We recently encountered cases where the corrected hydrotest pressure needed to be determined, and thought it would be useful to share the calculation process.  The cases in question were various tube ruptures potentially affecting a vessel in hot oil service.

API Standard 521, 6th Edition, provides a definition of the corrected hydrotest pressure (§3.1.22)¹  “Hydrostatic test pressure multiplied by the ratio of stress value at upset temperature to the stress value at test temperature”.  The standard explains further in §4.2.2:

Note that the corrected hydrotest pressure accounts for allowable stress differences of the material of construction between the overpressure scenario temperature (relieving temperature) and either the test temperature or the design temperature (usually higher than the test temperature).  Details and examples of the corrected hydrotest pressure are given in C.7.¹

A pressure vessel was constructed to ASME Section VIII using SA-515-70 carbon steel.  The vessel was constructed for a maximum allowable working pressure of 125 psig at a maximum design temperature of 700°F.  Its hydrostatic test pressure was 290 psig; however, this test pressure was performed at ambient temperatures.  For the tube rupture credibility criterion (API Standard 521, §4.4.14.2.1), we need to compare the potential pressure that the low pressure side may experience to the corrected hydrotest pressure of that low pressure side.¹  The vessel in question was in the low pressure side system of equipment and operated at temperatures near the design temperature, and thus a corrected hydrotest pressure was needed.

ASME Section II, Table 1A was used to obtain allowable stresses for the material of construction at various temperatures.²  Using API Standard 521 Equation C.41 (excerpt below), the corrected hydrotest pressure was found to be 262.5 psig.

• Hydrostatic Test Pressure (P_hydrotest) = 290 psi
• Allowable stress during hydrotest (σ_hydrotest) = 20.0 ksi (20,000 psi) at 100 °F
• Allowable stress during scenario (σ_scenario) = 18.1 ksi (18,100 psi) at 700 °F
• Corrected hydrotest pressure (P_corrected) = 262.5 psig

When looking for situations in which the corrected hydrotest pressure may apply, one clue is whether the reported hydrotest pressure is at the typical accumulation values; 50% for ASME Section VIII vessels constructed prior to 1998, or 30% for newer vessels.  If not, then look for design temperatures that result in lower allowable stress levels.  Another clue is the operating or relief temperature; temperatures greater than about 500 °F for the materials we encounter most often should be checked.

API Standard 521 Excerpt, Equation C.41¹