CSB releases Barton Solvents safety video

18 August 2008

The US Chemical Safety Board (CSB) has released a case study and safety video on the July 2007 explosion and fire at the Barton Solvents distribution facility in Valley Center, Kansas. The CSB found the most likely cause of the explosion - involving what is known as a non-conductive flammable liquid - was a static spark resulting from a loosely-linked level-measuring float within the tank. The spark ignited the air-vapour mixture inside the tank as it was being filled.

Damage to Barton Solvents storage tanks after explosions and fire

Non-conductive flammable liquids can accumulate and maintain static electrical energy which discharges more slowly than from more conductive liquids. In addition, some of these liquids can form ignitable vapour-air mixtures inside storage tanks which can explode if a spark occurs.

The CSB released a ten-minute safety video which features a computer animation depicting the sequence of events that led to the explosion and fire.

CSB Board Member William Wark said, “Our goal is to help companies understand the hazards associated with the kinds of flammable liquids that were stored and transferred at Barton Solvents. We believe our case study and the safety video will help accomplish that goal and prevent accidents.”

The July 17, 2007 explosion and fire led to the evacuation of 6,000 residents. CSB investigators found that on the day of the accident a tanker-trailer arrived to transfer Varnish Maker's and Painter's Naphtha into a storage tank. The CSB determined that the transfer equipment from the truck tanker to the storage tank likely was properly bonded and grounded to prevent the generation of static electricity. However, the CSB found, the float device inside the 15,000 gallon storage tank presented a hidden danger.

CSB Lead Investigator Randy McClure said, “When transferring liquids, it is standard industry practice to bond and ground storage vessels, tankers, and other equipment to prevent static discharges. But our investigation illustrates how normal bonding and grounding may not be enough to prevent ignition from static electric sparks.”

Inside the tank was a device used for measuring the liquid level, a metal float linked to a metal tape measure. The CSB determined that a static electrical charge in the liquid was generated by the flow of the solvent pumped from the trailer into the storage tank, and by stop-and-start filling which introduced air into the liquid, resulting in bubbles and turbulence.

At the same time, the space above the liquid was being filled with an explosive mixture of vapour and air. The CSB determined that the liquid flow and turbulence created by the filling of the tank likely resulted in the metal float accumulating a static electrical charge. As the float moved, a gap is believed to have formed within the linkage of the tape and the float. CSB investigators said a spark likely jumped between the metal parts and ignited the explosive mixture of vapour and air that had accumulated above the liquid.

The explosion blew the tank 130 feet into the air, and within moments two more tanks ruptured and released their contents. As the fire burned, the contents of nearby tanks were released and ignited, launching debris into the air where some of it struck a mobile home and a neighbouring business.
Board Member Wark said, “Several common flammable liquids are particularly susceptible to ignition by static sparks. Some of these flammable liquids can produce the optimal amount of vapour to fuel an explosion at normal temperatures inside a storage tank.”

Wark continued: “While we found the most likely cause of the Barton explosion was sparking across the float linkage, we emphasise that explosions can occur in tanks without faulty floats when there is a discharge from the build-up of static in the non-conductive flammable liquid itself.”

“The accident at Barton Solvents emphasises the need for accurate and detailed Material Safety Data Sheets, said Wark. “We found that while most MSDSs for this category of flammable liquids do warn about the dangers of accumulating static electricity because the liquids are poor conductors, the MSDSs do not warn specifically that they can be ignited in storage tanks. Companies should be aware that some of these flammable liquids can form an ignitable vapour-air mixture inside storage tanks.”