The thermal time constant of a big boiler, with its many parts, is long, minutes at least. The feedwater is already hot. You can cut the flames instantly but if you soon after shut off the steam outlet valves, you will blow a safety.
Blowing a safety valve is a very big deal. Expect a lot of paperwork.
I once saw a safety valve being tested. Fun, since I had nothing to do but watch. And listen. There were so many official witnesses they were hard to pack into the engine room.
The star of the show was the guy standing next to the valve with a wrench. He had a lot of interesting scars.