What is a dry pipe sprinkler system and where are they used?
Dry pipe systems are very similar to a wet pipe system. Instead of the pipes being filled with water, they are filled with pressurized air or nitrogen. Once the system is activated, the air is released, water travels through the pipes and is released out of the sprinkler heads that have been activated over the fire area.
A dry pipe system is typically installed in spaces that are subjected to temperatures below 40°F, such as parking garages, loading docks, or freezers. Areas like these that are exposed to low temperatures that can cause damage to water filled sprinkler.
Design challenges of dry pipe systems
While dry pipe systems are great for freeze protection, they do have some constraints that make their design a bit more difficult in areas where the water supply may be limited. For example, NFPA 13 §184.108.40.206.5 (2013 edition) states that the area of operation for dry pipe systems must be increased by 30% without revising the density. The reason behind this increase to the area is because there is a delay from when the sprinklers are activated to when the water reaches the fire. Wet pipe systems do not face this delay since the pipes are filled with water. Dry pipe systems, have to release the pressurized air or nitrogen before water is discharged, causing water delivery delays upon sprinkler head activation. During this delay, the fire will increase in size, thus triggering the need for a larger design area.
While an increased area of operation may seem like added protection, there are a few reasons why this can be troublesome for a system designer. An increase in area means more sprinklers are required to operate in order to provide the necessary amount of water over the design area.
Let’s look at a basic example of a parking garage with a dry pipe system:
- Hazard Classification: Ordinary Group 1
- Density/Area: 0.15(gpm/sqft) / 1500(sqft): 225 gpm
- Adjusted Density/Area: 0.15(gpm/sqft) / 1950(sqft): 293 gpm
This example would require at least 293 gpm to be applied over the design area, not including hose allowance or extra flow from heads being fed at a higher pressure. If a wet pipe system was able to be used, the area could have been maintained at 1500 sqft, with only 225 gpm being required. This may not seem like a tremendous difference but when a building’s available water supply is limited, a designer will try and utilize any possible alternatives to get the systems to work without needing to modify the existing water supply. What are the other options to a dry pipe system?
NFPA 13 §220.127.116.11.1 states that any portion of a sprinkler system subjected to freezing or cannot maintain temperatures above 40°F shall be installed as a dry pipe system or preaction system. Much like a dry pipe system, a preaction system is faced with the same design challenges such as an increased design area. For locations with inadequate water supplies, these two systems may not be feasible so alternative options may be needed.
One option would be to provide a listed antifreeze system. An antifreeze system is similar to a wet pipe system in terms of liquid-filled piping but instead of water, the pipes are filled with a mix of water and antifreeze solution. This mixture prevents the pipes from freezing when the temperatures drop towards freezing conditions. An antifreeze system requires special considerations when preforming hydraulic calculations, as the mixture has different friction loss characteristics when compared to water. Additional components must also be used, such as a backflow preventer between the antifreeze system and the remainder of the system (to prevent any of the solution to contaminate the water supply) and an expansion chamber (to compensate for thermal expansion of the antifreeze solution). An additional challenge in the use of antifreeze systems is that antifreeze products listed for fire protection use are required.
Heat-tracing is another method that can be used. This method is ideal for sections of wet pipe that run through unheated areas, rather than entire systems. Heat tracing is an electronic tape or coil that is typically wrapped around a pipe and energized to maintain a preset temperature above freezing. If heat tracing is used on fire protection piping, then it must be supervised by the building’s fire alarm system and must send out a supervisory signal if the heat tracing system experiences any type of failure.
If additional systems such as heat tracing or antifreeze are not desired, then one of the simpler options for freeze protection would be to heat the space. This can be achieved by providing adequate insulation around the pipes or walls of the space. One example where this may be a good alternative would be in an attic. If an attic is unheated, then wet sprinkler pipe cannot be run through the space. However, if the pipe is within conditioned space by having the attic insulation above it, or the space itself is insulated, then a wet system can be used there. Providing insulation and heat for a space may not be ideal for an owner but if protection is required and a location cannot support a dry pipe system, adding the heat and insulation may be the better option.
Lastly, another option is a dry sprinkler head. These sprinklers are typically a “normal” sprinkler head that are on a section of pipe that remains dry, even though it is connected to a wet sprinkler system. This allows the branch line to be in a heated space, while the dry sprinkler extends through a wall or ceiling into the non-heated space. These are commonly used in walk-in coolers, underneath outdoor overhangs, or in unheated vestibules where conditioned space is close by.
These are some alternatives to dry pipe system. Each option has its pros and cons, and not all may be suitable for every situation. When deciding which option to use to protect an area subject to freezing, it is important to consider all factors. Sometimes the long-term cost associated with certain alternatives may outweigh the benefits.
If you have questions on what type of sprinkler system is needed for an area, or alternatives to dry sprinkler protection, please contact our office at firstname.lastname@example.org.