What condition should be met for a double-walled tank to avoid air/soap testing?

For a double-walled tank, maintaining a vacuum in the interstitial space is crucial for avoiding air/soap testing. When there is a vacuum, it helps to ensure that the interstitial area is devoid of any air, which significantly reduces the chances of leakage or contamination. This vacuum creates a pressure differential that can be monitored, providing a reliable means of checking the integrity of the tank system without needing to resort to soap testing, which involves applying a soapy solution to check for leaks visually.

This condition is particularly beneficial because it is a proactive measure for leak detection. If a leak were to occur, the vacuum would be compromised, thus signaling the need for further inspection or corrective measures. This method ultimately improves safety and reliability in handling hazardous materials.

While pressurizing the tank, using special gauge models, or leaving the interstitial space empty may have their uses or implications within various contexts, they do not serve as effective means to eliminate the need for air/soap testing in the same way that a vacuum does. Hence, having a vacuum on the interstice stands out as the most effective condition for avoiding such testing.