Inside Information

BEM technology helps utilities gather data on condition and life expectancy in pressurized pipes as part of a risk-based, predictive approach to rehabilitation

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It’s relatively easy to assess the condition of gravity sewer pipes — just run a crawler camera through the line and record the defects.

 

Condition assessment is much harder in pressurized pipes like water lines and sewer force mains: taking these pipes out of service for inspection is highly disruptive, not to mention expensive.

 

Historically, cities and utilities have relied on predictive deterioration modeling to decide when to rehabilitate or replace pressure piping. Here, computer software is used to analyze pipe material and age curves, taking into account other factors such as soil conditions and break history. That method has often proven inaccurate.

 

Today, new technologies enable managers to get accurate “snapshots” of the actual condition of metal pressurized pipes — and therefore estimate remaining service life — by taking measurements from the pipe exterior.

One such technology is the Broadband Electromagnetic (BEM) eddy current system from Infra-Metrix LLC of Tampa, Fla. The system gives users an investigative tool that enables quick-turnaround sampling of pipe condition at a relatively low cost.

 

BEM produces a complete profile of a section of metal pipe, allowing users to gauge the metal thickness and check for conditions such as fractures or graphitization. Scans are performed while the pipe is in full operation.

BEM is not an all-in-one solution for condition assessment. Rather, it is designed to be a valuable part of an investigative program under a sound, risk-based approach to pressure pipe repair and replacement.

 

InfraMetrix president and founder William Di Tullio and project manager Robert Kerry demonstrated the technology on March 10 by way of a Web conference. As part of it, they shared results from an actual BEM inspection at a water authority in Florida.

 

Walk-around

BEM technology uses a series of six antennas in a configuration that looks like a foot-long ruler, 2 inches wide. Each antenna is 2 inches square and contains a coil of wire. In a typical investigation, a section of the pipe to be studied — typically about 6 feet long — is fully exposed by excavation. The antenna device is then used to take readings around the circumference.

 

First, a technician wraps the pipe with grid paper marked in 2-inch squares. The technician begins taking measurements by laying the antenna assembly along the crown of the pipe. An electric current is introduced to the antennas, in turn inducing a current in the metallic pipe.

 

The current is then shut off, and the antennas measure the amplitude and time of decay in the current in the pipe. The process takes about 5 seconds. Based on the measured data and the type of pipe material (previously entered), software calculates the pipe thickness.

 

The technician continues measuring around the entire pipe at 2-inch increments, using the grid paper as a guide. Once that is done, the technician moves 1 foot horizontally and repeats the procedure. While the pipe section is scanned, a display (Figure 1) continuously updates on the technician’s laptop computer roughly indicating the pipe’s condition.

 

The display shows green where the pipe is within 10 percent of its original thickness, yellow where the thickness has decreased by 10 to 30 percent, and red where the thickness has decreased by more than 30 percent.

 

Later, a more detailed data analysis yields what is essentially a topographic map of the pipe, showing the thickness at all points in the section studied. A BEM investigation typically looks at segments of pipe about 1,500 feet apart.

The BEM antennas can scan through coatings, linings or insulation up to 2 inches thick — direct metal contact is not necessary, nor is special cleaning of the pipe surface. The antennas are minimally affected by background electromagnetic noise, such as from nearby electric cables. The frequency can be modified to suit the pipe material or excavation conditions. The technology can detect loss of metal thickness as small as one millimeter. The antennas are water resistant, enabling their use in wet trenches as well as during wet weather.

 

Operation

Kerry and Di Tullio demonstrated the technology using data from an analysis performed for the Toho Water Authority in Kissimmee, Fla.

 

The authority, as part of an overall sewer system evaluation, tested BEM as a non-destructive way of assessing the condition of its wastewater force mains without taking them out of service. Managers chose to analyze two ductile iron force mains that carry a significant share of the sewage flow: a 30-inch line 11,700 feet long, and 20-inch line 6,000 feet long.

 

The excavations for BEM analysis — four on the 30-inch line and two on the 20-inch line — exposed 6 feet of pipe. “Because this was a demonstration, they were looking to keep costs down and get an idea of what the technology could do,” says Kerry. “There would have been more test pits if this had been a more comprehensive investigation.”

 

Both force mains were about 20 years old and had no external corrosion protection or internal protection. Both had typical sand bedding support, 4 to 5 feet of cover, and operating pressures less than 50 psi.

 

Using the grid method, InfraMetrix technicians took a total of nearly 2,500 individual measurements, each covering an area 2 inches square, on each 6-foot section of 30-inch force main. The investigation revealed an average thickness of 0.44 inch, a maximum of 0.48 inch, and a minimum of 0.33 inch in this pipe, which was 0.5 inch thick when new. The topographic profiles (Figure 2) showed the greatest thinning in the haunch areas (roughly 5 o’clock and 7 o’clock).

 

By determining the remaining wall thickness and knowing the pipe’s age, InfraMetrix was able to calculate the annual rate of wall loss and the time remaining at the current rate of loss (19 to 55 years) before the pipes reached the end of their design life according to American Water Works Association standards. The company was also able to estimate the life to failure (32 to 82 years).

 

BEM investigation concluded that there had been no significant deterioration of the pipe inverts. Pipe wall loss was related to internal corrosion. The average wall loss during the pipes’ 20 years of life had been 14 to 30 percent, and the average rate of loss was 0.0042 to 0.010 inch per year — not considered excessive for the application.

 

The InfraMetrix team concluded that the water authority should perform a detailed criticality analysis of its force mains, identifying those that are the most critical and conducting in-depth investigations to assess their condition.

 

Observer comments

Technologies like BEM enable cities and utilities to gather accurate data on pipe thickness at affordable cost. Before such techniques existed, the only reliable way to determine pipe wall thickness and estimate rates of erosion was to perform destructive testing, cutting a section (coupon) out of the pipe and measuring it. Naturally, that meant taking the pipe out of service and repairing the pipe before restarting flow.

 

The process of measuring with BEM is labor-intensive, and this would limit the technology to checking critical pipes at wide intervals, ideally at carefully selected locations where pipe wear would be likely to occur.

 

The technology appears to be a useful tool for assessing the condition of metallic pressurized pipe when used in the context of a complete assessment program.

 

Manufacturer comments

Kerry notes that while gravity sewers of VCP, concrete or PVC pipe may last 100 years and remain in good condition, pressurized pipes have shorter service lives. Sewer force mains, in particular, may have service lives of only 25 to 50 years because they are subject to a variety of stresses, including pressure differentials, pressure surges and the corrosive nature of wastewater.

 

InfraMetrix advocates using BEM technology in a program that starts with a criticality analysis. Such an analysis first determines which pressurized pipes are the most critical based on the consequences of failure: effect on the environment, commerce and public health and safety; cost of lost service; cost of repair; and others.

 

With a list of the most critical pipelines in hand, the next step is to consider the probability of failure, based on the existence of potential life-reducing factors. BEM can be used to analyze the points on those lines that appear the most potentially vulnerable to wear. Those include:

• High spots in the pipe or changes in horizontal direction — areas where the pipe may be subject to scouring.

• Locations with corrosive soils.

• Sites with fluctuating groundwater or saltwater.

• Areas with minimal cover and thus little protection for the pipe.

• Pipe sections near “live loads” such as railroad tracks or busy highways.

• Sites where other metallic pipes are nearby and galvanic corrosion may occur.

 

“If you test the worst spots and the pipe still looks OK, you have good confidence the rest of the pipe is OK, too,” says Kerry. “But if you test the worst places and confirm that the pipe is deteriorated, you know you have to make decisions on a course of action.

 

“If you’re making decisions based solely on pipe base material and age, you could be replacing a 50-year-old pipe that is actually still in good condition when another pipe somewhere else that in theory should last another 20 years is just about to fail. In other words, you could end up spending money to fix the wrong pipes.

 

“With BEM, you get a much better working knowledge of the condition of your metal pipes, and you get a basis on which to estimate remaining life. You can have much more confidence in your decisions and a more factual justification for capital budgeting.”

 

InfraMetrix is also applying the BEM technology to assess the risk of failure of other metal assets. “The same diagnostic, evaluation and prioritization procedures that we use for pipelines can also be applied to other ferrous components such as hydropnuematic and water storage tanks, buried vaults and light poles to help prevent costly failures that may affect public health and safety,” Di Tullio says.

 

“The economic constraints facing utility agencies make it necessary to encourage the development of new inspection and rehabilitation technologies and to be more open to using them. Innovations may provide solutions to the sustainability of our assets for the next generation of citizens.”



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