A History of Research and a Review of Recent Developments

66
Propellant, dust, gas and vapour
Explosions of domestic gas in the UK were running at about 100 per year
in the 1970s and causing about 10 deaths per year. An inquiry into serious gas
explosions, held in the late 1970s [3.16], noted that one-third of all explosions
were caused by gas escaping from distribution mains and service pipes on the
outside of buildings, and two-thirds from leaks at meters, appliances or
installations within buildings. External gas escapes were frequently connected
with the fracturing of cast-iron mains due to ground movement rather than
corrosion. Often escaping gas in winter could not be released upwards because
of frosted ground, and entered buildings through cellars or other service ducts.
It was natural at the time for suggestions to arise that natural gas represented
a greater explosive hazard than the old town gas, particularly as the countrywide
distribution of natural gas meant an all-steel high pressure network. This
comparison was investigated by an earlier inquiry [3.17], and it was concluded
that the hazards from the use of both gas systems were similar. Natural gas
has double the calorific value in BTU/ft 3 , a much greater distribution pressure
of about 30mbar (about three times), and a lower maximum burning velocity
(about half). Natural gas burner pressures are reduced by governing to about
21 mbar.
A domestic gas explosion is of the deflagration type, with a finite time
from ignition to maximum energy release. The pressure rise is relatively small
and direct injury due to this pressure increase (e.g. ear drum damage) is rare.
Gas explosion fatalities usually occur from flying or falling objects, or by
burns. Research has shown that from a total of 39 incidents, peak dynamic
pressures from town gas explosions averaged 13 KN/m 2 , and from natural
gas explosions 11 KN/m 2 . The air-rich mixture at which ignition occurs if a
source of ignition is present is about the same for both gases at approximately
7.5% by volume of gas. There are important differences in flame propagation
properties, however, with town gas having a propagation capacity about 5.5
times as great as for natural gas.
The effect of gas explosions in underwater tunnels is important, because
the tunnel walls would usually be designed to resist external loads due to
water and soil. In the event of an internal gas explosion the tunnel walls could
experience a load reversal for which they had not been designed. The pressures
generated by internal gas explosions have been investigated in Holland [3.18]
and from these experiments the pressure-time history has been recorded. The
duration of the overpressure plateau of 6 to 7 bar is governed by the time
needed to vent the overpressure. The peak instantaneous pressure of 25 bar
(2.5 N/mm 2 or 362 psi) represents a formidable internal load on the structure.
3.4 VAPOUR CLOUD EXPLOSIONS
Clouds of flammable vapour can be released accidentally from large process
plants in the chemical and oil industries, particularly when the flammable
materials are under pressure and above their atmospheric boiling points. If