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Fire Sprinkler Riser Corrosion

Fire Sprinkler Riser Corrosion (195 kb)

Up to this point much of the efforts to control oxygen corrosion in water based fire sprinkler systems have been focused on the overhead sprinkler system piping. By removing the oxygen and preventing its introduction by inerting the systems with nitrogen gas, corrosion can be completely controlled. Dry Pipe Nitrogen Inerting (DPNI) and Wet Pipe Nitrogen Inerting (WPNI) are proving to be the most effective solution for corrosion control in the overhead sprinkler system piping. The discussion in this paper, however, will focus on oxygen corrosion that is occurring in the fire sprinkler riser assembly itself. Recent work performed on the vertical piping in the risers have elucidated a new location within the fire sprinkler system where chronic oxygen corrosion is degrading not only the riser piping but also the system control valve assemblies and the riser check valve assemblies.

By evaluating physical evidence from many corroded riser pipe samples, it is possible to describe the corrosion mechanism as follows:

1. Corrosion is Oriented Vertically – the pattern associated with the corrosion by-product deposits and the actual metal loss indicate that the entire piece of pipe is filled with trapped air. The physical evidence indicates that the corrosion is longitudinal along the vertical axis of the pipe.

2. Evidence of Evaporation/Condensation – the underside of the riser check valve indicates that water is condensing and dripping off of the clapper. In this process water evaporates from the pool at the bottom of the pipe and rises up to re-condense at the top of the pipe. As the liquid condensate water then runs down the pipe dissolved oxygen from the trapped air reacts with the sidewalls of the pipe.

3. Oxygen Depletion Corrosion – the oxygen depleted zone in rolled groove connections shows evidence of oxygen corrosion. When the pipe ends become anodic, they become “hot spots” for rapid acceleration of the metal loss.

4. Under-Deposit Oxygen Acceleration – as the corrosion by-product builds on the vertical walls of the pipe the corrosion rate accelerates. The net effect is a continuous build-up of corrosion debris on the sidewalls of the pipe until a pinhole leak occurs.

5. Weld Seam Corrosion – the weld seam in the vertical piping is vulnerable to oxygen corrosion attack. Piping that is used in fire sprinkler systems is not heat annealed and as such is subject to weld seam attack as evidenced in the pictures of the metal loss on the piping.