To prevent damage in the interstitial space, the inner tank should not be pressurized above what level?

Maintaining the proper pressure in the inner tank of a double-walled underground storage tank (UST) is crucial for preventing damage and ensuring the safety and integrity of the tank system. Pressurizing the inner tank above a certain level can create excessive stress on the structural components of the tank and its surrounding interstitial space.

Setting the maximum pressure at 5 psig is based on industry standards and practices, which aim to balance sufficient pressure for leak detection and system performance while avoiding the risk of damaging the tank structure or causing failure within the interstitial space. Going beyond this pressure could lead to distortions, potential ruptures, or leaks, endangering the integrity of both the inner tank and the surrounding environment. This limit is established to ensure reliability and safety in operation, thus highlighting the rationale behind the correct answer.