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