Pressure Vessel under cyclic loading

The analysed Molecular Dehydration vessel (SK-30-V-01 A/B) was designed to withstand a thermal cyclic operation
Internal Pressure 6.89 MPa as operating pressure applied to internall surfaces. And thermal Cylclic applied at surfaces of inlet and outlet nozzle as per data shown:

The fatigue analysis was carried-out in accordance with ASME B&PV Code, Section VIII Division 2 Appendix 5 (2004 ed). As cycling loading is due to the thermal and pressure, the result shown in the table above were used for the fatigue life assessment.

The output quantity is the stress intensity (SINT-MPa). This is calculated by the program as the largest of the absolute values of differences between principal atreses S1,S2 & S3 (where: S1>S2>S3):

SINT = max {│S1-S2│,│S2-S3││S3-S1│}.

Thus, this is equivalent to Tresca stresses: S = S1-S3

The alternating stress intensity was calculated based on the maximum difference between conditions operating and stan-by operation:

Srij = 375.063 – 256.89 = 172.057 MPa

Correction Factor for increased temperature = Eo/Et

Eo = 207 GPa ( as used for S-N curve in the code )

Et = 189 GPa ( at temperature of 315 ^oC )

Thus, Eo/Et = 1.09

Srij_corr = 1.09 x 172.057 = 187.542 MPa

And Sa = 0.5 Srij

So, Sa = 0.5 x 128.809 = 93.3771 MPa

From S-N curve in the Code, number of cycles is 50⁵ cycles (13670 years).

Based on ASME B&PV Code, Section VIII Division 2 Appendix 5 (2004 ed), the expected fatigue life of Dehydration Vessel is over 25 years, based on steady state analysis