A dry sprinkler system is permanently charged with air and used to protect unheated buildings such as car parks or warehouses where there could be a risk of freezing temperatures. Water is held back by the dry installation control valve which is located in a warmer part of the building. When a sprinkler head operates and the air pressure in the system is released the dry valve opens and allows water to fill the pipe work and discharge from any open heads.
Pre-Action systems are designed for use in areas that are sensitive to water such as data processing or where sensitive goods are being protected such as irreplaceable books or documents. As with a dry system the pipe work is charged with air or an inert gas, this air pressure is monitored and the fire safety officer will be alerted if there is a drop in the standing pressure.
In a Type A installation:
A type A installation should be installed where considerable damage could occur if there were an accidental discharge of water. The opening of the installation control valve to allow water into the system is de-pendent on a fire signal from an independent detector such as a heat or smoke detector; this will activate a solenoid valve on the installation control valve allowing water into the system in anticipation of a sprinkler head operating. Once the system is charged with water it is ready to discharge water onto a fire if the sprinkler head is activated. Activation of a sprinkler head or damage to the pipe work alone will not result in water discharge from the system.
In a Type B installation:
A Type B installation is used where a Dry System is called for and the spread of fire is expected to be rapid. The installation control valve will open upon either receiving a signal from a detector or from a drop in standing air pressure due to a sprinkler head operating. This type of installation is in effect a Dry System but with the addition of the independent detectors to activate the system early. Unlike a type A system, any damage to pipe work or activation of a sprinkler head will result in water discharge from the system.
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