A double-walled tank shipped with a specific condition on the interstice need not be subjected to an air/soap test. What is this condition?

A double-walled tank that is designed with a vacuum condition in the interstice provides a layer of safety and integrity monitoring. When the space between the two walls of a double-walled tank is maintained at a vacuum, it essentially means that the air pressure inside the interstice is lower than the atmospheric pressure outside. This vacuum condition serves to detect potential leaks because if there is a compromise in the outer wall, the air pressure can change, making it more likely that a leak would be detected through other monitoring methods.

Since the vacuum essentially protects the interstice and keeps it clean and dry, it negates the need for additional testing methods, like the air/soap test, that would typically be used to detect leaks. This indicates that the structural integrity of the tank has been ensured through its design, and thus, testing for leaks using air or soap is unnecessary under these specific conditions.

This understanding is crucial for professionals involved in UST installation and retrofitting, as it highlights the importance of monitoring interstitial pressures and ensures compliance with safety regulations while also simplifying maintenance procedures.