I. Basic Structural Components

Steel expansion joints are devices used to compensate for pipeline displacements caused by thermal expansion/contraction, mechanical vibration, etc. Their core structure typically includes:

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

Steel expansion joints mainly fall into two categories:

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.

III. Key Design Considerations

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).

Pressure and Displacement Calculation

Cylinder wall thickness for design pressure (0.6–2.5MPa) is calculated by:t=2[σ]ϕ−ppD

( t = wall thickness, p = pressure, D = inner diameter, [σ] = allowable stress, ϕ = welding coefficient ).

Displacement must match pipeline thermal expansion (ΔL = αLΔT, α = linear expansion coefficient).

Seal Reliability Design

Gland bolts are tightened uniformly to avoid uneven packing stress; self-tightening seals (e.g., metal wound gaskets) for high temperatures.

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.