(146c) Catastrophic Explosion Investigation and the Dogma of Process Safety Management

Approximately 75%-85% of the nitric acid produced globally is weak nitric acid (~60 wt% HNO₃) used in the manufacture of nitrogen fertilizers. However, when applications require higher strength concentrations of nitric acid, additional processing is necessary due to the maximum-boiling azeotrope with water at HNO₃ concentrations near 70 wt%. One method to concentrate nitric acid to higher strengths is the direct processing method. In a direct processing unit, weak nitric acid is reacted with dinitrogen tetroxide (N₂O₄) and oxygen at high pressure. The reaction consumes the water in the weak nitric acid feed, while simultaneously producing additional molecules of HNO₃ to achieve higher strength nitric acid (> 98 wt%).

Operating a direct strong nitric acid concentrator combines several highly hazardous operations and materials. High strength nitric acid, dinitrogen tetroxide, and high purity oxygen are all strong oxidizers that can enhance the flammability of materials resulting in fires and explosions. Nitric acid is also highly corrosive. Both concentrated nitric acid (>94.5% by weight) and dinitrogen tetroxide are subject to the United States Occupational Safety and Health Administration Process Safety Management Standard, with threshold quantities of 500 pounds and 250 pounds respectively. Nitric acid concentrator reactors are also pressure vessels that operate at moderate to high pressure (~ 5 MPa / ~750 psi). As a result, a facility that produces high strength nitric acid using a direct process faces the formidable challenge of safely reacting a mixture of strong oxidizers, in a corrosive environment, at high pressure in a pressure vessel, under an enriched oxygen atmosphere, all while complying with the requirements of the OSHA PSM Standard.

A catastrophic explosion occurred at a US based facility that utilized a direct process to produce high strength nitric acid. The explosion occurred as the facility was restarting their concentrator reactor following an outage/turnaround. The explosion completely destroyed the reactor, and damaged several other processing units. Losses were estimated in excess of $250 MM, but luckily there were no serious injuries.

The ensuing explosion investigation addressed several key questions related to the incident and claims resulting from the loss. This effort included hazard analyses for a variety of conditions including high pressure processing units, oxidizers and oxygen enriched atmospheres, and corrosive chemicals. The investigation also reviewed the flammability of materials used to construct the reactor, which combined various grades of steel and aluminum in a multi-course pressure vessel to achieve both high strength and corrosion resistance. The investigation then evaluated the elements of the facility’s process safety management program that were intended to mitigate the risks posed by the hazards of the materials and equipment used in the process. This evaluation focused in particular on management of change, mechanical integrity, process safety information, incident investigation and process hazard analysis with emphasis on work performed on the concentrator reactor during the outage/turnaround that preceded the incident. Lastly, the investigation provided insight into a key philosophical (and contractual) question – are these types of incidents considered force majeure events (i.e. the so-call ‘act of God’ clause in many contracts).

This presentation will review the incident and the involved equipment, the investigation into the explosion’s cause, the facility’s process safety management program, and role of process safety management in risk mitigation of highly hazardous processing units. The presentation will also raise philosophical questions regarding whether such incidents are foreseeable, and within the control of a facility’s operations – two key elements of force majeure claims.