Summary of the Barton Solvents Facility Tank Explosion CSB Report
- İsa Ersoy
- Oct 24, 2024
- 8 min read
According to a report published in 2008 by the Chemical Safety Board (CSB), an industrial accident occurred at the Barton Solvents facility in Valley Center, Kansas, on July 17, 2007, which resulted in a fire and explosion. This incident led to the evacuation of 6,000 people. CSB outlined the possible causes of the accident.
The report reviews the Barton Solvents accident based on the latest standards of NFPA 77 (Recommended Practice on Static Electricity) and NFPA 30 (Flammable and Combustible Liquids Code). NFPA 77 provides guidance on identifying, assessing, and controlling static electricity hazards to prevent fires and explosions. NFPA 30 offers fundamental safety measures for the storage, handling, and use of flammable liquids. The Barton Solvents accident is analyzed within the framework of these standards.
One of the most common causes of fires detected in over 20% of cases examined in this category is negligence. The Barton Solvents accident, which involved a non-conductive liquid, is an example of this. The fire and explosion occurred during the transfer of Varnish Makers and Painters (VM&P) Naphtha into a storage tank. The final report indicated that grounding and bonding were not sufficient to prevent this incident. A loose connection in the float gauge detached, interrupting the grounding at that point, which created a potential for sparking.
On the day of the accident, a tanker arrived at the storage area and began transferring the flammable liquid into a storage tank with a capacity of approximately 66 m³. During this process, air bubbles from the pump reached the storage tank. The metal float, loosely attached to the liquid gauge inside the tank, shifted during the transfer of the flammable liquid, creating a gap. Static electricity accumulated in the float created a spark in the gap, acting as an ignition source for the liquid vapor-air mixture, resulting in an explosion. The explosion in the tank caused the other tanks containing flammable liquids to explode as well.
Following the explosion, the liquid in the tank spread across the storage area, with tank fragments propelled approximately 40 meters into the air. Some of these pieces struck nearby residential areas. The explosion launched the VM&P tank into the air, leaving behind a cloud of smoke and fire; the tank landed about 130 feet away. Within minutes, two more tanks exploded, spreading rapidly increasing fires, which concentrated in the earth containment area surrounding the tank farm. As the fire raged, other tanks were either over-pressurized or ignited, sending steel tank tops (10-12 feet in diameter), vent valves, pipes, and steel debris flying off-site and into the nearby community. About 20,000 gallons of flammable liquid were released into the spill site.
The Barton Solvents tank farm contained 43 above-ground storage tanks with capacities ranging from 3,000 to 20,000 gallons and heights of approximately 15 to 40 feet. A Chemical Safety and Hazard Investigation Board (CSB) investigation concluded that the initial explosion occurred inside a vertical above-ground storage tank filled with Varnish Makers and Painters (VM&P) Naphtha. This explosion happened shortly after the transfer of the final compartment of a tanker trailer containing VM&P Naphtha into the 15,000-gallon storage tank.
VM&P Naphtha is a refined petroleum solvent, typically composed of 55% paraffins, 30% monocycloparaffins, 2% dicycloparaffins, and 12% alkyl benzenes, with a flash point of 20-55°F and a boiling point of 203-320°F. Classified as a Class IB flammable liquid under NFPA 77, it has a flash point below 73°F and a boiling point of 100°F or higher.
Non-conductive flammable liquids, such as VM&P Naphtha, can accumulate static electricity during transfer and storage. Static sparks can ignite flammable vapor-air mixtures inside storage tanks. Due to its low electrical conductivity, VM&P Naphtha can build up dangerous levels of static electricity, creating a risk of ignition, particularly during transfers or when the liquid flows through pipes, valves, and filters.
Flammable liquids most likely to form ignitable vapor-air mixtures during tank filling at ambient operating temperatures are typically designated as Class IB or Class IC under NFPA 30. Fires occur when an ignition source, like a static electric spark, comes into contact with an ignitable vapor-air mixture. Conductive liquids like gasoline may not form ignitable mixtures in storage tanks because the vapor-air mixture is too rich (lacking oxygen). However, VM&P Naphtha, along with other Class IB flammables, can produce these mixtures during normal handling temperatures. Static electricity can also be generated by entrained air or water, splashing or agitation, and the suspension of sediment at the bottom of the tank. Non-conductive liquids like VM&P Naphtha dissipate static electricity slowly, increasing the risk of dangerous static accumulation that can spark inside tanks.
Since VM&P Naphtha is capable of forming a flammable vapor-air mixture, there was such a mixture present in the vapor space (headspace) of the tank. The stop-start filling process, along with air and sediment in the transfer lines and possible water in the tank, caused a rapid buildup of static charge within the VM&P Naphtha tank. The tank contained a float for the liquid level measurement system, which was likely loosely connected and could have created a spark during filling.
The rate of static charge generation during flow increases roughly with the square of the flow rate. A liquid is generally considered non-conductive if its conductivity is less than 100 pS/m (picoSiemens per meter). The conductivity of the VM&P Naphtha involved in the Barton incident was measured at 3 pS/m.
To determine whether an ignitable vapor-air mixture was present in the tank during the explosion, the Chemical Safety Board (CSB) tested the VM&P Naphtha involved in the Barton explosion. Results showed that at approximately 77˚F (25˚C) — the handling temperature of the VM&P Naphtha at the time of the incident — the vapor space of the tank likely contained an easily ignitable vapor-air mixture. Energy from a static spark would have been sufficient to ignite this mixture.
Tank Level Float Design
The tank liquid level measurement system float used by Barton included a loose connection at the float/band junction, which could slightly separate, interrupting grounding and creating a potential for sparking. The Chemical Safety Board (CSB) concluded that the turbulence and bubbles generated during stop-start transfer pumping not only caused a rapid buildup of static charge but also loosened the measurement band's connection to the float, leading to a potential spark.
Electrical testing on a sample tank level float showed that a loose connection could generate a spark with enough energy to ignite a flammable vapor-air mixture inside a tank. Although the CSB identified the loose connection of the float as the most likely source of the spark, the possibility of a spark from a "brush discharge" could not be ruled out. Brush discharges occur between a charged liquid surface and a grounded conductive object, such as a dip pipe, metal component, or even the tank wall. These can occur even if the equipment is properly bonded and grounded.
Bonding and Grounding
Bonding involves electrically connecting conductive objects like tanker-trailers and transfer pumps to equalize their electrical potentials and prevent sparking. Witnesses in the Barton incident reported that the tanker-trailer, pump, piping, and storage tank were bonded and grounded during the event. However, safety guidelines indicate that bonding and grounding measures applied to typical transfer and storage operations may not be sufficient for non-conductive flammable liquids, which accumulate static electricity and dissipate it more slowly than conductive liquids, necessitating additional precautions.
Static Accumulation in the Pumped Liquid
Barton transferred VM&P Naphtha from three separate compartments within a tanker-trailer into the VM&P tank. Air pockets were introduced into the filling lines and were transferred into the tank when the transfer hose was reconnected to the tanker-trailer after switching compartments. Studies have shown that static electricity rapidly accumulates during pump operations when non-conductive liquids are transferred to storage tanks. In this case, static charge buildup was likely exacerbated by air pockets (bubbling) and the possible presence of suspended sediment and water in the tank. Additionally, the VM&P tank was approximately 30% full at the time of the explosion, producing a liquid surface potential (voltage) close to the maximum expected value during filling.
According to NFPA 30 Flammable and Combustible Liquids Code, Section 6, Fire and Explosion Prevention and Risk Control:
Explosion hazards must be evaluated if any of the following conditions are present: Class I liquids (FP < 100°F (37.8°C)) are handled, transferred, or used in quantities exceeding the Maximum Allowable Quantities (MAQ).
Operations that generate sparks, such as welding and cutting, are not permitted in areas containing Class I liquids (FP < 100°F (37.8°C)) until a written permit allowing such work is obtained.
Areas, including buildings with potential for Class I liquid spills, should be appropriately monitored.
The following methods are permitted:
Personnel observation or patrol
Equipment to monitor operations that may indicate a spill or leak
Gas detectors for continuous monitoring of areas where facilities are unattended
A written emergency action plan, consistent with existing equipment and personnel, should be developed to respond to fires and related emergencies, and should include the following:
Procedures to follow in the event of a fire or the release of liquid or vapor, such as sounding the alarm, notifying the fire department, evacuating personnel, and controlling and extinguishing the fire.
Procedures and programs for conducting drills for these procedures.
Appointment and training of personnel to perform assigned duties, including review at initial assignment, when responsibilities or response actions change, and when duties change.
Procedures for the maintenance and operation of fire protection equipment and systems, drainage and containment systems, and distribution and ventilation equipment and systems.
Procedures for shutting down or isolating equipment to reduce, mitigate, or stop the release of liquid or vapor, appointing personnel responsible for maintaining critical facility functions, or shutting down and safely restarting the facility process after isolation or shutdown.
According to Section 9 of the NFPA 77 Recommended Practice on Static Electricity:
Charge separation occurs in areas where liquids flow from pipes, hoses, and filters; where splashing occurs during transfer operations; or where liquids are mixed or agitated. The larger the interface area between the liquid and surfaces, and the higher the flow rate, the greater the charge accumulation. Charges mix with the liquid and are transferred to receiving containers, where they can accumulate. The charge is generally characterized by the bulk charge density and the flow as a current in the container.
According to Section 12 of the NFPA 77 Recommended Practice on Static Electricity Hazards in Bulk Storage Tanks and Tank Vehicles: A liquid flowing into a tank can carry a static electricity charge that can accumulate in the tank. This charge can be detected as a potential above the surface of the liquid in the tank. The maximum surface potential obtained depends on the charge density of the incoming liquid and the size of the tank. Precautions related to filling rates and flow rates in this section should be taken in situations where a flammable atmosphere may be present in the tank. Generally, the flow rate guidelines for properly inerted tanks will not need to be followed.
Measures
Companies that handle, transfer, and store non-conductive flammable liquids, such as naphtha, toluene, benzene, and heptane, must take additional precautions to prevent incidents like the one in Barton.
Additional manufacturer guidance should be requested.
An inert gas, such as nitrogen, can be added to the tank headspace. If done correctly, this will make the tank headspaces unable to support ignition from a static spark, effectively reducing the likelihood of a flammable event (explosion). However, since this practice can create oxygen-deficient environments within the tanks, extreme caution should be exercised when opening tanks for routine inspections and maintenance.
Loose connections in tank level floats should be replaced.
An anti-static material can be added.
The flow (pumping) rate may be reduced.
Materials that can act as static accumulators and generate flammable vapor-air mixtures in storage tanks should be identified, and a warning should be added for them.
Conductivity test data should be provided for materials that can create flammable vapor-air mixtures and act as static accumulators in storage tanks.
References
CSB, (2008), Barton Solvents Static Spark Ignites Explosion Inside Flammable Liquid Storage Tank, https://www.csb.gov/barton-solvents-explosions-and-fire/
NFPA 30 Flammable and Combustible Liquids Code 2024 Edition
NFPA 77 Recommended Practice on Static Electricity 2024 Edition
Static Electricity Incident Review Final Report, Ioana Sandu, Francesco Restuccia, Ph.D. Department of Engineering King’s College, London, UK, August 2021, NFPA Research Foundation
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