Most states say you must be at least 15 feet away from a fire hydrant when parked.
So if you park closer than 15 feet near a fire hydrant or park in front of a fire hydrant you can be ticketed and for good reason because in case of a fire the fire hydrant needs to be accessible by the fire fighters and if you are parked in front of a fire hydrant the firefighters can break your window and run the hose through your vehicle to attach it to the fire hydrant.
Some places may allow you to park as close as 10 feet to the fire hydrant but most states say you must park 15 feet away from all fire hydrants and the streets should have signs near the fire hydrants stating how far away you're allowed to park.