The mechanism of lightning strikes on oil tanks
(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.
