“Wise men learn by other men’s mistakes, fools by their own”.
The massive explosion at a fertiliser plant near Waco Texas recently is another reminder of how the process industry fails to adequately control major accident hazards. Initial indications point to a warehouse fire that lasted for around 30 minutes before causing detonation of stored ammonium nitrate, resulting in an explosion equivalent to a magnitude 2.1 earthquake. The blast wave killed around 15 people including several firefighters, injured 160, and demolished 50-75 properties including a school and nursing home located within 150-250m from the site. Thankfully there appears to have been recognition of the hazard potential from the fire, as around 130 people were evacuated from the ‘demolished’ nursing home prior to the explosion, otherwise the death toll would have been far greater.
The accident occurred a day after publication of an investigation report from the US Chemical Safety Board on the Chevron Richmond Refinery fire in August 2012, showing that a high temperature carbon steel pipe had ruptured due to ‘sulfidation’ corrosion. The CSB report highlights that this failure mechanism is well know in the refining industry, and points to a failure to learn from previous incidents and install ‘inherently safer’ piping materials on this duty that would have avoided this type of corrosion. The report also highlights failures of the Process Hazard Analysis (PHA) revalidation team to identify the hazard and take remedial action, due to a lack of competence in metallurgy.
It would be tempting to conclude that the Waco accident was unforeseeable and that a number of unfortunate factors had conspired to cause the explosion. These types of events are thankfully rare, but companies handing ammonium nitrate should be aware of similar explosions, including one at a fertiliser plant in Toulouse France in September 2001. This event was thought to have involved a 300 tonne store of contaminated ‘reject’ material, that exploded killing 29, injuring around 2,500, and shattering windows up to 5 km away. In another accident described as the ‘worst industrial accident in US history’, a ship being loaded with 2,300 tonnes of Ammonium Nitrate in the Port of Texas caught fire and exploded, killing around 576 people and flattening 1,000 buildings.
Although it is too early to speculate on the causes of the Waco explosion, a common finding on similar accidents is a failure to carry out adequate PHA, a key aspect of an effective Process Safety Management system. The essential aspects of PHA are to identify credible hazards based on the properties of the substances being processed, select suitable control measures to reduce risks to a tolerable level, and then manage these control measures to achieve reliability throughout the lifetime of the facility. The UK Health and Safety Executive publishes a code of practice for the safe storage and handling of Ammonium Nitrate, reference INDG 230, and readily available on its’ website. This classifies AN as an explosive that can detonate when heated, with the risk increased if the material is contaminated. Its gives a number of requirements for safe storage, including locating AN either outdoors or within buildings that ‘will not burn’, and limiting the size of storage stacks to control the scale of an explosion.
Companies operating ‘major accident hazard’ facilities should critically review the competence of the teams carrying out PHA’s, where they are using established techniques such HAZID and HAZOP. These studies can appear fairly mechanistic in nature and can place a great demand on scarce skilled and experienced resources. A serious management shortcoming is to assume that less experienced teams will be capable of producing adequate results and conclusions. The team leader has a critical role and needs to be highly competent in all aspects of process safety, and capable of steering the team based on an in-depth understanding of the causes of rare ‘process industry incidents’. If the process industry is to achieve a step change improvement and avoid the human toll and negative image from accidents such as the explosion at Waco, it needs to take on board the adage “Wise men learn by other men’s mistakes, fools by their own”.
Image credit: The Bay Area News via flickr