TABLE OF CONTENTS For many years it was believed that a cloud of flammable vapor, mixed with air, would not explode in the open air. It was believed that such a cloud would only flash, without a damaging blast wave.
Then attention was drawn to a number of incidents in which such a damaging blast wave appeared to have occurred.
Then, in 1974, the vapor cloud explosion at Flixborough in the United Kingdom removed all doubt. The control room of the plant was demolished by the blast (killing the 26 people inside), and most of the plant itself was badly damaged by both the blast and the subsequent fires. (See Chapter 13.)
A typical sequence of events leading to a vapor cloud explosion, shown in Figure 5-11, is described below.
A leak develops of a liquefied flammable gas, or a flammable liquid held at pressure at a temperature above its atmospheric-pressure boiling point. A proportion of the escaping liquid flashes instantaneously into vapor, and much of the resulting spray of the unvaporized material absorbs sensible heat from the air and also evaporates effectively at once.
The vapor drifts with the breeze, mixing with the surrounding air, and the total mass of flammable vapor present in the cloud increases as the leak continues.
The vapor-air cloud reaches an ignition source, and in due course the concentration of the vapor rises above the lower flammable limit. The cloud ignites.
The flame front in the cloud accelerates (by a mechanism...
