Design of Safety Devices to Mitigate Explosions in Underground Vaults and Manholes

Results from a computer program are used to compare several alternative safety devices that can mitigate the effects of explosions in underground structures. After eliminating several candidate designs that are impractical or uneconomical, attention is focused on a flexible tether that can be attached to the manhole cover so that the cover does not become a dangerous projectile. An appropriate tether design is recommended which can restrain the cover when an explosion occurs in the structure. Types of explosions considered include an ignition of a combustible gas, an arcing fault, and the combination of an arcing fault followed by a gas explosion. Flexible tethers, such as traditional seat belt webbings, are shown to be superior to steel cables or chains. The tether material, length, and the amount of slack are shown to be critical factors in the design of an effective tether system. The effect of holes in the manhole cover that can vent the high-pressure gases inside the vault are shown to have a desirable effect on the size and geometry of the tether. An examination of the output is used to recommend characteristics of a tether system that will successfully mitigate the effects of an explosion, regardless of the source.