PipingStudy 



B31.3 uses higher allowable stress values than B31.1 (B31.1 uses an allowable of approx 117 MPa for A53B whereas B31.3 uses about 137MPa ).  for the same material ASME B31.3 uses relatively less allowable stresses 
B31.1 has an SIF on reducers.  B31.3 doesn't use SIFs on reducers 
B31.1 includes torsional stresses in sustain case loading.  B31.3 neglect torsion in sustain case laoding 
B31.1 explicitly defines calculation methods for sustained and occasional loads  B31.3 calculation methods are undefined for sustained and occasional loads 
B31.1 Code allowable stress is based upon a factor of safety of 4  B31.3 Code allowable stress is based upon a factor of safety of 3 
the B31.1 Code has a narrow scope  Power Plants and District Heating Plants. It focuses on the steam  water loop.  B31.3 has the greatest width of scope of any B31 Pressure Piping Code. That is why B31.3 created several "fluid services", each with specific rules. 
B31.1 on the other is about 40 years.  B31.3 plants generally have a plant life of of about 20 to 30 years. 
B31.1 Power code is designed to have a higher reliability since safety of thousand people and industries will be affected.  B31.3 Process code,with a lower factor of safety relatively, a lower reliability can be tolerated 
we dont compute SIF separately in B31.1,for example in B31.1 does not recognize extruded welding tee nor weldolet. If its considered then validity of using the SIF from B31.3.  In B31.3 the SIF are computed separately for each component 
B31.1 uses a simplified one SIF approach (the maximum) for in and out plane .In the B31.1 Code, the SIF used is the LARGER of the two (inplane as compared to outofplane)and this larger SIF is applied to both of the bending moments occuring about mutually perpendiculat axes.  B31.3 uses two separate values.The B31.3 Code applies the inplane SIF to the Inplane moment and the outofplane SIF to the outofplane moment (all SIF's calculated per Appendices "D" of both Codes). 
B31.1 simply warns deterious effect of the corrosion when joined in cyclic loadings.  B31.3 instruct the user to remove the Corrosion Allowance from the Z before making sustained and occasional stress calculations. 
Notes:
1) ASME B31.1 is written similer and it stays parallel with Section I of the ASME B&PV Code on most issues.
2) ASME B31.1 & ASME B31.3 ,both Codes spell out thier intended scopes and their rules are "valid" for the intended scope.ASME B31 Pressure Piping Codes are "voluntary consensus Codes". When a local building Code (which is the law jurisdiction ) requires the use of one of the B31 Codes, Only then does it have "the force of law".Always check with your local juridiction to see what piping Code is REQUIRED by the law.
3) Code in words dont speak of factor of safety as 4 or 3 ,for exapmle:The basic allowable stress S in B31.3 is typically based on lesser of one third tensile strength and two thirds yield strength. So the 1/3 tensile at least gives a safety factor of three for S.
4) Listed are few differences .Close comparison of ASME B31.3 and ASME B31.1 you will find vast differences in materials and fabrication.
5)
i = "Inplane"
o = "Outofplane""
These terms relate to the direction of the applied moments. See Figures 319.4.4A and 4B in B31.3.
The magnitude of stress in a component depends upon the direction of the applied moment. For example, an inplane bending moment on an elbow will create a higher stress in an elbow than will an outofplane bending moment. B31.3 recognizes this difference by specifying different SIFs for the two moments. B31.1 takes a more simplified (and conservative) approach by specifying only one SIF (the greater of the two) for both moments (actually, the SIF applies to all three moments as B31.1 also intensifies torsional moments).