What percentage of operating pressure is used for the underground hydrostatic test?

• A. 50%
• B. 100%
• C. 150%
• D. 200%

Answer: (C)

In a hydrostatic tightness test, you raise the system’s fluid pressure above normal operating pressure to stress the components and reveal leaks. For underground tanks, the test is typically performed at 1.5 times the maximum operating pressure. This level provides a safety margin: it’s high enough to expose weak seals and joints, yet not so high that it risks damaging the structure or exceeding design limits. Using only 50% of the operating pressure wouldn’t stress the system enough to reveal latent leaks, and testing at exactly the operating pressure wouldn’t push the system to expose hidden weaknesses. Going to 200% could over-stress and potentially harm components, so 150% is the standard, balanced choice.