Crude oil. The black blood of the earth. The precious vital fluid on which all modern life depends. These days there’s more crude oil being transported on North American railroads than ever before, and most of it is sour. What’s sour crude oil? I’m glad you asked Timmy.
All crude oils are a mixture of petroleum chemicals, dissolved gases, and trace minerals. The qualities of a particular crude oil depend on the exact mix of fluids, gases, and other contaminants that make it up. The mixtures of trace elements in a crude oil depend on the underlying geology of the field from which it is extracted. In the petroleum industry, all these variables are condensed into four ‘flavors’ on two axes. Crude oil can be light or heavy, sour or sweet.
Crude oil is classified as heavy or light based on its density & viscosity. Light crude oils are less dense. They have less wax and more of the lighter hydrocarbon fractions, making it flow more freely and easier to pump and refine. Heavy crude oils are denser. They are thicker, waxier, and harder to pump out of the ground; they require more processing to turn into useable fuels.
Crude oil is classified as sour or sweet based on its sulfur content. Sour crude oil has more than 0.5% sulfur, sweet crude oil has less than 0.42% sulfur. Sulfur must be removed from crude oil during the refining process, and can be reclaimed and used as a valuable chemical feedstock. But the more sulfur, the more refining needed before you get useable fuels.
Oil from some areas is usually sweet (West Texas) and from others are usually sour (Middle East).
The thing about sour crude oil is that the sourness comes from high amounts of sulfur. Sometimes, depending on the chemistry, crude oils with high sulfur content can emit hydrogen sulfide(H2S) gas during storage. H2S is a flammable gas, and toxic to inhale. Crude oil from Canadian and North Dakota oil sands tends to be this kind of sour crude. When a load of crude oil has so much sulfur in it that it emits H2S, then a special hazard communication is required by 49 CFR 172.327.
“A Bulk packaging used to transport petroleum crude oil containing hydrogen sulfide (i.e., sour crude oil) in sufficient concentration that vapors evolved from the crude oil may present an inhalation hazard must include a marking, label, tag, or sign to warn of the toxic hazard as follows
(a) The marking must be durable, legible and … readily visible and similar to the illustration …with the minimum dimension of each side of the marking at least 100 mm (3.9 inches…. The width of the border forming the square-on-point marking must be at least 5 mm. The marking must be displayed at each location (e.g., manhole, loading head) where exposure to hydrogen sulfide vapors may occur.
(b) The border of the square-on-point must be black or red on a white or other suitable contrasting background. The symbol must be black and located in the center of the square-on-point and be clearly visible as follows:
(c) As an alternative …, a label, tag, or sign may be displayed at each location (e.g., manhole, loading head) where exposure to hydrogen sulfide vapors may occur. The label, tag, or sign must be durable, in English, and printed legibly and of a size relative to the package with a warning statement such as “Danger, Possible Hydrogen Sulfide Inhalation Hazard” to communicate the possible risk of exposure to harmful concentrations of hydrogen sulfide gas.”
This special marking for sour crude oil was only created a few years ago [January 19, 2011, 76 FR 3367
] to deal with all the sour crude being pumped in the Bakken Fields of Dakota and Alberta. The hydrogen sulfide off-gassing, in addition to being a fatal health hazard, can also damage the pumps and seals of a bulk packaging or storage unit at an accelerated rate. Telling transporters they’re carrying sour crude lets them plan their maintenance schedules accordingly.