Pressure vessels designed for the internal pressure are required to be subjected to hydrostatic test after completion of their fabrication. Two methods are provided in the ASME Sec. VIII Div.2 to calculate the minimum test pressure. One method is based on the Maximum Allowable Working Pressure of the vessel or the MAWP and the other is based on the highest permissible pressure of the vessel at the test condition. This latter pressure is also known as the ‘calculated’ test pressure or the pressure based on the Maximum Allowable Pressure in new and cold condition or the MAP. In either case, the induced primary membrane stress, Pm, shall satisfy the below condition
where, Sy is the yield strength of the material at the test temperature.

It is common to notice in many User’s Design Specifications (UDS), the requirement to test the vessels at the ‘calculated’ test pressure.

In the year 2007, major changes were made to ASME Sec.VIII Div.2. Some changes that had relevance to the hydrostatic testing were the change in the basis of the allowable stress at room temperature to the yield strength for most materials, increase in the factor on the test pressure basis to 1.43 and the corresponding increase in permissible limits on the induced primary membrane stress to 0.95*Sy.

It is noted in pressure vessels, built with cylindrical shells and(/or) with hemispherical dished heads with materials whose maximum allowable stresses at room temperature were dependent on the yield strength, that the membrane stress induced when subjected to ‘calculated’ test pressure exceeded the Code limit of 0.95*Sy. Increasing the thickness of the shell and(/or) head did not result in containing the stresses below 0.95*Sy as such an increase in thickness resulted in proportional increase in the MAP values and therefore, further increase of the test pressure.

The objective of this paper is to first derive the actual membrane stress induced in the vessel subjected to the ‘calculated’ test pressure and provide an engineering solution to the problem of induced stress exceeding the Code limit in order to achieve Code as well as the UDS compliance.

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