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Explosion Venting
Venting is the one of the cheapest, most effective ways to relieve pressure in an explosion situation; however it is difficult to size the vent correctly to allow sufficient pressure relief, as it must allow sufficient outflow of burnt dust and air out to relieve the pressure being generated by the heat of the explosion. Usually venting will be designed such that the pressure can be kept below some upper value (that is lower than the strength of the vessel or building). If the powder in the process is toxic venting to atmosphere is not possible, however venting to a sealed area may be possible. Many different designs of venting are available for all kinds of processes, including hinged doors that can withstand many explosions, simple panels that are ejected, vent covers are usually attached to the process vessel with clips or rubber seals which are known to fail under the load of the explosion.

Vent Area Sizing
Vent size is dependant on:

Enclosure Volume
Enclosure Strength
Strength of Vent Cover
Burning Rate of Dust Cloud

There are some rules of thumb for sizing vents, you can use a vent ratio from this table:

Max Rate of Pressure Rise (bar/s) Vent Ratio (m-1)
< 345 ......................1 / 6.1
345 - 690................ 1 / 4.6
> 690...................... 1 / 3.1

The vent ratio is the ratio of vent area to vessel volume. You can also use the graph below to get an approximate size for your vents:

For large vessels, greater than 30m3, the ratio for vent sizing can be reduced, the reason being that it is unlikely that a vessel that large would be full of an explosive mixture, hence the pressure will not rise linearly with volume.

There are more accurate and complicated methods of vent sizing which can be found in most of the better books on dust explosions (both "Dust Explosions" by P. Field and "Dust Explosions in the Process Industries" by R. K. Eckoff have detailed chapters on vent design). The methods detailed contain accurate methods based on length to diameter ratios and special cube root laws.

Hazards Caused by Venting
Venting does minimise the probability of large equipment destroying explosions, but does provide some hazards:

Ejection of flames from vent opening
Emission of blast waves from the vent opening
Reaction forces on the equipment, induced by the venting process
Emission of solid objects (parts of the vessel, vent covers, etc)
Indoor venting can lead to secondary explosions
Fireballs can be ejected
The larger the vent and the lower the force required breaching it, the more hazardous it is, as this increase the chance of unburnt dust being ejected, which could then burn in the plant area as a fireball. Using vent ducts (ducts of the same area as the vent) can minimise some of these problems by allowing the explosion to take place in a sealed area, but take up a great deal of space and still require the pressure to be relieved somewhere else (at the end of the duct).



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