Dry barrel and wet barrel are the two kinds of Fire Hydrants. The wet barrel fire hydrant holds a constant water supply, while the dry barrel fire hydrant needs to have a valve release to let water in. Dry barrel hydrants are used in areas where water can be freeze in winters.
Pillar Hydrants are designed to provide rapid access to water in the event that a fire breaks out. Fire hydrants are connected to a large water supply through underground pipes. When firefighters arrive on the scene, they connect a hose to the side of the hydrant, open the fire hydrant valve & stating to draw water for fire fighting.