by Sara McCaslin Sara McCaslin No Comments

Metal O-Ring Seals Are Not Interchangeable

A metal O-ring is not simply a high-temperature replacement for an elastomeric or polymer O-ring. Rather, it is an engineered static-sealing component whose performance depends on several key characteristics, from surface finish to its orientation in the system.

In this article, we discuss when metal O-rings are commonly used, the three types of metal O-ring seals, and the information needed to correctly specify a metal O-ring.

When Elastomer O-Rings Reach Their Limits

An elastomeric O-ring can remain the better choice where conditions are moderate, and recoverability, cost, or installation tolerance matters more. Engineers begin considering metal O-ring seals as a viable option when there is …

  • High pressure or pressure cycling
  • High or low temperatures, including cryogenic service
  • Vacuum or controlled low-leakage systems
  • Aggressive media, radiation, or environments where elastomer aging is a concern
  • Static flanges, closures, valves, pressure vessels, and face-seal interfaces

Keep in mind that metal seals generally impose higher seating load and groove design requirements. 

Standard Metal O-Ring Seals

The standard hollow metal O-ring is a high-strength static-sealing option. It offers minimal springback compared with a spring-energized construction, which makes joint geometry, compression control, and surface quality especially important. This particular configuration works well when pressure, temperature, and vacuum conditions are challenging.

They are best for controlled, static assemblies. They do require a well-designed groove and adequate bolt or clamp load, and the selected tube material, plating, and coating must be matched to media, temperature, corrosion, and leakage requirements. Finally, they may be appropriate where system conditions favor robust metallic containment but repeated recovery is not the central need.

Standard metal O-ring seals are used in automotive exhaust systems, static flange seals in the oil & gas industry, and static seals for industrial machinery.

Spring-Energized Metal O-Ring Seals

Having an internal spring improves resilience and helps the seal maintain contact as the joint undergoes thermal changes, tolerance variations, or pressure cycling. This design typically offers enhanced flexibility and lower leakage in extreme conditions, including cryogenic, high-pressure, and vacuum environments. However, the spring does not eliminate the need for proper gland design; rather, it provides a controlled way to add recoverability to an otherwise metal-dominant sealing system.

Spring-energized metal O-ring seals are often used in high-pressure fuel systems, cryogenic sealing, wellhead seals in oil & gas, and steam turbine sealing in power generation.

Balanced Metal O-Ring Seals

Balanced metal O-ring seals are engineered for high-pressure, high-temperature applications. Pressure-assisted sealing is a balanced design that includes venting to enhance the O-ring’s sealing effect. The orientation of balanced metal O-ring seals is extremely important because the vented holes must be aligned with the system pressure to function properly. 

These balanced metal O-ring seals are used as pressure-vessel seals in aerospace applications, static seals for chemical-processing vessels, and pressure-activated seals for the oil & gas industry.

The Design Inputs Engineers Must Provide

What kind of design inputs are needed to spec a metal O-ring seal? 

  • Static or dynamic service
  • Pressure range, peak pressure, and pressure direction
  • Internal, external, or alternating pressure
  • Operating and thermal-cycle temperature range
  • Fluid, gas, vacuum, or cryogenic medium
  • Required leak rate and test method
  • Mating materials and corrosion environment
  • Available gland dimensions and installation load
  • Surface finish, flatness, and hardness
  • Reuse versus single-use sealing requirement
  • Material restrictions, including plating or coating compatibility

Remember that sealing performance is a system outcome: it cannot be predicted by seal type alone. So even with an excellent metal O-ring design, if the system itself is not accounted for, then the O-ring is not designed correctly.

Common Mistakes

Here are the most common mistakes when specifying a metal O-ring seal:

  • Treating a metal O-ring as a drop-in elastomer O-ring replacement
  • Ignoring pressure direction on a balanced seal
  • Underestimating seating load
  • Specifying a seal before defining allowable leak rate
  • Failing to account for thermal expansion of flanges and hardware
  • Using unsuitable mating-surface finish or damaged flange faces
  • Selecting a coating for lubricity without checking media and temperature compatibility

Conclusion

Metal O-ring seals are engineered static-sealing components, not drop-in replacements for elastomer O-rings. Standard, spring-energized, and balanced designs each suit different pressure, temperature, recovery, and leakage requirements. Successful selection depends on the entire sealing system: pressure direction, service media, thermal range, leak-rate target, gland geometry, seating load, mating surfaces, and material compatibility. 

For demanding high-pressure, high-temperature, vacuum, or cryogenic applications, Advanced EMC can help identify the appropriate metal O-ring configuration. Contact us today.

by Sara McCaslin Sara McCaslin No Comments

From Datasheet to Service Life: A Better Framework for Seal Qualification

In our previous article, we discussed the problems that can arise in high-performance seals when engineers focus too much on compressive stress when choosing a sealing solution. That leads to the question: How does one achieve a better characterization of seal behavior? Consider the following methods.

Compression Stress Relaxation (CSR) testing (ASTM D6147 / ISO 3384)

This approach measures retained sealing force over time under constant strain. CSR testing directly models what happens in a real gland rather than just measuring recovery after the fact. ISO 3384-1:2024 covers constant-strain conditions; ISO 3384-2:2019 extends this to alternating compression cycles, useful for simulating thermal cycling effects on effective gland compression. 

CSR is better aligned with force retention, but standard test results alone cannot predict installed life. A standard specimen compressed between parallel plates does not reproduce:

  • O-ring curvature and nonuniform strain
  • Actual gland squeeze and volume fill
  • Surface texture and groove corner effects
  • Internal pressure loading and pressure direction
  • Clearances, backup rings, and extrusion gaps
  • Media exposure, permeation, or fluid aging
  • Thermal gradients, dwell periods, and transient cycles
  • Dynamic motion or system vibration

CSR data can be an important input to a calibrated service-life assessment, but it is not a standalone life prediction. Correlation requires representative temperature, media, deformation, hardware compliance, pressure, and validation testing. 

Percent Retained Sealing Force (%RSF) 

This particular approach serves as a more directly actionable output than a compression set percentage. It answers the question engineers actually care about: how much sealing force is left after time and temperature have done their work. 

However, be aware that a normalized percentage can hide an inadequate initial sealing load. For example:

  • Material A retains 80% of an initially inadequate force.
  • Material B retains 65% of a substantially higher initial force.

Material B may retain more absolute force and provide the better seal. The design-relevant question is whether the absolute residual contact pressure/force exceeds the minimum required sealing threshold by a margin.

When using %RSF, report force decay or force retention together with the initial counterforce and a defined minimum acceptable residual force or contact-pressure threshold. A percentage alone can be misleading if the starting force differs between materials or geometries.

Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) evaluates an elastomer’s viscoelastic behavior by applying a small oscillating deformation while changing temperature, frequency, strain amplitude, or time. Its primary outputs are storage modulus, loss modulus, and tan δ\deltaδ. Here is what they tell us:

  • Storage modulus indicates the material’s elastic stiffness
  • Loss modulus reflects energy dissipation through internal molecular motion
  • tan δ\deltaδ, the ratio of loss modulus to storage modulus, indicates damping behavior

For sealing applications, DMA is especially useful for identifying temperature ranges where stiffness changes rapidly. A decrease in storage modulus at elevated temperature can reduce the contact force generated at a given gland squeeze and may reduce extrusion resistance under pressure. At low temperature, approaching a glass-transition or secondary-transition region can make the material less conformable, reducing its ability to accommodate surface roughness, dimensional variation, and thermal contraction. DMA can therefore help compare compounds with similar room-temperature hardness but that behave differently across the operating temperature range.

Keep in mind that DMA is a material-screening and diagnostic tool, not a direct seal-performance or service-life test. Test reports should state the test mode, frequency, strain amplitude, temperature ramp, specimen geometry, and the convention used to identify transitions, since reported transition temperatures can vary with method. Use DMA alongside compression stress relaxation, media aging, extrusion resistance, pressure cycling, and functional leakage testing on representative gland hardware.

Media-Representative Testing

Media-representative testing evaluates the seal while exposed to the actual service fluid or gas (or a technically justified simulant) rather than just relying solely on dry compression testing. The exposure should reproduce the relevant fluid composition, concentration, contaminants, temperature, pressure, dwell time, and fluid-aging or replenishment conditions. This is actually key because a material rated broadly as “compatible” may behave differently when it comes to real mixtures containing additives, water, cleaning agents, dissolved gases, oxidation products, or contaminants.`

Fluid exposure can cause issues such as shrinkage, swelling, plasticization and softening, extraction of compounding ingredients, hardening, cracking, embrittlement, and loss of mechanical properties. Such changes may alter both the installed squeeze and the force the seal retains over time. 

Standard liquid-exposure methods such as ASTM D471 evaluate changes in volume, mass, hardness, tensile strength, and elongation after immersion. This provides highly useful compatibility-screening data but does not, by itself, demonstrate leak-tight service performance.

For demanding applications such as those under discussion, it is wise to evaluate media-aged specimens using the properties that connect exposure to function:

  • Compression stress relaxation or retained counterforce
  • Dimensional change
  • Visible damage
  • Leakage performance in representative gland hardware. 

Where pressure, thermal cycling, or dynamic motion is relevant, it is also prudent to conduct exposure and functional testing under those combined conditions rather than aging the material separately in a static immersion vessel. The objective is to establish that the seal retains adequate contact pressure, dimensional stability, and structural integrity for the required service life—not merely that it survives a short, dry compatibility test.

Gland-Representative Geometry 

Material properties do not directly translate into installed seal performance unless the test configuration reflects the actual gland. Gland geometry controls the installed squeeze, stress distribution, contact width, available void volume, and deformation under pressure.

 Relevant factors include seal cross-section, groove depth and width, corner radii, installed stretch, squeeze, gland fill, clearance gap, pressure direction, and any backup-ring configuration. 

Excessive stretch can reduce effective cross-section and squeeze, while excessive gland fill can leave insufficient room for thermal expansion, media-induced swell, and pressure-driven deformation.

Representative testing should include nominal dimensions and tolerance extremes for the seal and hardware, including 

  • Cross-section variation
  • Groove dimensions
  • Concentricity
  • Runout
  • Thermal expansion
  • Pressure-induced housing deflection. 

A nominal-only test can miss low-squeeze conditions that reduce contact-force margin or high-fill conditions that increase assembly damage and extrusion risk.

Surface condition and pressure loading should also reflect the real assembly. Surface roughness, waviness, machining lay, scratches, edge breaks, pressure direction, and worst-case clearance gap can all affect leakage and extrusion. For high-pressure service, test the actual anti-extrusion configuration and verify functional leakage performance in representative hardware—not only material behavior in a parallel-plate fixture.

Focus on the Application

High-performance seals require more attention to the actual operating conditions than the compressive set. The methods we covered in this article can assist engineers in choosing the right sealing solution for high-performance applications.