Corrosion is a problem that must be addressed in both dry pipe and wet pipe sprinkler systems. In this article, we will discuss pipe corrosion in wet sprinkler systems and methods of preventing the corrosion.
The water that fills wet sprinkler pipes contains approximately 10 parts per million (ppm) of dissolved oxygen. All wet systems experience initial corrosion because of this dissolved oxygen. However, this corrosion is self-limiting as some of that dissolved oxygen will be consumed until the system is refilled with fresh, oxygenated water.
The primary catalyst of pipe corrosion in a wet system is air that has been trapped in air pockets of the pipes. These air pockets exist frequently within wet system pipes at high points of the piping. These trapped air pockets contain 20.9% oxygen, which can fully sustain electrochemical corrosion. As a result, pinhole leaks eventually develop at the location of the air pockets. These leaks have costly results including property and equipment damage, ongoing repairs to pipes or full system replacement, sprinkler head blockages, and an inoperable fire protection system that puts your people and assets at risk.
In order to prevent pinhole leaks, there must be less than 2% oxygen in the trapped space (reduced from the 20.9% that would typically be present). There are several preventative actions that can be taken: wet air vents, auto inert with nitrogen, and remove oxygen from the water. While each action can be taken individually, the best results occur when they are combined.
Wet Air Vents
Installing wet air vents is the most basic step you can take to reduce oxygen corrosion in your wet sprinkler pipes. Your Life Safety Partner will install the wet air vents at high points and other potential areas where air can be trapped, in order to provide a vent to remove the air. There are two types of wet air vents, single and dual. Both types remove the oxygen from the pipe, venting it outside the sprinkler pipe. A single vent removes the air and closes the valve when water reaches it, this water then drains back into the sprinkler pipe. The single vent will discharge any overflow so it should not be used in areas that are water sensitive. A dual vent will not discharge water so it is safe to install in water sensitive areas. As the pressure from the sprinkler pipe drops, any collected water in the air vents will automatically drain back to the sprinkler pipe through the secondary valve. The dual valve also creates redundancy, eliminating any failure concerns.
Auto Inert with Nitrogen
Filling the sprinkler pipes with high purity nitrogen gas prior to charging the system with water can ensure that corrosion in all trapped air locations is slowed. To do this to a wet sprinkler pipe system, you must first drain the fire protection system of all water and install an auto inert system. The fire sprinkler system is then filled with 98% pure nitrogen through the inert system. By displacing the trapped air with high-purity nitrogen, you minimize the source of oxygen to sustain electrochemical corrosion and corrosion cells have less chance of propagating.
You can use this technique in place of wet air vents, as installation of wet air vents may not always be possible due to lack of headroom or varying elevations, which make identification of trapped air pockets challenging or impossible. However, the best results for slowing corrosion occur when you combine the two solutions.
Removing Oxygen from a Wet Pipe Sprinkler System
Removing oxygen from a wet pipe sprinkler system is the best option to slow, or eliminate, internal wet system corrosion. In addition to the wet air vents, this method removes the oxygen from the trapped air pockets and reduces the dissolved oxygen in water to 1.0 ppm or less.
First, the wet pipe system is pre-purged with nitrogen. This changes the pockets of trapped air to pockets of trapped nitrogen gas, which does not act as a catalyst for corrosion like oxygen does. The sprinkler pipe is filled with deoxygenated water, 1.0 ppm or less of dissolved oxygen. This deoxygenated water, which is below the threshold of oxygen known to cause corrosion, stays in constant contact with the sprinkler piping. There is now not enough oxygen in the water to dissipate into the trapped nitrogen gas pockets, which protects the areas of pipe that would have corroded under normal circumstances.
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