“Tastes like heaven, burns like hell.” — Fireball slogan
Tequila is a far cry from the cloying cinnamon-flavored spirit called Fireball. But they both have one thing in common: they burn like hell. Literally. As do all spirits. For reference, the flash point of 80 proof (40% ABV) spirits, is 26 C (79°F), meaning that on a warm day, spirits at or above a typical strength will produce enough flammable vapor to form an ignitable mixture with air.
The personnel at José Cuervo’s La Rojeña distillery in Tequila, Jalisco, in Mexico, learned that firsthand on July 23, 2024. That is when an explosion and fire in a tank farm there killed six workers and injured two others.
A flammable mixture in air is not enough to cause an explosion, however. It takes an ignition source.
Possible Causes
Having no jurisdiction in Jalisco or anywhere else in Mexico, the Chemical Safety Board (CSB) will not be issuing an incident investigation report. We will not be getting their expert analysis of what caused the explosion and resulting fire. Instead, Civil Protection Jalisco and the Jalisco Institute of Forensic Sciences are investigating the explosion. A year after the tragedy, however, neither organization has released a report on the cause of the explosion and fire.
What has been reported?
Casa Cuervo, the company that owns and operates the distillery, stated at the time of the incident that it happened during maintenance work. JOIFF (originally the Joint Oil and Industry Fire Forum but now known as the International Organisation for Industrial Emergency Services Management) reported shortly after the incident that they had been told by Civil Protection Jalisco that “the disaster was triggered by the explosion of a tank car, which subsequently set ablaze three additional tank cars, each holding 219,000 liters of liquid.”
Photographs taken during the incident response show fixed vertical storage tanks. Another photograph shows a large crumpled vertical tank lying on its side some 660 meters (~2,000 feet) from where the explosion launched it. These are not rail car tanks, but fixed liquid storage tanks.
Powder & Bulk Solids reported on the explosion at the time of the incident, leading the folks at Dust Safety Science to include the incident in a list of recent combustible dust explosions. It was not a dust explosion. Although the process for producing tequila generates dust, that is not what the tank that exploded contained, nor what the tanks that burned contained.
So, what caused the incident?
The people I know who are knowledgeable about both process safety and spirits production speculate that the explosion was the result of hot work during maintenance on a vessel containing a flammable liquid, namely tequila, warm enough to generate an ignitable mixture in air.
Incidents Sparked By Hot Work
Sadly, there is a long history of incidents where hot work ignited deadly fires. The CSB lists thirteen incidents to which they deployed and investigated, beginning in 2001 and going through 2016. All involved maintenance work on tanks and almost all were fatal.
Some of the tanks contained liquids that were obviously flammable: methanol, with a flash point of 11 C (52°F), and vinyl fluoride (a gas at standard conditions), with an explosive range of 2.6% to 21.7% by volume in air. The work was outside the tank in those instances, however, and regrettably, no one recognized the potential for vapors to leak.
Some contained crude oil. The flash point of some crude oils can be over 120 C (~250°F), which is not normally thought of as a readily ignitable liquid. Some crude oils, however, have flash points as low as -20 C (-4°F), which are quite easy to ignite.
Most surprising, though, were the hot work-ignited incidents involving tanks storing liquids that most simply don’t think of as flammable: sulfuric acid, paper pulp, and in two instance, water. Yes, water. The water contained decomposing organic matter which released biogas, which in turn ignited during hot work.
In 2023, the National Fire Protection Association (NFPA), reported that there were about 3,400 fires per year involving hot work.
What Is Hot Work?
OSHA regulates hot work in 29 CFR 1910 Subpart O, Welding, Cutting and Brazing. Traditionally, these tasks comprised what was considered hot work. In the instances listed by the CSB, traditional hot work – welding, grinding, and use of a cutting torch – were the ignition sources. In the Process Safety Management (PSM) standard, 29 CFR 1910.119, OSHA defines hot work as “work involving electric or gas welding, cutting, brazing, or similar flame or spark-producing operations.”
In addition to welding, cutting, and brazing, hot work includes drilling into steel, striking objects with a sparking tool, performing any task that uses an open flame, such as thawing pipes or heating roofing materials, using power tools, using an internal combustion engine, or unplugging ordinary electrical equipment.
There was time when flash photography was considered hot work, because of the heat from flash bulbs, but photographic flash now depends primarily on LEDs, which do not create the kind of heat necessary to ignite vapors.
In the context of materials of construction, neither copper, brass, nor aluminum are sparking materials, but both carbon steel and stainless steel are.
This Is Not News
It is not news that hot work is an ignition source and that around flammable vapors, hot work can spark explosions and fires. Yet they keep happening. The challenge seems to be in recognizing hot work for what it is and in recognizing sources of flammable vapors. It is easy, in hindsight, to identify hot work as the ignition source for an incident and point to the tank as the fuel. To be useful, we must identify the ignition sources and fuel before the work begins.
Implement A Hot Work Program
Many facilities have effective hot work programs. However, many do not. Fortunately, there are online resources, such as the NFPA Hot Work Permit. While the OSHA standard has many requirements for performing hot work safely in 29 CFR 1910.252(a), there is one requirement that is essential:
Do not perform hot work on used drums, barrels, tanks, or other containers until they have been cleaned so thoroughly as to make absolutely certain that there are no flammable materials present or any substances such as greases, tars, acids, or other materials which, when subjected to heat, might produce flammable or toxic vapors. Any pipelines or connections to the drum or vessel shall be disconnected or blanked.
Be safe.
