When protecting vessels from explosions, one of the most common ways is to install explosion vents. The explosion vent is an engineered weak spot in the vessel that fails when the internal pressure reaches a certain point. This failure allows the pressure to be released in a controlled way so that the entire vessel doesn’t explode. The explosion vent needs to be installed so that the venting is away from any equipment, walkways, roads, picnic areas, etc.

Explosion vents are primarily used when the vessels are located outdoors, however they can be used indoors, if certain criteria is met. To install an explosion vent indoors, the vessel has to be located by an exterior wall. The vent would then be ducted through the wall, so in case of an explosion it is directed outdoors. An indoor explosion vent will usually need to be larger than an outdoor explosion vent. This is due to additional combustion of material in the duct and the inertia of the air mass in the ductwork.

The following is an explanation taken from FM Global 7- 76 Section 3.1.8

During the early part of the venting process, unburned dust is ejected into the duct ahead of the flame front from the vessel. When the flame front moves into the duct, dust starts to burn within the duct and generate additional combustion products. Those combustion products expand in ALL directions, thus slowing down or even reversing the flow out of the vessel and the pressure builds up within the vessel.

Inertia of the air within the duct also increases explosion pressure within the protected vessel. When the explosion vent opens and combustion gases first start flowing into the duct, those gases must push all the air out of the duct. During the time required to eject the air, the pressure continues to grow within the vessel because the combustion gases are obstructed from reaching the open atmosphere. In a long duct, that mass of air can delay the venting of the combustion gases enough to significantly increase the pressure in the vessel.

So if you’re installing a vessel indoors and you want to vent it outside, keep it as close to the outside wall as possible.

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