Project Description
Upon joining Wolf Creek post-graduation, I was offered a position as the Cathodic Protection (CP) System Engineer. I quickly learned that CP is a method of protecting underground piping and structures from corrosion. After learning a little bit more about the system, I accepted the position and began my journey into learning about this system. After attending a week long certification course, I realized there was major work that needed accomplished on the system. The Close-Interval Survey (CIS) that had been scheduled for later in the year was not only extremely necessary but several years late. This gave me the opportunity to take the lead on the project and accomplish it over the course of my first year.
Background Information
Corrosion is an electrochemical process that is a side-effect of the smelting process. During metal smelting, metal ore is heated and melted to extract that elemental metal from the mixture. Metal ore is a natural rock primarily composed of oxidized metal compounds, e.g. iron ore is primarily hematite (Fe2O3) and magnetite (Fe3O4). Smelting strips the oxygen from the iron atoms, leaving behind the elemental iron, which is called steel when it has a low carbon content. The resulting steel is in a much higher energy state than the iron ore, as energy was added in the form of heat to extract the steel. When given a path, the steel will seek its lowest energy state, i.e. it will revert back to ore. This path takes the form of a reduction-oxidation (redox) reaction, which is an electrochemical process in which a circuit forms between two metal electrodes of different electro potential values in contact with each other via a metallic path and an electrolyte. Current flows in the electrolyte between the two electrodes via cation exchange, or ion flow, which accounts for the oxidation of the higher energy electrode, while electrons flow in the metallic path.
Fig. 1. Corrosion cell between two metals.
All metals have a natural range of electro potentials determined by many mechanical and chemical factors. It is impossible to smelt ore until it is perfectly pure, and the imperfections account for differing potentials throughout a metal. Additionally, different metals will have completely different electro potential ranges post-smelting. For example, zinc is more electro negative (has a higher hydration enthalpy and is therefore energetically favorable) than steel, while steel is more electro negative than copper. Therefore, corrosion cells will occur between two different metals (e.g. copper and steel), as well as two different locations of the same metal (e.g. steel and steel). This same metal corrosion is common between old and new pipe, as the old pipe is more electro positive (lower energy) than the new pipe.
Energy can also be added to the pipe mechanically by bending the metal. Bends in pipe are acute locations for higher potentials due to this energy addition. With these and other corrosion methods it is impossible to stop corrosion from occurring to buried metal exposed to an electrolyte without applying a form of CP.
CP works by increasing the electro potentials of buried pipe and structures beyond the range of oxidizing potentials, thereby protecting the pipe from corrosion. This is done using sacrificial anodes, which are pieces of metal that are intended to corrode in order to provide a protective corrosion current to the piping and structures being protected. Sacrificial anodes may be metals such as zinc that are highly electro negative and will provide protection naturally; however, the method used at Wolf Creek, known as an impressed current system, involves using a DC power source to drive highly electro positive, slowly depleting carbon-based electrodes into oxidizing. Normally these anodes, being more electro positive, would be protected by the steel structures, but the DC current drives the process in reverse. This makes for extremely long lasting sacrificial anodes providing a controllable protective corrosion current.
Fig. 2. Circuit diagram of impressed current CP system
Fig. 3. Example of impressed current CP system used at a plant.
At Wolf Creek, Cu-CuSO4 (CSE) reference cells, or half-cells, are used to measure the pipe-to-soil potential of a location of interest to ensure it is adequately protected. A half-cell is a metal electrode (in this case copper) at equilibrium in an electrolyte (aqueous copper sulfate) that acts as a known potential, aka reference potential. A location of interest is a specific point on a protected structure that is being measured by the CSE for the purposes of determining if it is adequately protected. Structures are not protected evenly for several reasons and require optimally spaced CSEs to gain a picture of the overall protection of the buried components. Occasionally it is recommended that a Close-Interval Survey (CIS) is performed. A CIS is an at-grade survey in which a surveyor takes a measurement with a CSE directly above the pipe, records both the potential and the location, and then moves 3-10 feet further along the pipe and takes another. The result is a fine grain view of the pipe's protection levels and identifies any locations that are under protected and can even identify the cause.
Necessity of the CIS
For modern CP systems, a CIS is not always necessary to determine if the system is adequately protected. After my certification course, however, I learned that Wolf Creek's system was extremely outdated. For any CP system, and especially apparent in impressed current systems, there is intrinsic error in the soil-to-potential measurements known as IR drop. IR
drop is the voltage drop that occurs in the circuit due to the resistance of the soil and metal conductors and the current running through them. The CSE detects this IR drop and provides a falsely high potential reading. This would not be a problem, except that soil resistance is constantly changing, primarily due to water content but also from ion flow changing the composition of the soil. Since IR drop cannot be predicted, it causes error in the readings, making it impossible to know protection levels. To remove IR drop, the current must be stopped, thereby dropping the IR drop to zero. This, however, is problematic. As can be seen in Figure 4, the protection levels begin decaying instantly after removing protection. These facts gave rise to a surveying method known as interrupted surveying, or interrupted surveys. This method involves installing "interrupters", which are contactor relays (relays capable of disconnecting high
Fig. 4. Graphical representation of pipe-to-soil readings over time if current were removed from the circuit.
current sources) that interrupt the circuit long enough to take a reading before reconnecting the rectifier circuit. This interruption cycle allows IR free readings to be taken at locations of interest without losing the true protection level of the structure. Most modern CP systems are installed with interruption capabilities. Wolf Creek's system was not installed with interrupters as that was not the industry standard at the time, and still does not have interrupters installed into the rectifiers' circuits. Prior to the CIS performed in November 2020, IR free readings had not been taken on the system since 2008. This left the system vulnerable during that time as the readings being taken each year did not accurately represent the protection levels of the buried structures.
Preparing for the CIS
The CIS required several steps to be completed prior to mobilization. These steps included determining the pipe runs to survey, developing a Request for Service Labor, developing a Temporary Configuration Change (TCC), meeting with the vendor to discuss survey details and our work practices, and receiving and installing interrupters sent by the vendor.
Request for Service Labor
When I inherited the system, the previous CP engineer had contacted a vendor, Structural Integrity Associates (SI), to set up a time to do the CIS. He had also identified key pipe runs that should be surveyed, number one amongst those being our Essential Service Water (ESW) system, which consisted of 99% of the survey. SI had developed graphical interface software (GIS) used for the buried pipe program and could use the same software to locate the pipe and update the software with the CP information. Therefore, we decided to use SI for the survey exclusively. I immediately got in touch with the SI representative and got a copy of the CIS proposal that had been developed. To forego acquiring multiple bids, I wrote a sole source document to justify the use of SI. As the owner of our GIS and a company with a track record of state-of-the-art expertise, the justification was not difficult to write. In addition to the sole source form, a pre-requisition worksheet, a data input form, and a statement of work were necessary. To wrap it together, a checklist verifying the information accurate was completed. This was the first time I had dealt with this lengthy process and was able to rely on my more experienced coworkers for help and guidance.
Temporary Configuration Change (TCC)
The most interesting piece of the project was writing the TCC. A TCC is essentially an engineering disposition detailing the temporary modification and identifying critical characteristics of the modification that could impact the plant. This TCC was focused on the electrical engineering concepts of installing temporary interrupters into the DC output circuit of the rectifiers. As a mechanical engineer, I have experience with electrical concepts, but have never designed an electrical system. Designing the modification was a great opportunity for me to develop a broader skill set and become more knowledgeable about concepts with which I was largely unfamiliar.
The crucial piece of the modification was installing the interrupters into the rectifier circuit. A rectifier takes power from an AC power source and converts it to DC power using a diode bridge. We use rectifiers for our impressed current system as they are a reliable source of power that can be easily scaled up or down using the rectifier taps, which is a transformer function of the rectifier. Interrupters, or contactors, are relays designed for high power operation that one can encounter with CP rectifiers. Additionally, the relays must acquire GPS lock in order to synchronize the timing function of the relays. This allows all of the rectifiers to be interrupted simultaneously, completely removing the influence of a distant rectifier from the reading.
The relays to be sent to us were ADI-100's, solid-state relays contained in a box with the electronics capable of locking onto GPS and syncing with each other to perform the synchronous interruption. These normally closed relays allow power to flow through the circuit until receiving the signal to open the circuit, thereby removing power. The relay switching parameters were 140V, 100A, and 2000W. These parameters are used to keep from damaging the relays when installed into the rectifier output circuit. As the ADI-100's are solid-state, they can
Fig. 5. An ADI-100, the gray box, installed on the DC output of a rectifier using 4 gauge welding wire.
be used in series or parallel to split voltage or current, respectively, to reduce the strain on any one relay. This is due to extremely tight switching tolerances. As part of the TCC, I determined the number of relays that would be needed for each rectifier and the configuration (series or parallel) they would need to be in. As none of the rectifiers' design exceed 140V, the relays would not be configured in series, leaving the limiting factor as the 2000W parameter of the relays. To ensure that no relay interrupted more than 2000W, several of the rectifiers would require several relays in parallel on the output to satisfy this requirement. This design factor was the most crucial of the TCC, with power distribution block sizing and conductor sizing also contributing to the design.
Performing the CIS
Upon completing the TCC, the equipment needed for the CIS was relayed to MESA, who verified the number of relays necessary for the installation and confirmed they would have the number necessary available for the start of the survey. The survey had been scheduled for early November to align with MESA's availability and electrical maintenance shop resources
at the plant. When the time came to perform the CIS, MESA sent the required interrupters to us to install. It took approximately three days to install all of the interrupters. As can be seen by the slideshow to the right, the number of relays per rectifier varied from 1 to 12, depending on the rectifier. This may seem excessive, and indeed the surveyors admitted they had never seen that many interrupters needed for a rectifier, however, two of Wolf Creek's rectifiers provide 95% of the plant's protection via a remote anode bed consisting of 100 anodes. It was these two rectifiers that required the many interrupters.
The survey was conducted over a period of 5 days, which included the over-the-line survey of the indicated piping systems, measurements taken from the test stations positioned in key spots throughout the site, and adjustments to the system based on
preliminary CIS readings. The adjustments made to the system were to adjust the voltage level of the anode bed rectifiers down, as more energy than needed was being used to protect the site. There was actually piping being over protected, which can cause coating disbondment and hydrogen embrittlement. By adjusting the rectifiers down, less energy is being used to protect the system, anodes will last longer, and less maintenance will be necessary on the rectifiers in the future.
Conclusions
After the surveyors had demobilized, SI wrote a report for the system. Using the GIS software, they mapped the piping that had been surveyed, using color codes of green (within range), yellow (over protected), and red (under protected) to provide a visual representation of the protection of the system.
Fig. 6. Representation of protection levels for the areas surveyed during the CIS.
The effectiveness of the system was provided in the report. Additionally, the raw data of the survey was provided to us for further analysis. I was able to incorporate the CIS data into the annual report for the system, which includes a system description, the work we had performed throughout the year, and plans for the system going forward.