U3 ICC Tank Tightness Testing Practice Test

The interstice test relies on pulling a vacuum in the space between the inner and outer tank shells so that any leaks through seals or the interstice itself become detectable as a drop in vacuum or a rise in pressure. The required level is 15 inches of mercury because it provides enough differential pressure to reveal small leaks without overloading components. A lower level (5 or 10 inHg) may miss minor leaks, reducing test sensitivity. A higher level (20 inHg) could stress gaskets, joints, or the tank’s equipment and isn’t typically necessary for reliable detection. So, 15 inches of mercury gives the right balance of sensitivity and safety for accurately assessing interstice tightness.