A liquid tight container typically installed at a point where product piping from an aboveground storage tank transitions to underground piping. Other forms of transition sumps may accomodate piping from an UST tank to AST generators, or for piping that resides only below grade.

The transition sump exists to contain any contaminants that may leak from any piping or their connectors and to isolate and protect metallic components or equipment from the elements.

See also PEI/RP300 and Tank top sump.