Page 77 - Electrical Installation in Hazardous Area

Area classification for gases, vapours and mists 53

produce leaks if it cracks. As it is located in a recess there is little chance
of blowout and the leak geometry will be a crack in the ring. In this case
release velocities may be high due to the lack of any modlfylng elements
in the joint, Finally, the gasket is the most usual pipe joint and here there
are two possibilities. A normal Compressed Asbestos Fibre (CAF) gasket
is retained by pressure alone and at all but very low pressures blowout of
part of the gasket must be considered giving rise to a fairly large orifice.
The size of the blowout is normally related to the positioning of bolts on
the flange and a typical size can be quoted. To prevent such blowouts it is
possible to use a spirally supported gasket, which is a gasket fabricated of
similar material but with a spiral metal support wound within it. While this
effectively prevents blowouts there is still the possibility of leaks between
the pipe flanges and the gasket due to joint stress or relaxation. As gaskets
are usually used on larger pipes this will be larger than is the case for
compression fittings.
The above reflects the situation for pipe joints where routine or regular
breakage is not considered a possibility. Where such breakage is likely it
is normally because of human intervention due to such activities as the
spading-off of pipes for removal of process equipment and similar activities
to give sufficient confidence to allow entry of such things as vessels where
the possibility of leakage of flammable cannot be countenanced because of
other problems such as toxicity. In carrying out the exercise of spading-off,
human intervention in the close proximity of the flange is necessary and
the possibility of leakage of flammable materials is increased and must be
taken into account. Well over 50 per cent of flammable materials are toxic
at much lower levels than their lower explosive limit and the creation of
a significant explosive atmosphere in an area where personnel are likely
to be present at the time of creation is not acceptable. The procedures for
breaking pipes in such circumstances must take account of that fact and
the removal of flammable materials from the pipe before such activity is
necessary. This should dictate the breaking procedure and the procedure
adopted before the break activity itself. Such procedures should eliminate
the possibility of significant explosive atmospheres occurring at the time of
breakage of the joint.
While this is the case for toxic materials it should also dictate the proce-
dure for all joint breakages as it would result in so then no significant
explosive atmosphere at the time of joint breakage. It is, however, prudent
to acknowledge that such procedures are fallible and to take account of this,
a nominal Zone 1 of lm radius around the joint should be identified. It is
stressed that this will only apply to joint breaking which takes place on a
relatively regular basis (e.g., possibly more than once a month on average)
as less regular actions will result only in Zone 2 which is taken care of by
the random leak scenarios dealt with in this chapter.
Taking the above considerations into account it is possible to define
typical leaks which will cover most normal situations and simplify the area
classification process. In the following examples and tables the figures used
are as follows: