Explosion-proof (“Ex d”) joints — what they are, why they matter, and how to get them right

1) The simple idea (but a tricky reality)

An Ex-d enclosure is designed to contain an internal explosion and let the hot gases vent through controlled “flamepaths” (the joints) so the escaping gases cool and won’t ignite the surrounding atmosphere. The joint geometry — gap width, length and even surface finish — determines whether that venting gas will be safe. Getting those dimensions, tolerances and mechanical strength right is fundamental.

2) Types of joints and why you pick one over another

Common flameproof joints are:

• Plain flanged (flat) joints — a simple flange/gap. Easier to manufacture but sensitive to nearby rigid obstacles (hot gas jets can hit structures).

• Cylindrical / spigot joints — a spigot into a bore; inherently stiffer and less likely to open under dynamic load.

• Threaded joints — flamepath runs along the screw spiral; requires sufficient engaged threads (standards commonly require several full turns in contact).

• Labyrinth, serrated, conical and multi-section joints — special uses where extra interruption of the flamepath is needed.

Which to use depends on the gas group (I, IIA, IIB, IIC). Very reactive/fast-burning gases (hydrogen, acetylene — Group IIC) need joints and enclosures that withstand more demanding tests; historically that meant avoiding flat flanges for IIC and preferring cylindrical/threaded constructions. New machining techniques have extended options, but selection must align with certification and tested designs.

3) What actually makes a joint “work” (engineer’s version, short)

Three main physical effects stop ignition transmission through a flamepath:

1. Pressure drop cooling — high-pressure hot gases losing pressure as they jet through a narrow/long path, which lowers temperature.

2. Air entrainment / snuffing — a high-speed jet draws in cold external air and disrupts the flame front.

3. Heat transfer to metal surfaces — some heat is lost to the joint material (but this is a minor contributor compared with the first two).

Standards (IEC/EN 60079-1 and companions) quantify acceptable gap widths/lengths for different gas groups, enclosure volumes and joint types — and those tables are used during design and testing.

4) Design and testing realities — why a “calculated” joint can still fail

Internal explosion pressures are often predicted, but actual behaviour depends on enclosure geometry and contents; test labs run actual transmission tests because dynamic effects (flexing, pressure-piling) can open gaps you thought were safe. Some enclosures have shown pressures far above simple estimates during testing.

Rigid fasteners across a flange (screws, bolts) can interrupt the flamepath beneficially — but their placement, number and torque matter. Conversely, insufficient fastening or uneven torque during assembly can allow gas transmission.

5) Installation & maintenance: common practical rules

(Short, usable bullets you can give to technicians)

• Follow the manufacturer’s torque settings for flange/cap screws. Incorrect torque changes gap geometry under an explosion.

• Use the correct grease/assembly lubricant if manufacturer permits — non-setting, non-hardening greases (petroleum jelly, mineral-oil thickened soap, or specified silicone grades where allowed) can prevent seizure and corrosion. But: some greases (e.g., certain silicones) can poison gas detector sensors, and some grease/tape treatments are not permitted for IIC equipment — always follow the manual and IEC/EN guidance.
• Do not drill/tap additional cable entries in flameproof enclosures — use certified stopping plugs or factory-provided entry points. If an adaptor is required, only a single certified adaptor is allowed (no “adaptor on adaptor” stacking).
• Always ensure required thread engagement for threaded covers and cable glands (standards often specify minimum engaged threads). For large threaded covers, use a locking grub screw or similar to require a tool to open the lid.
• Inspect and clean joints with non-metallic brushes and non-corroding cleaners; remove loose corrosion before assembly and use compatible greases where allowed.

6) Certification and documentation — don’t shortcut this

Flameproof equipment isn’t just designed: it must be tested and certified to IEC 60079-1 (and related installation/maintenance standards such as IEC 60079-14 and IEC 60079-17). Test houses such as SGS-Baseefa perform transmission and strength tests and will often flag likely problem areas in advance — but ultimate responsibility for safe design and installation sits with the manufacturer and the installer. Keep design drawings, torque specs, grease/part lists, and test reports with the equipment.

7) Quick inspection checklist (on one page)

• Joints: no visible damage, corrosion or foreign materials.

• Fasteners: correct type and all present; torque per manual.

• Threads: required number of engaged threads (check depth and GO/NO-GO if possible).

• Gaskets: only manufacturer-supplied gasket types present; no soft flat gaskets inside flamepath.

• Cable entries: only certified glands/stopping plugs used; no extra drilled holes.

• Grease/Sealant: compatible, non-hardening grease where specified; none where forbidden (IIC).

• Clearance: flanged joints keep required minimum distance to rigid obstacles (as per installation tables).
  (Keep these inspection records and attach them to the equipment’s maintenance log.)

8) Common mistakes that keep showing up

• Relying on a hypothetical “calculated” gap without testing.

• Using weatherproofing fixes (thick sealant or improper gaskets) that alter the flamepath.

• Re-threading or drilling by installers who don’t follow certified procedures.

• Using incompatible grease near sensors (gas detection poisoning).

• Ignoring manufacturer torque and assembly sequences.

9) Final takeaway

The flamepath is a small part of the enclosure but the key to the whole protection concept. Good outcomes come from: the right joint type for the gas group, tight control of tolerances and machining, robust mechanical strength, proper testing/certification, and strict installation/maintenance discipline. Treat joint details as safety-critical engineering — document them, test them, and inspect them regularly.