17.2.3.3 A clean environment
The main goal of this measure is to avoid the formation of a volatile atmosphere on and
around the tank, under any circumstances. Such a volatile atmosphere could be formed
from the gases or vapours within the tank, and therefore several measures are applied
to ensure that this does not happen: (i) flammable air–vapour mixtures should be
avoided and (ii) all appurtenances should be kept in good working order.
It is quite common to observe that, during venting processes in manufacturing,
storage, compression, and chemical plants, explosive mixtures of gases or vapours
and air are released into the atmosphere. Such a condition makes lightning protection
almost impossible to achieve, because the distances over which such dangerous
mixtures can persist after release from outlets can reach several tens of metres,
which represents a serious condition as ignition (through flash-fires or explosions)
can be generated when a lightning channel is located near to or passes through the
outlet. For deliberate and unintentional venting processes, the following measures
should be fulfilled to minimize the risk of flash-fire and explosion:
† As far as possible, making use of the venting process, particularly during active
thunderstorms, should be avoided.
† Vents should be fitted with flame arresters to prevent eventual ignition in the air
being transmitted through the vent pipe into the container or installation.
† Where possible, vents should be made from non-conducting material to reduce the
probability of a lightning strike.