Cryogenic piping systems usually have vapor stops installed at all of the following locations except:

• A. Expansion joints
• B. Valve outlets
• C. Control box interruption points
• D. Solder joints

Answer: (C)

Vapor stops are placed at points in cryogenic piping where the pipe joints or connections could allow vapor to cross from one segment to another. Expansion joints, valve outlets, and solder joints each represent a potential leakage path in the flow path, so installing a vapor stop there helps contain vapor and maintain the isolation between sections as temperatures change or as pressure differs. A control box interruption point, however, is an electrical/service location, not part of the sealed fluid path, and does not present a cryogenic leakage pathway that would require a vapor stop. That's why this location is not typically equipped with a vapor stop.