Water Flow, Backflow, OSY, Tamper Switch

Wet system (fire sprinkler lines) is constantly filled with water as opposed to a dry system such as a pre-action system (we will talk about these later). The water in the sprinkler line is monitored by a pressure gauge usually located near the main fire riser and waterflow switch.

Also you will find a tamper switch located in strategic places as to separate the system into different sections as needed for maintenance and or emergencies. These will be typically located on each level of a facility. There are sure to be additional locations but it stickily depends on the layout of the facility and sprinkler system.

Another key item to point out in wet system installations is the Backflow Preventer also known as an OS&Y (outside screw and yoke). This contraption reduces the city back pressure from causing a false alarm signal at the water flow device.

To make things simple for this post we will assume we have a water flow switch on each floor off the facility. The water flow device is mounted onto the main portion of the riser before any sprinkler heads. Attached to the water flow switch is a large flapper that is inserted into the riser pipe. Once the system is filled with water and is holding pressure, the water flow paddle holds firm.

How it all works:

Once a sprinkler head reaches its destruction point it pops and opens up the valve attached the the fire sprinkler system. Now that the stored water has a place to escape it pours out with extreme pressure to suppress the fire. While this water is pouring out, it needs to be replaced. With the water pressure coming into the sprinkler system the water is replaced one for one. Once this cycle starts, the flapper behind the water flow switch is activated just like a light switch. As required by NFPA 72 the Water Flow switch shall initiate an alarm within 90 seconds. This adjustment can be achieved by tuning a small dial on the water flow device itself. In case you are wondering, the 90 second delay is to help prevent nuisance alarms from leaks, water surges, city back pressure, etc.  Also located on the water flow switch is two sets of dry contacts. One of these sets will be used for the 120VAC switch leg to the sprinkler bell and the other is to be used for the connection to the fire alarm control panel for the purpose of alerting the occupants and central station dispatch.  This is a very simple explanation of how a wet fire sprinkler system works. Let me know if you would like more details.  It is also nice to note that the movies always have it wrong. They seem to think that if one sprinkler head bursts that they all go. Nothing could be further from the truth.

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