Overview of lightning accidents in large oil tanks
Overview of lightning
accidents in large oil tanks
The lightning fire
accident of the outer floating roof tank requires two basic conditions: gas
leakage and discharge near the leakage. There are two forms of discharge: one
is direct lightning strike, and the other is induced lightning. The latter
usually occurs in ungrounded conductors or poorly grounded conductors. Judging
from the previous cases of lightning strikes in China, except for a gasoline
tank accident in 2005, evidence of induced lightning discharge was found, the
others all occurred when the two grounding wires of the floating roof were well
connected; Secondly, the accident location is almost all at the liquid level
above 70%-80%. Measured by the 30 m or 20 m rolling ball method, most of the
floating roof is in the exposed position. For example, four 50,000 cubic meters
of tanks in a petrochemical company are used with two 45 m high lightning
towers and tanks. Six 5-9m high lightning rods are combined for protection.
When the protection range is calculated according to the 30-meter rolling ball
radius, the floating roof has an exposed area of 44 square meters. When the 120
rolling ball radius is calculated, the exposed area of the floating roof is 928
square meters ( About 1/3 of the area of the floating roof); In 2011, the
"11.22" lightning fire accident, through analysis of the T031 oil
tank floating plate, suffered direct lightning strikes. Therefore, the
possibility of direct lightning strikes in previous domestic accidents is
higher.
From these accidents, the following
phenomena and conclusions have been preliminarily drawn:
1) Most of it occurs at high liquid level,
and direct lightning and induction lightning cannot be ruled out;
2) Combustible gas accumulates in the
secondary sealed cavity and reaches the explosion limit range;
(3) The conductive sheets are all "covered";
4) Large external floating roof oil tanks
have no high and medium frequency lightning current shunt facilities, and no
medium and low frequency lightning current shunt facilities.
5) Current measures still have loopholes in
lightning protection design and management, such as unqualified grounding
resistance, inconsistent contact resistance between conductive sheet and tank
wall, etc.
