How are flamepath gaps on an Ex d enclosure checked during inspection?
Abstract
Flanged joint gaps are measured with feeler gauges at several points around the fully bolted cover and compared with the manufacturer's documented maximum (or the IEC 60079-1 table). Joint faces are checked with the cover off; defects mean repair or replacement.
On flanged (flat) joints the gap is measured with feeler gauges at several points around the cover, with all bolts fitted and tightened, and the largest reading is compared with the maximum gap stated in the manufacturer's documentation. Where the documentation does not give a value, the inspector uses the maximum from the tables of IEC 60079-1 for the gas group, enclosure volume and joint width. Spigot, shaft and threaded joints are normally not measured unless there are signs of wear, distortion, corrosion or other damage, and then against the manufacturer's documents; any joint that is damaged or over the limit is repaired to the manufacturer's data under IEC 60079-19 or replaced.
Short answer
- What is checked: the gap of flanged joints against a documented maximum, and the condition of all joint faces (clean, undamaged, no paint, no unauthorised gasket or sealant).
- When: in the IEC 60079-17 inspection schedule for Ex d equipment, flange gap dimensions are a close and detailed inspection item, and the condition of the faces a detailed one (2007 edition schedule).
- How: feeler gauges inserted, without force, at several points around a closed, fully bolted flange; the largest reading counts.
- Limit: the manufacturer's documented maximum, which may be smaller than the standard's table value. Typical table values for a 25 mm wide flanged joint are 0.40 mm (IIA), 0.20 mm (IIB) and 0.10 or 0.04 mm (IIC, depending on volume).
- Result: a gap above the maximum, scratches or pits across the joint, or paint or sealant on the faces means the equipment is not compliant and must be repaired by a competent facility or replaced.
Where the maximum gap comes from
IEC 60079-1 sets the maximum gap for each joint type as a function of the minimum joint width and the free internal volume, in one table for Groups I, IIA and IIB and another for IIC. Some values for flanged joints, as published in the 2014 edition:
| Flanged joint width L | IIA | IIB | IIC |
|---|---|---|---|
| 9.5 mm, volume 100 to 500 cm3 | 0.30 mm | 0.20 mm | 0.10 mm |
| 25 mm, volume 500 to 2,000 cm3 | 0.40 mm | 0.20 mm | 0.04 mm |
| 25 mm, volume over 2,000 cm3 | 0.40 mm | 0.20 mm | 0.04 mm |
In IIC, flanged joints are permitted for acetylene only under extra restrictions. The table values are design limits, and the manufacturer may choose tighter ones to pass the explosion tests. In that case IEC 60079-1 requires an "X" certificate (or an advisory marking) and the conditions of use to give the joint dimensions, refer to a drawing, tell the user to contact the manufacturer, or state that the joints are not intended to be repaired. So the first step of any gap check is to find the documented value for that product, not to assume the table.
How the measurement is done
- Prepare. Obtain the permit and the manufacturer's data. A gauge check on a closed flange does not require opening the enclosure; inspecting the faces does, which needs isolation and normally a gas-free certificate.
- Check the bolts first. Every bolt must be present, of the specified type and tightened. A gap measured with a loose or missing bolt is meaningless.
- Clean the outer edge of the joint so dirt or paint at the lip does not hold the blade. IEC 60079-17 allows only non-metallic scrapers and non-corrosive cleaning fluids on flanges.
- Insert the feeler gauge at several points around the whole perimeter, especially midway between bolts and at corners, where covers tend to lift. Use the thinnest blade first and increase; never force or lever a blade into the joint, because that can scratch the faces.
- Record the largest value that enters, with location, and compare it with the documented maximum.
- At a detailed inspection, remove the cover and look at both faces over the full width: scratches or dents across the path, corrosion pits, hardened paint, sealant or a gasket that is not part of the design. Minor marks are judged against the manufacturer's data; damage across the width is not acceptable. Refit with the faces protected by non-setting grease or the corrosion inhibitor the manufacturer names, and re-measure.
Spigot, shaft and threaded joints. Their gap is hidden or, for threads, not a gap at all; cylindrical gaps are the difference between bore and shaft diameter, measured dismantled in a workshop. IEC 60079-17 (2007 text) treats checking their diametric clearance as normally unnecessary unless there is evidence of wear, distortion, corrosion or other damage, and then refers to the manufacturer's documents.
Example: A flameproof control station marked Ex db IIB T6 Gb has a flanged cover. Its instructions give a maximum gap of 0.15 mm. At a close inspection a 0.20 mm blade enters halfway between two bolts, next to a bolt that is not fully tightened. After that bolt is tightened to the specified torque, the largest blade that enters anywhere is 0.10 mm, and the reading is recorded. Had the gap stayed at 0.20 mm, the station would have been taken out of service for repair, even though 0.20 mm is acceptable under the general table for some IIB enclosures.
When a gap or face fails
A failed flamepath is a defect that removes the protection. The equipment is isolated or the area is made safe until it is corrected. Corrective action is either replacement of the cover or enclosure, or repair under IEC 60079-19 by a competent repair facility working to the manufacturer's drawings. Faces must not be filled with compound, painted, welded or re-machined on site, and gaps must not be "closed" by adding a gasket or sealant. After any repair, the affected joints are inspected again at the detailed grade and the result is recorded.
Common mistakes
- Using the standard's table instead of the product documentation. The documented value can be smaller.
- Forcing a blade or prising the cover with a screwdriver. Both damage the faces they are meant to check.
- Accepting paint in the joint because the gap reads small. IEC 60079-1 does not permit paint or powder coating on joint surfaces; it can hold the joint open in an explosion.
Related on excodex
- Flameproof enclosures (Ex d): how flamepaths stop an internal explosion
- Inspection of Ex installations under IEC 60079-17: initial, periodic, visual, close and detailed
- Common Ex inspection findings and how to fix them
- Can you paint, grease or seal the flange joints of a flameproof enclosure?
- How close can a wall or pipe be to the flange of an Ex d enclosure?
Standards and further reading
- IEC 60079-1:2014, flameproof enclosures "d": gap tables, documentation rules, ban on paint on joints (replaced by IEC 60079-1:2026; 2014 text incorporated by reference in US federal regulations): law.resource.org
- IEC 60079-17, inspection and maintenance; 2007 edition as adopted in India (IS/IEC 60079-17:2007), Table 1 and clause 5.1.1 on flameproof joints: archive.org. Current edition IEC 60079-17:2023, preview: standards.iteh.ai
- IEC 60079-19:2025 (edition 5), equipment repair, overhaul, reclamation and modification.
- IEC 60079-14:2024, initial inspection schedules in informative Annex O (preview of contents): standards.iteh.ai
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.