1. In most places a Structural Engineer does not get involved in a Piping issue. You must be asking this question for a reason.
2. You (as an engineering contractor) do not get to choose which Code to use. You do however, if you are the Client and you are stipulating the Code with which your Project is to be designed to.
if I do the stress analysis and the piping was for say refrigerated or cryogenic lines. The 31.3 provides materials and stresses down to -425. The 31.5 is similiar. So can I use either code or is there certain stipulations requiring one over the other
Check with the local jurisdiction (State or Province) to be sure what is required. The ASME B31 Codes for Pressure Piping are voluntary consensus Standards until they are mandated by the Building Codes of the local jurisdiction IN WHICH THE PIPING SYSTEM WILL BE CONSTRUCTED - then of course, they have the force of law.
Each of the ASME B31 Codes for Pressure Piping includes a very explicit scope. If you are designing a refrigeration piping system you would use the ASME B31.5 Code for Refrigeration Piping. If you are not designing a piping system associated with a refrigeration system, you could (with the owner's permission) use the ASME B31.3 Code for Process Piping assuming that the scope of B31.3 does not explicitly exclude your system.
