Fire Hydrants

Fire hydrants are a network of piping installed underground around the building with hydrant stand posts utilized in the building industry to minimize life and property loss by using pressurized water. Building services engineers do incorporate a minimum of a hydrant system in every building site. 

They are installed in easily accessible areas for firefighters and are not obstacles to pedestrians and vehicles. In this note, the design should be incorporated with the site water supply network.

Building services engineers do categorize fire hydrant system into two based on the piping network.

The Wet Barrel Fire Hydrants 

The Dry Barrel Fire Hydrants

The Wet Barrel Fire Hydrants system is the most efficient hydrant system commonly use in temperate climes where hard freezing is not an issue. The valves in this system are located above ground and can be controlled independently.

The Dry Barrel Fire Hydrants is used in cold climes using a single operating valve located below the ground in the riser base. These available valves control all outlets simultaneously when turned on. 

How to Design a Hydrant System

Designing a hydrant system is based on empirical analysis by considering the building hazard classification, tandem with the system pipe sizing, and the climatic condition.

Water supply systems in residential areas should be designed to deliver no less than 1000 GPM (3785 L/min) at each hydrant. This volume should increase based on the buildings’ required fire flows being protected in commercial and multi-story apartment zones. If the necessary fire flows are several thousand GPM, the needed flow will usually have to be met by two or more hydrants flowing simultaneously.

Designing a hydrant requires several operational elements which include:

  • How much water (GPM or L/min) is needed for fire fighting.
  • How many and what size hose connections are required.
  • Issues of clearance and Visibility.
  • Operating characteristics of the hydrants.
  • Amount of head (static pressure) that is present in the system.

Considerations should be given to the hydrant accessibility, obstructions, proximity to structures protected, driveway entrances, and other circumstances when determining their position on the site plan.

Fire hydrant piping network

A typical fire hydrant branch has a 6-inch diameter and gate valve on the branch as close as possible to the main.  In a commercial and industrial project, they must be installed on a minimum of 8-inch main to provide adequate fire flow.

hydrant pipe

Every fire hydrant should be capable of delivering a minimum flow of 500gpm with a residual pressure of not less than 20 psi.

In an upshot, a building services engineer must consider the following while designing for a hydrant system

  • Hydrants should be position at an average of 45m to one another in light hazard buildings, and 30m for ordinary and high hazards risks.
  • Clearance of 1.5m to 15m should be ensured between the hydrant and any obstructions such as external walls, sanitary, storm sewer, and 1m minimum to any equipment or facilities.

Fire hydrants have two nozzles, a 2 ½ inch hose nozzle, and a 4 ½ inch pumper nozzle. Nozzle ends are equipped with protective caps.

Maintenance and Inspection of Fire Hydrants

Hydrant inspection and maintenance are essential to ensure that the hydrant is operational during a fire. Flushing must take place at least once per year to prevent sediment build-up and secure operation. 

Fire hydrant inspection program must be exercised annually while considering the Pressure and Flow test, Unique FH identification number, Visibility, and the condition at isolation valves.

Fire Hydrants Flow Testing

A flow test is conducted according to local building and insurance recommendations to identify individual fire hydrants’ capacity under normal demand conditions. Flow is determined using two fire hydrants and reading the flow from a pitot tube at the test hydrant and the residual pressure residual at a reference hydrant connected to the same main. The flow rating is then determined based on a reference pressure of 20 psi. 

NFPA further recognizes the often functional differences in service provided by hydrant systems. They thus specify that non-municipal hydrants nozzle and bonnet caps be painted a color that distinguishes them from municipal hydrants. 

hydrants nozzle and bonnet

Hydrant working pressure

The minimum working pressure rating of any fire hydrant should be 150 p.s.i.

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