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November 19, 2011
Nonconductive flammable liquids: Bonding and grounding may not be enough to prevent a disaster

On July 17, 2007, 6,000 residents of Valley Center, Kan., were evacuated when a storage tank filled with varnish maker's and painter's (VM&P) naphtha, a material classified as a nonconductive flammable liquid, at the Barton Solvents facility exploded during routine transfer operations. A recent report released by the U.S. Chemical Safety Board (CSB), an independent federal agency that investigates industrial chemical accidents, found that the explosion was caused by a static spark resulting from a loosely linked level-measuring float in the tank. As part of its investigation, the CSB determined that the material safety data sheet (MSDS) for VM&P naphtha did not adequately convey the material's explosive nature and the precautions necessary to prevent ignition from static electricity.

After reviewing additional MSDSs for nonconductive flammable liquids, the CSB issued recommendations to federal OSHA and nonconductive flammable liquid manufacturers, requesting that information for MSDSs for the substances include improved warnings regarding hazardous static charges occurring not only when the materials are being transferred but also when these liquids are stored.

Static Electricity and Nonconductive Flammable Liquids

Static electricity is generated when liquids flow through pipes, valves, and filters as they are transferred. It can also be generated by splashing or agitation, as well as entrained water or air or the suspension of sediment from the bottom of a tank. This static electricity is generally dissipated quickly by most liquids. Nonconductive liquids, however, dissipate static electricity much more slowly and therefore pose an increased risk of static electric accumulation.

Static-accumulating flammable liquids include VM&P naphtha, cyclohexane, n-heptane, benzene, toluene, n-hexane, xylene, ethylbenzene, and styrene. (A more complete list of nonconductive liquids can be found in Annex B of the National Fire Protection Association's Recommended Practice on Static Electricity.) According to the CSB, normal bonding and grounding procedures used when transferring liquids may not be sufficient to prevent ignition from static electricity with nonconductive flammable liquids.

CSB Recommendations to Prevent Fires and Explosions

To reduce the potential of fire and explosion when handling nonconductive flammable liquids, the CSB recommends that workers follow these additional safety measures:

  • Obtain more detailed technical information on the liquids from the manufacturers that may not be found on the MSDS.
  • Purge storage tanks with an inert gas (such as nitrogen or carbon dioxide) to remove oxygen.
  • Add antistatic agents to nonconductive flammable liquids. Generally, antistatic agents need only be added at the parts per million (ppm) level. However, if materials are going to be used as part of a formulation, workers should first check to ensure the antistatic agents will not cause problems with the formulation.
  • Pump liquids more slowly to prevent static electricity buildup.
  • Ensure that storage tank level floats and any other tank appurtenances that could potentially cause a spark are effectively bonded.
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