Answer:

The focus is on the Axial Force column. 

 

There is a difference in the Axial Force reporting for Sustained Category combinations and Occasional Category combinations (Hydrotest combinations are created under Occasional Category):

 

1. For Sustained Category combinations (for example GR+Max P{1} in this case at node B13 of the model), the Axial Force column reports the combined effect of Longitudinal Pressure Stress effects and Gravity load case effects.
2. For Occasional Category combinations (for example Hydrotest-NL{5} in this case at node B13 of the model), the longitudinal pressure effects are not reported in the Axial Force column and simply combined axial force from Gravity and Pressure extension load case are reported

 

This is the reason for the differences in the Axial force for Sustained and Hydrotest combinations. However, the stress values for Hydrotest combinations are higher than the stress values for Sustained combinations:

 

                               ASME B31.3 (2016)  CODE COMPLIANCE              

               (Moments in N.m    , Force in N    )     (Stress in N/mm2   )                                   

 Point     Load                     Axial  In-Pl. Out-Pl. Torsion   S.I.F   Index/SIF  Eq. Load   Code   Code  

 name   combination                 Force  Moment  Moment  Moment  In  Out   Ax. Tor.  no. type  Stress Allow. 

 ------ ----------------------    ------- ------- ------- ------- ---- ---- ---- ---- ---- ----  ------ ------ 

 

 *** Segment B  begin ***                                                      

 

 B13    GR + Max P{1}             2248.08    6.32    1.55    0.35 1.00 1.00 1.00 1.00  23a SUST    5.90 132.12 

        Hydrotest-NL{5}             29.88    6.12    1.64    0.31 1.00 1.00            23a  OCC    8.97 217.19 

 

So in the stress calculations, the 1.6 factor is considered for both pressure extension forces and moments, as well as the longitudinal pressure stress when calculating Hydrotest stresses. It is just due to the difference in the reporting for the Axial column between the Sustained and Occasional stress categories which results in difference in values.