Hazardous Area Classification

The Explosion Protection Chain

The six links from hazard identification to work control, and why a failure at any one link defeats the others.

13 min

Activities: 0 of 2

One chain, many owners

An explosion needs three things at the same place and time: a flammable substance mixed with air inside its flammable range, and an ignition source with enough energy. Hazardous area classification (HAC) does not remove any of these by itself. It is a map. It says where a flammable atmosphere may exist, how often, and of what type. Every later decision uses that map.

In this course, the whole system is treated as a chain with six links. Each link has a different owner, and each owner usually trusts the link before it without checking. That trust is the weakness. A perfect equipment selection is worthless if the zones it was selected against are wrong. A perfect drawing is worthless if the portable radio in a contractor's pocket is never challenged.

The six links

  1. 1

    1

    Avoid and reduce

    Remove release sources where possible: fewer flanges, welded joints, closed drains, better seals. Process design owns this link.

  2. 2

    2

    Classify

    Identify release sources, grade them, assess ventilation, set zone type and extent, gas group and temperature class. Recorded in a basis document and drawings.

  3. 3

    3

    Select

    Choose equipment whose protection level, group and temperature rating match the zone, group and temperature class together.

  4. 4

    4

    Install

    Install to the certificate and to the installation standard, including glands, cable types, earthing and intrinsically safe loop calculations.

  5. 5

    5

    Inspect and maintain

    Keep the protection intact for the life of the plant through initial, periodic and sample inspection and controlled repair.

  6. 6

    6

    Control work

    Control what people bring and do: permits, gas testing, portable equipment, vehicles, temporary plant, static.

Two cases where one link failed

Case 1: the right box in the wrong place. A new gas metering skid was installed with equipment certified for a Zone 2, gas group IIA area. The skid design was correct for the original drawing. But six months earlier a hydrogen-rich stream had been routed nearby, and nobody revised the classification. The selection link was done perfectly against a classification link that was already out of date.

Case 2: the perfect drawing nobody used. A refinery unit had a well-founded, recently revised classification. During a turnaround, a contractor sited a diesel generator and a welding set next to a pump bay that was Zone 1. The planners never looked at the drawing; it was filed with the design records. The classification link was sound; the work control link was missing.

Questions an advanced practitioner asks at each link

A unit has a correct classification and correctly certified fixed equipment, but a contractor brings an uncertified generator into a Zone 1 area. Which link of the chain has failed?

Put the chain in order

Arrange the links from first to last.

  1. 1Classify the areas
  2. 2Install to the certificate
  3. 3Select equipment
  4. 4Control work in the area
  5. 5Avoid and reduce release sources
  6. 6Inspect and maintain
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