Transfer valves were a component of the prototype Romulan drone-ship, a stealth vessel equipped with holographic technology on its outer hull.

When Commander Charles "Trip" Tucker and Lieutenant Malcolm Reed boarded the drone-ship, they manually opened the vessel's starboard transfer valve. Their actions resulted in a problem with one of the Romulan ship's fuel tanks. (ENT: "Babel One")

It is likely that the drone-ship also had a port transfer valve.