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I. Three Core Elements

The explosion-proof performance of flameproof joint surfaces mainly depends on the strict control of three parameters:

Joint surface width (L): Refers to the length of the shortest path that flames must pass through to spread from the inside of the enclosure to the outside. The width must be sufficiently long to ensure that flames are fully cooled and extinguished when passing through the gap. The standard specifies different minimum width values for equipment with different volumes and explosion-proof grades (e.g., IIA, IIB, IIC).

Joint surface gap (i): Refers to the distance between the opposing surfaces of the joint (a diameter difference for cylindrical structures). The gap must be sufficiently small to prevent the direct ejection of explosive flames. The gap value is also related to the equipment volume and explosion-proof grade; generally, the IIC grade (e.g., for acetylene environments) has the strictest requirements (e.g., ≤0.04mm).

Surface roughness (Ra): The average surface roughness Ra of the joint surface shall not exceed 6.3 µm. Excessively rough surfaces will accelerate wear and lead to an increase in the gap, while an overly smooth surface is conversely unfavorable for flame cooling.

II. General Technical Requirements

Anti-rust treatment: The joint surfaces must be subjected to anti-rust treatment (e.g., coating with anti-rust oil). Painting or plastic spraying is strictly prohibited unless it can be proven that the coating does not affect the flameproof performance.

Electroplating layer limitation: If electroplating is applied to the joint surfaces, the thickness of the plating layer shall not exceed 0.008mm. If exceeded, a flame transmission test shall be conducted and verified in the unplated state.

Fastener requirements: For bolt-connected joint surfaces, all bolts must be complete and tightened, with spring washers compressed flat. The number of engaged screw threads must meet the standard requirements (usually no less than 5 threads).

III. Special Structural Requirements

Quick-acting doors/covers: For quick-opening structures, the effective length (width) of the joint surface is generally required to be no less than 25mm.

Moving parts (e.g., rotating shafts): For moving parts such as control levers or rotating shafts, the joint surface gap shall take dynamic wear into account, and the surface roughness is recommended to be controlled at Ra≤3.2 µm.

Bonded joint surfaces: For joint surfaces sealed with adhesives (e.g., observation windows), a sufficient bonding width must be ensured (usually ≥10mm, depending on the volume), and the structure shall be equipped with mechanical reinforcement measures.

IV. Summary

The core of flameproof joint surfaces lies in cooling flames through the narrow gap effect. Any factor that causes an increase in the gap, a reduction in the width, or surface damage may result in the "loss of flameproof performance" of the equipment, rendering it ineffective in explosion protection.