I. Basic Structural Components

1. Main Cylinder

• Material: Commonly Q235 carbon steel, stainless steel (e.g., 304, 316L), selected based on medium temperature, pressure, and corrosiveness.
• Function: Serves as the main frame, bearing internal pressure and external loads while guiding internal moving parts.

2. Expansion Sleeve

• Structure: A slidable sleeve nested inside the main cylinder, forming a telescopic structure.
• Features: Surface often polished or coated (e.g., galvanized, chrome-plated) to reduce friction and enhance wear/seal resistance.

3. Sealing System

• Packing Box: Located at the interface between the main cylinder and sleeve, filled with flexible seals (e.g., graphite packing, rubber O-rings).
• Sealing Principle: Gland compresses the packing to create radial sealing. Design considers the packing’s temperature/pressure resistance and anti-aging properties.

4. Connection Components

• Flange: For pipeline connection, following standards like GB, ANSI, JIS, with material matching the main cylinder.
• Welding End: Directly welded to pipelines for high-pressure or non-detachable scenarios, requiring NDT (e.g., X-ray) on welds.

5. Limit Device

• Structure: Composed of limit bolts and stop rings, installed outside the main cylinder.
• Function: Restricts maximum displacement to prevent seal failure or structural damage from over-stretching/compression.

II. Typical Structural Types and Design Differences

1. Sleeve Expansion Joint

• Features: Simple structure, strong axial compensation, suitable for straight pipelines.
• Design Key Points:
  • Clearance between sleeve and cylinder is precisely controlled (0.5–1mm) to avoid leakage or jamming.
  • Limit bolts are sized for maximum displacement to prevent overload fracture.

2. Bellows Expansion Joint

• Features: Uses metal bellows for axial/lateral/angular compensation, with excellent sealing.
• Difference from sleeve type:
  • Bellows made of multi-layer thin steel, deforming to absorb displacement without sliding seals.
  • Requires guide supports to prevent bellows buckling.
    
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III. Key Design Considerations

1. Material Selection
   • Stainless steel (e.g., 316L) for temperatures >300℃ or corrosive media; carbon steel for normal-temperature water.
   • Sealing packing must be compatible with the medium (e.g., PTFE for acid fluids).
2. Pressure and Displacement Calculation
   • Cylinder wall thickness for design pressure (0.6–2.5MPa) is calculated by:$t=\frac{pD}{2[\sigma ]\varphi -p}$
     ( $t$ = wall thickness, $p$ = pressure, $D$ = inner diameter, $[\sigma ]$ = allowable stress, $\varphi$ = welding coefficient ).
   • Displacement must match pipeline thermal expansion (ΔL = αLΔT, α = linear expansion coefficient).
3. Seal Reliability Design
   • Gland bolts are tightened uniformly to avoid uneven packing stress; self-tightening seals (e.g., metal wound gaskets) for high temperatures.
4. Vibration and Anti-Detachment Design
   • Stop ring thickness ≥ cylinder wall thickness, ≥4 limit bolts, preventing sleeve detachment from seismic/vibration loads.

IV. Structure-Application Matching

• Large-diameter pipes (DN≥1000mm): Prefer sleeve type for low cost and easy maintenance.
• Complex conditions (high temp/pressure/multi-directional displacement): Use bellows type with balance rods or hinge structures.
• Buried pipes: Add anti-corrosion coatings (e.g., epoxy powder) and waterproof packing.

V. Industry Standards

• China: GB/T 12772 (flexible cast iron pipes), GB/T 12777 (metal bellows expansion joints).
• International: ASME B16.5 (flanges), EJMA (Expansion Joint Manufacturers Association).