Page 78 - Electrical Installation in Hazardous Area

54 Electrical installations in hazardous areas

CAF6 gasket
Release aperture 4 x m2 (25 mm x 1.6 mm)
Release velocity high (unimpeded) or low (impeded)
This aperture is based upon a piece of gasket being ejected from a joint
between two fixing bolts. The dimension is chosen to cover all but small
bore pipes where it may be pessimistic.
Spirally supported gasket
Release aperture 2.5 x m2 (50 mm x 0.05 mm)
Release velocity high (unimpeded) or low (impeded)
This aperture is based upon relaxation of the joint as parts of the gasket
cannot be ejected. Again, it is sized to be valid for all but small bore
pipes where it may be restrictive.
’0’ Ring
Release aperture m2 (1 mm x 1 mm)
Release velocity high (unimpeded) or low (impeded)
Here release aperture is based upon a crack in the ‘0’ ring.
Compression joint
Release aperture lop6 m2 (0.1 mm x 10 mm)
Release velocity low (impeded)
This is based on a relaxation in the metal/metal of a compression joint
and will be of low velocity as its path is labyrinthine around the olive.
Screwed joint
Release aperture 5 x m2 (small and labyrinthine)
Release velocity low (impeded)
Release is based upon exit via the threads of the joint.

While being, to a degree, arbitrary the above figures are fairly conserva-
tive and thus have a low likelihood of being smaller than what actually
happens in practice. The hazardous areas produced by their use is likely
to be larger in the vast majority of cases to that which actually occurs in
practice. It must be stressed that if local knowledge provides more accu-
rate figures for the particular situation then, if larger, these should be used.
Likewise, although the release velocities are given as high for gaskets and
’0’ rings any obstruction either within the leak orifice after its smallest point
or close to it will sigruficantly reduce that velocity and unless such obstruc-
tions can confidently be ruled out dispersion by the wind should always
be considered.

3.3.3 Typical extents of Zone 2 from pipe joint releases

Gas or vapour release

The hazardous area typically created by a gas or vapour release is given
in Fig. 3.2 and values for its extent for both high velocity and low velocity