What must all tanks be outfitted with?

All tanks must be outfitted with pressure relief valves to ensure safety and compliance with regulations. Pressure relief valves are crucial because they allow excess pressure to escape from a tank when the internal pressure rises above a predetermined level. This is essential in preventing tank rupture or explosions, which can occur when gases or liquids expand due to temperature increases or other factors. The presence of a pressure relief valve helps maintain the integrity of the tank and protects the surrounding environment from potential hazardous leaks or spills.

The other options, while important components in various contexts, do not universally apply to all tanks. Vacuum relief valves are specific to situations where vacuums might form, double wall configurations are typically used for environmental protection rather than being a requirement for all tanks, and emergency shutoff valves, while critical for some systems, may not be necessary for every type of tank. Thus, the requirement for pressure relief valves stands out as a fundamental safety measure applicable across a broad range of tank types.