Image showing the assembly of the RFT tool

Inspection Advancements for PCCP – Addressing the Mode of Failure

In spring 2022, the patent for wire break detection inspection methods quietly expired. This method is known as the ‘Near Field Testing Electromagnetic Technique.’

This should be great news for private companies and public utilities, as we are beginning to see sole source agreements end and competition begin. The two most common and accepted electromagnetic techniques used for municipal pressure pipe inspections are ‘Near Field Testing’ (NFT) and ‘Remote Field Testing’ (RFT). Electromagnetic techniques are defined by the location of the detector and receiver coils, which are the core components performing the inspections. While there is considerable literature on the deployment of NFT in PCCP pipelines, there has been little published on the deployment of RFT.

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Near Field Testing and Remote Field Testing

For NFT, the detecting solenoid and exciting solenoids are within in the same pipe diameter, so there is no axial separation. For RFT, the detecting solenoid and exciting solenoids are two to three pipe diameters apart. Below are the patent illustrations for both techniques:

NFT requires less power, due to the proximity of the sensors and is a simpler technique. NFT is adept and proven at finding and quantifying prestressing wire breaks in “AWWA C301 Prestressed Concrete Cylinder Pipe, Steel-Cylinder Type.” NFT cannot effectively scan the cylinders of pipes and it cannot identify the clock position of wire breaks/defects. There are usually only one or two detectors on NFT tools, so they can therefore be quite small due to requiring less power and less detectors.

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RFT used to be called Remote Field Eddy Current (RFEC) or Remote Field Transformer Coupling (RFTC) but was standardized as RFT by ASNT several years ago. Since the tools must, by definition, have axial separation between the excitor and receiver coils by two to three pipe diameters and need to cover the circumference with detectors, the tools are by definition larger. RFT Tools can scan the cylinder of all ferromagnetic cylinder type pipes (Steel, DIP, CIP) as well as concrete pressure pipes (PCCP, BWP). RFT can also sense wire breaks in PCCP pipes.

Inspecting PCCP by Focusing on Modes of Failure

Now that we have covered the tech, let’s focus back on PCCP inspections at a high level. Traditionally, PCCP has been inspected with NFT to identify wire breaks, fiber optics for wire break monitoring as well as visual and sounding inspections (usually via manned entry). These options have served well to date, as utilities have seen sharp declines in pipeline failures (which can be quite catastrophic from PCCP pipelines). There are additional tools that we at PICA Corp. use for PCCP inspections such as LiDAR sensors, RFT for cylinder inspection and handheld EM Bracelet Probe devices. LiDAR can quantify spalling and joint separation conditions, RFT scans the cylinder and a handheld EM scanner can scan closer to the pipe joints and at butterfly valves that can interfere with scan coverage. The aforementioned inspection methods cover several common failure mechanisms, but that is not the same as the failure mode. The failure mode of PCCP is loss of the pipe’s ability to act in compression.

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In May 2023, PICA Corp. released the findings of a years’ long, R&D effort for PCCP inspection. This research and development was funded by (University of Tennessee, WRF, Trinity River Authority, San Diego County Water Authority, GLWA, TRWD). By scanning the entire cylinder accurately with an RFT method, we not only identify and locate defects such as localized corrosion, but we can also identify a loss of preload in the cylinder. We performed four different phases of RFT tests 1) test bed experiments, 2) expanded test bed experiments, 3) application of measurement model to buried pipe and 4) destructive testing. See Study Phase Table:

PCCP pipes study phase table

The conclusion of our study and experiences inspecting prestress taught us several things. Preload is detectable and is another data set available for PCCP asset owners. Corroded wires are typically at one clock position, whereas hydrogen embrittled wires tend to not be grouped together. Hydrogen embrittled breaks may not result in a loss of preload (a mode of failure). RFT can differentiate between corroded and hydrogen embrittled wires.

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Kristopher Embry is the vice president of client services at PICA Corp.

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