by Erin Van Ruiswyk

Since the first automatic fire sprinkler system was patented in 1872, almost 150 years have passed along with countless developments to these sprinkler systems. Fire protection today uses a variety of sprinkler systems for different applications – primarily wet, dry, and preaction systems. The evolution of the individual sprinklers has also seen some pretty dramatic progress.

One very important development in fire sprinkler history came in the 1930s, when the dry sprinkler was introduced. Dry sprinklers are designed to be used in environments that are subjected to freezing temperatures. They operate similarly to standard sprinklers, but they include a sealed extension nipple (see Figure 1). Since the seal that keeps out water is at the end of the extension, the entire barrel is kept dry until the sprinkler operates, so that no freezing will occur in the sprinkler that could prevent proper operation.

Figure 1. Basic components of a Dry Sprinkler and Standard Sprinkler.

In some wet systems that utilize dry sprinklers, most of the piping is in a heated environment but the protected area is subject to freezing. One example of this situation would be protecting a freezer at a restaurant. In these cases, a dry sprinkler is needed to make the thermal transition between the freezing deflector and the water in the system piping. The air that is being cooled and warmed at opposite ends of the sprinkler creates a temperature gradient inside the pipe (see Figure 2). Depending on the system environment, NFPA 13 2016 Ed. Table 8.4.9 has specific requirements for how long the exposed barrel must be. A longer barrel length generally offers more freeze protection.

Figure 2. Freeze protection offered by dry sprinkler attached to a wet system.

Dry sprinklers are sometimes also required for dry systems, as stated in NFPA 13 2016 Ed. 7.2.2. Generally, a dry system is pressurized with air rather than water so the use of a dry sprinkler may not seem necessary. However, these requirements are in place because residual water from testing and maintenance can still pose a threat in freezing conditions.

In addition to their installation differences, there are some testing differences for dry and standard sprinklers. NFPA 25 requires that most standard sprinklers are tested after 50 years in service, but currently, dry sprinklers must be tested after only 10 years. This more conservative requirement is in response to a high failure rate that dry sprinklers used to have when O-ring water seals were more prominent. UL no longer lists O-ring dry sprinklers, and more modern sprinkler technology has proven to be much more effective.

At Dyne, we test all sprinkler types including dry and standard sprinklers. Regardless of the sprinkler type, our testing includes evaluating the appearance and determining a response time. If you have any questions about testing sprinklers with Dyne, please contact us at lab@dyneusa.com or (800) 632-2304.