Thursday, May 16, 2013

Fire Sprinkler - Dry Pipe Systems

A deluge and pre-action fire sprinkler system are both part of the dry pipe family.  Dry pipe systems are typically installed in areas where water is subject to freezing such as unheated facilities and parking garages.  Unlike the wet pipe fire sprinkler system (filled with water) a dry pipe system is filled with compressed air from a source such as an electric compressor or nitrogen bottle.

Like wet pipe fire sprinkler systems, dry pipe systems also have a valve clapper and alarm port.  The dry pipe valve clapper is held closed by a pressure differential between air pressure and water pressure.  Supervisory pressure switches are typically used to measure the pressure on the system side.  These pressure switches can send either a low or high pressure signal to the panel.  The fire alarm control panel or pre-action panel will then turn on a pump or release pressure depending on the signal (low or high pressure).  This ensures that the correct amount of pressure is present in the system at all times.

In the event of an actual fire and a fire sprinkler head activates, the air pressure in the system is released.  When the air pressure drops to the trip point (usually around 10 psi below normal), the dry pipe valve opens and allows water to flow in from the main source to fill the distribution pipes and out of the activated fire sprinkler head.  Once the water is flowing through the clapper valve, the alarm port is exposed to the water.  At this time a pressure switch senses the increase in pressure and sounds an alarm.

Dry Pipe Valve

A dry pipe valve is located above the main system control valve and must be installed in areas not subject to freezing.  The whole purpose of the dry pipe valve is to ensure water does not enter the distribution piping and freeze.  It is a one way clapper that has a small amount of water covering it known as the priming water.  This priming water helps to seal the valve seat and has an immediate chamber that connects the valve to the alarm line.

Pressure Switches

There are alarm pressure switches and supervisory pressure switches both used in dry pipe fire sprinkler systems.  Both of these pressure switches monitor air pressure on the system side of the dry pipe valve.  The alarm pressure switch measures pressure change within a system and sends alert signals the the main fire control panel or pre-action system panel.  The supervisory air pressure switches monitor air on the system side of the dry pipe valve and perform two different functions.

1.  Monitor low air pressure and prevents accidental operation of the dry pipe valve due to low air pressure from an air compressor failure or leaks in the system.

2.  Monitor high air pressure to prevent damage to the clapper and gaskets in the dry pipe valve.

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