What must be completely open when filling or dumping a tank to prevent damage?

When filling or dumping a tank, the vents must be completely open to prevent damage. Proper venting is crucial for maintaining the correct pressure within the tank during these processes. If the vents are blocked or not adequately opened, pressure can build up inside the tank due to the incoming liquid or the displacement of air. This pressure buildup could lead to structural failure of the tank, leaks, or other hazardous situations.

Additionally, having open vents ensures that air can escape or enter the tank freely, allowing for a smooth flow of liquid without creating a vacuum or backpressure that could disrupt the operation or damage the tank. Proper venting not only protects the integrity of the tank but also promotes safe handling practices during the filling or dumping processes.