1. Main Types of Flexible Connections in Piping Systems
Flexible connections are primarily divided into two major categories: Non-Metallic Flexible Connections and Metallic Flexible Connections.
①Non-Metallic Flexible Connections
Rubber Flexible Connections (Flexible Rubber Joints)
The most common type, including single sphere, double sphere, concentric reducer, eccentric reducer, etc.
Plastic Flexible Connections: Usually made of PTFE (Teflon) or other engineering plastics, offering excellent corrosion resistance.
Fabric (Canvas) Flexible Connections: Often used in ventilation and dust removal ducts to absorb minor displacements and isolate vibration. They offer good temperature resistance but low pressure ratings.
②Metallic Flexible Connections
Metal Bellows Hoses: The most mainstream type, consisting of a stainless steel bellows, flanges/fittings, and a wire braid (for protection and pressure containment). Based on the bellows profile, they are further classified as Annular Bellows (more flexible) and Helical Bellows (higher pressure rating).
Metal Hinged and Gimbal Type Expansion Joints: Used to absorb large angular or lateral movements, with a more complex structure.
2. Rubber Flexible Joint vs. Metallic Flexible Connection: Core Differences
| Comparison Aspect | Rubber Flexible Joint (Rubber Expansion Joint) | Metallic Flexible Connection (Metal Bellows Hose) |
| Core Material | Natural/Synthetic Rubber (e.g., NR, EPDM, NBR), reinforced with nylon cord/steel wire. | Austenitic Stainless Steel (e.g., 304, 316L), with a very thin bellows wall. |
| Working Principle | Utilizes the high elasticity, low modulus, and flexibility of the rubber itself to deform and absorb movement. | Utilizes the elastic deformation (extension, compression, bending) of the metal bellows to absorb movement. |
| Compensation Direction | Multi-directional: Can simultaneously absorb axial, lateral, and angular movement. High tolerance for installation misalignment. | Type-dependent: General types allow multi-directional movement, but specialized types (hinged, pressure balanced) are designed for large movements in specific directions. |
| Vibration & Noise Damping | Excellent. Rubber is an excellent damping material, effectively isolating and absorbing vibration and structure-borne noise from pumps, compressors, etc. | Poor. Metal transmits vibration and noise. It primarily reduces stress by compensating for movement, not by isolating the source. |
| Temperature Range | Limited. Typically -30°C to +80°C (special rubbers up to +120°C+). Prone to aging at high temperatures. | Very Wide. From cryogenic -270°C to high temperatures up to +800°C (with appropriate material selection). |
| Pressure Rating | Low to Medium. Typically suitable for PN10, PN16, PN25 ratings. High pressure requires special reinforced designs. | High. Through multi-ply construction and wire braid reinforcement, it can withstand very high pressures (PN40 up to several hundred Bar). |
| Corrosion Resistance | Depends on rubber compound. Must be selected based on media (e.g., EPDM for acids/alkalis, FKM for oils). Susceptible to ozone and UV aging. | Excellent. Stainless steel is inherently resistant to many media. Surfaces can be polished or coated. |
| Media Purity | Can be an issue. Rubber may leach微量 substances or swell with certain media. Not suitable for ultra-pure water, food, semiconductor, etc. | Excellent. The inner wall can be electropolished to ensure high cleanliness. It is the only choice for ultra-clean systems. |
| Service Life | Limited. Affected by fatigue, ozone, thermal aging. Typically 5-10 years, requiring periodic replacement. | Long. With correct design and selection, fatigue life can reach thousands to tens of thousands of cycles. More durable. |
| Fire Resistance | Poor. Rubber is combustible and will burn in a fire, causing system failure. | Excellent. Stainless steel is non-combustible. It is mandatory for fire protection systems and critical防火 areas. |
| Cost | Low. Relatively low material and manufacturing costs. | High. High material (stainless steel) and manufacturing (welding, hydroforming) costs. |
3. Summary of Advantages, Disadvantages, and Application Scenarios
Rubber Flexible Joints
Advantages:
①Excellent vibration isolation and noise reduction, the preferred choice for protecting pumps and equipment.
②Strong ability to compensate for combined movements, easy installation, high tolerance for pipe misalignment.
③Good electrical insulation, preventing electrochemical corrosion.
④Low cost, economical.
Disadvantages:
①Low temperature and pressure limits.
②Prone to aging, relatively shorter service life, requires maintenance.
③Not suitable for ultra-clean, high-temperature, high-pressure, or high fire-safety requirement applications.
Typical Applications:
Building water supply and drainage systems: Pump inlet/outlet for vibration and noise absorption.
HVAC chilled/condenser water systems. General plant service water, low-pressure air lines. General media pipelines in wastewater treatment plants.
Metallic Flexible Connections
Advantages:
①Resistant to high temperature, high pressure, and corrosion, reliable performance.
②Long service life, good fatigue resistance.
③Wide range of compatible media, from ultra-pure water to steam and corrosive chemicals.
④Fireproof and explosion-proof, high safety rating.
Disadvantages:
①Almost no vibration/noise isolation function; may need to be used with rubber isolators.
②Stricter requirements for installation accuracy and movement direction (except for special types).
③High cost.
④Thin-walled bellows require protection against mechanical damage and pitting corrosion.
Typical Applications:
High-temperature steam/thermal oil lines: Compensating for thermal expansion.
Chemical/Petrochemical process lines: Transporting corrosive or high-temperature media.
Gas pipelines, fuel oil lines: Where fire safety is required.
Ultra-pure water, semiconductor, biopharmaceutical pipelines: Ensuring media purity.
Aerospace engine ducting: Withstanding high temperature/pressure, compensating for complex movements.
In simple terms:
Need vibration damping, noise reduction, and low cost? -> Choose Rubber Flexible Joints.
Need high temperature/pressure resistance, fire safety, ultra-pure media, or long-term durability? -> Choose Metallic Flexible Connections.
In practical engineering, they are often used in combination. For example, at a pump discharge, a Rubber Flexible Joint might be installed first for vibration isolation, followed by a Metallic Flexible Connection to compensate for pipeline thermal expansion, thereby leveraging the maximum advantages of each type.