What Is Gasket Stress?
Gasket stress is the compressive force per unit area applied to a gasket by bolt load to create and maintain a leak-free seal. Two distinct stress conditions govern gasket performance: seating stress (the minimum stress to initially deform the gasket into the flange surface) and operating stress (the residual stress on the gasket after internal pressure acts on the flange assembly).
| Term | Symbol | Definition | Unit |
|---|---|---|---|
| Seating stress | y | Minimum stress to initially compress and seat the gasket (no pressure) | psi or MPa |
| Gasket factor | m | Ratio of minimum operating stress to internal pressure | Dimensionless |
| Operating stress | Sg(op) | Residual gasket stress during pressurized operation | psi or MPa |
| Bolt load (seating) | Wm2 | Total bolt force required to seat the gasket | lbs or kN |
| Bolt load (operating) | Wm1 | Total bolt force required during operation | lbs or kN |
Seating Stress (y-Factor)
Seating stress is the compressive load applied during bolt-up, before the system is pressurized. It must be high enough to deform the gasket material into the microscopic imperfections on the flange face, filling every scratch and tooling mark to create a continuous seal path.
| Gasket Type | y-Factor (psi) | y-Factor (MPa) |
|---|---|---|
| Rubber sheet | 100-200 | 0.7-1.4 |
| PTFE (virgin) | 500-1,000 | 3.4-6.9 |
| CNAF (compressed fiber) | 1,600-3,700 | 11-25.5 |
| Flexible graphite | 1,800-3,700 | 12.4-25.5 |
| Spiral wound (graphite filler) | 10,000 | 69 |
| Kammprofile | 10,000-15,000 | 69-103 |
| RTJ (soft iron) | 18,000 | 124 |
| RTJ (stainless steel) | 26,000 | 179 |
A gasket that is not seated properly at bolt-up will leak during operation, regardless of how much pressure is applied. Under-torqued bolts are the most common cause of gasket leaks.
Gasket Factor (m-Factor)
The m-factor defines the minimum ratio of gasket stress to internal pressure that must be maintained during operation. If internal pressure is P, the gasket must maintain a minimum operating stress of m x P to prevent leakage.
| Gasket Type | m-Factor |
|---|---|
| Rubber sheet | 0.5-1.0 |
| PTFE (virgin) | 2.0-3.0 |
| CNAF (compressed fiber) | 2.0-3.0 |
| Flexible graphite (with insert) | 2.5-3.0 |
| Spiral wound (graphite filler) | 3.0 |
| Kammprofile | 3.0-3.5 |
| RTJ (metal) | 5.5-6.5 |
Higher m-factors require higher bolt loads. RTJ gaskets have the highest m-factor because the metal-to-metal seal demands significant residual compressive stress to resist the hydrostatic end force.
Bolt Load Calculation
The required bolt load is the greater of:
- Wm1 (operating) = H + Hp, where H = hydrostatic end force and Hp = m x P x gasket contact area
- Wm2 (seating) = y x gasket contact area
The number and size of bolts must provide enough cross-sectional area (Am) to carry the greater of Wm1 or Wm2 at allowable bolt stress per ASME Section II, Part D. This calculation is performed during flange design per ASME B16.5 or ASME Section VIII (pressure vessels).
Proper gasket stress requires correctly sized stud bolts torqued per the ASME bolt chart using the multi-pass star pattern recommended by ASME PCC-1.
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