A hose is attached to the fire hydrant, then the valve is opened to provide a powerful flow of water, on the order of 350 kPa (50 lbf/in) (this pressure varies according to region and depends on various factors including the size and location of the attached water main). This hose can be further attached to a fire engine, which can then use a powerful pump to boost the water pressure and possibly split it into multiple streams. The hose may be connected with a threaded connection or a Storz connector. Care should be taken not to open or close a fire hydrant too quickly, as this can create a water hammer which can damage nearby pipes and equipment. The water inside a charged hoseline causes it to be very heavy and high water pressure causes it to be stiff and unable to make a tight turn while pressurized. When a fire hydrant is unobstructed, this is not a problem, as there is enough room to adequately position the hose.
Clapper valve
Most fire hydrant valves are not designed to throttle the water flow; they are designed to be operated full-on or full-off. The valving arrangement of most dry-barrel hydrants is for the drain gate valve to be open at anything other than full operation. Usage at partial-opening can consequently result in considerable flow directly into the soil surrounding the hydrant, which, over time, can cause severe scouring. A hose with a closed nozzle valve, or fire truck connection, or closed gate valve is always attached to the hydrant prior to opening the hydrant