Acid tank overflow line

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. 

Figure 5-12 A large acid tank fails due to blocked vent and a blocked overflow line.

With a newly installed precautionary blind in the overflow line, the hidden blind in the vent line, and no additional overpressure devices, the tank was destined to fail. As liquid was pumped into the vessel the inerts had no place to go and the tank was pressurized to destruction. To the best of everyone s knowledge and the evidence, this hard-to-detect blind may have been inserted over a year prior to the incident for a previous internal inspection of the acid tank. Obviously the mechanic(s) who removed the other blinds a year or so before and on this occasion did not notice this one. 

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. 

Figure 7.16 A large fiberglass acid tank fails due to blinded vent line and blinded overflow line.

The stainless steel nitraror of the process is equipped with a motor driven steel propeller, vertical cooling coils and an overflow tube to separator. A two-way valve, which connects to the mixed nitric-sulfuric acid line, or may be opened to the drowning tank, is fitted centrally to the bottom of the nitrator. Brine at ca -5° is circulated at controlled rate around the cooling tubes in the annular space outside the nitrator. 

Before the acid truck was unloaded, acid started to overflow and pour through the six-inch (15 cm) line into the scrubber. The alert truck driver quickly responded. He abruptly shut the delivery valve on his truck. Unexpectedly, the partial vacuum created by the siphoning action of the overflowing liquid, exceeded the tank s vacuum rating and the storage tank was totally destroyed (see Figures 2—7, 2—8, and 2—9). [2]... 

The product and spent acid overflowed into a lead-lined separating tank. The nitration product contained 1-2% of dinitrotoluene. 

---