At Christmas in 2008 the US Chemical Safety Board (CSB) issued a report and a video explaining what caused the explosion that destroyed the Barton Solvents tank farm in Kansas in 2007: a float alarm with inadequate earth bonding.
The report and video also contain recommendations on preventing a similar explosion.
Barton Solvents was a wholesale distributor of solvents and industrial chemicals that used a modern tank-farm facility on the outskirts of a small Kansas town.
A delivery of VM+P Naptha (Varnish Maker’s and Painter’s Naptha) was pumped from several compartments of the road tanker, with the tanker properly earthed, using a bottom-entry delivery line to the storage tank.
Air introduced into the delivery hose between the deliveries from each tanker section caused some agitation and surface disturbances in the storage tank.
Within the tank, a centrally mounted liquid level monitoring device used a tensioned earthed metal tape to track a large metal float.
This tape was wrapped around a metal pin on the float, which also acted as the electrical connection.
Agitation of the liquid-surface caused the electrical connection between the tape and the float to become intermittent.
The float collected a significant amount of charge from the agitated surface of the insulating liquid.
Air bubbles and agitation mixed vapour and air layers, above the liquid surface, to create a flammable vapour mixture.
A spark between the float and the earthed metal tape caused an ignition, destroying the tank.
Several adjacent tanks also ruptured, with the subsequent fire destroying the whole tank farm and causing the nearest town, of 6000 people, to be evacuated.
The fire was contained in the earthen spill containment area surrounding the tank farm, but various tank-tops and vent-valves were propelled up to 400 feet from the site.
The CSB made many recommendations to help prevent this type of accident from happening again.
It was agreed that an intermittent electrical connection between float and tape was not sufficient: a separate bonding cable between the two was proposed as an additional safety measure.
The MSDS (material safety data sheet) for the VM+P Naptha did not specifically warn of the potential danger of flammable mixtures in the storage tank.
Unlike petrol tanks, where normally within the tank a fuel-rich atmosphere makes the vapour non-flammable, chemicals such as Naptha can create dangerous conditions inside storage tanks.
The CSB said the same problem can occur with cyclohexane, xylene, benzene, n-hexane, n-heptane, ethyl benzene, styrene and toluene.
Additionally, the site operators should consider various further approaches to reducing potential hazards with such liquids.
These might include: lowering the pump delivery speed to one metre/sec to reduce charge build up; use of an inert gas blanket in the ullage space of the tank during such transfers; and the addition of an anti-static additive to the chemical.
The video can be found at http://uk.youtube.com/watch?v=tVZzdtnZaJkandfeature=channel_page which also provides links to other CSB video reports.
Thanks go to Dr Jeremy Smallwood at Electrostatic Solutions in Southampton, which is the main UK centre for electrostatic hazard assessment, for highlighting this report.
Jeremy has collected further investigation reports and examples, which can be consulted via http://www.electrostatics.net.