What is the hydrotest pressure per UG-99?

• A. MAWP × 1.25
• B. MAWP × 0.8
• C. MAWP × 1.0
• D. MAWP × 1.5

Answer: (A)

Hydrostatic testing checks that a vessel can hold its service pressure and stay leak‑tight, by applying pressure above the operating limit to reveal flaws without overstressing the structure. For vessels covered by UG‑99, the required test pressure is 1.25 times the MAWP. This means you push the test to a level higher than the maximum allowable working pressure to exercise the joints and walls, but still within the code’s allowable stress during testing. So, multiplying MAWP by 1.25 gives the correct test pressure. If the MAWP were, for example, 200 psi, the hydrotest pressure would be 250 psi. Other factors used in different contexts exist, but UG‑99 specifies this 1.25× margin.