A water flow switch in an automatic fire sprinkler system is an electro-mechanical device that is installed typically in the main (above an alarm valve) or in a branch of the sprinkler system that is used to detect water flow and therefore the activation of a fire sprinkler head or remote test valve.
A flow switch is typically mounted on the side of a pipe where water flow is expected to be detected in the event of a sprinkler head activation. When water moves past the flow switch in the pipe the pressure of the water moves an integrated paddle (a component of the flow switch) in the direction of water flow. This movement activates a switch that triggers an alarm signal at a fire detection and alarm system.
More modern flow switches may also feature a retard mechanism that helps reduce the incidence of false alarms.
For the purposes of this article and the use of flow switches in an automatic fire sprinkler system, we will focus on two variations of a wet pipe sprinkler system;
The most common type of automatic fire sprinkler system used in commercial applications (as compared to residential apartments or nursing homes) is one where the fire sprinkler system is fitted with either a water-motor direct brigade alarm or a water pressure switch.
In these systems, the sprinkler system is fitted with an alarm valve that includes a water-motor direct brigade alarm or a pressure switch, either connected to the alarm valve (detects a rise in pressure) or a pressure switch connected to the sprinkler installation (detects a fall in pressure).
Where a water-motor direct brigade alarm or a pressure switch is fitted it acts as the primary signal for the activation of an signal that is transmitted to the fire brigade.
In these systems, a water flow switch may also be fitted a s a secondary alarm signalling device. In this circumstance the flow switch alarm activation signal is typically used to indicate the location of water flow in the fire sprinkler system. The signal may also be used to operate a local annunciation device such as an emergency warning system or trigger the operation of the heat and smoke control measures of mechanical ventilation systems.
The alternative to the aforementioned system arrangement is an automatic fire sprinkler system where one or more flow switches are used as the primary device to activate an alarm signal that is used to summon the fire brigade.
The two most common types of systems or applications of this arrangement are as follows;
In a Class 9c, Aged Care Building or Class 2 / Class 3 Residential Buildings, we find a wet pipe automatic fire sprinkler system installed and fitted with a flow switch as the primary alarm signalling device. In these scenarios there is no water-motor direct brigade alarm or alarm pressure switch fitted.
Where the building requires two or more automatic fire sprinkler systems due to its size or other requirements, then each automatic fire sprinkler system will be fitted with its own water flow switch.
The cost saving benefits of a Combined Hydrant / Fire Sprinkler System typically mean that these systems may not be fitted with a traditional sprinkler alarm valve. In these systems the building is fitted with a combined hydrant / fire sprinkler system water supply.
On each floor of the building or in each fire zone a monitored sprinkler isolation (stop) valve is fitted along with a flow switch. In this case, the water supply in the protected area may be isolated independently of the entire fire sprinkler system.
Under normal conditions when water flow is detected off the branch of combined main water supply serving the automatic fire sprinkler system an alarm signal is activated indicating;
These systems are very common in buildings such as apartment buildings.
In each of the aforementioned Class 9c, Aged Care Buildings, Class 3 Residential Buildings or Combined Hydrant / Fire Sprinkler Systems there are always exceptions to the rule, and these descriptions are general in nature and actual system configurations and operation from one building to another may change.
In summary, an automatic fire sprinkler system is fitted with a flow switch to detect the flow of water for one or each of the following two reasons;
It can be difficult for a person who is not familiar with the various types of automatic fire sprinkler systems in a building to determine the function of their flow switches.
In these cases, it may be beneficial to clearly identify the operation of a flow switch in the building. This can be done on the zone block diagram, with a label on the Fire Detection Control and Indicating Equipment ("FDCIE") or with a label on the flow switch.
One of the methods we employ is to use the background on the zone label on the FDCIE in a different colour with alarm signals that call the fire brigade. We typically use YELLOW background with black text on the zone label to signify a non brigade calling alarm signal where a brigade calling signal is a WHITE background with black text.
It may come as a surprise to people not familiar with the operation of a fire detection and alarm system that not all fire alarm signals automatically call the fire brigade. The most common of which are the following;
In these cases, the alarm signal is used for other purposes other than a brigade calling signal.
Australian Standard AS 1851:2012 sets out the requirements for the routine servicing of fire protection systems and equipment. In Table 22.214.171.124 - Six Monthly Routine Service Schedule, Automatic (Wet Pipe) Sprinkler Systems, the Standard specifies the following requirements for the routine inspection and test of a water flow switch;
|Item No.||Item||Action required and pass/fail requirement||Result||Pass/Fail||Comments|
|2.2||Floor/zone indication test (where fitted)||
Your fire protection maintenance contractor should be conducting a test of each flow switch and recording the results on a six-month basis. The results of these test may also indicate flow switches that are not working or others that may be too sensitive.
Flow switches that are not working correctly or that are too sensitive may be the source of unplanned nuisance) alarms, also known as false alarms.
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