Metal Hollow O-Rings: Principles, Advantages, and Installation Grooves

The metal hollow O-ring is a circular sealing ring made from metal tubing, typically with a circular cross-section (though oval, rectangular, etc., are also possible). It functions by deforming elastically under compression to fill the micro-gaps between sealing interfaces, thereby achieving a seal. Unlike elastomeric seals, it is specifically designed to address sealing challenges in extreme environments involving high temperature, high pressure, high vacuum, and strong corrosion.

​I. Working Principle: Elastic Deformation and Line Contact Sealing​

The sealing principle of the metal hollow O-ring is based on the classic "line contact" mechanism, which involves the following key steps:

1. ​Preload and Line Contact:​​

   • When flanges or connectors are tightened with bolts, the groove cover applies an axial compression force to the metal hollow O-ring.

   • This force causes the wall of the hollow metal ring to undergo ​elastic deformation​ (flattening), pressing its outer wall tightly against the bottom of the sealing groove and the cover plate, forming a continuous "line contact" sealing band. This initial contact provides the basic static seal.

2. ​System Pressure-Assisted Sealing (Self-Energizing Effect):​​

   • When the internal system pressure increases, the medium pressure acts on the inside of the O-ring, either through vent holes in the groove base or directly.

   • The pressure further forces the O-ring to expand outward, pressing its sealing lip (the contact edge formed by flattening) more tightly against the opposing sealing surface. ​The higher the system pressure, the greater the sealing contact pressure. This "self-energizing effect" significantly enhances sealing reliability.

​Difference from Solid Metal O-Rings:​​

• ​Solid metal O-rings​ primarily rely on large compression forces to plastically deform the metal to fill gaps. They require high sealing forces and are often not reusable after disassembly.

• ​Hollow metal O-rings​ primarily rely on the elastic deformation of the tube wall, requiring much lower sealing forces and offering better elastic recovery, resulting in superior reusability.

​II. Core Advantages​

Based on its working principle and metal material, metal hollow O-rings offer the following outstanding advantages:

1. ​Extremely Wide Temperature Tolerance:​​ Its most notable advantage. Depending on material choice (e.g., Inconel, stainless steel 316, Hastelloy), it can withstand extreme temperatures from ​​-250°C to over 1000°C, far exceeding any polymer sealing material.

2. ​Excellent High-Pressure and High-Vacuum Sealing Performance:​​ Capable of withstanding ultra-high pressures of hundreds of Megapascals (MPa). In high-vacuum systems, the very low gas permeability of the metal itself ensures excellent vacuum integrity.

3. ​Outstanding Corrosion Resistance:​​ Special alloys can resist various strong acids, alkalis, organic solvents, and high-temperature oxidation environments.

4. ​No Creep or Relaxation, Long-Lasting Seal:​​ Metal materials do not creep or age like plastics or rubbers at high temperatures, maintaining a stable sealing force long-term and preventing leakage due to relaxation.

5. ​Reusability:​​ With correct design and installation, as long as permanent crushing (over-compression) does not occur, it can be disassembled and reused multiple times, reducing long-term maintenance costs.

6. ​Radiation Resistance:​​ Suitable for environments with radiation exposure, such as the nuclear industry.

​III. Installation Groove Design and Key Parameters​

Groove design is critical for the proper functioning of metal hollow O-rings. The core objective is: ​to provide precise compression space for the O-ring while restricting its lateral flow.​​

​1. Groove Types​

• ​Open Groove (Most Common):​​ The groove is machined into one flange face, and the other is a flat sealing surface. Suitable for most high-pressure static seals.

• ​Closed Groove (Two-part Groove):​​ The groove is formed by machining a half-groove into each flange face. Facilitates O-ring positioning and installation but requires high machining accuracy.

​2. Key Groove Dimension Design​

Groove dimensions are closely related to the O-ring's free diameter (OD) and wall thickness (WT). The following are basic design principles for circular cross-section hollow metal O-rings (specific dimensions should refer to standards or supplier recommendations):

• ​Groove Width (W):​​

  • Should be slightly larger than the O-ring's free diameter (OD) to accommodate it. Typically, ​W ≈ OD + (10% ~ 20%) OD.

  • The width must not be excessive, otherwise, the O-ring may deform excessively under pressure and be extruded into the gap, causing damage.

• ​Groove Depth (D):​​

  • This is the ​most critical​ parameter, directly determining the O-ring's ​compression ratio.

  • ​Compression Ratio = (OD - D) / OD × 100%​​

  • For static seals, the recommended initial compression ratio is typically between ​15% and 30%​. Too low a ratio can cause leakage; too high can crush the O-ring, causing loss of elasticity and making it non-reusable.

  • Groove depth (D) must be less than the O-ring's free diameter (OD).

• ​Groove Surface Roughness:​​

  • The surface roughness of the sealing contact faces is crucial. Typically, ​Ra ≤ 0.8 μm​ is required to ensure good line contact and minimize leakage paths.

• ​Groove Corners:​​

  • The junction between the groove bottom and sidewall should have a suitable radius (small chamfer) to prevent stress concentration and scoring of the O-ring.

​3. Compression Control: Percent Compression​

After installation, the height of the O-ring is compressed. This degree of compression is measured by "percent compression".

• ​Percent Compression = (Free Height - Compressed Height) / Free Height × 100%​​

• For standard circular cross-section hollow metal O-rings, ​20% - 30% compression​ is a common and effective range. This must be ensured by precisely calculating the groove depth and selecting appropriate gaskets or spacers.

​IV. Typical Applications​

• ​Aerospace:​​ Engines, fuel systems, high-temperature pipelines.

• ​Petrochemicals:​​ High-temperature, high-pressure reactors, valves, pipe connections.

• ​Nuclear Industry:​​ Nuclear reactors and related equipment.

• ​Vacuum Furnaces, Semiconductor Equipment:​​ Chamber door seals requiring ultra-high vacuum maintenance.

• ​Supercritical Fluid Equipment.​​

​Conclusion​

The metal hollow O-ring is one of the ultimate solutions for sealing challenges in extreme conditions. Its successful application depends heavily on ​correct material selection​ (matching the medium and temperature), ​precise groove design​ (controlling compression), and ​clean, professional installation. When selecting and using them, it is highly recommended to consult in detail with professional suppliers or standard specifications (such as the AMS series) to ensure fail-safe operation.

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