CSB Calls For Improvements to MSDSs for Nonconductive Flammable Liquids
The U.S. Chemical Safety Board (CSB) released a case study and safety video on the July 2007 explosion and fire at the Barton Solvents distribution facility in Valley Center, Kan. The CSB found the most likely cause of the explosion -- involving what is known as a nonconductive flammable liquid -- was a static spark resulting from a loosely-linked level-measuring float within the tank. The spark ignited the air-vapor mixture inside the tank as it was being filled.
Nonconductive 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 vapor-air mixtures inside storage tanks which can explode if a spark occurs.
The CSB released a 10-minute safety video which features a computer animation depicting the sequence of events that led to the explosion and fire. The video, titled "Static Sparks Explosion in Kansas," is available for downloading free of charge at www.safetyvideos.gov.
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. Eleven residents and one firefighter sought medical attention. Fire destroyed the facility.
CSB investigators found that on the day of the accident a tanker-trailer arrived to transfer Varnish Maker's and Painter's Naphtha (VM&P 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 vapor 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 vapor 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 neighboring business.
Material Safety Data Sheets, or MSDSs, communicate hazard information on chemical products. The CSB determined the MSDS for the VM&P Naphtha did not adequately describe the explosive hazard or the precautions necessary to prevent ignition from static electricity. Most of the MSDSs for the flammable solvents supplied to Barton indicated that the solvent could accumulate a static charge, which could spark and ignite vapor. But the MSDSs did not warn that the solvent could form a highly explosive vapor-air mixture inside a storage tank.
The CSB reviewed 62 MSDSs for some of the most widely used nonconductive flammable liquids in industry, such as VM&P Naphtha, hexane and toluene. Most failed to recommend specific precautions beyond bonding and grounding.
The CSB issued recommendations to OSHA and others to improve required information contained in MSDSs to include addressing nonconductive flammable liquids which are routinely shipped to distributors such as Barton. The CSB also recommended that six major oil and chemical industry associations ask their member companies to improve the warnings on the MSDSs of flammable liquids because these materials can accumulate static electricity.
The board recommended companies handling the liquids should take additional safety measures, such as:
- Obtain more detailed additional technical information on the liquids from manufacturers that may not be found on MSDSs.
- Purge storage tanks with an inert gas to remove oxygen.
- Add anti-static agents to the liquids.
- Pump liquids more slowly.
- Verify that storage tank level floats are effectively bonded.