VALVE CHAMBER CASE STUDY NO. 1

Valves are a common location for corrosion to occur in water piping systems. Frequently, they are located in manhole chambers where the pipe emerges from the soil. These chambers are generally filled with water and soils that run into the chambers from the road runoff. In addition water from ditches and along the water main enters in openings where the main enters the manhole. Leaks into the manhole chambers can be reduced, but, with the loading of traffic and shifting of the chamber in the frost cycles, it becomes very difficult to completely eliminate the inflow of water.

A recent repair was carried out on a valve that had been buried for five years. The soil in this area is approximately 800 to 900 ohm-cm and creates a very active condition. This 8-inch valve was epoxy-coated and had a zinc anode attached to the top body by an exothermic weld. The anode was intended to slow corrosion of the uncoated bolts used to assemble the valve. This type of valve is a two-part casting and is fabricated with a sealing ring and 5/8-inch diameter bolts around the flange. When properly assembled, the bolts create a continuous electrical circuit.

This valve failed as a result of a combination of ½-inch diameter bolts holding the top body instead of 5/8-inch bolts and corrosion affecting the undersized ½-inch bolts until they failed. Figure 1 shows a corroded bolt. The corrosion took place on the lower section of the valve where water and wet soil accumulated. As the valve was epoxy coated, the corrosion occurred mainly on the 12-inch coupling bolts and on the undersized ½-inch bolts that held the valve together. These bolts were carbon steel and not coated. The undersized ½-inch bolts did not provide electrical continuity between the top body and the valve as with the larger bolts, resulting on the lower part of the valve being unprotected. In the valve system, bolts are generally the first to be effected by corrosion. In chambers, it is sometimes very clear as to the cause of the corrosion, and, in the early stages, corrective action can be taken.

Figure 1-Corroded 12 inch coupling bolt

Chambers in these areas are found with bolts heads completely corroded. Bolts are also found with the stem reduced to pencil lead thickness. Some rungs of ladders for access to these chambers have completely disappeared. These are some of the problems encountered during rehabilitation of these valve chambers that, in some cases, were installed in the mid 1980's. Various methods are being used to replace corroded material using a material that is both more corrosion resistant and of higher strength. During the last couple of years, mild steel bolts have been replaced with 304 stainless steel bolts in some specific areas that need protection. This was the first step in corrective maintenance. The possibility exists that corrosion in these chambers could have an effect on these bolts. For this reason the valves in one of our chambers completely protected with a wax-based tape. A wax-based primer was also used to coat the bolts and reduce the effects of corrosion on the material when the chambers were flooded. A process/procedure is being developed for use when rehabilitating any valve chamber. This procedure will be a three-step process based on initial evaluation of the chamber. Each chamber will be placed in to one of the three categories and, based on these categories, the required cathodic protection will be applied. In the chambers, different corrosion types were found, including crevice corrosion and galvanic corrosion along with general corrosion of the aluminum ladder.

It is difficult to cathodically protect against crevice corrosion. This type of corrosion was also found on couplings and bolts with the valves. Replacement with more corrosion resistant materials or the use of protective coatings or sealants to keep the area dry is the cost-effective means of controlling crevice corrosion.

In some of the valve chambers, there were aluminum steps that decayed as a result of the chloride/phosphates.  

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