The Houston Chronicle is reporting that the four DuPont workers who died earlier this month during a methyl mercaptan leak at the LaPorte, TX facility were asphyxiated (died because of lack of oxygen) and did not die because of methyl mercaptan poisoning. To be sure, this is a technical difference because the relative lack of oxygen was due to the large amount (23,000 lbs) of methyl mercaptan released into the building.
This raises a number of interesting questions, most of which I would assume that the CSB is asking and/or answering:
• Did anyone at the site know about the magnitude of the ‘leak’ before any of the four employees entered the building? If they did, this should have been a full-blown hazmat response team involved, not four piecemeal employees responding to a ‘leaking valve’.
• If no one knew the magnitude of the leak, why not? That amount of material should have registered as a very significant change in tank/vessel level/weight. That alone should have set off significant alarms on site.
• Was this a leak from a process vessel or a storage tank? There would have been different types of safety instrumentation and safety controls on a storage tank for a toxic substance like methyl mercaptan that should have been able to been able to isolate the leak without anyone entering the building.
• What type respirators were the four wearing when they entered the building. There was at least one news report early on that talked about one of the brothers trying to share his air with his overcome younger brother. So at least one person had a supplied air respirator. For the others to die of asphyxiation that would seem to indicate that they were wearing cartridge respirators which don’t help at all if there is a lack of oxygen in the atmosphere.
From the news reports to date we can guess at some of the answers to the above questions. It would seem that as many as three individuals entered the facility wearing cartridge type respirators. This argues that the people in charge of the unit were not aware of the magnitude of the leak. No one in the chemical industry with a modicum of sense or training would send anyone into a building where that volume of toxic, volatile chemical had spilled without doing serious gas testing, both for the presence and concentration of the toxic gas and the oxygen levels in the air.
From the news reports available to date I suspect that something like this happened (entirely supposition on my part and I have never seen the facility):
A gas detector goes off in part of the building that people are not normally working in. The gas detector reports the presence of methyl mercaptan but not the level. Two employees in proper PPE for a small leak (probably a cartridge respirator) are sent to investigate/fix the problem. When communications are lost with those individuals another person is sent in to find out what happened and is also overcome. A senior operator goes in wearing an air supplied respirator to rescue his younger brother who was one of the three original responders. He reports the extent of the problem and is then overcome while trying to resuscitate his brother with his supplied air respirator. A properly informed response team is then formed and deployed, too late to rescue the four earlier responders.
The Question Not Asked
The Chronicle article mentioned above also reports that the LaPorte facility also manufactures and uses methyl isocyanate (of Bhopal and Charleston, WV fame). This is a deadly toxic chemical and much less ‘friendly’ than methyl mercaptan. I would like to think that a facility run by DuPont that handled toxic chemicals like this had properly instrumented control systems and safety systems that would prevent incidents like this.
Giving DuPont the initial benefit of doubt there is another ugly possibility that should also be considered, was this accident the result of a cyber-attack on the facility. This could explain why a stand-alone chemical detector alarm could go off and no one in the control room could see the cause for the alarm or detect the extent of the leak. That would explain the completely inadequate preparation done to enter the facility to correct the ‘leaking valve’; all indicators would have shown a minor leak, not a massive release.
I am not sure if the Chemical Safety Board has control system security experts on the team. If not, they may not be able to detect the incursion that would have precipitated this incident. I would propose that the following might be non-technical indicators of a possible attack:
• Discrepancies in the amount of methyl mercaptan reported on site (via the automated inventory management system) before, during and after the attack;
• Discrepancies in the indicated flow rates of methyl mercaptan reported by the process control system versus the computed rate of the leak;
• Discrepancies between the control system commands initiated by the operator (either in the data historian or the operator testimony) and the control states of the equipment recorded in the data historian.
If any of those discrepancies are noted, the CSB ought to call in the ICS-CERT fly-away team to help investigate the potential for control system problems (‘problems’ covers a multitude of sins) contributing to or as the source of the massive methyl mercaptan release.