What Is a Pre-Action Sprinkler System?
A pre-action sprinkler system is typically a modified version of a “dry” system. Where standard sprinkler systems always contain water in the pipes just behind the sprinkler heads, a dry system only contains air until certain conditions indicating the presence of fire are met, at which point water is quickly pumped into the system to suppress the fire. A pre-action system utilizes an electrically operated valve — the pre-action valve — to hold back water from the piping until flames, heat or smoke associated with a fire are detected.
How Does a Pre-Action Sprinkler System Work?
A pre-action sprinkler system prepares to act when a fire detection system identifies a developing fire inside a facility and electrically opens the pre-action valve through the use of a solenoid switch, which allows water to flow into the piping system. (Note that in freezer warehouses and other below-freezing situations where the pipes must be kept dry unless fire suppression is definitively called for, a specific type of system known as a double interlocked pre-action system is typically employed to mitigate against accidental tripping of the fire detection system, as detailed in the “Types of Pre-Action Sprinkler Systems” section of this article.)
The types of fire detection systems that trigger the pre-action valve include:
- Thermal detectors, which react to temperature increases
- Smoke detectors, which react to combustion products or flaming/smoldering fire conditions
- Flame detectors, which react to a fire’s infrared radiation and/or ultraviolet light
Once the pre-action valve has opened, the pre-action sprinkler system acts much like a typical sprinkler system — when the temperature-sensitive solution contained within any individual sprinkler head detects heat at a predetermined temperature (typically 155 degrees Fahrenheit or higher, depending on the setting appropriate to a particular facility), the glass bulb holding back the water bursts and water flows through the sprinkler to extinguish the fire.
Types of Pre-Action Sprinkler Systems
In order to deliver the optimal performance for a particular facility, there are several types of pre-action systems available, including non-interlock systems, single interlock systems and double interlock systems.
The non-interlock pre-action system acts as a typical pre-action system when a fire detection system reacts before a sprinkler head does. In cases where a sprinkler operates first, the system responds like a dry pipe system to fill the pipes with water that exits through the activated sprinkler. Because the valve opens to allow water flow to the piping the moment that the detection system is tripped, a non-interlock system allows for very rapid response to fire — the moment that the sprinkler head reacts, water flows. The chief drawback of the non-interlock system is that if a sprinkler head is activated due to damage, the safeguard feature typically offered by a pre-action system is nullified.
A single interlock pre-action system centers around a deluge valve that is tripped by a fire detection system, which then fills the piping system with water that discharges through any activated sprinklers. This system introduces some protection against accidental discharge, as a broken sprinkler or pipe will not cause the valve to open — only the fire detection system can open the deluge valve and start the flow of water.
The most sophisticated pre-action sprinkler system is the double interlock pre-action system. In this type of system, which also uses a deluge valve, air or gas is kept pressurized within the sprinkler piping until such time as a sprinkler is activated. If this pressure drop is accompanied by a tripping of the fire detection system, then the valve will open to deliver water through the piping to the activated sprinkler or sprinklers. Should onlyone of these conditions be met, the valve will remain closed to prevent accidental tripping. The pressurized system offers the additional benefit of providing warning about any leaks in the piping.
Typical Applications of Pre-Action Sprinkler Systems
Pre-action systems are best suited to any area at risk for serious water damage due to damaged sprinkles and/or piping. Computer rooms, data centers, cell tower stations, telephone switching stations, server farms, telecommunications equipment and high-voltage electrical components must be protected from accidental water discharge. Similarly, artworks, libraries and archived paper records call for this additional protection.
In any type of cold or frozen storage facility, a double interlock pre-action system is a must to guard against accidental discharge of water and to ensure that water will flow in the event of a fire. While this type of system can be relatively costly, the price is still significantly lower than that which will be incurred through any accidental water discharge!
Pre-action systems offer protections not afforded by a typical dry system, which will quickly release water should a sprinkler head accidentally open. This factor alone should give any facility designer or manager pause.
An additional, oft-overlooked advantage of pre-action systems is that they ensure protection against the dangers posed by pipes going out of pitch. While no one ever sets out to put a pipe off of its pitch, which prevents it from draining properly after testing (trapping water that can freeze and expand inside the pipe), there are many situations in which this happens by accident. A construction worker or pest control specialist may sit on what appears to be a unmovable section of pipe while engaged in a task. An electrician may stand on a pipe to gain access to a higher area or hang components from it. Someone installing a security camera may place a ladder against a pipe. Even foundation shifting due to a building settling or seismic activity can affect pipe pitch. We have seen this type of incident occur time and again, with the ramifications becoming clear when wintry conditions set in and freeze the standing water, which expands and bursts through the pipe.
In both my own as well as Valley Fire Protection Systems’ experience, pre-action systems more than justify their costs due to their unparalleled ability to prevent accidental water discharge that can prove so devastating to facilities ranging from libraries and cultural institutions to cold storage warehouses to high-tech and financial businesses. As part of our ongoing mission to inform and protect our customers, I thank you for reading this piece and welcome your questions or feedback.
About the Author: Tom Hartel is President of Valley Fire Protection Systems, LLC, located in Batavia, IL. For further information, please visit www.valleyfire.com or call Tom at 630-761-3168.