It seems to me that the vendor is justified in saying that installing a mixer to the tank needs no special treatment. API 650 Code mandates a 40% increase of the end plate thickness and nozzle reinforcement for a mixer application. That should be adequate to buttress the mixer—-so long as the tank fabricator has indeed been told to comply with this requirement.

Having said that, it is important to mention API Codes do not invoke as much authority, for example, as ASME Codes. More often than not owners write revisions to supplement their own needs. Take the ExxonMobil’s Global Practices on API650/653 Codes, you will find there that nozzle stress calculations for a mixer are mandatory. Your situation fits.

So…just go ahead to make an Appendix P calculation in accordance with the Tank Program. If the dynamic loading is giving you some concern, always you can double the external mixer loads to account for the so-called Dynamic Load Factor (DLF). Then, your nozzle stress analysis should come out alright….I think!