Fire protection is extremely important in all types of buildings, given the potential consequences of uncontrolled fire and smoke. Automatic sprinkler systems are among the best measures you can implement to ensure occupant safety and protect your property.
Based on their configuration, fire extinguishing systems can be classified into four main types, and the best option depends on the specific conditions of each project:
- Wet piping systems
- Dry piping systems
- Deluge systems
- Pre-action systems
This article will focus on dry pipe sprinkler systems, including their applications, advantages, and disadvantages.
How does a dry pipe sprinkler system work?
According to NFPA 13, a dry pipe sprinkler system is one that uses automatic sprinklers connected to a network of pipes with pressurized air or nitrogen instead of water. The pressure retains the water thanks to a dry pipe valve, which is a special type of valve designed for this application. Pressure is released when one of the sprinkler heads activates, opening the dry pipe valve and allowing water into the sprinkler piping. For comparison, a wet pipe system is permanently filled with water, which is released as soon as a sprinkler head opens.
Get a code-compliant sprinkler system design for your building.
Dry pipe sprinkler systems get their name because the pipe is typically empty of water, filling only when one or more sprinklers activate in response to heat.
- The dry pipe valve is designed to be pressure held and opens when the pressure in the sprinkler piping drops to 6 psi.
- Because the piping is dry, there may be a delay of up to 60 seconds while water passes between the valve and sprinkler. A disadvantage of dry pipe systems is that the fire can spread for a somewhat longer period of time before the affected zone is flooded with water.
- An alarm line is connected to the valve assembly and activates when the valve opens.
It is very important to keep the dry pipe valve and associated equipment in an area where the temperature does not drop below the freezing point of water. Otherwise, the supply may be blocked with ice and water will not flow even if the dry pipe valve opens.
Operating Procedure of a Dry Pipe Valve
Dry pipe valves are installed at the end of the sprinkler riser, separating the pressurized piping network from the water supply.
- As explained above, these valves are held back by a pressure difference – sprinkler piping generally has 5 times the pressure of the water supply to keep the valve closed. A typical system side pressure is 40 psi.
- On some dry pipe valve models, there is a small amount of priming water to achieve a better seal.
- There is an intermediate chamber with a water flow switch and an alarm port, which can be connected to a hydraulic motor gong on the exterior of the building. Typically, this section of piping is water-free and not pressurized.
- NFPA standards require pressure-type flow switches in dry pipe sprinkler systems. Reed-type switches are not permitted as the water flow is very strong when the valve opens and can damage the blades of reed-type switches.
The sprinkler system pressure must be reduced to 6 psi to open the dry pipe valve, but there is a low pressure switch that sends an alarm if the pressure drops 10 psi below the rated value. When this switch activates, it does not necessarily mean that a sprinkler has opened – the drop can be caused by problems such as leaks and compressor failure, and the system must be inspected to correct the problem before the dry pipe valve opens. NFPA 72 requires that air pressure be monitored in order to detect a change of 10 psi below normal air pressure.
There are also many accessories available for dry pipe valves, with functions that improve system operation.
- Accelerators help the dry valve operate faster, as its name implies. They redirect airflow in the sprinkler piping to reduce pressure more quickly, decreasing valve opening time. In other words, they reduce the time it takes to deliver water to the affected area after the sprinkler is opened.
- Air maintenance devices maintain adequate pressure in sprinkler piping, supplying more air when a small loss of pressure is detected.
- Air dryers remove moisture from inside sprinkler piping, as their name implies.
When is a dry sprinkler system recommended?
Because dry pipe sprinkler systems are not filled with water, they are the best option in applications with freezing temperatures. As stated in NFPA 13, these sprinkler systems are intended for applications where the ambient temperature cannot be maintained at a minimum of 40°F at all times. Dry pipe sprinkler systems can be commonly found in areas prone to freezing, such as the following:
- Unheated warehouses
- Exposed parking garages
- Loading docks
- Commercial freezers
- Unconditioned attics
- Water-sensitive storage areas
The main advantages and disadvantages of dry pipe sprinkler systems are summarized in the following table:
|
BENEFITS |
DISADVANTAGES |
|
1) Suitable for industrial and commercial spaces where piping is exposed to freezing temperatures. 2) The system is less vulnerable to rust, as under normal conditions there is no water inside the pipes. 3) Due to the lack of water in the piping, surface condensation and dripping are less likely. This is a key advantage in areas where water-sensitive materials are stored. |
1) Technical complexity: Dry pipe sprinkler systems require control equipment and air pressurization devices not found in wet pipe systems. 2) Increased cost of ownership: Dry pipe systems are more expensive due to their technical complexity, and are also more demanding in terms of maintenance. 3) Inflexible Design: The capacity of a dry pipe system cannot exceed 750 gallons, providing little flexibility for additions. 4) Slower response: As water must pass from the valve to the sprinklers, there may be a delay of up to 60 seconds after the sprinkler opens. Fire has more time to spread and cause damage. |
Testing and maintenance procedures
Because dry pipe systems have many interacting components that must respond appropriately when there is a fire, testing and maintenance are vitally important.
- The low pressure switch is normally tested with a lever-type bleed valve.
- The flow switch is usually tested by opening the alarm test valve (normally closed), allowing water to flow directly to the switch without opening the dry valve.
- If there is a shutoff valve between the dry pipe valve and the alarm pressure switch, it must be supervised in accordance with NFPA 72.
Conclusion
Dry pipe systems are more expensive to install due to their technical complexity. The pipes themselves must also have a specified slope angle to prevent water accumulation, and as a result, their installation is more labor intensive compared to a wet pipe system. However, dry pipe systems allow automatic fire protection in areas where a wet pipe system would be ineffective due to freezing.
NFPA 13 and other related standards are very demanding, but this is justified as they deal with fire protection. If you own or manage commercial properties in New York City, please note that Local Law 26 makes the installation of sprinkler systems mandatory for many occupancies by July 1, 2019. With a professional assessment, you can detect fire vulnerabilities in your property and also determine whether you are subject to LL26/04.
