Dry Hydrant

A dry hydrant turns a pond, lake or stream into something a fire crew can use: a fixed pipe, usually 15 cm (6 in) or wider, running from below the water surface to a capped standpipe on the bank where an engine couples a suction hose. It is called dry because nothing stands in it between uses. The component is only the pipe and its fitting; the pump, the hose and the crew belong to whoever responds.

Two things decide whether it will draw. The first is depth at the intake. The strainer is set against the level the water falls to in a drought, not its usual line, because the season a fire supply is wanted is the season the source is lowest: about 0.6 m (2 ft) clear of the bottom and 0.6 m below that low line, in a source holding at least 950 cubic metres (250,000 gallons). The second is the lie of the ground: an engine draws by suction and can only lift so far, so a pond far below the road cannot be drafted at all. Access decides siting more than distance from a house: a loaded engine must arrive in mud and snow as well as dry weather.

Most of what goes wrong is invisible. Silt eats the clearance under the strainer, so the hydrant loses its draw years before the pond looks shallow; depth is watched at the intake, not the deepest point, and slowing erosion upstream is the cheap defence. Because the pipe runs under suction rather than pressure, a joint leaking air breaks the prime with nothing to see; only drawing water finds it. A periodic test with the responding service is the real maintenance, plus back-flushing the strainer and keeping the road open. The pipe drains back to the source after use, which is what stops it freezing; a valve or low point added to ease priming undoes that. Installation is cheap next to most water infrastructure and often repays itself in lower insurance premiums; a portable pump on the same standpipe covers the time before a crew arrives.