What does interstitial space testing validate in double-wall tanks?

• A. It checks exterior paint condition.
• B. It validates the tank's structural integrity via the outer shell.
• C. It confirms the interstitial space is intact and can contain leaked contents without environmental release.
• D. It measures the tank's payload capacity.

Answer: (C)

In double-wall tanks, the space between the inner tank and the outer shell is a safety buffer called the interstitial space. This test checks that this space is intact and capable of containing any leaked liquid if the inner tank ever develops a breach. The goal is to ensure that leaked contents are confined to the interstitial area and can be detected before they reach the environment, providing a secondary containment function.

This is why the interstitial space test is about containment and detectability: it verifies the gap remains sealed and that any potential leak would be captured rather than released outside the tank system. Exterior paint condition or the outer shell’s structural strength is either unrelated to this containment function or addressed by other tests, and payload capacity has nothing to do with leaks between walls.