Connected pipe with different hazard level

[818what]

In a plant there are always lots of small pipe branch off from a big pipe. In my understanding if the there is a separation valve between the small pipe and the big pipe, then the hazard level can change after the valve, so the small pipe can be a lower hazard level. If the small pipe is in hazard level E then theoretically this section don't need design verification and inspection. However in piping design of the big pipe(say in hazard level C or B) we still need to account for the loading from the small hazard level E pipe. I am trying to save the cost on design verification so my question is: can I model the big pipe up the valve, and account for the loading from small pipe by:
a). Build a full pipe model, obtain the force and moment at the separation point, and then insert them into the truncated model at the same point? Like we get the nozzle load from pipe stress analysis and then do WRC 107 & 297 for vessel nozzle. or
b). Extend the small pipe a little bit, say after some constrains in X, Y, Z directions, further pipe with have limit effect on the connection so I can stop somewhere.

Thanks for your opinion.

 

[KevinNZ]

You need to agree on the method with your design verifier. We tend to do your b) option.

 

[818what]

Thanks KevinNZ!