Protection for Structures Having Explosives

Introduction:

Lightning is a natural phenomenon of sudden discharge of charges from highly charged cloud to ground. They are very destructive. Besides the danger to people, it may cause damage to structure as well. If a structure contains highly inflammable materials or explosive materials, special care should be taken while designing the proper lightning protection system for them. IEC 62305-3 explains about the precautionary steps and design considerations of lightning protection system for structures with risk of explosion.

Zones of protection:

Based on the configuration of explosive material and the duration of the flammable atmosphere present, IEC 62305-3 classifies the structures into different zones and the details are as follows.

1) Zone 0: Place in which an explosive atmosphere consisting of a mixture of air
and flammable substances in the form of gas, vapour or mist is present
continuously or for long periods or frequently.
2) Zone 1: Place in which an explosive atmosphere consisting of a mixture of air
and flammable substances in the form of gas, vapour or mist is likely to occur in
normal operation occasionally.
3) Zone 2: Place in which an explosive atmosphere consisting of a mixture of air
and flammable substances in the form of gas, vapour or mist is not likely to occur
in normal operation, but if it does occur, will persist for a short period only.
4) Zone 20: Place in which an explosive atmosphere, in the form of a cloud of
combustible dust in air, is present continuously or for long periods, or
frequently.
5) Zone 21: Place in which an explosive atmosphere, in the form of a cloud of
combustible dust in air, is likely to occur in normal operation occasionally.
6) Zone 22: Place in which an explosive atmosphere, in the form of a cloud of
combustible dust in air, is not likely to occur in normal operation, but if it does
occur, will persist for a short period only.

Structures containing solid explosive materials:

While designing the lightning protection system for structures containing explosive materials, we should have special care on the sensitivity of the materials in the configuration in which the explosive is used or stored.

There are some explosive materials which are very sensitive to the rapidly changing electric fields due to lightning impulses. Hence an isolated external lightning protection system is preferred for such applications.

Surge Protection Devices (SPD) shall be provided as part of LPS for all locations where explosive material is present. To prevent the accidents due to spark from SPD, it is recommended to fix the device outside the structure where explosive material is stored.

Structures containing hazardous areas:

For the structures containing hazardous areas, all the parts of external LPS shall be at least 1m away from the hazardous areas. If a structure is made of metallic roof sheets with at least 5mm thickness steel or equivalent, the metallic roof itself will act as a natural air termination system. Soil covered tanks and piping does not require any air termination system. The SPDs are always recommended to be fixed outside the hazardous area.

Structures with areas defined as Zone 2 and Zone 22 may not require additional protective measures. If the structure is made of metal sheets with adequate thickness as specified by standard, separate air-terminal and down conductor system are not required.

For structures with areas defined as Zone 1 and Zone 21, the additional protection than Zone 2 and 22 is provided with the help of isolating spark gaps and insulation pieces. Explosion protected isolation spark gaps can be used to avoid disruptive discharge.

For structures with areas defined as Zone 0 and Zone 20, the temperature rise during the lightning strike also may cause explosion. Hence if the thickness in not adequate separate air terminal should be installed. Zone 20 and 20 includes the protection measures of the zones mentioned above.

Earthing System:

Type B earth arrangement (ring earthing) is preferred for all lightning protection systems for structures with danger of explosion. The earth resistance values for such structures with risk of explosion shall be as low as possible and should not be greater than 10Ω.

The pipe networks above the ground level inside the production unit but outside the
process units should be connected with earth at every 30m intervals.

Conclusion:

Since the lightning is discharge of huge amount of electric charges, the electric field in the atmosphere will change suddenly at the time of lightning strike. In the case of structure with explosives, this sudden change in electric field itself can cause explosion. Hence the lightning protection system and SPD for such structures with risk of explosion should be designed with due care in such a way that, in case of direct lightning flash, there are no smelting or spraying effects except at the point of strike.

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