Carbon Graphite Blistering in Mechanical Seal Faces

Carbon graphite blistering is a persistent challenge in mechanical sealing systems. These defects reduce sealing reliability, increase leakage risk, and often lead to premature seal failure. Understanding how and why blistering occurs is essential for improving performance in demanding applications.

Understanding Blister Formation

Blistering appears as raised, shiny areas on the carbon graphite sealing surface. These features originate from subsurface fractures that evolve into bubbles surrounded by cracks. As damage progresses, the blister detaches, leaving a pit that compromises sealing integrity and increases friction and heat generation.

Primary Causes of Blistering

Blister formation results from a combination of startup conditions, process fluid properties, and thermal effects. These factors interact at the seal interface, making blistering a system level issue rather than a single material defect.

Startup Conditions and Shear Stress

The highest risk of blister initiation occurs during seal startup. Breakaway torque is greatest at initial rotation, and thin fluid films combined with high viscosity fluids generate elevated shear forces. These stresses can create microfractures in the carbon graphite surface within seconds, forming initiation sites for blisters.

Influence of Process Fluids

Fluid properties strongly affect blistering behavior. High viscosity fluids increase shear forces, while certain chemistries increase traction at the sealing interface. Carbon graphite materials also contain inherent porosity that allows low viscosity fluids to penetrate below the surface.

In hydrocarbon applications, fluids that vaporize under heat can expand within this porosity. This expansion lifts the surface material, forming blisters that worsen as friction and heat increase.

Thermal Cycling and Heat Effects

Thermal cycling accelerates blister growth. Once a blister forms, it creates a localized high friction area that generates additional heat. Uneven thermal expansion and repeated temperature changes during startup and shutdown promote crack propagation and surface degradation.

Material Selection and Resistance to Blistering

Material choice plays a critical role in blister resistance. Resin impregnated carbon grades are more susceptible due to curing shrinkage, residual porosity, and higher thermal expansion. Metal impregnated grades offer improved performance.

Antimony impregnated carbon graphite provides enhanced resistance by reducing porosity through expansion during solidification. This improves structural integrity and limits blister growth under thermal and mechanical stress.

Mitigation Strategies

Reducing blister risk requires a comprehensive approach. Selecting low porosity, thermally conductive carbon graphite grades with good lubricity is essential. Seal design should promote effective heat dissipation and uniform contact.

Operational practices such as controlled startup, preheating, optimized lubrication, and moderate surface roughness further reduce blistering risk by minimizing shear stress and heat buildup.

Conclusion

Carbon graphite blistering is a well understood failure mechanism driven by mechanical stress, fluid behavior, and thermal effects. While it cannot always be eliminated, its occurrence can be significantly reduced through informed material selection, effective thermal and fluid management, and disciplined operating practices. Addressing blistering at both the material and system levels improves seal reliability and extends service life.

This blog is based on the Sealing Sense Article “Carbon Graphite Blistering in Mechanical Seal Faces”. Click here to read the entire article.