Oxygen corrosion in fire sprinkler systems is caused by a number of things, but can be reduced and controlled if it’s managed correctly. The introduction of oxygen gas is the primary driver of corrosion related failures in water-based fire sprinkler systems. In order for the corrosion reaction to take place in these systems, oxygen gas and liquid water must be present and make intimate contact with steel piping.
Factors that accelerate the corrosion reaction and increase leak frequency include:
- More air introduction, more corrosion (direct, linear relationship)
- Higher temperatures increase the oxygen corrosion reaction rate
- Trapped pools of water in dry (pre-action) systems are the primary location for corrosion
- Trapped air pockets (21% oxygen) in wet pipe systems are the primary location for corrosion
- Compressor run frequency is directly tied to oxygen introduction to dry (pre-action) systems
- Compressors add acidic condensate moisture to the fire sprinkler system
- Thin walled sprinkler piping will exhibit through-the-wall oxygen pits faster than thicker walled piping
Oxygen corrosion in fire sprinkler systems causes two (2) primary problems:
- The reaction of trapped oxygen gas with steel piping creates a void (pit) in the metal surface that over time with repeated introduction of oxygen will penetrate the pipe wall to cause a leak.
- The oxygen corrosion reaction produces water insoluble iron oxide (hematite) debris which can create obstruction risk over time within the fire sprinkler system piping. In extreme cases, the accumulation of corrosion debris can completely clog the sprinkler piping.
Managing Corrosion: Wet Pipe Fire Sprinkler Systems
Reducing and controlling oxygen corrosion in wet pipe fire sprinkler systems can be accomplished by reducing or eliminating the periodic introduction of oxygen rich air to the fire sprinkler system. Air is most often introduced to wet pipe systems during tenant modifications and to perform code-mandated system testing. Unfortunately, the process of repairing corrosion related leaks in fire sprinkler system piping exacerbates the problem because the process actually adds more air to the system when it is taken out of service to perform the repair.
The most cost-effective approach to reduce the cumulative effect of oxygen corrosion in wet pipe fire sprinkler systems is the installation of an automatic air venting device. These simple devices automatically vent air while the system is filling. This results in reduction of accumulations of trapped air at the high points of the system’s piping. Although the automatic air vents cannot completely stop the corrosion, they can certainly reduce the cumulative effect of oxygen corrosion. The greater the amount of ventilated air, the less the corrosion will take place.
The only way to completely stop oxygen corrosion in wet pipe fire sprinkler system is through a process called Wet Pipe Nitrogen Inerting (WPNI). In this approach, nitrogen gas is used to dilute and displace oxygen from the fire sprinkler system piping before it is filled with water. This process eliminates the opportunity for oxygen gas to cause corrosion.
Managing Corrosion: Dry and Pre-Action Fire Sprinkler Systems
Corrosion can only occur in dry pipe systems if liquid water is present within the system piping. Water can be introduced to the piping in several different ways:
- Initial hydrostatic testing of the system during commissioning
- Condensate water introduced by the pressure maintenance compressor
- Periodic code-mandated system testing
Unfortunately, it does not take very much liquid water to support the oxygen corrosion reaction. Low point drains and pitching can remove some of the water from the system piping, but never enough to completely stop the oxygen corrosion. In the case of condensate moisture from the compressor, new water is introduced every time the compressor cuts in to maintain the system pressure.
There are two (2) ways to stop corrosion in dry pipe systems:
- Completely remove all of the water
- Completely remove all of the oxygen gas
The concept of removing all water from the system piping and keeping it out is impractical and virtually impossible; however, it can be accomplished by using nitrogen gas. Dry Pipe Nitrogen Inerting (DPNI) is a process for diluting and displacing oxygen gas from the dry pipe system. The process includes using nitrogen gas to maintain the system pressure within the piping. DPNI has three fundamental components:
- Nitrogen generator
- Integral venting device
- Fill and purge breathing controller in the nitrogen generator
The fill and purge pressure cycling is used as means to displace air in the fire sprinkler piping, because it is the only process that can completely remove the oxygen gas before it has a chance to cause corrosion. Simply injecting nitrogen gas at the riser and then venting it at the end of a branch line, a process called sweep purging, will not displace the oxygen in the extremities of the piping.
Using the fill and purge breathing process allows for placement of the integral venting device directly on the riser. During the fill portion of the cycle, the system pressure is raised by injecting nitrogen gas. The purge portion consists of nitrogen diluted gas flowing toward the vent. After a fixed number of cycles over a 14-day period, all of the oxygen gas is removed and the piping is completely nitrogen inerted.
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