What hole size should be assumed for a secondary grade release?

There is no single correct value. IEC 60079-10-1 gives suggested hole cross-sections for secondary grade releases in a table in its Annex B, which depend on the type of component (for flanged joints, for example, the type of gasket) and on the assumed leak conditions. The assessor selects the value that matches the actual component, its service conditions and the maintenance and inspection behind it, and records the reason. Because the release rate is proportional to the hole area, this choice decides the size of the zone more than almost any other input.

Short answer

• Source of values: IEC 60079-10-1:2020, Annex B (clause B.5 on hole size and source radius, Table B.1 with suggested hole cross-sections for secondary releases). The same table is retained in the 2025 draft of edition 4. Take the numbers from the edition you work to.
• Two choices: the component type, and the expected behaviour of the opening: a leak that stays small on a well-maintained joint, or one that can grow, for example by erosion or a gasket failure.
• Justify it: age, vibration, thermal cycling, corrosion or erosion, operating pressure against rating, leak testing and the inspection regime all argue for a smaller or larger hole.
• Alternatives: EI 15 uses hole sizes from the Energy Institute's risk-based method; any other basis needs equal justification.
• Out of scope: ruptures and catastrophic failures are not covered by area classification at all.

Why the hole size matters so much

For a gas leak at the pressures found in most process plant the flow through the hole is choked, and the mass release rate is directly proportional to the hole area. The zone extent then grows with the release rate, and the degree of dilution falls as the release rate rises. A tenfold larger assumed hole can move a source from Zone 2 NE (negligible extent) to a Zone 2 of several metres, or from medium to low dilution in a room.

The Tommasini, Pons and Palamara comparison of European and North American methods (IEEE, 2014) makes the same point from the other side: in the hypothetical volume (Vz) of the earlier IEC method, which the paper discusses, the volume is proportional to the cube of the source radius, so small differences in the assumed hole have large effects.

How to choose the value

1. Identify the component exactly. A spiral-wound gasket, a compressed fibre gasket and a ring-type joint behave differently; so do a compression fitting, a threaded connection and a valve stem packing.
2. Decide how the opening could develop. A static, leak-tested joint with no moving parts that is far below its pressure rating justifies a value at the small end. A joint subject to vibration, frequent thermal cycling, erosive or corrosive service, or poor maintenance justifies a larger value.
3. Check the maintenance and inspection evidence. The assumption only holds while leaks are found and repaired quickly. A plant with no leak detection or repair routine cannot claim the smallest values.
4. Consider the consequence of being wrong. For a zone that will drive the choice of equipment near a control room air intake or a public boundary, a more cautious value is often prudent.
5. Record the basis on the source of release schedule: component, hole area, discharge coefficient, pressure and the justification.

A published worked case shows the reasoning. The 2023 HySafe paper by Torrado and co-authors, on the hydrogen generator of a refuelling station (gas at 30 barg), used 0.025 mm² from Table B.1 for compression fittings of 12 to 25 mm outside diameter. Its justification: no moving parts, pressure and leak testing, automatic pressure drop tests at every start-up and recurring tests in operation, a hydrogen detector that shuts the system down at 25 % of the LFL, and fittings rated far above their working pressure.

What hole size does not cover

The secondary grade release is a leak that is not expected in normal operation and is short when it happens. Line ruptures, vessel failures and other catastrophic events are excluded from the scope of IEC 60079-10-1 and belong in a process hazard analysis. Choosing a "severe failure" hole for every flange does not turn area classification into a consequence study; it only enlarges the zones.

Common mistakes

• Choosing the hole size to reach Zone 2 NE. The value must come from the component and its condition, not from the desired answer.
• One hole size for every source. Different components and services need different values.
• Ignoring liquid leaks. The hole size also sets the liquid release, the pool and the evaporation rate.
• Never revisiting the assumption. If the leak history shows frequent leaks, the hole size, and perhaps the grade, must change.

Related on excodex

• Estimating the extent of a hazardous zone: release rates and dilution
• Grades of release: continuous, primary and secondary
• Are flanges and valves always secondary grade sources of release?
• What does 'Zone 2 NE' (negligible extent) mean?
• Standards space: IEC 60079-10-1, gas library: methane

Standards and further reading

• IEC 60079-10-1:2020, Classification of areas, explosive gas atmospheres (Annex B, clause B.5, Table B.1): iteh.ai sample
• prEN IEC 60079-10-1:2025, draft of edition 4 (contents): iteh.ai sample
• D. Torrado et al., Zone negligible extent: example of specific detailed risk assessment for low pressure equipment in a hydrogen refuelling station, HySafe 2023: hysafe.info
• Energy Institute, A risk-based approach to hazardous area classification, 2nd edition: energyinst.org
• Energy Institute, EI 15, 5th edition (2024): energyinst.org
• R. Tommasini, E. Pons, F. Palamara, comparison between European and North American approaches, IEEE Trans. Ind. Appl., 2014: iris.polito.it

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.