Pipelines operate most efficiently when there is a steady, uninterrupted stream of product moving from refinery to market. If there are gaps between batches of product, pumps have to work too hard to keep the pipeline contents under pressure and moving forward.
Because a variety of refined products move through the same pipelines, some mixing occurs where the trailing end of a batch of one product meets the leading edge of the next batch of product in the line. If the two products are similar, such as different grades of gasoline, this "interface" can be incorporated into the lower-grade product.
When two dissimilar products, such as diesel fuel and gasoline, come into contact, the blended product is called "transmix." The transmix is collected separately, and then trucked back to a refinery for reprocessing.
In some cases, pipeline operators will separate especially sensitive products, such as jet fuel, from other pipeline contents using spherical plastic "pigs" to prevent transmix.»next