PROTECTING OIL TANKS FROM LIGHTNING

Lake Wakatip Mail, Issue 3714, 12 October 1926, Page 7

PROTECTING OIL TANKS FROM LIGHTNING

A recent oil lire in California, which resulted in the death of two men and the destruction of more than 10,000,000 dollars in oil and property, was attributed to lightning. The lightning hazard is well known to the oil industry, and studies of protective methods have been made. Oil storage tanks, says the Scientific American ’ (Now York, August), are necessarily so large that reinforced concrete, rather than metal, must usually be used. These concrete reservoirs are frequently 500 ft in diameter and 30ft deep. As many as twenty-five or more sometimes make up a “ tank farm.” The tops are of wood, often covered with felt. Frequently tho space between the surface of the oil and flic top of the tank becomes filled with an explosive mixture of air and oil vapors. In general, no lightning protection is used. F. W. Peek, jun., consulting engineer of the General Electric Company at Pittsfield, Massachusetts, speaking at the annual meeting of the American Petroleum Institute in Los Angeles, described experiments which had been made in the Pittsfield Laboratory with artificial lightning and miniature oil tanks, and told how protection from direct hits by lightning can be secured. The following notes from Mr Peek’s address are given by the paper named above: — “ In considering oil tanks, two cases must be kept in mind—the tank containing explosive mixtures and the tank free from such gases. “ Where there arc explosive mixtures the protection problem is exceedingly difficult. Just as a considerable voltage may appear on electrical transmission lines a mile or more from the lightning discharge, because of ‘ wireless ’ or induction elfect, so may sparks be caused by voltages induced in metal parts of the tank, even though the tank is not struck directly by the lightning. A spark at 500 volts, caused by a lightning stroke several miles away, could ignite the mixture and cause great damage. “ If a tank were made completely of insulating material there could bo no spark, but there are always pipes, nails, conducting strips, and so on, more nr less isolated. Grounded wires strung over the tank, as in the case of a protected transmission line, would reduce the voltages. But the only safe method where there is explosive gas is a complete metal tank, inside of which no sparks can occur. Even then, however, there is a danger when projections extend from the surface.

“ Where explosive mixtures of gases can bo kept from the lank, the chance of puluced voltage causing trouble is small. Direct hits, however, could set the oil-soaked roof on fire. There arc a number of principles, discovered in the Pittsfield Laboratory, that can be applied to prevent the tank from being struck. It has been found, for instance, that lightning never strikes the ground nearer a conducting rod than four times its height. IT the lightning does not strike the rod, then it hits the ground some distance away. A number of rods properly placed about the tank would either take the hits or cause the lightning to strike, the ground some distance away. Grounded wires strung over tho tank afford protection against direct hits.”