ONE OF THE ISSUES that building owners and facility managers face is corrosion in sprinkler system piping. Corrosion can be a costly problem due to potential water damage from leaks and the need to replace piping. It can also cause blockages in the piping network, which can lead to an ineffective sprinkler system during a fire event.
Sprinkler pipe corrosion can take on many forms, including oxidation, microbiologically influenced corrosion (MIC), and galvanic corrosion. The technical committees responsible for NFPA 13, Installation of Sprinkler Systems, have been looking at ways to eliminate, or at least reduce, corrosion in sprinkler systems for several revision cycles, including the most recent cycle devoted to the 2016 edition of NFPA 13. The concept will be the subject of a certified amending motion at NFPA’s annual technical meeting in Chicago in June.
In recent cycles of the standard, the discussions have centered around MIC in dry systems. Major discussion points for reducing corrosion in dry systems included the need to eliminate all of the water from these systems after testing/activation and the use of alternative gases like nitrogen for charging these systems. During the most recent revision cycle, however, the corrosion discussion shifted to wet systems. When looking at wet systems, the focus is the opposite of that for dry systems: limiting the amount of air in the system as opposed to the amount of water.
One method for eliminating air trapped in the system is through the use of system air vents. The technical committee on sprinkler system installation criteria discussed this topic in great detail at both the first and second draft meetings. The initial discussion led to the development of the pipe venting task group, which was charged with studying the issue of system air vents and proposing language for inclusion in the standard.
As part of its research, which included an in-depth review of public inputs submitted on this topic, the task group concluded that venting the trapped air in a wet system can have a positive impact on a system in several ways, including decreased water delivery time, reduced alarm ring delay, reduced water flow alarm cyclic ringing, and reduced corrosion activity.
The major benefit and critical talking point during the committee meetings was the potential for reducing corrosion activity. Air trapped in pressurized sprinkler system piping results in an increase in the concentration of dissolved oxygen in the piping. There is a linear relationship between the amount of dissolved oxygen in a system and corrosion rates inside the piping. Evidence of this internal “trapped air corrosion” is typically found at the air-water interface. These findings led to a revision to the standard requiring that a single air vent be installed for each wet system.
The counterpoint to the new requirement is that the newly required air vent does not guarantee that all of the air in the system will be eliminated; depending upon the building geometry and sprinkler design, it is possible to trap air in many locations in the system, meaning a single vent may not be effective. Therefore, some stakeholders believe that the requirement adds cost to the property owner and the design team without any assurance that there is a long-term benefit to the system.
If you would like to be part of this discussion at the upcoming annual technical meeting, please visit the NFPA Conference and Expo website.
MATT KLAUS is principal fire protection engineer at NFPA and staff liaison for NFPA 13, 13R, & 13D.