VALVE CASE STUDY NO 2

On Innes Road, in the eastern section of the new City of Ottawa, in late 1999, a chamber, for which the valves had just been rebuilt, was selected for study. This chamber, during its brief ten-year lifetime, had been contaminated with phosphates and chlorides resulting in the bolts corroding to the point where the valves in the chamber were leaking. This chamber housed a 24-inch valve and 16-inch valve along with an air drain-out. All of the bolts on these valves required changing (Figure 2). They were replaced with stainless steel bolts, which, while they should be more corrosion resistant than carbon steel,1 will still require monitoring, because they will be exposed to crevice corrosion conditions.

Figures 2A and 2B-corroded bolts from manhole showing susceptibility to crevice corrosion

Figure 3 shows how the aluminum ladder rungs had completely corroded to the point where they could no longer be used. Considering the limited number of times that this chamber was to be entered, it was decided to leave the ladder in this corroded state and enter with a restraining device and / or portable ladder.

Figure 3-Corroded aluminum ladder rungs

At this site, a wax tape coating was tested and supported with magnesium anodes. It took approximately four hours to clean out the chamber, and to wire brush and coat the valves and the air drain out. Within hours after finishing in the chamber it was again filled with water.

Early the following summer, the wax tape coating was inspected and found to be holding up very well (Figure 4). The bolts showed no signs of corrosion, and the magnesium anodes were still electrically continuous. With this type of protection, very little corrosion can be anticipated, and limited maintenance is required other than normal operating maintenance and inspection of the chamber for valve integrity.

Figure 4-Tape wrapped air valve after one-year exposure in valve manhole chamber

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