Overview of lightning accidents in large oil tanks

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Issue Time: Mar 5,2021

Summary

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.