I. Structural Composition

• External pressure bellows: Wave structure facing outward, with the pressure acting from the outside to the inside.
• Pressure-retaining sleeve: Encloses the bellows to withstand external pressure.
• End connections: Flanges or weld ends for pipeline integration.
• Guide brackets: Maintain axial movement and prevent lateral deflection.

II. Core Working Mechanism

1. Pressure Transmission Path
   • When internal pipeline medium expands (e.g., thermal elongation), the bellows is subjected to axial tensile force.
   • External pressure (from the sleeve) counteracts the internal expansion force, allowing the bellows to deform axially.
2. Displacement Compensation Process
   • Thermal expansion scenario:
     • Pipeline length increases by ΔL due to temperature rise.
     • External pressure forces the bellows to expand axially, absorbing ΔL through wave elongation.
   • Schematic formula:$\Delta L=n\times {\delta }_{\text{axial}}$
     • $n$: Number of bellows waves
     • ${\delta }_{\text{axial}}$: Axial displacement per wave

III. Pressure Balance Mechanism

• Key feature: The external pressure design converts internal pressure thrust into compressive force on the bellows, eliminating axial thrust on the pipeline.
• Force analysis:
  • Internal pressure $P$ generates axial thrust $F=P\times A_{\text{eff}}$ (where $A_{\text{eff}}$ is effective area).
  • External pressure sleeve counteracts $F$, ensuring net force on the pipeline is zero.

IV. Operational Advantages

V. Engineering Application Example

• Scenario: A 200°C steam pipeline (DN300, L=80m, carbon steel) with ΔT=180°C.
• Calculation:
  1. Thermal displacement: $\Delta L=12\times 10^{-6}\times 80\times 180=172.8\text{mm}$.
  2. External pressure bellows with 12 waves, each wave allowing 15 mm axial displacement:$\text{Total compensation}=12\times 15=180\text{mm}\ge 172.8\text{mm}.$
• Installation: Use guide supports every 10–15m to maintain axial movement, eliminating fixed anchors.

VI. Design Considerations

1. External pressure rating: Must exceed maximum operational pressure to prevent bellows collapse.
2. Guide system: Rigid guides ensure pure axial displacement, avoiding lateral loads.
3. Pressure test: Hydrostatic testing at 1.5× working pressure to verify structural integrity.