ASME Pressure Vessel Hydrostatic Test (UG-99) Calculator
Calculate shop hydrostatic proof test pressure per ASME Section VIII Div 1 UG-99(b), lowest stress ratio (LSR), pneumatic test pressure (UG-100), and test temperature limits.
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Mandatory Test Pressures
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Frequently Asked Questions
Why does ASME Section VIII Div 1 use 1.3 × MAWP for hydrostatic tests?
Prior to the 1999 Addenda, ASME specified a 1.5 multiplier based on a design safety factor of 4. When ASME increased allowable design stresses by adopting a design factor of 3.5 on tensile strength, the hydrotest multiplier was revised to 1.3 (1.3 / 3.5 ≈ 1.5 / 4.0), maintaining the exact same proof stress level on the vessel steel without exceeding yield.
Why is test water temperature strictly regulated by UG-99(h)?
At cold temperatures, ferritic carbon steels undergo a ductile-to-brittle transition. Under intense hydrostatic test pressures (130% of design), cold metal can shatter catastrophically via brittle cleavage fracture without plastic warning. UG-99(h) recommends maintaining liquid test temperature at least 30°F (17°C) above the vessel Minimum Design Metal Temperature (MDMT).
Why is pneumatic testing (UG-100) restricted to 1.1 × MAWP?
Compressed gas stores enormous compressible elastic potential energy. In the event of a seam tear, compressed air expands instantaneously like an explosive blast, posing severe lethal fragmentation hazards. Water, being virtually incompressible, releases nearly zero explosive energy upon rupture, allowing a higher test pressure (1.3×) with vastly superior personnel safety.