Page 141 - Electrical Installation in Hazardous Area

Calculation of release rates and extents 107

4.2 Release of liquid below its atmospheric boiling point

The release of a liquid below its atmospheric boiling point from an orifice
will take the form of a jet or, where the upstream pressure is sufficiently
high, a mist. The classic formula for release of a liquid from an orifice or
nozzle is as follows:
G = Ca[2al(P - 105]0.5 kg/s4
where G = Mass release kg/s
a = Orifice area m2
p = Upstream pressure N/m2
C = Discharge coefficient -
~1 = Liquid density at
atmospheric conditions kg/m3

As before, C has a maximum value of around 0.8 and therefore the equa-
tion becomes:
G - 1.13 a [al(P - 105]0.5 kg/s (Equation 4.13)

If no mist is formed then no significant hazardous area will surround the
leak (unless liquid is further contained around the point of leakage) and
a liquid jet will exit the orifice, forming a pool of liquid where it reaches
the floor or ground. The time taken for this jet to reach the ground can be
calculated using Newton’s Laws of Motion:
Using v = u + at, where v is the final velocity, u is the initial velocity,
and a is the acceleration due to gravity, the following is the case for a jet
released at an angle above horizontal:
v sin@-@, =o
where Q, = Angle of release above horizontal
g = Gravity acceleration (9.81 m/s2)
tl = Time to apex of jet (final velocity = 0 at apex)

Alternatively:
t] = vsin@/g Secs
Using the relationship v2 = u2 + 2gS, where S is the vertical distance above
the release point which the apex reaches,
(v sin - 2 x g x s = o
s = (vsin al2/2g m
If the initial release is at height h then the total height becomes [h+
(v sin Q,)*/2 g] and the time for the jet to reach the ground or floor can be
calculated using the relationship;

S = ut + 0.5gP
where u = 0