When designing a sprinkler system one of the first decisions a designer has to make is what type of sprinkler system should be installed. Types of sprinkler systems permissible by NFPA 13, Standard for the Installation of Sprinkler Systems, are wet, dry, preaction, and deluge. Other types of extinguishing systems, such as clean agent or water mist, are addressed by other standards. When selecting the appropriate sprinkler system type it is important to first understand the differences between the systems and then to understand how these differences can be beneficial, or detrimental, under certain conditions. Selecting the wrong system type can be costly.
Wet Pipe Systems
Wet pipe sprinkler systems are the most common. In this system the sprinkler piping is constantly filled with water. When the temperature at the ceiling gets hot enough the glass bulb or fusible link in a sprinkler will break. Since the system is already filled with water, water is free to flow out of that sprinkler head. Contrary to what Hollywood would have you think, not all sprinkler heads will operate at once in this type of system. The temperature around that specific sprinkler head needs to be high enough to break the glass bulb or fusible link that is holding water back. Once that happens, water will immediately start flowing from only that head.
Wet pipe sprinkler systems are the most reliable and cost effective. Therefore, they should be the first type considered when selecting a sprinkler system. However, there are times when a wet pipe sprinkler system may not be appropriate. One of the major factors in determining if a wet pipe system can be used is the temperature of the space to be protected. Will all areas of the building where the sprinkler piping is located be conditioned to at least 40OF (4OC) or greater? If the answer is yes, then there is no risk for the water in the piping to freeze and a wet system is the preferred method. However, if the answer is no, an additional study may need to be done to determine if an engineer can prove that although the temperature could drop below 40OF (4OC) it will never drop low enough for the water to freeze. If the temperature of the space cannot be guaranteed to eliminate the risk of freezing water, then a different system type should be chosen.
Dry Pipe Systems
Dry pipe systems are very similar to wet pipe systems with one major difference. The pipe is not constantly filled with water. Instead, the water is held behind a dry pipe valve usually some distance away from where the sprinklers are located. Like a wet pipe system, when the temperature at the ceiling becomes hot enough, the glass bulb or fusible link of the sprinkler breaks. However, in this case, water isn’t immediately available because the pipe is not water filled. Instead, air is released from the now open sprinkler head. This creates a drop in pressure causing the dry pipe valve to open and water to fill the system. Water will then flow from the open sprinkler head. Since there is a delay between sprinkler operation and water flow, the size of dry pipe systems is limited. The size limitation is intended to minimize the amount of time water delivery is delayed.
A dry pipe system is a great option for unconditioned spaces, or locations where the temperature of the space cannot be guaranteed to be high enough to prevent water in the system from freezing. It is important to note that a least the portion of the building where the water comes in and the dry pipe valve is located will need to have temperatures hot enough to prevent freezing.
Of all the sprinkler system types perhaps the most complicated is the preaction system. There are three different types of preaction systems, a non-interlock system, a single interlock system, and a double interlock system. The main difference between preaction systems and wet and dry pipe systems is that a specific event (or events) must happen before water is released into the system. This might sound similar to a dry pipe system, but the differences lie in what event triggers the release of the water:
For a non-interlock system: the operation of detection devices OR automatic sprinklers
For a single interlock system: the operation of detection devices
For a double interlock system: the operation of detection devices AND automatic sprinklers
To better explain how these types of systems work, we’ll walk through an example using a room that is protected with sprinklers fed from a preaction system. In addition to sprinklers, the room has complete automatic heat detection. Typically, the detection system, will have a lower temperature rating than the sprinklers. This will help ensure that the detection system activates before a sprinkler head operates. In this case, heat detectors that have a rating of 135OF will serve as our detection system, and the sprinklers will have a temperature rating of 165OF.
In a non-fire event, such as accidental damage to a sprinkler head that results in the glass bulb breaking, the system would fill with water in a non-interlock system, and water would flow from the broken sprinkler head. The same situation in a single interlock preaction system would not result in waterflow because the broken glass bulb will not trigger the system to be filled with water. Only the operation of detection devices will result in a water filled system for a single interlock system.
In the same room, the non-interlock and single interlock systems operate very similarly if there was a fire event. The heat detectors should activate first since they have a lower temperature rating. For both a non-interlock and a single interlock system, the activation of the heat detectors would result in the system filling with water. Then, if the temperature continues to rise, a sprinkler will operate. Since the “event”, heat detection, has already happened, the system is filled with water, and we would expect it to act like a traditional wet pipe system. In this same situation, a double-interlock system will not fill with water upon the activation of the heat detection. Instead, the system will only fill with water after the activation of the heat detection system and the operation of a sprinkler head. Therefore, a delay in water delivery similar to what is seen for dry pipe systems will occur. For this reason, double interlock preaction systems have similar size restrictions as dry pipe systems, whereas non-interlock and single interlock are just limited to 1000 sprinkler heads per preaction valve.
Additional considerations, other than temperature, may lead to the selection of another type of permitted sprinkler system. In some cases, there may be a desire to minimize the risk of water damage or to prevent the accidental filling of the system. In these cases, a single or double interlock system may be the preferred option. A single interlock system may be beneficial in museums, computer rooms, or similar settings where water damage is a concern. This would eliminate the risk of accidental water flow if a sprinkler head was damaged. Although NFPA 13 does not specifically prohibit the use of double interlock systems in these types of spaces, the double interlock preaction system was not developed for these situations. It was intended for use in freezer storage warehouses, or in similar situations where the accidental presence of water in the piping system will lead to expensive remediation. It is important to consider the delay in water delivery that occurs with a double interlock preaction system before selecting that system type. If it is used in a museum or similar type of environment, the delay in water delivery would allow the fire to continue to grow which could result in additional sprinklers opening. In turn, this could increase the water damage and result in a larger portion of the building being involved.
Deluge systems are similar to preaction systems in that they use another type of detection for operation. However, the biggest difference is that deluge systems use open sprinklers or nozzles. Instead of getting water flow from individual heads that have operated, once water fills the system, water will flow from every sprinkler head. Much like a preaction system, a deluge valve will keep water from filling the system until the operation of another type of detection system, such as smoke detection. Once that detection system is activated, water not only fills the system but flows from the open sprinklers or nozzles.
Another consideration in the selection of the type of sprinkler system is the level of hazard being protected. If protecting an area of very high hazard, such as aircraft hangers, a deluge system may be the most suitable.
Each system type has its own unique benefits. It is important to consider the pros and cons of each system type when selecting which sprinkler system is appropriate for your specific environment. An entire building may be protected with a combination of systems. For example, one of the more common designs in the Northeast is to protect the portions of the building that are conditioned with a wet pipe system and to use dry pipe systems in the attic and other unconditioned areas. Combining different types of systems for full building protection allows the designer to consider each unique environment and apply the most appropriate system type to that space without sacrificing what is best for other areas of the building.