A double-walled tank shipped with which condition on the interstice need not be subjected to an air/soap test?

When a double-walled tank is shipped with a vacuum condition in the interstice, it does not need to undergo an air/soap test. This is because a vacuum creates a negative pressure in the interstitial space between the inner and outer walls of the tank. If there is any breach or leak in the inner wall, the vacuum will be compromised, and air will enter the interstice. The presence of air in a vacuum would indicate a problem, effectively serving as a leak detection mechanism without the need for additional testing methods like an air/soap test.

This testing is typically employed for other conditions, such as those that involve air or liquid, where the integrity of the double-walled tank must be validated against potential leaks. However, a vacuum inherently provides a reliable indication of the tank's condition, which is why it is exempt from the air/soap testing requirement.