What Is Bolt Relaxation?
Quick Answer: Bolt relaxation is the gradual loss of clamping force (preload) in a bolted joint over time, even without any external loosening. It is caused by creep in the bolt material, embedding of contact surfaces, and gasket compression settling. Relaxation is the leading cause of flange leaks in high-temperature piping systems.
Causes of Bolt Relaxation
Three distinct mechanisms contribute to bolt load loss in flanged piping joints:
| Mechanism | Description | Typical Load Loss | Time Frame |
|---|---|---|---|
| Elastic interaction | Tightening one bolt reduces load on adjacent bolts | 10-30% per pass | During assembly |
| Embedment relaxation | Microscopic surface irregularities on flange faces, gasket, and nut bearing surfaces flatten under load | 5-10% | First 24 hours |
| Creep relaxation | Bolt material slowly deforms under sustained stress at elevated temperature | 5-25% (temp dependent) | Weeks to months |
| Gasket creep | Gasket material compresses and thins permanently under load | 5-15% | First thermal cycle |
| Thermal cycling | Differential expansion between bolt, flange, and gasket | Variable | Each cycle |
Creep Relaxation by Temperature
Creep becomes significant for carbon and alloy steel bolts above approximately 300 degC. The higher the operating temperature, the greater the relaxation rate:
| Bolt Grade | Material | Creep Onset | Relaxation at 400 degC | Relaxation at 500 degC |
|---|---|---|---|---|
| A193 B7 | Cr-Mo steel | ~300 degC | 5-10% | 15-25% (near limit) |
| A193 B16 | Cr-Mo-V steel | ~400 degC | 2-5% | 5-10% |
| A193 B8 Cl.2 | SS 304 strain-hardened | ~350 degC | 8-15% | Not recommended |
| A286 | Ni-Fe superalloy | ~500 degC | < 3% | 3-8% |
Mitigation Methods
The ASME PCC-1 guidelines for bolted flange joint assembly recommend the following measures to minimize relaxation:
- Multi-pass tightening: Tighten bolts in a minimum of 3 cross-pattern passes to counteract elastic interaction. ASME PCC-1 specifies a star pattern sequence.
- Controlled torque or tensioning: Use calibrated hydraulic bolt tensioners instead of impact wrenches for critical joints. Refer to the bolt torque charts for target values.
- Hot re-torque: Retighten bolts after the system reaches operating temperature (first thermal cycle) to compensate for gasket and embedment settling.
- Proper gasket selection: Use gaskets with low creep relaxation characteristics, such as spiral wound gaskets with graphite filler or Kammprofile gaskets. Avoid compressed fiber gaskets in high-temperature service.
- Belleville (disc spring) washers: Install under the nut to maintain a spring-loaded clamp force that compensates for minor relaxation.
- Correct bolt grade: Select a bolt grade with adequate creep resistance for the service temperature.
Effects of Relaxation on Joint Integrity
When bolt preload drops below the minimum gasket seating stress, the gasket loses its seal. This is especially critical on raised face flanges with spiral wound gaskets and on high-temperature systems where simultaneous bolt creep and gasket creep compound the load loss.
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