Gasket Seating Stress Explained For Industrial Flanges

April 7, 2026

A gasket can be chemically compatible with the process fluid, rated for the operating temperature, and still leak. Most of the time, the reason traces back to something that never came up during selection: Gasket Seating Stress.

Gasket seating stress is the minimum compressive load per unit area that must be applied to a gasket during assembly to achieve an initial seal. Without reaching that threshold, the gasket never properly conforms to the flange faces, and no matter how carefully the material is selected or the torque is applied, it will not compensate. A gasket that is not properly seated will leak.

In simple terms, gasket seating stress determines:

• Whether a gasket can form an initial seal
• How well it conforms to flange surfaces
• Whether leakage occurs immediately after installation
• Long-term joint reliability under pressure and temperature

The challenge is that seating stress requirements vary significantly between gasket types, and the available bolt load in a given flange configuration is finite. The two have to match. When they don’t, the result is premature leakage, repeated maintenance calls at the same joint, and eventually an unplanned shutdown that traces back to a specification decision made long before the gasket was ever installed.

Gasket Type vs Seating Stress

Understanding Spiral Wound Flanges and Their Applications

A spiral wound flange connection is defined by the gasket at its center: the Spiral Wound Gasket itself. These connections appear throughout petroleum refining, chemical processing, and power generation, wherever elevated pressure classes and thermal cycling demands rule out softer sheet gasket materials. They are the workhorse joint of high-integrity process piping.

Understanding how seating stress functions in a spiral wound flange assembly starts with the gasket’s construction. The alternating layers of formed metallic strip and soft filler material, wound in a spiral, create a semi-metallic structure that compresses under bolt load and springs back elastically as operating conditions change. That recovery behavior is what makes spiral wound gaskets reliable in thermally dynamic services, but it depends on achieving adequate seating stress at installation.

The inner and outer rings of the gasket play distinct roles in this process:

• The outer centering ring locates the gasket within the bolt circle and acts as a compression stop, preventing the winding from being over-compressed during installation. It defines the seating geometry.
• The inner ring protects the winding from buckling inward under internal pressure and shields it from turbulent process flow. It supports the integrity of the sealing element once the joint is in service.

Seating stress requirements for spiral wound flanges vary based on the winding material and filler combination. PTFE-filled windings require lower seating stress than flexible graphite-filled types. Graphite-filled windings, however, deliver superior recovery under thermal cycling, making them the preferred choice for high-temperature steam and hydrocarbon services. PTFE filler is suited for aggressive chemical environments where graphite would be attacked.

One of the most common errors in spiral wound flange assemblies is failing to verify that the available bolt load in the flange configuration is sufficient to achieve the required seating stress across the full gasket contact area. ASME B16.20 governs dimensional requirements for spiral wound gaskets. Specifying the correct pressure class for the flange rating is the starting point, but it does not substitute for verifying actual Bolt Load Delivery during installation.

Spiral Wound Gasket Factors Affecting Seating Stress

Everything You Should Know About Expanded PTFE Sheet Gaskets

Expanded PTFE Sheet Gaskets are specified wherever process chemistry poses a risk to other materials. They resist virtually every industrial chemical short of molten alkali metals and elemental fluorine, seat at lower stress levels than most gasket materials, and tolerate flange surface irregularities that would compromise harder alternatives. In aggressive chemical services, they are often the only practical option.

The low seating stress requirement is a genuine advantage in the field. Because expanded PTFE conforms readily under moderate bolt load, it can achieve effective sealing on flanges with minor surface imperfections or corrosion pitting. This is especially valuable in older piping systems with corroded or pitted flanges, where reconditioning before every replacement is not always feasible.

That conformability, though, is also the material’s primary limitation.

Expanded PTFE is susceptible to creep relaxation under sustained compressive load. Without constraint, the material continues to compress slowly over time, reducing effective gasket thickness and, with it, the contact stress maintaining the seal. In high-pressure or elevated-temperature applications, creep can be severe enough to open a leak path before the next scheduled maintenance window.

Several design approaches address this:

• Biaxially oriented PTFE formulations resist creep better than standard grades by aligning the polymer structure in two directions, improving load retention under sustained compression.
• Filled PTFE compounds incorporate inorganic fillers to increase stiffness and reduce cold flow, particularly in higher-temperature applications.
• Envelope and encapsulated designs use a PTFE facing bonded to a more rigid core material, providing chemical resistance at the sealing surface while the core handles compressive load.

Installation discipline matters more with expanded PTFE than with most other gasket types. Because the material conforms so readily to flange surfaces, any contamination on those surfaces, including residual old gasket material, oils, or loose particles, can become embedded in the seating interface and create a persistent leak path. Flange faces must be thoroughly cleaned before installation, and the gasket should be positioned flat and centered without folding or stretching.

A Complete Guide to Spiral Wound Gaskets

The Spiral Wound Gasket is the most widely specified semi-metallic gasket in process industry piping. Its construction gives it a combination of properties that no single-material sheet gasket can match: the structural strength of metal, the sealing conformability of a soft filler, and the elastic recovery that maintains seal integrity through repeated thermal cycles.

From a gasket seating stress perspective, spiral wound gaskets occupy a middle ground. They require more bolt load than expanded PTFE or flexible graphite sheet gaskets to achieve initial seating, but significantly less than solid metal ring joints. This positions them as compatible with a broad range of standard ASME pressure classes – typically Class 300 and above – without requiring special high-load flange configurations.

Several variables govern spiral wound gasket seating performance in practice:

• Winding material determines temperature ceiling and chemical compatibility. 316 stainless steel is the standard for general process service. Alloy 625, Hastelloy C-276, and titanium are available for more aggressive environments.
• Filler material controls seating stress requirements and recovery behavior. Flexible graphite filler provides superior recovery under thermal cycling and suits steam, hydrocarbon, and refinery services. PTFE filler reduces seating stress requirements and is preferred in chemical and pharmaceutical applications.
• Gasket thickness affects compressibility and recovery. Thicker windings provide more elastic range but require more bolt load to seat. Standard thickness for most applications is 0.175 inches, with thicker options available for challenging flange conditions.
• Pressure class governs the centering ring dimensions and the load capacity that the gasket is designed to work within. Mismatching the gasket pressure class to the flange rating is a straightforward path to under-seating.

Recovery is what distinguishes spiral wound gaskets from softer alternatives in thermally demanding services. As temperature cycling causes bolt loads to relax and flanges to contract, the metallic winding springs back to maintain contact with the flange face. This behavior is not unlimited, but it substantially extends reliable service life in applications where a compressed fiber or soft sheet gasket would progressively lose contact stress over successive cycles.

Durlon’s ETG Gaskets are engineered for applications where conventional spiral wound gaskets reach their performance limits, delivering reliable seating and recovery across demanding pressure and temperature conditions.

Key Uses and Benefits of Rubber Sealing Rings

Rubber Sealing Rings serve a different part of the pressure and temperature spectrum than semi-metallic and engineered polymer gaskets. They seat at very low stress levels, typically below 200 psi, conform readily to irregular flange surfaces, and deliver reliable sealing in utility, water, low-pressure steam, and mild chemical services where their temperature and pressure limitations are not factors.

With rubber sealing rings, the seating stress problem is not getting enough load onto the gasket but putting too much.

Unlike metallic or semi-metallic gaskets, elastomeric materials can be damaged by over-compression. When bolt load exceeds the material’s compressive limit, the rubber extrudes from the flange interface, permanently reducing the effective sealing area. This leads to immediate leakage or, more insidiously, to a joint that appears tight after installation but fails progressively as the extruded material loses contact. Most rubber sealing ring installations specify torque limits as much to prevent over-compression as to ensure adequate seating load.

Elastomer selection determines service compatibility:

• Nitrile (NBR) handles hydrocarbons, oils, and fuels effectively and is widely used in refinery utility services.
• EPDM provides good resistance to steam, hot water, and many dilute acids and alkalis, making it the standard choice for water treatment and general industrial piping.
• Neoprene offers weather and ozone resistance for outdoor or above-grade piping, along with moderate chemical resistance.

Temperature is the hard constraint. Most standard elastomers are rated to 250°F or below for continuous service, with specialty compounds extending to 300-350°F. For process services at elevated temperatures, rubber sealing rings should be replaced with materials that retain their mechanical properties and seating characteristics at the actual operating temperature, not just the ambient installation condition. Specifying an elastomer gasket based on ambient temperature and then operating the line at process temperature is a predictable path to early failure.

How Flex Seal Gaskets Are Used in Different Systems

Flex Seal Gaskets, a category that includes flexible graphite sheet gaskets and composite flexible materials with metallic reinforcement, are engineered to bridge a gap that neither soft sheet gaskets nor semi-metallic types fill entirely. They provide better recovery and thermal performance than compressed fiber or PTFE sheet materials, while seating at lower stress levels than spiral wound gaskets require. That combination makes them widely applicable across demanding industrial process services.

Flexible graphite materials seat at moderate stress levels, generally 1,500 to 3,000 psi, depending on formulation and thickness. Once seated, flexible graphite maintains residual stress effectively because the material recovers well under thermal cycling, expanding slightly as joints cool and contract to maintain flange contact. This recovery behavior is more predictable than compressed fiber materials and more forgiving of flange surface variation than harder alternatives.

In practice, flex seal gaskets are used across a wide range of systems:

• High-pressure steam services benefit from flexible graphite’s ability to withstand repeated thermal cycling without the progressive stress loss that plagues compressed fiber materials.
• Hydrocarbon process lines in refineries and petrochemical plants rely on flexible graphite for its resistance to oils, solvents, and the wide temperature swings that occur during startup, shutdown, and process upsets.
• Heat exchanger tube sheet applications where differential thermal expansion between shell and tube creates shear forces at the gasket periphery are well-served by flexible graphite’s combination of conformability and recovery.
• Flanged vessel connections at elevated temperatures, where elastomeric and compressed fiber materials would degrade, are another common application.

One important service boundary to understand: standard flexible graphite begins to oxidize above approximately 850°F in air-containing environments. This oxidation progressively reduces the gasket material and, with it, the contact stress maintaining the seal. For applications approaching or exceeding this threshold, inhibited flexible graphite formulations specifically designed to resist oxidation are available. Material datasheets should be reviewed carefully to confirm the upper service temperature for the specific product and application conditions.

For plants managing Fugitive Emissions compliance obligations, flexible graphite materials offer an additional benefit: low permeability. The material’s dense, layered structure limits the migration of process fluid through the gasket body, supporting leak-tightness at the low emission thresholds required by EPA LDAR programs and similar regulatory frameworks.

Gasket seating stress is not an abstract calculation. It is the physical condition that determines whether a flanged joint actually seals. Getting the right material into the right flange is the starting point, but verifying that the available bolt load can actually seat that material is what makes the specification mean something. Skipping that verification step is one of the most common and most avoidable reasons that well-specified gaskets fail in service.

Proper Seating Stress Is What Makes a Seal Work

For technical support with gasket material selection or seating stress calculations for your specific service conditions, reach out to Durlon’s team. We support engineers and maintenance professionals across a broad range of demanding industrial applications.