What Turns On a Fire Pump?
A fire pump pushes more fire-fighting water into the building's sprinkler system. The pressure has to be kept up, so the controller for the pump is a fancy pressure switch.
Tall buildings are too high for water to be pushed to the top by the water tower. That means they require some sort of pump to get the water to the top in case of fire.
By Douglas Krantz
Fire Pump Controllers are Pressure Switches
Fire pump controller boxes are large, and the only people who should even open the controllers are authorized sprinkler service people. The thing is though, fire pump controllers are simple water pressure controlled switches.
Fire Pumps Push Water to the Top of Tall Buildings
To start with, it's obvious: if a building is taller than a water tower, the water pressure from the water tower isn't going to be enough to push fire drenching water to the top of the tall building. As a matter of fact, the water tower's pressure isn't even enough to keep the sprinkler system effective above the 4th or 5th floors.
Taller buildings, to get the water to the higher levels, need more water pressure at the ground level than what normal city water pressure provides. To get water to the upper floors of a building, with enough pressure to effectively slow down or stop a fire, the building has to use its own water pump (Fire Pump).
Main Fire Pump Pressure Switch
The purpose Main Fire Pump Pressure Switch of the main fire pump is to push quantities of fire suppressing water up to the upper floors of a building.
The turn-on for the main fire pump is a water pressure switch. When there's a fire, sprinkler heads are going to activate and spray water. This loss of water in the sprinkler system causes the pressure to drop. When the pressure drops, the main fire pump turns on.
Electric Fire Pump
Inside the cabinet for an electric fire pump is switch gear with big enough contactors (relays) to turn on and off the electricity to run the electric motor, and possibly switch it to an alternative electrical source.
There's also monitoring for off-normal conditions of electric power, and, on newer systems, a history readout.
Diesel Fire Pump Controller
Diesel fire pump controllers also have a battery charger and monitor extra points like fuel levels and engine performance. Newer systems also have a history readout.
But the main function of the diesel pump controller is to watch the pressure of the sprinkler system and to turn on the pump if the pressure starts to fall.
Delayed Turn Off of the Main Fire Pump
Of course, the main fire pump, electric or diesel, has a delay so the pump stays on for a while once it activates. There actually is a reason for this delay.
One way of looking at the sprinkler system is, if a single sprinkler head starts spraying water, the open sprinkler head is basically a small hole (about a centimeter in size) in the sprinkler system. When the hole leaks water, it diminishes water pressure in the sprinkler system.
When the pressure drops, the main fire pump turns on, pushing a lot of water into the sprinkler system. The main fire pump almost immediately brings the water pressure back to the normal stand-by pressure.
At this point, without the delay, the pump would turn off.
But then, because of the small water leak (one open sprinkler head), the pressure in the sprinkler system would drop again and the fire pump would shortly turn on again, just long enough to bring the pressure up again.
In essence, the fire pump would cycle on and off.
This cycling on and off can cause problems with an electric fire pump motor, and it's really bad for a diesel fire pump motor.
The main purpose of the delay is when a fire is small, the fire pump stays on.
Jockey Pump - - Maintains the Pressure at Stand-By
While the sprinkler system is in stand-by mode (no fire), even at the top of a tall building, water pressure needs to be preserved. The pressure is kept up using a smaller second pump, called a Jockey Pump
. It isn't good enough to use for fire suppression. As a matter of fact, it usually isn't even monitored.
The jockey pump's sole purpose is to keep the water pressure up to normal in the sprinkler system when the sprinkler system is just standing by, waiting for a fire.
The on and off of the jockey pump is exclusively controlled by a pressure switch. When the sprinkler system's water pressure starts to decrease, the jockey pump turns on to bring the pressure back up; when the sprinkler system's water pressure returns to normal, the jockey pump turns off.
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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