An expansion joint in ductwork accommodates the expansion and contraction that occurs in the ducting material due to temperature changes. 

1. The function of Expansion joints

Expansion joints serve a crucial role in industrial ductwork by facilitating the controlled expansion and contraction of ducts. Their primary function is to accommodate thermal movements within the ductwork system. They mitigate thermally induced stresses by allowing thermal movements in a relatively unrestricted manner. Expansion joints contribute to the overall flexibility and structural integrity of the system which is essential in preventing the build-up of stress that could otherwise lead to damage in equipment, ductwork, and supporting structures.

2. Types of Expansion Joints

  • Non-metallic Expansion Joints:
    • Constructed from fabric or rubber-like materials.
    • Provide flexibility to accommodate thermal movements.
    • Non-metallic expansion joints offer effective sealing within the bellows area, ensuring airtightness.
    • Slight leakage may occur in clamping areas, especially in corrosive fluid conditions.
  • Metal Expansion Joints:
    • Made from light-gauge convoluted metal.
    • Offer durability and strength in managing axial expansion and contraction.

The type of expansion joint chosen for each duct section is a critical decision, with non-metallic expansion joints providing optimal isolation due to their low stiffness in all directions. Metal expansion joints, while suitable for isolating in the axial direction, come with trade-offs such as increased vibration transmission and thermal restraint.

3. Optimal Placement for Performance

The placement of the Expansion joint is designed to;

  • Minimize exposure to significant transverse movements for optimal performance.
  • Mitigate potential issues, such as flue-gas blockage or flow liner-induced shear forces.
  • Enhance effectiveness and longevity by considering the location of expansion joints in relation to potential movements within the ductwork.

Expansion joints, while serving a crucial purpose, come with high costs and maintenance demands, often require repair & replacement every few years.

Figure 1: Placement of Expansion Joint

4. Limitations and Cold Offsets

  • Expansion joints have specific movement limitations defined by design and specifications.
  • Cold offsets can be incorporated to manage limitations and reduce movement severity.
  • Metal expansion joints typically have more stringent limitations than non-metallic counterparts.

During the early stages of ductwork layout planning, it is of utmost importance to clearly define the type of joint intended for each duct section. Additionally, establishing the specific deformation and temperature limits for each joint is crucial. This early decision-making process ensures that the chosen joints align with the expected movements and temperature variations within the ductwork system.

The designer of expansion joint must carefully plan the placement of expansion joints to exert control over where unbalanced pressure loads are transmitted to the supporting structure. This strategic placement also considers factors like ease of erection, installation sequence of structural steel and duct, and friction loads generated at support points during ductwork expansion and contraction. The designer needs to establish early on the type of joint, deformation limits, temperature limits, and insulation arrangements for each duct section.

Novelty Structures offers metallic expansion joints for ducting projects.

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