When it comes to fire protection systems for buildings, there are three basic models. Firstly, there is the common, traditional wet-pipe model, which most buildings have these days. With this model, water is constantly under presser within the piping, closed off by the heads of the sprinkler, and is ready to deploy at any time. Secondly, there is the less common dry-pipe sprinkler system model. This model is used where temperatures drop below 40 degrees Fahrenheit – that is, close to where water may freeze – and does not contain any water in the piping itself. Rather, the pipe is charged with air under extreme pressure. The water is held back with a dry-pipe valve, and when fire or heat is detected by the sensors, the sprinkler heads pop open, thereby releasing the pressurized air, which in turn opens the dry-pipe valve so that water then flows through the piping and out the sprinkler. Now, pre-action sprinkler systems are similar to the dry-pipe systems just explained. That is, in this model, the piping is not filled with water, but is instead filled with air (often times nitrogen) under pressure. However, in pre-action systems the air pressure is normally less than in the dry-piping model.
Checks & Balances
In more detail, with a pre-action sprinkler system, two different actions must occur before the sprinklers discharge water. First, the system must detect a fire (normally by way of smoke or by way of heat) and then release the pre-action valve allowing the water that is held back to flow into the piping system. Once this occurs, the water is not yet discharged onto the potential fire, but rather a wet-pipe model system has essentially been reached. Secondly, each individual sprinkler head must then release to allow the water to flow out of the pipes into the building. Therefore, you might think of these pre-action systems are containing a few checks and balances before releasing potentially damaging water into the building. It is for this reason that these systems are generally used in areas in buildings where computers or other technology is largely present, or in museums and other buildings housing valuable objects.
Pros & Cons
Pre-action sprinkler systems thus come with advantages, but they also come with disadvantages. The advantage of them is, as mentioned, that two different operations must deploy before the water is released into the room, thereby limiting the chance of accidental discharge. For it is indeed possible, during a false alarm, for one of the operations to deploy, without the second one deploying, thereby saving the building from costly water damage. There are, however, a few disadvantages of pre-action sprinkler systems. Firstly, since two operations need to deploy for the water to be ejected there is a slightly higher chance that this system may not deploy water during a fire, as compared to other systems. Secondly, since this system contains the most complex components, purchasing and installing it will drive your costs up. And finally, since this system is one of the more complicated ones, modifying and upgrading an already installed system throughout the facility will likewise be a bit more costly than the other systems.