Closed-loop return of cargo vapour from ship to shore (or shore to ship) during loading to prevent atmospheric venting.
Regulatory detail & full definition
Required at terminals where VOC emissions are regulated (US OPA-90 Section 4115; EU VOC Directive). The vapour return line connects via a flame arrestor and detonation arrestor; the ship's IGS pressure is balanced against shore receiver. Critical for chemical, MTBE, and gasoline cargoes.