Fire protection is extremely important in all types of buildings, considering the potential consequences of uncontrolled fire and smoke. Automatic sprinkler systems are among the best measures you can deploy to ensure occupant safety and to protect property.
Based on their configuration, fire sprinkler systems can be classified into four main types, and the best option depends on the specific conditions in each project:
Wet pipe systems
Dry pipe 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 the NFPA 13 standard, a dry pipe sprinkler system is one that uses automatic sprinklers connected to a piping network with pressurized air or nitrogen instead of water. Pressure holds back water thanks to a dry pipe valve, which is a special type of valve designed for this application. The 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 one sprinkler head opens.
Get a code-compliant sprinkler system design for your building.
Dry pipe sprinkler systems get their name because the piping is normally empty of water, only filling up when one or more sprinkler heads activate in response to heat.
The dry pipe valve is designed to be held back by pressure, and it opens when the pressure in the sprinkler piping drops to 6 psi.
Since piping is dry, there can be delay of up to 60 seconds while water travels between the valve and the sprinkler head. A disadvantage of dry pipe systems is that fire can spread for a slightly longer time before the affected zone is showered with water.
An alarm line is connected to the valve assembly, and it activates when the valve opens.
It is very important to keep the dry pipe valve and associated equipment in an area where temperature will 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 fire sprinkler riser, separating the pressurized piping network from the water supply.
As explained above, these valves are held back by a pressure difference - the sprinkler piping generally has a pressure 5 times higher than the water supply to hold the valve closed. A typical system side pressure is 40 psi.
In 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 water motor gong outside the building. Normally, this piping section has no water and is not pressurized.
NFPA standards require pressure-type flow switches in dry pipe sprinkler systems. Vane-type switches are not allowed, since the water flow is very strong when the valve opens and it can damage the paddles of vane-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 pressure drops 10 psi below the nominal value. When this switch activates, it does not necessarily mean that a sprinkler head has opened - the drop can be caused by issues such as leaks and compressor failure, and the system should be inspected to fix the issue before the dry pipe valve opens. The NFPA 72 standard requires air pressure to 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 implied by their name. They redirect air flow in the sprinkler piping to reduce pressure faster, lowering the opening time of the valve. In other words, they reduce the time required to deliver water to the affected area after a sprinkler head opens.
Air maintenance devices keep adequate pressure in the sprinkler piping, by supplying more air when a small pressure loss is detected.
Air dryers remove moisture inside the sprinkler piping, as implied by their name.
When Is a Dry Sprinkler System Recommended?
Since dry pipe sprinkler systems are not filled with water, they are the best option in applications with freezing temperatures. As stated in the NFPA 13 standard, these sprinkler systems are intended for applications where the ambient temperature cannot be kept 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:
Exposed parking garages
Water-sensitive storage areas
The main advantages and disadvantages of dry pipe sprinkler systems are summarized in the following table:
1) Suitable for industrial and commercial spaces where piping is exposed to freezing temperatures.
2) The system is less vulnerable to rusting, since there is no water inside the pipes under normal conditions.
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 ownership cost: Dry pipe systems are more expensive due to their technical complexity, 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: Since water must travel from the valve to the sprinkler heads, there may be a delay of up to 60 seconds after a sprinkler head opens. The fire has some extra time to spread and cause damage.
Testing and Maintenance Procedures
Since dry pipe systems have many interacting components that must respond properly when there is a fire, testing and maintenance are of vital importance.
The low pressure switch is normally tested with a lever-type bleeder valve.
The flow switch is usually tested by opening the alarm test valve (normally closed), allowing water flow directly to the switch without opening the dry valve.
If there is a shut-off valve between the dry pipe valve and the alarm pressure switch, it must be supervised according to the NFPA 72 standard.
Dry pipe systems are more expensive to install due to their technical complexity. The pipes themselves must also have a specified pitch angle to prevent water accumulation, and as a result their installation is more labor intensive compared with a wet-pipe system. However, dry pipe systems allow automatic fire protection for areas where a wet-pipe system would be ineffective due to freezing.
The NFPA 13 and other related standards are very demanding, but this is justified since they deal with fire protection. If you own or manage commercial property in New York City, note that Local Law 26 makes sprinkler system installation mandatory for many occupancies by July 1, 2019. With a professional assessment, you can detect fire vulnerabilities in your property, and also determine if you are subject to LL26/04.
At New York EngineersChicago EngineersNew Jersey Engineers, we search for simple, eloquent solutions to complex problems. We minimize construction costs by eliminating the extraneous and focusing on the overall efficiency for the most streamlined designs.