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Source of Release by Calculation Vs Annex ZA Examples

The model solutions provided in AS/NZS 60079.10.1 Annex ZA, are generic and conservative. These do not specifically allow for different gas or vapour types, other than “lighter” or “heavier” than air. With the exception of Ammonia, no consideration has been given to the properties of specific gas or vapour types, particularly common gases with unique properties, such as Hydrogen.


A client’s site Hazardous Area Classification was conducted many years ago using the classification by example method, whereby blanket zones were created and encompassed many items and types of equipment. Having such a large zonal extent had many flow on effects, most of which increased operational costs.

More items of equipment being included in a Hazardous Area result in:

  • More certified equipment required (much more expensive than normal commercial grade equipment)
  • The more certified equipment due to the larger Hazardous Areas, the more inspections were required within a given time frame
  • The more inspections required, tended to require more Hazardous Area competent personnel
  • Greater restrictions on access with uncertified equipment and a greater need for hot work permits for some maintenance and inspection activities.


A tailored solution for the specific, predominant flammable gases and vapours, using the Source of Release Method via calculation, was performed using available data on site, such as:

  • Stream Tables
  • Process Flow Diagrams
  • Piping and Instrumentation Diagrams
  • Equipment Specification and Data Sheets
  • Mechanical General Arrangement Drawings of Vessels and Equipment
  • Normative Standard references (such as EI/IP 15) within AS/NZS 60079.10.1
  • Other internationally recognised standards for particular equipment
  • Technical papers from credible sources (Health and Safety Executive in the UK)

These calculations were performed for outdoor installations, using AS/NZS 60079.10.1 Section B.8, Calculation No. 6 as a guide for calculating the Hazardous Area volume (and hence radius of the Hazardous Area) generated from a given source and rate of release, for a given gas or vapour and under specific conditions. The time taken to develop the methodology will apply to all sites with little further development time, for a given installation.  These calculations are summarised within a Source of Release Table (SoRT) which can be used as the basis for drafting Hazardous Area Classification drawings.


Primary Sources of Release

In general; it was found that there was approximately a 50% reduction in size of Zone 1 areas, predicted by the examples in AS/NZS 60079.10.1 Annex ZA for such sources of release.

Example: Zone 1 areas previously classified using Annex ZA were predicted to have a radius of 1 metre, where with the Calculated value was typically 0.53 metres.

Secondary Sources of Release

It was also found that up to a 90% reduction in the extent of Zone 2 Hazardous Area could be had, over that predicted by the examples in AS/NZS 60079.10.1 Annex ZA for such sources of release.

Example – Zone 2 areas previously classified using Annex ZA were predicted (under some conditions) to have a radius of 1.5 metres, where the Calculated value was typically 0.14 metres.


While the re-classification by calculation has predicted significantly reduced zone extents, these reduced extents are yet to be accepted by site, with particular reference to agreed realistic/worst case release rates for Secondary Sources of Release from:

  • Flanged Joints (assumed to use spiral wound gaskets – verbal instruction from the site)
  • Valve Packing
  • Threaded Joints including instrument connections.

Further data is being sourced and an on-site discussion of acceptable, conservative release rates is yet to occur. The results of this discussion will crystalise the methodology and provide a set of rules for the classification of the remaining site.


Developing the methodology and researching the data to allow calculation of the Hazardous Area zone extents required a significant investment of time by Bluefield staff as well as Client staff. However, now that the methodology has been developed, subsequent areas can be analysed with a similar speed to referring to the Annex ZA examples, but providing more accurate, fit for purpose results.