The scene was an oil refinery in an industrial suburb outside Chicago, some 20 years ago. A 50-ft-high, 10-ft-diameter tank contained mostly propane at 200 psi and 100F. An operator noticed a gas plume issuing propane from a crack in the tank and tried to close the inlet valve, but the crack quickly extended from 6 inches to 2 ft. He called the firemen and ducked behind a stub wall, where he found safety.
The tank fractured around a circumference, and the escaping propane ignited. The top 40 ft of the tank soared upward, powered by the escaping gas, traveled two-thirds of a mile, and came down on a central power distribution station, thereby knocking out power to the entire refinery. The blazing tank-projectile, followed by electrical pyrotechnics, resembled a war scene. The local residents were actually calm about the affair, having already witnessed a number of impressive industrial accidents.
Major loss of human life and limb resulted from the refinery fire. The estimated cost of the incident was a quarter of a billion dollars, mostly in lost business. The size of the loss guaranteed the involvement of high-powered lawyers and a large cast of experts. I was retained by the company that designed the refinery.
The bottom 10 ft of the tank contained a water-amine solution, somewhat akin to aqueous ammonia, and the top 40 ft was propane. The tank was designed to absorb hydrogen sulfide from the propane.
The innards of these absorbers are subject to aqueous corrosion, and some ten years previous to the accident a "strake," or strip of steel extending all around the tank had been welded in near the water line. The fracture ran in the heat-affected zone near the lower circumferential weld. The fracture was of the brittle-cleavage type and extensive additional cracking of the steel had occurred in regions near the fracture. There was a bluish iron sulfide deposit on the fracture surface, a result of the reaction between the steel and hydrogen sulfide in the scrubber. It was a clear case of hydrogen embrittlement.
Hydrogen embrittlement mostly occurs in high-strength steel. Ordinary steel and non-ferrous materials are usually immune. The hydrogen usually comes from the operating environment, but may also be introduced during plating processes and the like. No one is sure of the mechanism, but it probably involves formation of fine platelets of metal hydride, which act as crack nuclei. Failure takes place by progressive cracking over the course of months or years. The ultimate result is sudden, often catastrophic failure.
The steel in the scrubber contained about 0.24 percent carbon, and as rolled was a low-strength steel. But, the welding process raised the temperature in and near the weld to bright red heat, so that air cooling produced martensite, which is very hard stuff. Martensite may be softened (tempered) by a post-weld heat treatment. In this case the carbon content was just below the level that would have made such heat treatment mandatory.
So what went wrong? My client had designed many refineries and the people who constructed, operated, and repaired the scrubber were well qualified. As far as I know a calamity of this type had never happened before. Something must have been different.
The difference was the Arab oil boycott. In 1974 the Arab Gulf producers sharply cut back on crude oil production, leading to panic buying and long gas lines in this country. Oil refineries got replacement supplies of oil from wherever they could. Some of this crude was "sour," which is higher in hydrogen sulfide than the "sweet" Arab Gulf oil it replaced. The sourness propagated down the refinery stream into the amine absorber. The high hydrogen-sulfide level in the propane caused hydrogen embrittlement in the heat-affected zone, which probably would not have occurred had sweet Arab oil been processed. The crack started (in all probability) with the sour oil in 1974 and continued over the years, finally ending in disaster.
I believe that the lawyers got their heads together to decide which party had to write out what size check. The heirs and dependents of the victims were surely compensated according to the dollar values then appropriate for death and crippling. Money is, of course, cold comfort for the loss or maiming of a loved one. I suspect that the welding rules were rewritten to require post-weld heat treatment for steels such as those in this case.