Page 111 - Electrical Installation in Hazardous Area

Calculation of release rates and the
extents of hazardous areas

Methods for the determination of release rates and the extents of areas of
explosive atmosphere (hazardous areas) created by any given leak or leaks
have been the subject of much discussion and are recognized as a difficult
area of technology. There are several schools of thought and, as a result, it
has proved impossible to settle upon a method which has full confidence
of all relevant interests. This has resulted in the lack of any nationally or
internationally agreed method and so those involved are left to select a
procedure acceptable to them from a variety of industry-based and other
sectorial codes. Several attempts have been made to include bases for calcu-
lative procedures in national and international codes but disagreements as
to their accuracy have almost always resulted in a failure to agree on their
inclusion. Nonetheless, those involved in the classification of hazardous
areas have always had to use some form of estimation/calculation to justify
the hazardous areas which they specify in each particular case and it must
be remembered that, provided the methods used produce results similar
to those recorded in subjective assessment, which has in the past been the
only possibility, the use of a limited form of calculation as described in this
chapter may be justified. As will be seen this chapter is not concerned with
absolute accuracy but merely with a gauging operation and the assumptions
made are all aimed at giving overestimates rather than underestimates.
The problem experienced in any method of area classification is the lack
of information from experimentation on mixing of releases with air when
such releases are accidental rather than deliberate. This causes problems in
both the subjective approach to area classification and in proving that the
mathematical approach is totally valid.
Most of the mathematics which has existed for some time is based upon
calculation of fluid flow from orifices specially designed to ensure the
maximum flow for minimum effort (e.g., situations where flow or mixing is
required for operational reasons rather than as a result of an unwanted leak).
The last serious attempt to produce a mathematical approach was during
the production of BS 5345, Part 2' (1975-82) and, although a relatively
comprehensive set of formulae based upon classical theory and experimen-
tation were proposed, those involved did not feel able to recommend the
approach in a national document, principally because of the possibility of
misuse by those not sufficiently expert or failure of such people to iden-
tify the limitations of such formulae. The approach was, however, used
in at least one company code2 and, in general, gave results which had