(The busyness has got to me again lately, with no posts for nearly 4 weeks – apologies.)
Location classes in AS 2885 are based on the land use within the “measurement length”, defined as the distance at which the radiation from an ignited full bore rupture is 4.7 kW/m2 (the level at which an unprotected person will receive very serious burns in under a minute). Because this definition is based on full bore rupture there are occasional questions about whether this definition applies to a “No Rupture” pipeline (one that meets the requirement of Clause 4.7.2 in Part 1 of the Standard).
The answer is that the measurement length applies to ALL pipelines, regardless of their potential failure mode, because the possibility of catastrophic consequences from full bore rupture might have been the reason for selecting a “No Rupture” design in the first place. You can’t adjust the measurement length based on failure mode because that upsets the logic behind this.
It is perhaps easy to confuse two separate distances: the measurement length (used to determine location class) and the consequence distance for failure from a specific threat (used for risk evaluation). There is nothing inconsistent in the following (in fact this should be the norm):
- Assign the location class based on the measurement length (full bore rupture radiation distance)
- Establish a “No Rupture” design if required by the resulting location classification
- Evaluate risks based on whatever radiation distances are applicable to the failure modes from any uncontrolled threats that exist at that location (e.g. pinhole due to corrosion, puncture due to mechanical damage puncture, rupture only if it is a credible failure mode)
Of course much same logic applies to reviewing the risks to an existing pipeline that happens to meet the “No Rupture” requirement in a high consequence area.