Sunday, January 6, 2013

New Flow Alarm Switches may need to be designed for use with Variable Speed Fire Pumps

When water pressure is low in an area and a large sprinkler system is being used often times you must use a large Pump to boost the pressure and flow rate of the available water to a sprinkler system.
Just like the Fire dept must hook up a fire engine to a hydrant at a working fire for adequate water flow .
Controller for Fire Pump made by Firetrol
There are two types of Fire pumps for sprinklers available Diesel powered  combustion engine  pumps and electric pump's backed up by a diesel powered generator. They are monitored continually for any problem that would prevent them from operating such as a switch misplaced , loss of power , low fuel etc.  and are run weekly for 15 minutes  and exercised to keep them ready.

Typical diesel fire pump
The pumps are automatically brought on line when they sense a flow of water in the sprinkler system  and stay on line  for as long as needed to fight the fire.

Flow switch showing internal components Manufactured by System Sensor

Flow switch with cover in place
When a sprinkler head activates beside the fire pump coming on the buildings fire alarm also comes on and activates as well starting up horns and strobe lights and sending a signal direct  to the authorities or thru a  3rd party central station . In a wet system where the pipes are always filled with water the fire alarm activates by means of a flow switch as pictured above made by System Sensor   a pretty simple device which is strapped to the pipe a small hole drilled into it and a flexible plastic flapper inserted which unfolds  and then senses water movement which causes the flap to move which activates a electromechanical switch in the flow mechanism. To prevent false alarms as water pressure can go up and down in a community depending on daily usage activity's.  a  Mechanical Retard Mechanism is used and can be set from 0-60 seconds the water must flow and move the paddle and handle  for the set time period before it activates an effective way to prevent false alarm trips .
 But that is where the problem starts recently on a job we ran into to a problem with a new generation  variable speed pump.
In older systems the pumps where one speed  period and you had a steady continual flow which was not always a good thing some times you could hit a situation where you had too much or too little flow depending on how many heads activated and how much water was available . To solve this problem  they recently  developed variable speed pumps  which match the flow rate to the volume of water needed.  But when we went to test recently  a brand new multiple wet and dry pipe system in an extremely large distribution warehouse  the first Wet Stand Pipe activated as designed . when the inspectors test port was opened at the end of the piping system which is designed to simulate one head activating the pump came on with in 5 seconds and started providing flow then with in 30 seconds the fire alarm activated ,perfect test. Then on to Wet pipe 2# the pump comes on and a minute and half goes by no fire alarm activation? OK whats going on we know this flow switch was working because when the pipes where first filled it went into alarm. So after checking several things we found out that on pipe 2#  the small flapper handle in the flow switch was not staying in position
as the first test and was modulating up and down with the pump as it went up and down in speed. the problem this wet pipe 2# was  much smaller piping set up than other system and was allowing more variation in the pump speed. To get around the problem we set the retard mechanism to 10 seconds  and the alarm sounded the Fire Marshal was worried and rightly so .What if off only one head went off on a weekend when facility is closed  and switch did not sense it ? The Sprinkler Tech ,Fire Marshal  and myself  running and witnessing  the test have never in 30+ years we each have in experience in the field run across this before but now that its a possible problem it needs to be addressed and possibly a new switch and standards put in place to prevent just such a scenario from happening.
While variable speed pumps make sense there could be very major water damage problems should a sprinkler  head fail and activate start flooding the building and fire alarm not sense it and while any time the pump runs it starts a supervisory signal to central station it could be misinterpreted as just a pump test  and just logged instead of some one getting a call to find out whats going on. UL and NFPA need to be looking into this matter but we will see as usual how long it takes for them to respond to a very dangerous potential happening . By the way I now they read this blog.

Once again we have a situation where we bring new technology on board with out thinking thru all its ramifications 

Wet Pipes 1# 2# at far left Pre -Action  Dry Pipes 1#2# at Right

1 comment:

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