Use of New Allowable Stresses for Rerating or Determining Minimum Required Thickness of Pressure Vessels

By Ray Chao

The ASME Boiler and Pressure Vessel Code Section VIII, Division 1, 1999 Addenda (hereafter referred to as “code”), and Code Case 2290 provide higher allowable stresses than those given in the previous editions of the code. This has permitted rerating of pressure vessels that could not be rerated before, or allowed pressure vessels that have thinned below the originally specified corrosion allowance to remain in service. However, the pressure vessel designers should be cognizant of certain restrictions prior to undertaking the task of performing the necessary calculations for rerating a pressure vessel, or determining the minimum required thickness of a pressure vessel for continued service.

The new maximum allowable stresses of the code and Code Case 2290 are based on a safety factor of 3.5 on tensile strength, among the criteria used in establishing maximum allowable stresses. Since, in a majority of the cases, the design of a pressure vessel is governed by the tensile strength, the new maximum allowable stresses are about 14 percent over those given in earlier editions of the code, which were based on a factor of 4 on tensile strength. Accordingly, in these cases, one could expect at least a 14 percent increase in the maximum allowable working pressure (MAWP) if an existing pressure vessel is rerated using the latest edition/addenda of the code. Likewise, an existing pressure vessel may remain in service even if it has corroded beyond its originally required minimum thickness, if sufficient wall thickness remains after considering the higher allowable stress.

In rerating pressure vessels or determining the minimum required thickness of a pressure vessel, however, one must adhere to the rules of the National Boiler Inspection Code (NBIC). NBIC allows repairs or alterations of pressure vessels to be performed to the edition/addenda used for the original construction or a later edition/addenda most applicable to the work. However, in rerating or calculating the minimum required wall thickness of a pressure vessel using the higher allowable stresses of the code or Case 2290, several restrictions have been imposed by NBIC, as stipulated in NBIC Interpretation 98-14. Among these are:

The last of the above is the most restrictive as any vessels constructed prior to the 1968 edition/addenda of the code may not take advantage of the higher allowable stresses. Unfortunately, rerating and excessive corrosion often involve pressure vessels of pre-1968 vintage.

For vessels where the use of higher allowable stresses is permitted, special attention should be given to the design of flanges. The code rules on the design of bolted flanged connections are based on stresses without any considerations for flange stiffness. Nevertheless, flange designs in accordance with the code rules have generally been satisfactory. When the use of higher allowable stresses was permitted in Code Case 2290, it was stated that the maximum allowable stress values obtained from this Case are not recommended for the flanges of gasketed joints or other applications where slight distortion can cause leakage or malfunction. Indeed, the same concern existed for vessels designed in accordance with Division 2 rules where the use of Division 1 allowable stresses is required for the design of flanges. Although the 1999 addenda is silent on this issue, it may be prudent to continue the practice of using the lower allowable stresses for flange designs.

It should be noted that the allowable stresses for the commonly used pressure vessels steels are no longer the same up to a design temperature of 650°F as was the case before. In some cases, a reduction in maximum allowable stresses now starts at 500°F. Therefore, a design temperature of 650°F should not be specified if it is not required by the process. Also, those responsible for pressure testing of vessels should be aware that the 1.5 factor for hydrostatic testing and 1.25 factor for pneumatic testing have been reduced to 1.3 and 1.1, respectively, for vessels designed to the higher allowable stresses. It is important that a record of the design basis used for each of the vessels be kept and referred to each time a pressure test is contemplated so that the correct test pressure is applied.