Which types of double-walled tanks do not require a pre-installation air/soap test?

The correct answer is that tanks with a liquid filled interstice do not require a pre-installation air/soap test. This is due to the design and functionality of these double-walled tanks. A liquid filled interstice is designed to provide a secondary layer of containment, which inherently offers a greater level of leak detection and environmental protection.

The liquid within the interstice can be monitored for changes in levels, which indicates the presence of any leaks. This feature negates the need for a pre-installation air or soap test, which is typically a requirement for tanks without such interstitial monitoring capabilities. Thus, these tanks have a built-in system to effectively manage the integrity of the tank without the need for additional testing prior to installation.

Other options, like tanks without a liquid filled interstice, tanks that are under 10 feet, and tanks located above ground, do not enjoy the same level of safety assurance and monitoring that comes with a liquid filled interstice. Therefore, pre-installation testing is crucial for these configurations to ensure that no leaks or integrity issues exist before they are placed into service.