Operation of wet pipe sprinkler systems and fire detection systems

Operation of the typical wet pipe sprinkler system and a fire detection system are discussed in the following paragraphs.

1. Wet pipe sprinkler system operation. From figure 7-1 it can be seen that city potable water serves as the source for fire water. A shutoff valve is provided in the fire riser just inside and above the building floor. Above the shutoff valve is a double check valve installed to prevent back flow into the city water system. This valve is followed by a shutoff valve with outside access having an indicator to indicate whether it is open or closed. The valve is normally open with a chain and padlock applied to prevent tampering. A tamper switch is also installed to sound an alarm if and when the valve is tampered with by unauthorized personnel. Above this valve is an alarm check valve which permits flow to the sprinkler system and has connections for a drain, pressure gages and the retard chamber which operates the water flow motor driven alarm placed outside of the building. Test connections, shutoff valves, check valves, vents and drains are located as required to flush and test various components of the system on a schedule established with NFPA 25: Standard for the Inspection, Testing and Maintenance of Water Based Fire Protection Systems. One leg of the tee allows fire water to be provided by a set of fire department connections located outside of the building. A check valve is incorporated to prevent backflow through the fire department connection. The fire department connections are redundant to the city water supply and provide another source of water for the wet pipe sprinkler system. The riser then connects to branch lines which serve sprinkler lines. In the sprinkler lines are fusible link sprinkler heads located as required by the NFPA, local codes and the insurance company. The fuse is usually chosen to melt at 135°F, however other temperatures are available. The number, type and location of sprinkler heads is determined by the fire occupancy rating. This rating considers the type of structure, the materials present in the area, the number of personnel and the liability resulting from a fire. In the wet pipe system, heat from a fire will melt the fuse in the sprinkler allowing water to flow from that head. The purpose of the water is to slow the fire until professional firefighting assistance can arrive. Also provided in the sprinkler lines are a high point vent and a drain at the far end of the system. The vent allows the system to relieve air to assist with draining the sloped sprinkler and branch lines and to assist in filling the system with water. The drain at the far end of the system permits verification of water flow and flushing of the system.
2. Fire detection system operation. The fire detection system uses smoke detectors and heat detectors to detect fire in a facility. Heat detectors are either fixed temperature or rate-of-rise. Fixed temperature detectors are activated when the sensor inside the detector reaches a set temperature. This temperature is normally 135°F for common areas and 195°F for mechanical or boiler type rooms. Rate-of-rise detectors are activated when temperature rises to a set temperature within a set period of time. An electrical signal interruption from these detectors, pull stations, tamper switches on fire riser valves and/or flow switch on the fire riser is interpreted by the control panel and processed. The panel will activate local alarm/strobe lights mounted inside and outside of the facility, and an alarm at the fire department. The panel face displays the location and type of sensor which sent off the alarm. The backup power supply includes a solid-state automatic switch, battery and charger.