Gasket Factor⚠ unverified
Mechanical / Joints · Compute the required gasket seating pressure
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| m | m | — | 1.0 | Gasket maintenance factor (dimensionless) |
| y | y | Pa | 1.0 | Minimum gasket seating stress |
| P | P | Pa | 1.0 | Internal design pressure |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | Sg | Pa | Required gasket seating pressure, in pascals (Pa) |
The science & history
Understanding the Parameters
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Seating stress $y$ — the stress needed to initially crush the gasket into the flange surface finish so it conforms and seals. It is a property of the gasket material/type (soft rubber ~ a few MPa; spiral-wound and metal gaskets tens to hundreds of MPa) and dominates at low pressure.
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Gasket factor $m$ — the multiplier on operating pressure: to stay sealed, a residual gasket stress of at least $m\,P$ must remain after internal pressure tries to blow the flange open. Soft gaskets have $m \approx 1$–$2$; hard metal gaskets need $m \approx 3$–$7$. It captures that stiffer gaskets require proportionally more clamp to keep contact.
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Internal pressure $P$ — the operating pressure the joint must hold. The maintenance term $mP$ grows with it, so high-pressure joints are gasket-clamp-governed.
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Two conditions — a real flange must satisfy both seating ($S_g \ge y$, at assembly) and operating ($S_g \ge mP$, under pressure); the larger governs the bolt load. The sum form $mP + y$ is the ASME simplification for the required design stress.
Derivation (Approaching a Proof)
Two distinct sealing conditions must hold:
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Seating (assembly). Before any pressure, the bolts must crush the gasket enough to make it conform to the flange faces. This needs a minimum stress $y$ over the gasket contact area.
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Operating (under pressure). Internal pressure $P$ acts to separate the flanges and to blow through the gasket. To keep a leak-tight contact, a residual stress proportional to pressure, $m\,P$, must remain on the gasket after the pressure-induced unloading.
The ASME Boiler & Pressure Vessel Code (Section VIII, Div. 1, Appendix 2) combines these into the required design gasket stress
$$S_g = m\,P + y,$$
which is then multiplied by the effective gasket area to give the bolt load, and checked against the seating-condition bolt load ($y\times$ area). The bolts are sized for whichever governs.
Dimensional check. $m\,P + y = (\text{–})\cdot\text{Pa} + \text{Pa} = \text{Pa}$ — a stress; both terms share units, so the sum is well-formed.
History and Development
The $m$ and $y$ gasket constants were introduced in the ASME code in the 1940s and tabulated for every common gasket type — the empirical backbone of flange design for most of the 20th century. They are admittedly approximate (they don't capture creep, relaxation, or leak-rate directly), and modern gasket standards (EN 13555 / ROTT test data) provide more physically grounded parameters. But $S_g = mP + y$ remains the familiar first-pass sizing rule that ties sealing to bolt load in pressure-vessel and piping joints.
Related Concepts: Bolt Spacing, Bolt Preload, Member Load External, Pressure Vessel Design, Bolt Proof Load
Notes: ASME Appendix 2 model. Two checks: seating ($S_g \ge y$, assembly) and operating ($S_g \ge mP$, under pressure) — larger governs the bolt load. $m$ dimensionless (~1–2 soft, ~3–7 metal); $y$ in Pa. Empirical; EN 13555 gives modern data.