Thursday, May 16, 2013

Fire Sprinkler - Wet Pipe Systems

With fewer components and less installation time, a wet pipe fire sprinkler system is the most common system you will come across.  A wet pipe fire sprinkler system has a series of piping systems or branches that are fed from the main riser.  The term wet pipe system comes from the fact that the fire sprinkler system is always filled with water.  Behind each sprinkler head is water waiting to be released upon activation.  With that said, you will not typically see wet pipe fire sprinkler systems in areas exposed to freezing temperatures such as parking garages, etc.

The operation of a wet pipe fire sprinkler system is rather simple.  The sprinkler heads contain either a fusible glass bulb or metallic heat sensor.  Heat from a fire will cause the fusible glass bulb to burst or the metallic heat sensor to separate which then opens the small valve on the fire sprinkler head causing water from the piping to discharge and extinguish the fire.

Wet pipe fire sprinkler systems are a fixed temperature type installation, meaning each sprinkler head is is activated only if exposed to a predetermined temperature as low as 155 degrees F or as high as 286 degrees F.

Below are three types of wet pipe fire sprinkler systems with additional information on their operation.

Wet pipe fire sprinkler system with and Alarm Check Valve

When water enters the fire sprinkler system it becomes non-potable and unsuitable for drinking.  To keep this water from entering back into the city water system, an alarm check valve is used.  An Alarm Check Valve is a small and simple device consisting of a spring, valve clapper and an alarm port.

Withing the alarm check valve, the spring holds the valve clapper closed allowing water to flow in only one direction.  Once the system is filled, the water remains in the piping until an individual or multiple sprinkler heads are activated.  Once a sprinkler head is activated, water is discharged and needs to be replaced within the piping.  In order for this to happen, the valve clapper opens allowing the water to flow in to the system and out of the activated sprinkler head.  As the alarm port becomes exposed to water pressure, it causes a water motor gong, mechanical horn or audible/visual device to sound.  A more common way to initiate an electric bell or fire alarm horn/strobe, is a water flow device.

To prevent city water surges from causing false alarms, a retard chamber is used.  A retard chamber is a metal reservoir that holds the water during a brief city water surge.  A drip valve at the bottom side of the reservoir allows the water from the surge to slowly drain out.  If a true fire activates a sprinkler head, the drip valve is unable to drain the water fast enough causing the flapper on the water flow device to move thus activating an alarm condition.

Wet pipe fire sprinkler system with Maintained Excess Pressure

In this type of wet pipe fire sprinkler system, excess pressure is pumped in just above the alarm check valve to hold the clapper down even during a city water surge.  This type of wet pip fire sprinkler system eliminates false alarms caused by city water surges.

There are two ways to measure pressure loss in this type of system.

1.  Connect a pressure switch to the system above the alarm check valve.  If as sprinkler head is activated, the pressure switch will detect a loss in pressure and sends an alarm.  Usually a supervisory pressure switch is used to monitor if there are leaks within the system.

2.  Connect a pressure switch to the alarm port on the alarm check valve.  If a sprinkler head is activated, the alarm check valve opens.  When the valve opens, the alarm port is exposed to the incoming water supply.  The pressure switch will detect a change in pressure and will send an alarm.

Wet pipe fire sprinkler system with a straight pipe riser

This type of system will have a straight pipe riser located downstream from the check valve or backflow preventer and are the most common type of wet pipe system.  The vertical supply pipe or riser, does not have a retard chamber or alarm port like others.  Instead you will find a riser check valve.

A riser check valve utilizes a vane type water flow switch to monitor water flowing within the wet pipe fire sprinkler system.  When continuous flow of water equal to 10 gallons per minute occurs, the water flow switch activates an alarm condition after a predetermined amount of time.  This time delay can be selected on the actual water flow device.  This time delay is to help eliminate false alarms caused by city water surges.

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