contact area. With Sb fixed, everything depends on the ratio n × Ab / Ag, and that ratio is set by the flange standard, not by the person tightening the joint.
- Small sizes get the most. A small flange still needs four bolts of a practical size, but its gasket ring is short. A lot of bolt area pushes on a small ring, so gasket stress runs high.
- Large, low-pressure sizes get the least. Gasket area grows with the diameter, but Class 150 bolts stay comparatively small. On big Class 150 flanges, the same bolt stress may not seat the gasket.
- Steps between sizes. Bolt count and bolt size change in steps, so stress jumps up and down from one NPS to the next instead of following a smooth curve.
- Higher classes carry more bolt area. More and larger bolts on the same gasket raise gasket stress, which is why one bolt stress can crush a gasket in one class and under-seat it in another.
What to do about it
ASME PCC-1-2026 Appendix O makes the same point: using one bolt stress for every joint (for example 50 ksi) can leave some gaskets under-compressed and can overload small flanges or stainless bolts (O-1.1). The appendix instead starts from a target gasket stress and works back to the bolt stress each joint needs, then checks that bolt stress against minimum and maximum limits (O-4). Switch the chart to Same gasket stress for every size to see that approach, then use the Gasket Stress Target Torque Calculator for a specific joint.
Get gasket stress limits from your gasket manufacturer and nut factor from tests or the Nut Factor (K) Guide. The default windows shown here are examples only. This page does not check flange stress, which PCC-1 Appendix O also covers.