Carbon steel cylinder corrosion allowance 3mm, can the corrosion allowance of carbon steel pipe take less than 3?

Consult:
Carbon steel cylinder corrosion allowance 3mm, can the corrosion allowance of carbon steel pipe take less than 3?
The design institute will provide a blueprint for reaction kettle and process according to the map.
Figure tube corrosion allowance 3mm, a nozzle nominal thickness is 3mm
There are several takeover nominal thickness of 3.5mm.
Self calculation, take the corrosion allowance 3, all but.
We only received blueprints and no calculation books. How do we deal with this situation?