Pipeline Electrical Continuity: An In-Depth Guide to Evaluation Techniques
Embarking on a journey to ensure the longevity and reliability of buried pipelines involves critically examining their electrical continuity. Understanding a pipeline's electrical continuity is paramount, especially when considering implementing cathodic protection systems to safeguard against corrosion. In this blog article, we dive into the complexities of determining electrical continuity by exploring corrosion experts' methods to test for continuity along existing pipeline alignments.
Pipeline Electrical Continuity
The electrical continuity of an existing buried pipeline refers to its ability to conduct electrical current along its entire length without interruption or significant resistance. This continuity is important for various reasons, particularly in the context of cathodic protection (CP) or corrosion monitoring. To ensure effective corrosion protection, the pipeline must have good electrical continuity. If there are interruptions or discontinuities along the electrical path, the protective current may not flow uniformly, leading to localized areas of corrosion.
Common reasons for interruptions in electrical continuity include:
High Resistance Joints - Flanged joints have a higher resistance than welded joints; therefore, the electrical current may not travel as far on a pipeline with flanged joints.
Electrical Shorts - If a pipeline is electrically shorted to a concrete vault along the alignment, current may go to the vault if it is a lower resistance circuit, rather than continue along the pipeline.
Insulating Joints - Some pipelines have insulating joints, which are designed to isolate different sections of the pipeline electrically. While these joints serve a purpose in certain situations, they can also disrupt electrical continuity.
Pipe Repair Welds - Joint repair welds on a pipeline may have different electrical properties than the surrounding material, potentially affecting the continuity. Likewise, the electrical continuity will be affected if the pipeline has flanged joints, then a welded joint is added in.
Electrical continuity testing is usually not required for welded steel pipe except at bolted connections where bond wires have been installed. Welded steel pipe is inherently electrically continuous, so additional bond wires are unnecessary. Concrete cylinder and ductile iron pipe use bell and spigot joints, and the joints must be bonded with insulated copper wire to provide electrical continuity through the joint (bonding clips are not recommended as they can fail due to pipe settlement). The number, length, and wire gauge for joint bonds depend on the pipe type and pipe diameter.
Electrical continuity testing determines whether a pipeline or structure is electrically continuous, and two acceptable test methods are described in this blog article:
Attenuation Survey (primary test method)
Linear Resistance
Attenuation Survey Method
The attenuation survey method only applies to pipelines equipped with an impressed current cathodic protection (ICCP) system, whether installed permanently or temporarily. Using a portable rectifier, a temporary ICCP system is installed on a pipeline without a permanent CP system. Typically, this survey is conducted concurrently with the pipe-to-soil potential survey. The assessment method relies on a subjective analysis involving the interpretation of pipe-to-soil potential measurements in both the “on” and “instant off” states.
The effectiveness of an impressed current system, assessed by V&A based on its ability to provide corrosion protection meeting NACE SP0169 criteria, is contingent on the longitudinal resistance of the pipeline. CP current attenuation refers to the diminishing current flow to the pipeline with increasing distance from the current source. Joint bonds play a crucial role in reducing the longitudinal resistance of the pipeline and ensuring its electrical continuity.
A properly designed impressed current CP system can provide corrosion protection to a concrete-coated pipe for miles in each direction from a rectifier. The protective reach is even more substantial for steel pipes with a dielectric coating. If the pipeline is sufficiently electrically continuous, the “instant off” potential profile for the pipeline will meet NACE criteria for corrosion protection, sometimes many miles away from the rectifier, as shown schematically in Figure 1.
Figure 1: Potential CP Attenuation with Distance from Rectifier (Under Ideal Conditions)
As the longitudinal resistance increases, usually due to discontinuous or high-resistance pipeline joints, CP current flow to the pipeline will attenuate quickly. The length of the pipeline protected by the CP system decreases proportionally to the longitudinal resistance of the pipeline. A pipeline bonded for electrical continuity but with undersized bonding jumpers will have a rapid attenuation at a relatively short distance from the rectifier. Joint bonding must be sized appropriately for each pipeline, and one bond wire or one bond jumper may not be sufficient for a large-diameter bar-wrapped concrete cylinder pipe (BWCCP). The CP current will polarize the pipeline, but the “instant off” potentials will not satisfy the NACE criteria for corrosion protection, as shown schematically in Figure 2.
Figure 2: Potential Attenuation with Distance from Rectifier (Undersized Bonding Jumpers)
A pipeline with a discontinuous pipeline joint due to a missing or broken bond wire can be recognized by a significant drop in the “instant off” potential from one test station to the next. The CP current has negligible influence on the pipe-to-soil potential beyond this point, as shown in Figure 3.
Figure 3: Potential Attenuation with Distance from Rectifier (Electrical Discontinuity)
Determining the appropriate size and quantity of joint bonds for a pipeline involves detailed calculations based on the steel cylinder's length, diameter, and thickness. The selection of joint bond type and quantity aims to fulfill a criterion specifying that the resistance of these bonds should fall within the range of 100% to 150% of the resistance of a single section of the pipe.
Ensuring the pipeline's electrical continuity is imperative for a CP system's efficiency. CP impresses current from the anode bed to the pipeline, and the current travels along the pipeline, returning to the rectifier to complete the electric circuit. Given the tendency of electric current to follow the path of least resistance, it predominantly flows through the pipe sections that maintain electrical continuity with the rectifier.
The installation of joint bonds serves two crucial purposes: maintaining the electrical continuity of the pipeline and diminishing the longitudinal resistance. While a pipeline is initially bonded for electrical continuity during construction, instances of high resistance or discontinuity in pipeline joints may occur after the pipe has been in operation, especially if the pipe experiences movement or deflection. If the pipe deflects beyond the strength of the joint bonds, the welded connections between the bonds and the pipe may break, resulting in the pipeline becoming discontinuous. This discontinuity in pipeline joints is a common factor contributing to the ineffectiveness of CP systems.
Linear Resistance Method
Some pipelines have installed test leads, permitting a more in-depth continuity testing survey. The test stations consist of two pairs of test wires. The section between the two test stations is the section of pipe that is the test span for measuring longitudinal current flow along the pipeline (this might include a pipeline section previously identified as a possible discontinuity using the attenuation survey method). By measuring the voltage drop (with polarity) and current across the test span, it is possible to calculate the resistance and direction of current flow in the pipeline.
To determine the resistance or continuity of the pipeline segment, a test current is applied across the test span on one pair of wires, and the resulting voltage drop is measured on the second pair of wires. The resistance of the pipe section is calculated from a variation of Ohms’s Law, resistance equals voltage (potential) divided by test current (R = V/I). The measured resistance is compared to the theoretical resistance, which is calculated using the physical dimensions of the pipeline and bonding wires. For guidance, if the measured resistance is less than 150% of the theoretical resistance of the pipeline, the pipeline has adequate electrical continuity for cathodic protection.
The theoretical resistance of a pipeline can be calculated using the following equation:
R=ρL/A
where: R is the linear resistance of the structure for length (ohms)
ρ is material resistivity (ohm-centimeters)
L is the length of the pipe section between the test stations
A is the cross-sectional area of the pipeline (centimeters squared)
At V&A, we are passionate about protecting infrastructure assets from the harmful effects of corrosion. Our corrosion specialists assist water and wastewater clients nationwide in evaluating potential corrosion activity on critical infrastructure assets and developing appropriate strategies for corrosion mitigation. We are available to answer your corrosion-related questions and provide project planning support. Feel free to contact one of our corrosion engineers at the link below.
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