When does liquid surge tend to be at its worst?

Surge is worst at intermediate fill levels because the liquid has room to move freely while still carrying significant mass. A completely full tank cannot surge — the liquid has nowhere to go (and pressure relief valves prevent overfilling). A nearly empty tank surges less because the mass involved is small. The peak surge region is roughly 25% to 75% fill, with maximum often near half full. Loading practice for tank operations sometimes encourages either full or near-empty loads to minimize surge effects. Drivers who repeatedly deliver to multiple destinations face the worst conditions after the first delivery, when the tank may go from full to partially empty along the rest of the route.