When tanks are filled with liquid and lie above the waterline, what should the cross-connection valve be?

When tanks are filled with liquid and are positioned above the waterline, having the cross-connection valve open is critical for ensuring a proper balance of pressures and levels within the system. When the valve is open, it allows for the equalization of pressures between the tanks and any adjacent spaces, which can help prevent issues such as overpressurization or creating a vacuum that might lead to structural failures.

If the valve were closed, it could result in pressure differences that could ultimately compromise tank integrity, lead to overflow, or create dangerous situations if a tank became too full or empty. Keeping the valve open enables a controlled flow and ensures that liquid can move freely between connected tanks or compartments, maintaining overall vessel stability and safety while in operation.

The other options would not provide the necessary conditions to prevent various operational issues related to tank filling and pressure management.