V&A Newsletter Focus On

Background

The West County Wastewater District in Contra Costa Co., CA, owns and operates the Tara Hills/Giant Road force main, a pressurized, 3.25-mile-long sewer transmission pipeline located in Richmond, California. The alignment of the force main follows land contours and changes in elevation. The District relies solely on this transmission line to transport wastewater; no redundant conveyance system exists.

Following a series of corrosion-related failures, the District carried out repairs by replacing a downstream segment of pipe and installing clamps. Because there was concern that possible loss of pipe wall section might compromise the force main reliability – possibly to the point where the entire pipeline might need replacement – V&A was retained to perform a condition assessment of the facility, identify areas of the pipeline with internal corrosion, and evaluate various alternatives for repair.

As part of the overall sewer force main evaluation, V&A conducted nondestructive UT (ultrasonic testing) of the force main to evaluate the internal corrosion and to determine remaining pipe wall thickness. UT testing is a preferred alternative to cutting out sample sections of pipe for visual evaluation. UT testing requires no facility shut-down and is cost effective because it can be used to create a highly detailed report and may reduce the need for costly pipeline repair or replacement. In this case, the amount of pipe to be replaced was reduced from as much as 3.25 miles (the entire facility) to 1,800 feet.

Project Approach

The selection of locations to be excavated for testing was based on V&A’s previous extensive experience with pressurized wastewater piping systems. Most of the testing sites were located at peak elevation points, where force main systems are most subject to the collection of atmospheric hydrogen sulfide and related corrosion due to the periodic or continual presence of air/oxygen. Several other locations were also excavated in order to obtain a representative sampling of thickness measurements along the force main alignment. A total of 39 locations were excavated for the evaluation and UT.

The Tara Hills/Giant Road force main is an 18-inch cement-mortar-lined ductile-iron pipe. A nominal pipe wall thickness of 0.410 inches was referenced from the original manufacturer’s specifications. V&A established the minimum thickness criteria of 0.340 inches for this project by subtracting the manufacturer’s pipe-casting tolerance and the corrosion allowance from the nominal thickness.

A total of 1,600 UT measurements were taken. Of the 1,600 measurements, 70 of them, located at 10 separate excavation/testing sites, were below the minimum thickness criteria. In addition, all of the measurements below the minimum thickness criteria were located on the upper half of the pipe. At one excavation site, a series of measurements was conducted along the circumference of the pipe in order to plot a simulated section view representative of the interior surface profile of the pipe (Figure 1). The section view shows that approximately 22 percent of the wall section had deteriorated due to internal corrosion.

Outcome

As expected, the ultrasonic thickness data showed a consistent correlation between the decreased wall thickness measurements and the peak elevations of the force main. This data further substantiate V&A’s experience that maximum internal corrosion degradation occurs at areas where a periodic or continual air/oxygen volume occurs. Degradation of the interior surface extended in the downstream direction possibly due to turbulent flow conditions, allowing atmospheric hydrogen sulfide to form, which results in sulfuric acid corrosion. The graph below (Figure 2) illustrates the average thickness and the low measurement recorded at each excavation location.

Based on the UT measurements, the elevation profile of the force main shows sections identified for repair coinciding with peak elevations and extending in the downstream direction. The UT results identify areas that have a higher likelihood of failure due to internal corrosion. Furthermore, the UT testing results assist in prioritizing the repair efforts.

The segments in need of replacement totaled only about 1,800 feet, or about 10 percent of the total force main length, and V&A’s project report offered a range of repair alternatives for the District’s consideration. These included new material options for replacement; a review of current rehabilitation techniques; and the alternative of doing nothing. Finally, V&A presented the pros and cons associated with each alternative, including future development and economic factors.

Nondestructive Ultrasonic Testing

In a recently completed condition assessment project, the use of UT (ultrasonic testing) to determine wall thickness allowed the engineers to reduce the extent of anticipated pipeline replacement by nearly 90%.


Vol 1. No 4, October 2003 *Focus On . . .*		for previous articles, visit the Infrastructure Preservation News archives and V&A's web site at www.vaengr.com
Nondestructive Ultrasonic Testing In a recently completed condition assessment project, the use of UT (ultrasonic testing) to determine wall thickness allowed the engineers to reduce the extent of anticipated pipeline replacement by nearly 90%. Background The West County Wastewater District in Contra Costa Co., CA, owns and operates the Tara Hills/Giant Road force main, a pressurized, 3.25-mile-long sewer transmission pipeline located in Richmond, California. The alignment of the force main follows land contours and changes in elevation. The District relies solely on this transmission line to transport wastewater; no redundant conveyance system exists. Following a series of corrosion-related failures, the District carried out repairs by replacing a downstream segment of pipe and installing clamps. Because there was concern that possible loss of pipe wall section might compromise the force main reliability – possibly to the point where the entire pipeline might need replacement – V&A was retained to perform a condition assessment of the facility, identify areas of the pipeline with internal corrosion, and evaluate various alternatives for repair. As part of the overall sewer force main evaluation, V&A conducted nondestructive UT (ultrasonic testing) of the force main to evaluate the internal corrosion and to determine remaining pipe wall thickness. UT testing is a preferred alternative to cutting out sample sections of pipe for visual evaluation. UT testing requires no facility shut-down and is cost effective because it can be used to create a highly detailed report and may reduce the need for costly pipeline repair or replacement. In this case, the amount of pipe to be replaced was reduced from as much as 3.25 miles (the entire facility) to 1,800 feet. . Project Approach The selection of locations to be excavated for testing was based on V&A’s previous extensive experience with pressurized wastewater piping systems. Most of the testing sites were located at peak elevation points, where force main systems are most subject to the collection of atmospheric hydrogen sulfide and related corrosion due to the periodic or continual presence of air/oxygen. Several other locations were also excavated in order to obtain a representative sampling of thickness measurements along the force main alignment. A total of 39 locations were excavated for the evaluation and UT. The Tara Hills/Giant Road force main is an 18-inch cement-mortar-lined ductile-iron pipe. A nominal pipe wall thickness of 0.410 inches was referenced from the original manufacturer’s specifications. V&A established the minimum thickness criteria of 0.340 inches for this project by subtracting the manufacturer’s pipe-casting tolerance and the corrosion allowance from the nominal thickness. A total of 1,600 UT measurements were taken. Of the 1,600 measurements, 70 of them, located at 10 separate excavation/testing sites, were below the minimum thickness criteria. In addition, all of the measurements below the minimum thickness criteria were located on the upper half of the pipe. At one excavation site, a series of measurements was conducted along the circumference of the pipe in order to plot a simulated section view representative of the interior surface profile of the pipe (Figure 1). The section view shows that approximately 22 percent of the wall section had deteriorated due to internal corrosion. Outcome As expected, the ultrasonic thickness data showed a consistent correlation between the decreased wall thickness measurements and the peak elevations of the force main. This data further substantiate V&A’s experience that maximum internal corrosion degradation occurs at areas where a periodic or continual air/oxygen volume occurs. Degradation of the interior surface extended in the downstream direction possibly due to turbulent flow conditions, allowing atmospheric hydrogen sulfide to form, which results in sulfuric acid corrosion. The graph below (Figure 2) illustrates the average thickness and the low measurement recorded at each excavation location. Based on the UT measurements, the elevation profile of the force main shows sections identified for repair coinciding with peak elevations and extending in the downstream direction. The UT results identify areas that have a higher likelihood of failure due to internal corrosion. Furthermore, the UT testing results assist in prioritizing the repair efforts. The segments in need of replacement totaled only about 1,800 feet, or about 10 percent of the total force main length, and V&A’s project report offered a range of repair alternatives for the District’s consideration. These included new material options for replacement; a review of current rehabilitation techniques; and the alternative of doing nothing. Finally, V&A presented the pros and cons associated with each alternative, including future development and economic factors.