Are flanges and valves always secondary grade sources of release?
Abstract
Usually, but not always. A flange or valve is secondary when it is not expected to leak in normal operation and a leak would be found and repaired quickly. Joints opened routinely, valve packings known to weep and open-ended valves used in normal work are primary.
Usually, but not always. A flanged joint or a valve is a secondary grade source when it is not expected to release in normal operation and any leak would be infrequent, short and repaired. It is a primary source when a release is expected in normal operation: a joint that is opened as part of routine work, a valve packing that is known to weep, or an open-ended drain, vent or sample valve that operators use.
Short answer
- The grade follows the expected frequency and duration of the release, not the type of component. IEC 60079-10-1 defines a secondary release as one not expected in normal operation and, if it happens, infrequent and short.
- A tight gasketed flange or a valve stem packing in good condition, under a working inspection and repair regime, is the textbook secondary source.
- The same component becomes primary when it releases as part of normal operation: routine opening, sampling, draining or venting, or a packing that is expected to weep.
- A component that leaks persistently and is left that way does not meet the "infrequent and short" condition. Repair it rather than reclassify it.
- The grade decides the zone type only together with ventilation; the hole size assumed for the leak decides the extent.
Why flanges and valves are normally secondary
A gasket, a valve stem packing or a bonnet seal is designed to be tight. The International Electrotechnical Vocabulary, in its note on normal operation, gives broken pump seals and failed flange gaskets as examples of failures that need repair or shutdown and therefore are not part of normal operation. That is exactly the definition of a secondary release: something that can go wrong, but is not expected, and does not last because it is noticed and fixed.
This is why most of the area in a typical process plant is Zone 2. The many flanges, valves and small-bore fittings each count as a secondary source, and their small Zone 2 envelopes merge into one. Leak-tight welded pipe in good condition is usually not counted as a source of release at all.
For the extent of the zone, IEC 60079-10-1 gives suggested hole cross-sections for secondary releases in an annex table, which depend on the type of joint or gasket. The 2020 edition is current and edition 4 was circulated as a draft in 2025, so take the values from the edition you work to.
When a flange or valve becomes primary
The test is whether the release is expected during normal operation. Normal operation includes start-up and shutdown, so tasks that are part of them count too.
| Component | Usually | Primary when |
|---|---|---|
| Gasketed flange, permanent joint | secondary | it is opened routinely in normal operation (for example a filter housing cleaned every shift, a hose or spool connected at every batch) |
| Valve stem packing or gland | secondary | the valve type or service is known to weep in normal operation |
| Drain, vent or sample valve to atmosphere | secondary while closed and capped | operators open it in normal work, for example to drain water or take samples |
| Relief valve | secondary at the outlet if it lifts only on a fault | it is expected to lift in normal operation |
| Threaded or small-bore connection | secondary | it is disconnected routinely, such as an instrument connection used for checks |
Maintenance on a line that has been emptied, isolated and purged is usually controlled by a work permit and temporary precautions, not by the permanent zone drawing. Routine operator tasks are different: they belong in the source list and are often forgotten because they do not appear on piping drawings.
A single valve can also produce two releases. A packing that weeps slightly in normal service is a small primary release, and a blown packing is a larger secondary release. Each is assessed, and the zones are combined: typically a small Zone 1 close to the stem inside a wider Zone 2.
From grade to zone
The grade is only the first input. IEC 60079-10-1 combines it with the degree of dilution and the availability of ventilation:
- Secondary release, medium dilution: Zone 2 whatever the availability.
- Secondary release, high dilution and good or fair availability: non-hazardous (Zone 2 NE).
- Secondary release, low dilution, for example in a closed room or a pit: Zone 1 and even Zone 0.
So a flange inside a poorly ventilated enclosure can need Zone 1 equipment even though the flange itself is a secondary source.
Example: A solvent filter is opened every shift to clean the basket. Its body flange is a primary source during that task, so a Zone 1 is drawn close to the filter, sized from the vapour released when the housing is opened. The six permanent flanges on the same line stay secondary and give Zone 2. Replacing the filter with a duplex design that is drained to a closed system before opening can turn the primary source back into a secondary one.
Common mistakes
- Grading by leak size. A large possible gasket blow-out is still secondary if it is not expected in normal operation. Size goes into the release rate and the extent.
- Calling a known weeping valve secondary. Persistent leakage fails the "infrequent and short" condition.
- Leaving operator tasks out of the source list. Draining, sampling, filter changes and hose connections are typical primary sources.
- Forgetting the ventilation step. Secondary does not automatically mean Zone 2 indoors or in a pit.
Related on excodex
- Grades of release: continuous, primary and secondary
- Typical hazardous area examples: tank vents, pumps, compressors and loading points
- Is a pump seal a primary or a secondary grade of release?
- What hole size should be assumed for a secondary grade release?
- Standards space: IEC 60079-10-1
Standards and further reading
- IEC 60079-10-1:2020 (edition 3.0), classification of areas, explosive gas atmospheres, with definitions of the grades of release and an annex of examples and suggested hole sizes: preview
- International Electrotechnical Vocabulary 426-28-07, normal operation (non-electrical equipment), with notes on minor releases and on seal and gasket failures: electropedia.org
- Draft edition 4 of IEC 60079-10-1 (CDV 2025), contents list showing the grade examples and the hole size table: preview
- HSE, Hazardous area classification and control of ignition sources: hse.gov.uk
- Choi and Byeon, Specific process conditions for non-hazardous classification of hydrogen handling facilities, Safety and Health at Work, 2021, reproducing the zone table of IEC 60079-10-1 edition 2 (2015), which the 2020 edition keeps as Table D.1: koreascience.or.kr
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.