Page 134 - Electrical Installation in Hazardous Area

1 00 Electrical installations in hazardous areas

Because several assumptions have been made and the point of change from
one equation to the other (Equation 4.1 to Equation 4.3) the results of the
two equations will be seen to vary but only by less than 5 per cent at the
point of changeover and this will result in a small variation in the actual
hazardous area defined. The changeover from Equation 4.1 to Equation 4.3
is quite rapid at 2 x 105 N/m2 and, because of the small variation produced
at critical pressure, utilizing either equation at the pressure related to the
critical pressure ratio produces an acceptable result for mass of gas released
in view of the type of exercise in which we are involved. It is, however,
recommended that Equation 4.3 is used up to and including pressures of
2 x 105N/m2.
Calculation of the extents of hazardous areas is dependent upon the
velocity of air at the point of leakage and the quantity of air which is avail-
able as, if sufficient air is not available, the leak will steadily increase the
amount of flammable gas or vapour in air at any point and progressively
increase the extent of the hazardous area throughout the period of leakage
until equilibrium is reached. Therefore, the calculative methods described
in this chapter (with the exception of those given in 4.4) are generally only
valid in areas which are very well ventilated and where a large amount of
unconfined air is available. This tends to limit the application of the equa-
tions given for calculating the extent of hazardous areas to outdoor areas
or other areas with similar levels of ventilation.
If the release velocity of the flammable gas is high compared to the
typical wind or ventilation velocity in the circumstances described above,
the release will provide all the necessary energy for the necessary mixing to
achieve the necessary dilution but, if the release velocity is low, then mixing
will be due to the local wind conditions. In Europe these are normally
considered to be of the order of 0.5 - 2 m/s. Because of the nature of release
the effective cross-sectional area of the gas jet is normally less than the orifice
cross-sectional area and the release velocity is higher than might at first be
thought. This effect is, of course, taken care of in the derivation of Equa-
tions 4.1 and 4.3. A further problem is that the gas or vapour expands as
soon as it leaves the leak orifice and it can be assumed that at, and above,
an upstream pressure equal to the critical pressure the release velocity can
be demonstrated to be at least at sonic velocity (e.g., the velocity of sound
in the gas or vapour in question in the prevailing conditions). The gas or
vapour will expand almost instantly on release and will leave the release
orifice at around the speed of sound in the gas or vapour which velocity is
given by the following equation:

V, = [SRT/M]o.5 m/s
where SR = ratio of specific heats

V, = 108[T/M]0.5 m/s (Equation 4.5)
The distance which the jet of gas or vapour will travel before mixing
with air causes the resultant mixture to fall below the lower explosive limit