CSB Incident No. 16, reactivity with air, self reactivity, incompatibility. NaK used as thermometric fluid spilled into a furnace. Personnel were not aware that NaK reacts with air to form KO<subscript>2, which can decompose violently in the presence of hydrocarbons. During cleaning operations, mineral oil was used to wash the furnace walls, resulting in an explosion due to incompatibility with the KO<subscript>2. This reactivity and compatibility information is available from a Level 1 search.

CSB Incident No. 32, water reactivity. A process to produce alkyl benzene for detergents had been altered by replacing aluminum chloride with Al powder without going through management-of-change procedures [12]. Level 1 information would have shown that Al powder can react exothermically with water. Following this substitution, the reactor was overfilled and the bottom outlet became fouled with solid catalyst residue. Because tests showed that the residue reacted with water, it was decided to inject ambient temperature water to dissolve the solids. Instead, personnel injected continuous steam and within minutes the reactor vessel exploded. Because the reactivity tests had been performed only at ambient temperature, the exothermic reaction of aluminum powder with hot water or steam to form hydrogen was not identified. This incident teaches the importance of management of change, a need for testing at normal and likely upset conditions, and the importance of pressure generation as part of an overall hazards assessment.

Additional explosion hazards due to combustible materials can include:

• Releases of flammable gases that disperse and encounter ignition sources [13];
• Aerosols produced from leakage of industrial fluids to ambient pressure from higher pressures [14]; and
• Dust accumulations that can form airborne dust clouds that disperse and explode [15].

Level 3 — detailed reactivity tests
Screening tests satisfy or at least greatly simplify minimum requirements for a hazard assessment. In some cases based on Level 1 and Level 2 information, however, additional quantitative information using more detailed computations or experimental tests may be required to reduce risk to an acceptable level. Computational tests include quantum methods such as HF, G2 and B3LYP [16], while experimental methods include tests under adiabatic conditions using adiabatic calorimeters such as the APTAC or ARC [4]. These tests require increased resources, but they can explore sensitivities of the chemicals and provide more accurate values of reaction heats and activation energies for estimating hazard evaluation parameters, such as time to maximum reaction rate, which are useful for risk quantification and reduction. The following example from the CSB 167 incidents illustrates the value of  Levels 2 and 3 testing.

CSB Incident No. 1, self reactivity, thermal decomposition. Nylon polymer in a process vessel had decomposed to carbon dioxide and other gases. Because the foamy polymer fluid had blocked vessel tubing, operators opening the vessel were not aware of the trapped pressure. The partially unbolted cover blew off and the released hot fluid ignited. Following the incident, adiabatic calorimetric testing showed that the thermal decomposition of the polymer was endothermic with significant pressure generation (Figure 1) [17]. Closed-cell testing at Levels 2 and 3 over a temperature range that included the primary and secondary decomposition reaction systems could have identified and measured pressure formation behavior, including maximum pressure and pressure generation rate. Other reactivity characterization information for hazard assessment is onset temperature for significant decomposition, the difference between the main process temperature and the decomposition onset temperature and, from Level 3, the time to the maximum pressure generation rate under adiabatic conditions.

Figure 1

Thermal decomposition of a nylon polymer generated significant pressure that ultimately led to a fluid release and fire.

This incident demonstrates the importance of pressure as a hazard indicator and the danger of relying on a single indicator, such as the overall reaction energy, for hazard classification.

Tools you can apply
Before hazards can be controlled, they must be identified. The multi-tiered approach and methods we have outlined can play a key role in identifying reactivity risks, without requiring extensive resources. The cited CSB incidents underscore the value of the approach. Basic reactivity hazards should have been uncovered from a Level 1 information search in all but the last incident case study. If Level 1 information had been employed, followed by Level 2 screening for hazard identification and initial characterization, the number and severity of incidents should  have been greatly reduced. Level 3 tests for more detailed characterization require greater resources, but just the knowledge that additional information beyond Level 2 is needed helps to determine whether a process can be performed economically at a manageable level of risk or must be modified or abandoned. Additional information is available from many sources including the periodically updated websites of OSHA (www.osha.gov/SLTC/reactivechemicals/), and the Mary Kay O’Connor Process Safety Center (http://process-safety.tamu.edu).

Process safety courses

Sam Mannan and William Rogers co-teach a course concerning reactive chemical hazards as part of a continuing education program provided by the Mary Kay O’Connor Process Safety Center, Texas A&M University, College Station, Texas. For more information concerning available courses, call (979) 458-1863 or visit http://process-safety.tamu.edu.

References