Brussels Air: LAX to IAD to NAP Is 1.5h at IAD sufficient given change of planes, but both United?

It appears that you are flying into and out of Washington Dulles on a United Airlines plane (regardless of the codeshare). At Dulles, all United flights (excluding some United Express flights which this will not be) arrive and depart from terminals C and D - which are really just two ends of the one terminal.

Walking between any two gates within these terminals shouldn't take more than about 10 minutes. United officially states that you need to be at the gate for your departing international flight 30 minutes before departure (although realistically will not offload you until at least 15 minutes before), so that gives you around an hour which is plenty of time presuming your inbound flight is on time.

As both flights are on United, any checked-in luggage will be checked all the way through, so there is no need to collect it in Dulles, and no need to re-clear security. (Note that this is true even if you are booked on 2 separate tickets due to both flights being on UA)

The Minimum Connection Time (UA) wiki lists IAD Domestic-to-International as 35 minutes. 1:30 is plenty.