Characteristics of No.6 Fuel Oil and the Related Fire Hazards of Welding

Number 6 fuel oil is a thick, syrupy, black, tar-like liquid. It smells like tar, and may even become semi-solid in cooler temperatures. No. 6 fuel oil, also known as bunk oil, bunker oil, or black liquor, is a petroleum product consisting of a complicated mix of hydrocarbons with high boiling points. It is a "leftover" product of crude oil after the more valuable hydrocarbons have been removed. Manufacturing companies use it as fuel for steam boilers and power generators. It is generally bought in large quantities and stored in large tanks, either above or below the ground. Since No. 6 fuel oil is so thick and viscous, users heat the oil up before they burn it. Heating makes the oil flow more easily, reducing pump demands and allows spray nozzles in the burner to turn the oil into a mist for maximum burning efficiency. The oil is typically heated to anywhere from 150 to 250 degrees Fahrenheit.

Cool or cold No. 6 fuel oil is surprisingly stable. Don't try this at home, but a burning match or a shower of sparks would probably not be sufficient to ignite the fuel oil. On the other hand, when No. 6 fuel oil is heated, problems can arise. Flashpoint is defined as the temperature at which a liquid gives off enough flammable or combustible vapors to sustain a flame. The flashpoint of No. 6 fuel oil is approximately 150 degrees Fahrenheit. If the vapors mix in the correct proportion to air, an ignition source as small as a stray welding spark can cause a violent explosion. The tanks blow up when flame from an accidental ignition source traces a path of vapor up into the empty space in a tank. This headspace is actually filled with flammable vapors if the contents of the tank are above their flashpoint. No. 6 fuel oil is a complicated mixture, and it sometimes contains hydrocarbons with flashpoints significantly lower than 150 degrees. For this reason, No. 6 fuel oil will sometimes give enough flammable or combustible vapors to start a fire when the oil is stored at ambient temperatures. When these vapors burn inside the tank, they produce enormous pressure. Such explosions have been known to tear storage tanks off their foundations, causing oil-spills and expensive fires.

Some Things to Look For
When investigating a fuel oil #6 tank explosion or fire (often both) that is associated with welding, there are certain characteristics to look out for. These guidelines will help you get a handle on exactly what went wrong.

1. Fuel storage tanks must have some way to vent out excess pressure to keep from exploding in case of fire. Most tanks have emergency pressure relief valves that open when the pressure in the tank gets too high. Some tanks will have intentionally weak seams between their roofs and sides instead. These seams are designed to break when pressure gets dangerously high. A tank like this will still have a small air vent so that it can be filled and emptied.
(Factory Mutual Engineering Corp., Loss Prevention Data 7-88, Storage Tanks for Flammable and Combustible Liquids, July 1976.)

2. Figure out whether or not the tank was heated, and if so, how high the temperature was. Remember that the flashpoint of No. 6 fuel oil is around 150 degrees Fahrenheit.
(American Petroleum Institute, API Publication 2216 Ignition Risk of Hydrocarbon Vapors by Hot Surfaces, 1991)

3. Regulations require that any fuel oil #6 tank that is heated over 140 degrees Fahrenheit must have a flame arrestor on its relief vent. A flame arrestor is a device with a network of small passages that prevent fire from entering a fuel tank. If the tank had no flame arrestor, or if the flame arrestor didn't work, then the tank design may not have been appropriate for its job.
(National Fire Protection Association, NFPA 30 -Flammable and Combustible Liquids Code, 1993.)

4. Inquire about the level of fuel in the tank before it exploded. A nearly empty tank will have a large headspace, resulting in a larger, more powerful explosion. Even a tank that is over half full has significant headspace.
(National Fire Protection Association NFPA 325 -Fire Hazard Properties of Flammable Liquids, 1985)

5. Some tanks have a floating roof design that eliminates headspace all together. The roof of the tank floats on the surface of the fuel. Often, these tanks will have a second, immovable roof as well to protect against rain or snow. Flammable vapors could still leak out and accumulate between the two roofs.
(National Fire Protection Association, NFPA 30 -Flammable and Combustible Liquids Code, 1993)

6. Measure how close the tank was to the nearest building. Section 30, chapter 2 of the NFPA codes contains very specific minimum distance regulations, based on the size, construction, contents, and pressure of the tank.
(National Fire Protection Association, NFPA 30 -Flammable and Combustible Liquids Code, 1993)

7. Obtain a copy of the hot-work permit issued for the welding job. This document is often very significant in a subrogation case. Chances are good that the cause of the accident was a violation of one of the basic safety rules outlined on the permit. Check to see that the form was filled out correctly, and whether or not all the necessary safety precautions were followed.
(American Petroleum Institute, API Publication 2009 -Safe Welding and Cutting Practices in Refineries, Gas Plants, and Petrochemical Plants, 1988)

A No. 6 fuel oil explosion and fire can cause a painful financial loss. The fuel oil tanks are usually quite large, expensive manufacturing buildings and machinery are generally located somewhere nearby. Welding on or near a heated tank is a major cause of these fires and explosions. Managers, supervisors, and the welders themselves share the responsibility to ensure safety before, during and after the welding job.

Suggested Reading
1. National Fire Protection Association, NFPA 30, 1993 Flammable and Combustible Liquids Code. Chapter 2.
2. Factory Mutual Engineering Corp., Loss Prevention Data 7-88, Storage Tanks for Flammable and Combustible Liquids, July 1976. Page 1, paragraph 2.b.
3. Underwriters Laboratories of Canada ULC-S603.1 M -Standards for Galvanic Corrosion Protection Systems of Steel Underground Tanks for Flammable and Combustible Liquids, 1982.
4. Amerada Hess Corporation, Material Safety Data Sheet for No. 6 Fuel Oil (all sulfur grades), January 18, 1994
5. National Fire Protection Association NFPA 325 -Fire Hazard Properties of Flammable Liquids, 1985
6. American Petroleum Institute, API Publication 2216 Ignition Risk of Hydrocarbon Vapors by Hot Surfaces, 1991
7. Oil Companies International Marine Forum, The Flammability Hazards Associated with the Handling, Storage and Carriage of Residual Fuel Oils, 1989.
8. Texaco Material Safety Data Sheet No.6 Fuel Oil Data Sheet, 1995