A Hidden Blind Surprises the Operators

This 12-ft. (3.7 m) diameter and 24-ft. (7.3 m) high tank was equipped with a fill line from the process, a 4-inch (10 cm) overflow line, a 3-inch (7.5 cm) vent line and a 6-inch (15 cm) vacuum relief device. Overpressure protection was intended to be supplied by the vent system piping or the overflow line. The designers did not include a high-maintenance pressure-relief device, because the 3-inch (7.5 cm) valve-free vent line was the overpressure device relieving into the scrubber. Neither the vent line nor the overflow line were equipped with block valves. The 4-inch (10 cm) overflow line was routed to a chemical collection/treatment sewer. 

The fiberglass acid tank was out-of-service because it had just been washed. If the level in the acid tank was ever overfilled, the 4-inch (10 cm) overflow line would relieve into the adjacent chemical collection sewer. The sewer was scheduled for maintenance. As a precaution to protect the individuals assigned to repair the chemical collection sewer, an operations supervisor authorized the maintenance crew to blind the 4-inch (10 cm) overflow line. The operations supervisor provided instructions to the operators to limit the liquid level within the acid tank to a well-defined maximum. 

When isolating a hydrochloric acid tank, it is common practice (and the preferred practice at this plant) to use a two-piece blind. A sheet of Teflon is placed toward the acid supply for corrosion resistance and this is backed up with a steel blind for strength. 

The tank was returned to service receiving a weak acid. The operators observed what appeared to be a rapidly rising liquid level. The level sensing device was actually reading the increasing pneumatic pressure as well as the slowly increasing liquid level as production was routed to the unvented vessel. Within a short time, the tank blew apart at the base. The tank fabricator estimated a failure of this nature could be reached as low as 2.5 psig (17 kPa). 

Several probable causes were speculated and investigated. Since ambient temperature was just below freezing, it was very easy to conclude that water froze in the small water quenching scrubber, seating the vent. It took just a few minutes to reach that conclusion, but the culprit—a blind found sort of hidden in the 3-inch (7.5 cm) vent tine flanges— was a major contributory cause. 

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