Section 7.2 of the 2020 edition explicitly distinguishes between obstructed (beams, grating) and unobstructed structural members. If you design spray systems assuming unobstructed ceilings but your plant has open grating, the water spray will never reach the protected vessel.
API RP 2030 is primarily aimed at:
The primary goal isn't always to "put out" the fire, but rather exposure protection . It details how to apply water to surfaces to prevent "boil-overs" or vessel ruptures (BLEVEs). api rp 2030pdf