If you are studying a Fire Heater, then API 560 is the correct code to be used, as it covers this equipments (in your case table 8 & table 9 for manifolds).

As Mr. dclarkfive says, normally the manufacturer is the one approving loads (in projects, at the beginnig, we request the timesxAPI we will require for heaters).

Ask them how many timesxAPI that nozzle withstand. That will give you the approval.

Regards,

PG