Page 138 - Electrical Installation in Hazardous Area

104 Electrical installations in hazardous areas

A figure for G of the mixture can now be calculated using the original
released mass of gas plus the mass of air which is included in the mixture
at the point of obstruction.

V (gas) = 0.082GT/M (gas)
V (Mixture) = V (Gas) x [100/%Gas in Mixture] m3 (Equation 4.10)
G (Mixture) = V (Mixture) x 12.19M (Mixture)/T kg (Equation 4.11)

It is now necessary to calculate a new lower explosive limit (LEL) and
this can be done simply by the following calculation:

LEL (Mixture) = LEL (Gas x [100/%Gas in Mixture) YO (Equation 4.12)

These new figures can then be inserted in Equation 4.7 and a distance to the
edge of the hazardous area calculated. This distance, added to the distance
between the point of release and the obstruction, will then define the extent
of the hazardous area in the direction of the obstruction.
A further problem occurs when the release is close to the ground as
the dispersion is affected. There are no mathematical procedures for deter-
mining the effects of this but the following are typical of the procedures
adopted.
First, whatever the release pressure (e.g., above or below that defined
by the critical pressure ratio), provided that the lower limit of explosive
atmosphere is less than 3m from the ground then Equation 4.7 should be
used in all cases for extent of the hazardous area and the horizontal limits
should be projected to the ground. (Chapter 3 deals with the results of this
in more detail.)
Second, where the lower extremity of the explosive atmosphere is within
1.5 m of the ground the drift release equation (Equation 4.7) should be used
and, again, the extremities of the explosive atmosphere projected to ground
level but in this case the ground footprint should be multiplied by 1.5.
(Chapter 3 deals with the results of this in more detail.)
Third, where the lower extremity of the explosive atmosphere touches
the ground the extent of the hazardous area should be that produced when
the result of the drift equation (Equation 4.7) is multiplied by 1.5 to take
account of the distorting effect of the ground upon dispersion of gas in all
directions. (Chapter 3 deals with the results of this in more detail.)
The above gives a calculative approach which will effectively deal with all
normal circumstances where gas or vapour is released, provided that gas or
vapour does not have a density dramatically different from air. In the case of
sonic releases which are not impeded, and therefore utilize Equations 4.1
and 4.6 density will not have a significant effect for any normal circum-
stance (including hydrogen) as the energy in the released gas by virtue
of its velocity will overcome any significant effect of density differences,
but where releases become subsonic (typically where Equations 4.3 and 4.7
are relevant) differences of density will become increasingly relevant. It is
difficult to determine the exact effect of these, but as a rule of thumb the