An automatic sprinkler system is a network of pipes and regularly spaced sprinkler heads normally located at roof/ceiling level that is connected to a source of water. In most configurations, the sprinkler head is sealed closed by a mechanism (typically a glass bulb) that is designed to release when heated to a set fixed temperature, allowing water to discharge, strike the sprinkler head’s deflector and form an umbrella-like spray pattern over the fire. Each sprinkler head operates independently. In most cases a flow switch is integrated with the system. When the flow switch detects the flow of water, it sends an electronic signal to the building alarm system which then sounds an audible alarm and notifies the fire department that the system has been activated.
Wet Sprinkler System
The wet sprinkler system is the most common sprinkler system. The sprinkler piping is always filled with water under pressure. When a sprinkler head activates, the water immediately discharges and continues to flow until the system is manually shut off. This is considered to be the simplest and most reliable type system, but must never be exposed to freezing temperatures.
Dry Sprinkler System
The dry sprinkler system is designed to protect unheated areas where there is a risk of water freezing in the pipes. The piping is filled with pressurized air or nitrogen which holds a remote valve, also known as a dry pipe valve, closed. This valve is located in a heated area and prevents water from entering the system until released. The release is triggered by a drop in air pressure when one or more sprinkler heads are actuated by the presence of fire allowing the pressurized air to escape. Water then enters the pipe, flowing through the open sprinklers onto the fire.
Preaction Sprinkler System
Preaction sprinkler systems are designed to protect water sensitive environments such as computer rooms, archival vaults, libraries and fine art storage rooms. A preaction fire sprinkler is similar to a dry sprinkler system given that water is not normally contained within the piping. The water is prevented from entering the piping by an electrically controlled preaction valve which is actuated by a smoke, heat or flame detector.
For the system to discharge, two independent actions must occur. First, the detection system must sense smoke/fire, thereby opening the preaction valve to allow water into the piping. Second, water is discharged onto the fire as each activated sprinkler head opens when the temperature rises above the set fixed temperature of each head. Since this system requires both actions to discharge, there is a delay for the water to be discharged thru the sprinkler head(s); however, the chance of an accidental discharge is unlikely. For this reason it is considered not to be as reliable as a wet sprinkler system. Also, additional maintenance is needed to service both the preaction valve and the detection system.
Some preaction systems are setup with a double interlock. This system has pressurized air or nitrogen in the piping. This prevents water from entering the piping if the preaction valve is tripped accidentally. Also, the piping can be monitored for leaks. This interlock system is commonly used in areas where freezing is possible such as freezer warehouses.
A foam system is generally used to protect aircraft hangars, petrochemical plants or any facility that stores flammable or volatile liquids. The system normally consists of foam concentrate, a foam concentrate storage container, water supply, proportioning equipment, distribution piping and discharge heads. This system is similar to a wet sprinkler system, but instead of discharging water, foam is discharged. The foam is produced when water and a pre-determined proportion of foam concentrate are mixed. The foam contains special agents that will extinguish the fire and will form a blanket to prohibit oxygen from “feeding” the fire. Also, the blanket serves as a vapor barrier to prevent re-ignition.
There are several types and concentrations of foam available. Every hazard is unique and will dictate the type of foam and hardware required.
A fire pump provides the needed water pressure to meet the hydraulic requirements of a sprinkler system when the local municipal water pressure is not sufficient. Most common applications include high-rise buildings, large warehouses where a relatively high terminal pressure at the fire sprinkler head to flow a large volume of water is required or where the fire protection water supply is stored in a ground level tank.
An electric motor, diesel engine or a steam turbine can power a fire pump. For the electric motor, a back up generator is generally required by local building codes.
A fire pump automatically starts when it detects a drop in water pressure of the sprinkler system. The drop in pressure is usually caused by a sprinkler head opening or if firefighting connections (valves) are opened.
To avoid accidental activation of the main fire pump due to pressure loss from minor leaks within the system, a jockey pump is installed. The jockey pump is connected in parallel with the main fire pump to keep the system fully pressurized during normal non-fire conditions. When a sprinkler head discharges or a fire department connection is opened, the main fire pump detects the rapid loss of pressure and immediately starts, thereby supplying the sprinkler system with the large volume of water needed. At that point the jockey pump is switched off.