As I returned from the technical committee meeting for NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, I began talking to a gentleman at the airport who owns and manages several multi-family residential buildings throughout the United States. Upon sharing what I do for a living, he asked me matter-of-factly, “Why would anyone install a wet system when you could install a dry pipe system?” 

Thinking that a dry pipe system discharges a powder and not water (this would describe a dry chemical system typically seen in commercial kitchen applications), he believed this choice lowered his risk when tenants accidentally damage sprinklers and the system discharges. However, I explained to him that when the additional appurtenances, fluid delivery time requirements and higher hydraulic losses are taken into account, a cost-risk analysis may prove that the wet pipe system was, in fact, the right choice all along.

Outside of the fire protection industry, similar questions are often prompted by misconceptions and misinformed end-users. The portrayal of sprinkler system performance emphasized by the entertainment industry, media and other sources of public information providing exaggerated and incorrect information about sprinkler systems and how they operate perpetuates this problem. 

While this article focuses on preaction systems, my co-traveler’s question highlights the need to educate other technical professionals on the use, application and types of systems.

System Types

There are three types of preaction systems, and NFPA 13, Standard for the Installation of Sprinkler Systems, 2019 Edition (NFPA 13-2019) describes them in section 8.3.2:

8.3.2 Preaction Systems. 8.3.2.1 Preaction systems shall be one of the following types: (1) A single interlock system, which admits water to sprinkler piping upon operation of detection devices; (2) A noninterlock system, which admits water to sprinkler piping upon operation of detection devices or automatic sprinklers; or (3) A double interlock system, which admits water to sprinkler piping upon operation of both detection devices and automatic sprinklers.

Of the three types mentioned above, single and double interlock preaction systems are the most common. Noninterlock preaction systems are almost exclusively seen in the European Union. Therefore, this article will further focus on the single and double interlock preaction systems mentioned in NFPA 13-2019 section 8.3.2.1(1) and 8.3.2.1(3) as they are more prevalent.

Both single and double interlock preaction systems require the operation of a detection device to release water into the system. For individual interlock systems, a single detection device must operate, where double interlock systems require both the actuation of a detection device and a sprinkler (electric/pneumatic) or two detection devices (electric/electric) to operate before water being released into the system by the preaction valve (deluge valve).

System Applications

Preaction systems may be one of the most over-specified systems. A wet system may suffice where a preaction system is installed and a double interlock preaction system may be requested when only a single interlock system is needed to accomplish the intended objective of protection against inadvertent water discharge. Each additional action adds another mechanical or electrical operation, which may delay or prevent the sprinklers from operating as intended, thus reducing the reliability of the system. 

That’s not to say wet systems should always be used. Preaction systems do provide value to designers, owners and tenants who wish to add another level of surety that sprinklers will only operate in the event of a fire. They want to protect the valuable contents within a building that could be damaged by the output of water. 

Some occupancies, such as those containing historical content, critical communications systems or cold storage that may benefit from a preaction system include: Art museums; Cold storage facilities; Data centers; Federal Aviation Administration facilities (control towers); Laboratories using chemicals; and Paper records storage.

In these situations, a preaction system can be a valuable risk mitigation approach to help avoid the loss of the contents or the continuity of operations and communications, and increased maintenance costs involved with resetting a system exposed to freezing temperatures. A simple case of aquaphobia or hydrophobia should not be the deciding factor to move from using a wet system to a preaction system.

Detection Devices and Operation

General requirement. It is critical in the implementation of a preaction system to have appropriate detection devices at the ceiling level and other areas where detection would be needed. Per NFPA 13-2019:

8.3.1.6 Location and Spacing of Releasing Devices.

8.3.1.6.1 Spacing of releasing devices, including automatic sprinklers used as releasing devices, shall be in accordance with their listing and manufacturer’ specifications.

8.3.1.6.2 The release system shall serve all areas that the preaction system protects.

8.3.1.6.3 Where thermal activation is utilized, the activation temperature of the release system shall be lower than the activation temperature of the sprinkler.

Detection should activate before sprinklers for the system to work as intended. In data centers, unique airflow patterns may require detectors to be installed beneath cable trays between computer racks. If a pilot sprinkler system (a secondary system connected to a dry or wet pilot actuator) is being used for the detection of a fire, a lower temperature-rated sprinkler should be installed as the pilot. 

Where electric detection devices are used, their activation temperatures are required to be lower than the activation temperature of the sprinklers as well. The intent behind this requirement is to allow the preaction valve to trip, which pre-primes the sprinkler system piping with water before a sprinkler fusing, thus reducing water delivery time.

Refrigerated spaces. Additional precautions need to be made for refrigerated areas. The special requirements can be found in NFPA 13-2019, section 8.8. If dry pilot detection is utilized, compressed air in the refrigerated space could create condensation and freeze the line. 

Due to this risk, it is essential to install drum drips in low points and drain them daily. If an electronic means of actuation is chosen, heat detectors are limited to a fixed temperature type — unless the detectors are specifically listed for use in refrigerated areas: 8.8.2.8 Fire Detection for Preaction Release; 8.8.2.8.1 Detectors for Preaction Systems; 8.8.2.8.1.1* The release system shall be designed to operate prior to sprinkler operation, unless detectors meet the requirements of 8.8.2.8.1.2; (A) Detectors shall be electric or pneumatic fixed temperature type with temperature ratings less than that of the sprinklers; (B) Detection devices shall not be rate-of-rise type; 8.8.2.8.1.2 Where the system is a double interlock preaction system or single interlock preaction antifreeze system, detection devices shall be permitted to be any type specifically approved for use in a refrigerated area if installed in accordance with their listing requirements and NFPA 72.

Auto defrost in a freezer may set off a rate-of-rise detector in a nonfire event and cause the system to trip unnecessarily.

Cross-zone detection. One final misconception results from the confusion of cross-zoned detection with double interlocking actions. Cross-zones consist of two independently spaced detection arrangements tied into the same control panel. 

Upon the actuation of at least one detector in each zone, the control panel will send a signal to open a solenoid valve, releasing pressure on the preaction valve’s diaphragm, allowing water to flow into the system. Each detection zone must completely cover the protected space per NFPA 72.

Two types of control panels can be used for cross-zoned detection systems. A conventional panel (Figure 1) requires each detection arrangement to be wired on a separate circuit while addressable control panels (Figure 2) allow detectors to be programed on different zones on the same signaling circuit.

A common misconception is that a cross-zoned detection system is indicative of a double interlock preaction system. This is not the case. A cross-zoned detection system is only one event for detection purposes and can be used in all three types of preaction systems; however, crossed-zoned detection should be used sparingly since this further increases the time before water is applied to a growing fire and reduces the reliability of these systems.

Preaction systems can be an effective tool for a design engineer, but only if used for the right application. Just because a system has more bells and whistles than another does not necessarily mean it provides a higher level of protection. When it is practical, use a wet system. When it becomes necessary to provide additional actions to prevent accidental discharge, use the appropriate type of preaction system.