Hydrocarbon Pipelines

You should find someone within your organization that knows what they are doing OR pay a third party independent consultant to verify.

Sorry to "avoid" the answer/question, but what you are asking is too specific/detailed/involved to rely on an internet forum.

Besides, with the very limited information that you have given there is no way anyone can give you an appropriate answer, other than - 1. Check the design code (mechanical) or 2. Possibly check with Crane TP-410 (fluid flow wise) 3. Confirm with P&IDs

If you are talking about pipe internal diameter only, (not material or schedule or anything else) then look at the velocity. Limit the velocity in your pipelines to below that specified in your standards.

Uhmmm... Check the submitted documentation for correctness?