The ASME Code Section VIII, Division 1, does not have mandatory rules for the calculations of vessel component thickness for hydrostatic test condition. The requirement for such calculations comes from the customer in its vessel specification. As the test is for short duration at ambient temperature, the customer specification often requires that no components of the vessel to be subjected to above the 90% of the material yield strength during the test.
The pressure due to liquid head is always added to the design internal pressure and the total pressure is used to calculate the thickness in the UG-27 (shell) and UG-32 (heads). The Code allowable stress value at the design temperature is used in the formula. The operating liquid may not be always full of the vessel. For example, consider a tall distillation tower, where liquid level of sp. gr. greater than water is at the maximum just below the lowest down comer, which could be only one-sixth or less of the tower height.
In the hydro test, the vessel is filled with water (sp. gr. 1.0) at room temperature. Using the allowable stress at the design temperature would be too conservative and design will be uneconomical. The test condition is very short and the allowable stress used in this case is from 0.8Sy to 0.9Sy or as per the client specification. The ASME Code Section VIII, Div. 1, does not have a mandatory allowable stress value in this case.
A typical calculation procedure is to design the vessel for its specified design conditions (UG-22). After establishing component final thicknesses and the hydrostatic test pressure (UG-99), compute the stress level in each component for the test pressure plus the applicable hydrostatic head using the Code respective formula with un-corroded (new) thickness. Compare the computed stress value with the 90% of the yield strength (or as specified by the customer). Increase the thickness of a component if necessary. If you are using a computer program like COMPRESS or PV Elite, option is provided for such calculations.