Why Are There Waterflow Alarms When No Water is Flowing?
Unless there's some way of removing air when a building's fire sprinkler system is worked on, the air stays in the system. Air is compressible, so when water pressure goes up due to city water pressure changes, water does flow into the system.
Sprinkler systems are long pressure tanks. When water is drained out, they are full of air. When flooded with water again, the air usually does not have anywhere to go and just gets compressed, making the sprinkler system a long compressed air tank.
By Douglas Krantz
Between building trades, there's a natural line of demarcation. Sprinkler pipe fitters don't like working on a fire alarm system; they evacuate buildings. Fire alarm technicians don't like working on a sprinkler system; they make messes.
Building maintenance personnel don't always understand this line of demarcation, but when things go wrong, they do the best they can to figure out who to call.
False Waterflow Alarms
When the fire department arrives and the sprinkler system appears normal, but the fire alarm system keeps going into false waterflow alarms, it's natural to call the fire alarm company to fix whatever problems are causing the false alarms. At issue here, however, is that often times the cause of false waterflow alarms
is really air bubbles trapped in the sprinkler pipes.
Air Entering the Sprinkler System
Any time the sprinkler system is drained by the sprinkler company for maintenance, air fills up the pipes in the sprinkler system. Once the maintenance is finished, water is let in back to fill the pipes. The air, which is trapped inside the pipe, gets compressed.
Whereas the air will slowly bleed out of the pipes through microscopic cracks, this bleeding can take months. In the meantime, the compressed air is a problem.
Conflict Between a Real Fire and Compressed Air
Normally, when a sprinkler head is heated by a fire, a link melts to allow water to spray out. At this point, water pushes past the flow switch
When the city water pressure bumps higher (sometimes on a daily basis), and there's air bubbles in the pipes, water pushes past the flow switch to further compress trapped air.
Now, a flow switch cannot tell the difference between water flowing to a sprinkler head because of a fire, and water flowing to compress air bubbles. All it can tell is that water is flowing.
Conflict with the Air Bubbles
This temporary waterflow problem with trapped air is recognized, and is partially dealt with. To prevent the switch from activating because of trapped air, there's a retarder (damper) to delay the switch action. This delay is often :45 seconds. But too much trapped air and this delay may not be long enough to prevent false alarms.
However, there's another consideration to delay length. Because fires spread fast, and the alarm needs to be sounded quickly, most fire marshals don't want this delay to be longer than :45 seconds.
When there's too much trapped air, this all becomes a false alarm conflict.
The Fire Alarm Technician Arrives on Site
When it's the fire alarm technician that's called first, on arrival full diligence is needed
. Remember, the fire alarm technician has to give the building owners an effort because, after finishing, the fire alarm technician is still going to "hold-out-the-hand" for payment.
Ask about recent sprinkler maintenance history. If the system had been drained and refilled recently, that might lend a clue.
But check out the fire alarm system anyway. Look for intermittent shorts, broken or loose switches, lose or broken connections, and fire alarm panel troubles.
Finally, if no fire alarm issues can be found, the best a fire alarm technician can do is explain the compressed air bubble troubles to the building's representative so the explanation can be passed on to the sprinkler company, and then leave.
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms
. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.
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