Verification of Hazardous Area Classification Drawings During Plant Modifications

Hazardous area classification drawings are usually accurate on the day they are issued. After that they drift. A pump gets upgraded, a vent gets rerouted, a sample point appears, a louvred wall gets clad, and the zone boundaries on the drawing stop describing the plant. The installed electrical equipment may still match the drawing exactly. The real problem is that the drawing no longer matches the release sources and ventilation that decide where an explosive atmosphere can form. An inspector who checks equipment only against the drawing will pass an installation that is wrong. Either you get ignition-capable equipment sitting in a zone that is now more onerous than documented, or you get an expensive over-specification programme when someone finally finds the error.

This article looks at how modifications and maintenance activities invalidate existing classifications, and what an Ex inspector or integrity engineer should verify in the documents and out in the field.

Standards and requirements context

The framework for gas atmospheres rests on a small set of IEC documents. National adoptions such as AS/NZS 60079 in Australia and New Zealand carry the same content into local law.

• IEC 60079-10-1 covers classification of areas for explosive gas atmospheres. It is the basis for identifying release sources, grades of release, ventilation effectiveness and resulting zones.
• IEC 60079-14 covers electrical installations design, selection and erection. It requires equipment to be selected against the classification, including gas group and temperature class.
• IEC 60079-17 covers electrical installations inspection and maintenance. It defines initial, periodic and sample inspection regimes.
• IEC 60079-19 covers equipment repair, overhaul and reclamation. It matters here because repairs can alter the protection concept.
• IEC 60079-0 sets the general requirements for Ex equipment.

On top of these, regulators and owners add their own requirements. The UK DSEAR regime is one example. The Australian utility standard HA-ST-10 and industry guidance on verification dossiers follow the same logic. The dossier pulls together the classification, the equipment register and the inspection records, so that each can be traced back to the others.

For inspectors, the key point is that the drawing is a derived document. It is built from the release source register, the ventilation assumptions and the properties of the substances handled. Change any of those inputs and the drawing is potentially invalid, whether or not anyone has redrawn it.

How changes invalidate a classification

Published industry commentary treats drawings that are not updated after a plant change as a systematic failure mode. That covers new equipment, modified process lines and altered ventilation. The mechanisms below are the ones I see most often in the field.

Changes to release sources

A classification zone exists because a release source exists. Anything that adds, removes or changes the grade of a source can change the zone:

• New flanges, valves, instrument tappings, sample points or drain and vent connections.
• Replacement of a sealed pump or compressor with one using a different sealing arrangement.
• Conversion of a closed drain to an open drain, or a fixed vent to a pressure-relief discharge to atmosphere.
• Change of operating pressure, temperature or throughput on an existing line, which alters release rate and the extent of the cloud.

Changes to the fluid

A feedstock switch, a new chemical injection or a different product slate can change the gas group and the temperature class requirement. It can also flip whether the fluid is lighter or heavier than air. A heavier-than-air vapour shifts where a zone reaches below grade, into pits and trenches, and along drainage routes. Equipment that was correctly certified for the old fluid can be wrong for the new one, and nothing on the nameplate will tell you.

Changes to ventilation and layout

Ventilation is a classification input, and it is the one most easily altered without anyone thinking of it as a hazardous area change. Examples include:

• Cladding or partially enclosing an open structure.
• Adding a new skid, tank, bund wall or screen that obstructs airflow.
• Modifying or decommissioning building HVAC, or changing pressurisation of an adjacent room.
• Relocating a door, louvre or dampers on a compressor or analyser house.

Open-air assumptions that held for a pump on a bare slab may not survive a new structure built beside it.

Maintenance and repair activities

Maintenance changes the as-built condition, not the design. Typical examples:

• Replacing a flameproof enclosure cover with a non-original cover or fasteners.
• Substituting cable glands or blanking elements with uncertified or different-type items.
• Repairing flamepaths or reclaiming equipment without the controls expected under IEC 60079-19.
• Replacing a motor or luminaire with a like-for-function unit whose certification, gas group or temperature class is lower.
• Temporary equipment brought in for a shutdown and left in place.

These matter for classification verification because the dossier then describes equipment that is no longer the equipment installed.

Comparison: what each change affects

What inspectors should verify

Start with the basis, not the drawing

The classification basis document should name the governing standard, its edition, the jurisdiction and the assumptions. If the basis is missing or predates the modification, you cannot verify the drawing, and you already have a finding before you leave the office. Next, check that every zone on the drawing traces back to a release source in the register, and that every register entry appears on the drawing. Orphan zones and unzoned sources both point to uncontrolled change.

Cross-check three records

1. The classification drawing and data sheets, including the vertical extent of zones above and below the source.
2. The equipment register or verification dossier, with certificate, protection type, gas group and temperature class per item.
3. The management-of-change records for the period since the last classification review.

Any modification in the third record that touches process, venting, drainage, structures or ventilation should have a documented classification review. If it doesn't, treat the associated drawing as unverified.

Walk the plant against the drawing

A field walk-down is essential, and it should deliberately look for:

• Sources that are on the plant but not on the drawing.
• Equipment installed in a zone boundary area where the boundary was drawn for an earlier layout.
• Obstructions, cladding or enclosures that change ventilation.
• Drains, trenches and pits where heavier-than-air vapours could accumulate.
• Equipment whose nameplate, certificate or condition does not match the dossier.

Safety during verification

Most of this work is observational. If verification means opening enclosures, joints or process equipment, though, it needs a permit and the following controls: isolation of the process and electrical supply, depressurisation, verification of zero energy before work starts, lock-out/tag-out, hazardous-area precautions for tools and temporary equipment, gas testing before and during the work, and safe venting or purging to a defined location. Do not open an Ex enclosure in a classified area unless the supply is isolated and the area is confirmed gas-free under the permit.

Illustrative scenario

This scenario is constructed for illustration and is not drawn from a reported event.

A gas-processing site replaces a pump with a unit that has a different sealing arrangement, and adds a sample point on the discharge line. The project closes out with updated piping drawings but no classification review, because the change was classed as like-for-like. Separately, a weather screen goes up beside the pump to protect operators, restricting airflow on one side.

During a periodic inspection, the inspector finds that the classification drawing predates both changes. The register has no entry for the sample point. A walk-down shows the screen. The basis assumes open-air natural ventilation, which no longer holds on one side of the pump. The nearby junction box was selected for the old, smaller zone.

The inspector would:

1. Raise a finding against the classification drawing, not just the junction box.
2. Request a reassessment of release sources and ventilation by a competent person against IEC 60079-10-1.
3. Hold judgement on equipment suitability until the revised zones are issued, then re-verify each affected item against its certificate.
4. Update the register and dossier, and close the gap in the management-of-change process that let the changes pass without review.

The corrective action here is a revised classification followed by equipment verification. Swapping the junction box alone would not fix it.

Inspector checklist

• Confirm the classification basis names the standard, edition, jurisdiction and assumptions, and is dated after the last relevant modification.
• Trace every drawn zone to a release source in the register, and every register source to a drawn zone.
• Compare the management-of-change log against the classification revision history for gaps.
• Confirm the drawing shows zone extents vertically as well as horizontally, including pits, trenches and drains.
• Check that ventilation assumptions match current field conditions, including new structures, cladding and HVAC changes.
• Confirm the fluid properties used in the basis match the current process, including gas group and temperature class.
• Verify installed equipment markings and certificates against the dossier for every item in or near modified areas.
• Look for repaired or substituted components, such as covers, fasteners, glands and blanking elements, and check records of repair against IEC 60079-19 expectations.
• Identify temporary equipment left in classified areas after shutdowns.
• Record all discrepancies against the drawing and the dossier, with an owner and a close-out route.

Conclusion

A zone classification is only as current as the last review of its inputs. Plant modifications and maintenance work change those inputs, often without anyone recognising it as a hazardous area event. Treat the drawing as a derived document. Verify the basis, trace zones back to release sources, reconcile against the management-of-change record, and then confirm everything in the field.

Your next step: take your most recently modified unit, pull its change records for the period since the last classification review, and run the checklist above. Any modification without a documented classification review should trigger reassessment before the next periodic inspection. The management-of-change procedure also needs amending so that it cannot close without a classification decision.