Here’s How the Colonial Pipeline Carries Multiple Fuels at Once

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One less-explored aspect of the unfolding Colonial Pipeline Co. ransomware disaster is how a single pipe is able to carry premium gasoline and regular gasoline and home heating oil and jet fuel and diesel fuel. You’d think the fuels would mix into an unusable soup, but they don’t. The savings from being able to use one pipe instead of five to carry five types of fuel are enormous. 

Colonial’s website has a diagram that explains how it works. Fuels are put into the pipe one at a time. A batch of regular gasoline might come right after a batch of premium. When the fuels come out the end of the pipe and are put into separate storage tanks, the cutover from one tank to the other is staged to protect the quality of the higher-grade product. Premium can’t have any regular gasoline mixed in. On the other hand, it’s all right to have a little premium mixed into the regular. Similarly, home heating oil and jet fuel are similar enough that they tend to be run down the pipe consecutively.

Other fuels are so incompatible that it’s impossible to cut over from one to the other. The blend at the boundary between them is called intermix. When intermix—also known as transmix—comes out the end of the pipe, it’s “stored separately and re-processed into a useful product,” Colonial says. The Environmental Protection Agency has detailed rules on which fuels can be mixed into which other fuels and in what quantities.

The ideal amount of mixing of incompatible fuels in a pipeline is, naturally, zero. “Transmix generates a lot of cost to shippers and pipeline operators because it must be stored, transported, and reprocessed to decontaminate and separate the mixed products,” says XOS, a unit of Danaher Corp. that makes analysis equipment. “Pipeline operators have reported transmix costs of many millions of dollars per year.”

For physics buffs, one fascinating aspect is that turbulent flow in the pipe helps keep the fuels separate. That’s surprising because ordinarily turbulence promotes mixing. The reason is that when the flow in the pipe is turbulent, there’s little difference between the speed of flow at the center of the pipe vs. closer to the walls. It moves along relatively uniformly. In contrast, in smooth “laminar” flow, the speed is much higher at the center than toward the sides, so the fuel in the center of the pipe races ahead and penetrates the batch that’s ahead of it. Fortunately, it’s almost impossible to maintain laminar flow in a fuel pipeline. Keeping a fuel pipeline pressurized during a shutdown also helps minimize mixing of adjacent fuels, according to a manual for military personnel from GlobalSecurity.org.

The ability of Colonial and other pipelines to carry multiple fuels is a fascinating—albeit well-established—bit of technology. It’s a pity that it takes a crisis like this one to bring attention to it.

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