Fire hydrants, or plugs, are active protection devices that allow emergency services such as the fire brigade to quickly tap in to a local water supply.
A typical above-ground pillar hydrant consists of a cylindrical, capped and valved drum standing proud of the pavement. This drum acts as a portal between the below-surface mains pipeline and the emergency service’s hoses, governing the rate of flow.
Water is accessed via the hydrant with a special five-sided wrench, which allows the valve covers to be removed. Once firefighting hoses have been connected to the valves – of which there are typically three (one large main opening and two smaller, side-mounted subsidiary ones) – water is drawn up through manipulation of what is commonly known as the stem nut. This nut acts in much the same way as domestic tap handles, allowing water to be streamed slower/faster or stopped.
As hydrant water is sourced from low-pressure municipal sources – typically around 3.5-5.6 kilograms per square centimetre (50-80 pounds per square inch) – to gain the adequate propulsion it must be filtered through pumps, which are located on emergency vehicles like fire engines. This dramatically increases the water’s pressure, allowing for safer and more effective long-distance spraying.
There are two main types of hydrant: dry barrel and wet barrel. Dry-barrel hydrants are the more common and are so named as they don’t allow water to stay in the upper section (ie the drum above the ground). This prevents the water freezing when the temperature drops.