(2) Indirect lightning strike
If a lightning strike occurs near the tank, some current will flow on the outer skin of the tank shell and flow down through the floating roof to the ground on the other side of the tank shell. Compared with the storage tank directly struck by lightning, the energy of the discharge current passing through the storage tank is much smaller at this time. Like a direct lightning strike, any discontinuity in the current path will cause an arc in the gap.
The current spreads out from the lightning connection point, including to the storage tank, upwards and on the storage tank, and downwards along the far side, as shown by the typical current flow lines and arrows. This current flow plan is only suitable for fast high current pulses. Continuous current flows only along the ground and bottom of the tank.
(3) sparks to fire
The reason why sparks are generated is because: due to the driving current on the storage tank or close to the storage tank flowing through the floating roof (through the conductive sheet or through any contact between the floating roof and the shell intentionally or unintentionally) Any lightning strikes of other metals have a tendency to generate electric current, so sparks are most likely to cause a tank fire on the outer floating roof tank due to lightning.
The following considerations should be made for air gap sparks:
1) The air gap spark appears in such a place, that is, there is a small gap between the conductive objects in the part, where lightning generates a sufficiently large voltage, which can cause air or steam-air mixture in the air gap Electrical breakdown has occurred.
2) If it is in the range of flammable mixture, an air gap spark with an energy higher than 0.2 mJ is sufficient to ignite the steam-air mixture of the product.