Page 61 - Electrical Installation in Hazardous Area

Area classification 37

be part of the design process and to significantly affect the design process
in order to minimize the explosion risks. This means that hazardous areas
should be eliminated as far as is possible, leaving only those which are
absolutely necessary and introduce only an acceptable level of risk. To this
end, area classification should begin on a process plant, for example as soon
as process-line diagrams are available and should be constantly refined
throughout the design process, influencing all stages of process design. The
following is typical of the type of argument which is addressed in the
progress to a final area classification.
Is the quantity offlammable material small? In some places, such as labo-
ratories, only small amounts of flammable material are normally present
and the operation of these areas is such that security against explosion
can be produced by other methods (e.g., utilization of the continuous pres-
ence of trained staff together with special operating procedures). In such
circumstances area classification may not be appropriate. These cases will,
however, be few and the objective is to achieve the required level of security
by other means. It is necessary to act with extreme caution when selecting
such areas. Where an alternative to area classification cannot be justified
and significant amounts of flammable material can be identified as present,
a further question must be asked.
Can the flammable material be released? If the flammable material cannot
be released in such a way that it can mix with air (either inside a process
vessel or externally) then an explosive atmosphere cannot be formed and
a hazardous area cannot exist. This is, however, very unlikely and it is
usual that where such materials exist some form of release will usually
be present (an exception to this rule is where, for example, an all-welded
pipeline passes through an area in which case a release within the scope of
area classification will not be considered).
How does the release occur? If release is possible within the range of releases
associated with area classification it then becomes necessary to determine
the nature of release. (Is it, for example, from the free surface of a liquid in
contact with air, due to a pipe joint failure or due to a moving seal all of
which are examples of releases within the scope of area classification.) The
identification of the type of release will allow it to be graded as described
earlier in this chapter.
Having identified the release as to its grade (continuous, primary or
secondary) it is then necessary to determine if the Grade can be reduced
(e.g., primary to secondary, etc.). The objective should be to produce the
smallest number of releases of the lowest aggregate grade possible taking
account of practical operational considerations. There is a balance here
in that the product has to remain saleable in general or its manufacture
becomes pointless. Such an argument does give a little flexibility but cannot
be used to allow a coach and horses to be driven through the general
approach given in this guide as there comes a point where, on safety
grounds, the manufacturing or other activity becomes unacceptable. Large
unconfined Zones 0 and Zones 1 are not, for example, generally accept-
able whatever the operational justification. It should also be remembered