What Is Gasket Creep Relaxation?

Quick Answer

Gasket creep relaxation is the gradual loss of gasket stress over time as the gasket material flows (creeps) under sustained bolt load, reducing the compressive force that maintains the seal. Creep causes the gasket to thin permanently, bolt stretch to decrease, and the effective gasket stress to drop below the minimum required for a leak-free joint. All gasket materials creep to some degree, but soft gaskets (PTFE, CNAF) are far more susceptible than metallic types.

Creep and relaxation are related but technically distinct phenomena. Creep is the time-dependent deformation of the gasket under constant load. Relaxation is the time-dependent decrease in gasket stress (and therefore bolt load) at constant gasket thickness. In practice, both occur simultaneously, and the combined effect is called “creep relaxation.”

Creep Susceptibility by Gasket Type

Gasket Type	Creep Susceptibility	Typical Relaxation (% of initial stress lost after 1,000 hrs)
PTFE (virgin sheet)	Very high	40-60%
Expanded PTFE (ePTFE)	High	30-45%
CNAF (compressed fiber)	Moderate to high	20-35%
Flexible graphite (with insert)	Moderate	15-25%
Spiral wound (graphite filler)	Low to moderate	10-15%
Kammprofile	Low	5-10%
RTJ (solid metal)	Very low	<5%

PTFE gaskets are the most creep-prone because PTFE cold-flows even at room temperature under modest loads. This is why pure PTFE sheet gaskets are generally limited to low-pressure utility services. PTFE envelope gaskets with a graphite or fiber insert perform better because the insert resists creep.

Factors That Accelerate Creep

Factor	Effect
Higher temperature	Accelerates molecular flow in all gasket materials
Higher initial stress	Increases rate of deformation (especially in soft gaskets)
Thicker gaskets	More material volume to creep; thinner gaskets resist better
Thermal cycling	Repeated heating/cooling compounds creep with each cycle
Vibration	Micro-movements progressively reduce bolt load
Bolt relaxation	Bolt steel also creeps at elevated temperatures (above 350 degC for B7)

Consequences of Excessive Creep

When gasket stress drops below the minimum operating stress (m x P, where m is the gasket factor and P is internal pressure), the joint begins to leak. Creep-related leaks typically develop gradually; days, weeks, or months after initial bolt-up; making them difficult to diagnose. They are often mistakenly attributed to faulty gaskets or incorrect torque rather than the underlying creep mechanism.

Mitigation Strategies

Strategy	How It Works
Use thinner gaskets	Less material volume = less total creep; use 1/16” instead of 1/8” where possible
Hot re-torque	Re-tighten bolts after first thermal cycle to compensate for initial creep
Disc spring (Belleville) washers	Maintain bolt load as gasket thins by providing spring energy
Higher-grade gaskets	Spiral wound or Kammprofile instead of soft gaskets
Bolt tensioning (not torque)	More accurate initial load reduces risk of under-loading
Live loading	Spring assemblies on critical joints (heat exchangers, pressure vessels)

Pro Tip

ASME PCC-1 recommends hot re-torque (retightening at operating temperature) for joints in service above 200 degC (392 degF). Perform re-torque within the first 24 hours of reaching operating temperature, following the same star-pattern sequence as initial bolt-up. This single step eliminates most creep-related leaks.

Manage creep relaxation by selecting the right gasket type, applying proper bolt torque with A193 B7 stud bolts, and planning for re-torque on hot services.

Read the full guide to gasket selection