API 610 specify allowable forces and moments for pump nozzles larger than 2" inclusive, but for nozzles less than 2" don't say nothing ¡¡...What do we do in this case ?...What allowables use for 1.5" diam. for example ? .....

Thanks

For nozzle sizes less than 2" you can safely take allowable loads as per 2" nozzle. That's what vendors usually give.

Regards.