What is a bonded seal?

What Is a Bonded Seal?

A bonded seal (also called a “bonded washer seal” or “combination seal”) is a dual-material sealing component designed to create a leak-proof barrier in threaded or flanged connections. Unlike an O-ring (which is a single elastic ring), a bonded seal combines two parts:

A rigid metal outer ring (e.g., steel, stainless steel) that provides structural support and controls compression.

An elastic elastomeric inner layer (e.g., rubber, silicone) that conforms to gaps and forms the actual seal.

These two parts are “bonded” together (via adhesives or molding) into a single, easy-to-install component. Its primary role is to prevent leakage of fluids (oil, fuel, water) or gases (oxygen, compressed air) in high-pressure, high-vibration, or threaded connections—where O-rings alone may fail due to extrusion (being pushed out of gaps) or uneven compression.

1. Core Structure & Working Principle

To understand how bonded seals work, let’s break down their design and functionality:

a. Structure Details

Component	Material Example	Purpose
Metal Outer Ring	Stainless steel, carbon steel, aluminum	– Maintains shape under high pressure/torque.- Prevents the elastomer from being “extruded” (squeezed out) of gaps between mating parts.- Distributes clamping force evenly across the seal.
Elastomeric Inner Layer	Silicone, NBR (nitrile rubber), FKM (fluororubber)	– Deforms to fill micro-gaps between threaded/flanged components.- Creates a flexible, leak-proof barrier against fluids/gases.- Resists chemical degradation from the medium (e.g., oxygen, oil).

b. How It Seals

When installed (e.g., between a bolted flange or a threaded fitting):

The metal outer ring aligns the seal and limits how much the elastomer can compress (preventing over-deformation).

As the connection is tightened (e.g., with a bolt), the elastomeric layer is compressed between the metal ring and the mating surfaces.

The elastomer deforms to fill even irregular gaps (e.g., thread imperfections, surface roughness) that a rigid metal washer or a single O-ring couldn’t seal.

The metal ring acts as a “shield,” stopping the elastomer from being pushed out of the gap (a common failure mode for O-rings in high-pressure threaded connections).

2. Key Materials for Bonded Seals

Material selection depends on the application’s pressure, temperature, and the fluid/gas being sealed. Below are common combinations, including options for oxygen systems (linking back to your prior interest):

Metal Ring Material	Elastomer Material	Key Properties	Typical Applications
Stainless Steel (304/316)	Silicone	Corrosion-resistant, oxygen-compatible, wide temperature range (-60°C to 230°C).	Oxygen tanks, medical gas fittings, food processing.
Carbon Steel	NBR (Nitrile Rubber)	Low cost, oil/fuel resistance.	Automotive engines (oil lines), hydraulic systems.
Aluminum	FKM (Fluororubber)	Lightweight, high-temperature resistance (up to 260°C), chemical resistance.	Aerospace (fuel lines), industrial ovens.

Critical for oxygen equipment: Bonded seals used in oxygen systems (e.g., high-pressure oxygen tank valves, POC pressure regulators) require stainless steel outer rings (to avoid corrosion) and silicone/FKM inner layers (to prevent reaction with oxygen, which could cause ignition). Oil-based elastomers (e.g., some NBR grades) are strictly avoided here.

3. Applications: When to Use Bonded Seals (vs. O-Rings)

Bonded seals excel in scenarios where O-rings are less reliable—especially in threaded connections or high-pressure systems. Below are key use cases, including overlaps with oxygen-related devices:

a. Oxygen & Medical Gas Systems

Oxygen Tank Valves: Threaded connections on high-pressure oxygen cylinders use bonded seals to prevent catastrophic leaks. The metal ring resists the extreme pressure (up to 3,000 psi in medical tanks), while the silicone elastomer ensures a tight seal.

POC Pressure Regulators: Bonded seals seal the connection between the POC’s compressor and pressure regulator, preventing oxygen leakage and maintaining consistent flow rates.

b. High-Pressure Industrial Systems

Hydraulic/Pneumatic Lines: Threaded fittings in hydraulic machinery (e.g., construction equipment) use bonded seals to withstand pressures up to 10,000 psi—O-rings here would be extruded out of gaps.

Chemical Processing: Bonded seals with FKM elastomers seal pipes carrying corrosive chemicals (e.g., acids), as the metal ring protects the elastomer from chemical attack.

c. Automotive & Aerospace

Engine Oil Lines: Threaded oil fittings in car engines use bonded seals to resist vibration and high temperatures, preventing oil leaks that could damage the engine.

Aerospace Fuel Systems: Bonded seals with aluminum rings and FKM elastomers are used in jet fuel lines, as they are lightweight and resist fuel degradation.

d. Plumbing & Heavy Machinery

Water Pumps: Flanged connections in industrial water pumps use bonded seals to seal against high water pressure and prevent rust (via stainless steel rings).

4. Advantages Over O-Rings & Other Seals

Bonded seals solve key limitations of O-rings and traditional washers:

Resists Extrusion: The metal ring stops the elastomer from being squeezed out of gaps in high-pressure or wide-gap connections (a major O-ring failure mode).

Easy Installation: Unlike O-rings (which can slip out of place during assembly), bonded seals are a single component that stays aligned with threaded fittings.

Uniform Sealing: The metal ring distributes clamping force evenly, ensuring the elastomer compresses consistently—even on rough or uneven surfaces.

Durability: The metal outer ring protects the elastomer from physical damage (e.g., scratches from bolts) and extends the seal’s lifespan.

5. Maintenance & Replacement

Like O-rings, bonded seals are wearable components and need replacement over time. Signs of failure include:

Leakage: Hissing (in gas systems) or visible fluid stains around the connection.

Elastomer Damage: Cracking, hardening, or swelling of the inner elastomeric layer (from aging or chemical exposure).

Metal Ring Damage: Rust, bending, or deformation (which breaks the bond with the elastomer).

Replacement guidelines:

Always match the new seal’s material to the original (e.g., silicone elastomer for oxygen systems).

Ensure the metal ring’s size fits the bolt/fitting (misalignment causes leaks).

Avoid over-tightening: Excessive torque can damage the elastomer or metal ring.

6. O-Ring vs. Bonded Seal: Key Differences

To summarize how bonded seals compare to O-rings (your prior topic):

Feature	O-Ring	Bonded Seal
Structure	Single elastic ring	Dual-material (metal + elastomer) bonded component
Best For	Smooth, low-to-medium pressure gaps (e.g., POC sieve bed connections)	Threaded/high-pressure connections (e.g., oxygen tank valves, hydraulic lines)
Extrusion Resistance	Low (risk of being squeezed out in wide gaps)	High (metal ring prevents extrusion)
Installation	Prone to slipping out of place	Easy (single component, self-aligned)

Conclusion

Bonded seals are specialized sealing components that combine the flexibility of elastomers (like O-rings) with the structural strength of metal. They are critical for high-pressure, threaded, or vibration-prone systems—including oxygen tanks and POC regulators—where O-rings alone can’t reliably prevent leaks. By choosing the right material combination and maintaining them properly, bonded seals ensure long-term safety and efficiency in demanding applications.

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