There are two basic types of hydrants commonly used in distribution systems: 1) the DRY-BARREL type; and, 2) the WET-BARREL type. Most hydrants are of the dry barrel COMPRESSION type. See the figure below.
The major components of the hydrant are these: 1) the inlet; 2) the main valve; 3) the hydrant barrel; and, 4) the head.
The operating valve on dry barrel hydrants is located at the bottom. A drain hole located in the valve assembly makes it possible to drain all the water from the hydrant after use. This prevents water from freezing in the hydrant. On wet-barrel types of hydrants, the operating valve is at the outlet and for this reason these hydrants are limited to climates where freezing is not likely.
There are a number of fire hydrant types available depending on the level of fire fighting service needed. Two, three, and four nozzle arrangements are available. The two-nozzle, one pumper connection arrangement is the most common. Nozzles are usually 6.5 sm (2.5 inches) in diameter with pumper nozzles having a diameter of 11.4 cm (4.5 inches). Caps over the nozzle ends protect protect the threaded nozzle ends and safeguard the water system from contaminants entering through the nozzles.
Hydrants are generally installed with gate valves on the hydrant pipe lead. Hydrant connections off distribution mains are normally 15 cm (6 inch) diameter with the gate valve installed in the line. Valves should always remain open. Lengths of the connecting pipe should be minimized to reduce friction losses and, to a lesser extent, stagnant water conditions.
Some hydrants are designed for easy disassembly and allow the valve and stem to be lifted without removing the head. Break away features are offered on some hydrants. Break away flanges allow the hydrant head assembly to break away from the lower hydrant barrel. If hit by a car, for example, the break away flange and stem coupling shear and separate from the rest of the hydrant assembly. Repair to the hydrant is done by replacing the break away flange and coupling. See the “break-away flange” figure below.
