This article describes the components of a cathodic protection rectifier, and when to use oil cooled cathodic protection transformer rectifiers vs. air cooled rectifiers.
When it comes to cathodic protection power supplies, conventional transformer rectifier circuits have long been employed by the cathodic protection industry for impressed current CP systems. These power supplies (commonly referred to as rectifiers in the CP world) consist of three main components; the transformer, the rectification stack, and a cabinet to house these components. The transformer takes the input AC voltage on the primary side and controls the output AC voltage on the secondary side. The rectification stack, typically silicon diode stacks which have largely replaced older less efficient selenium stacks, convert the AC input wave form into a DC wave form by cycling the AC flows in one direction and blocking in the other. Additional components typically include circuit breakers, fuses, voltage and current output meters, lightning arrestors, surge suppressors, transformer tap bars, and monitoring systems.
The majority of these Rectifiers are housed in air-cooled NEMA 3R enclosures – these enclosures are typically constructed of hot dipped galvanized steel, aluminum, stainless steel or painted steel. NEMA 3R enclosures are intended for outdoor use. They provide a degree of protection against falling rain and ice formation but are not completely water tight or weather proof and could be subjected to beating rain or streams of water, under certain conditions, entering the enclosure. This is the most common type of rectifier enclosure in the industry.
When and Where to Use Oil Cooled Cathodic Protection Transformer Rectifiers
For some applications; however, the use of air cooled NEMA 3R enclosures is not recommended or not suitable. The three most common reasons not to use air-cooled NEMA 3R enclosures are:
Rectifier transformer size is too large to support an air cooled enclosure. For a small percentage of impressed current CP systems where the power requirements (measured in DC Watts) are sufficiently high that the cooling capacity of the enclosure is insufficient for the heat generated by the transformer (typically anything more than 12kW for single phase and 18kW for three phase.)
Severe environment locations where high humidity, dust or other situations could shorten the life of a standard air cooled rectifier. Marine and near shore applications often fall into this category.
The enclosure must be in a hazardous classified location requiring Class 1 Div. 2, Group D compliant enclosure – commonly referred to as Explosion Proof.
For these applications, oil cooled cathodic protection transformer rectifiers are typically specified. As implied in the name, the oil cooled rectifier utilizes an enclosure that has a sealed reservoir which houses the transformer and transformer tap bars and is filled with a special transformer oil. The transformer oil provides better heat transfer and dissipation and the larger case facilitates improved heat removal.
It is very important to note that standard oil cooled rectifiers are NOT explosion proof. For an oil cooled rectifier to be considered Explosion Proof, the components that are not immersed in the transformer oil reservoir must be housed in special Explosion Proof fixtures. Simply specifying oil cooled when ordering a rectifier does not satisfy the requirements for locating the rectifier in a hazardous Class 1 Div.2 location without also including the additional provisions required for the explosion proof fittings.
To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote for cathodic protection materials or related construction services, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.
MATCOR is pleased to announce that we are now capable of performing Helium leak testing on our full range of linear anode products as an optional testing service. This is a common practice among companies and product developers that provide products that could potentially leak gas or that require water tightness. Products commonly leak tested include refrigeration lines, vehicle brake lines, and devices that contain potentially harmful or deadly substances. Helium is the second smallest element (Hydrogen is the smallest), which means that it is valuable for leak testing. Smaller molecules naturally can find smaller gaps or defects from which to leak. Unlike hydrogen, however, helium is a noble gas and is therefore unreactive due to its complete valence electron shell. As a result, helium is the most viable gas for use in leak testing.
MATCOR has enjoyed an outstanding record as the world’s leading supplier of MMO anodes/Titanium linear anodes with over 25 years of linear anode experience supplying our industry leading SPL™ family of linear anode products for pipelines, tanks and other applications around the world. Our patented automated injection molded Kynex® connection technology has an outstanding track record with no known connection failures since this technology was introduced in 2009.
We do, however, see some client specifications calling for 100% connection testing and helium leak testing is the most effective means to test an entire anode assembly.
At MATCOR, we pride ourselves on being a world class manufacturer of unique cathodic protection systems and AC mitigation systems. Our anode systems offer you longer life, lower total installed cost, and are safer and easier to install than many conventional anode solutions. We have earned a reputation for exceptional manufacturing quality—but all companies say their products are world class and have exceptional quality, right? What makes MATCOR different? What does it mean to be exceptional?
At our state of the art Chalfont, Pennsylvania manufacturing facility we have developed a culture of quality. That is not to imply that we are perfect or that we don’t occasionally make a mistake; we are not perfect. However, we HAVE embraced, through our ISO Certified Quality Management System, a systematic approach towards excellence. So, while everyone aspires to do a quality job, our manufacturing team’s quality culture is based on perspiration—we work relentlessly to do a quality job for YOU by embracing the key tenets of quality.
Through our Manufacturing Quality Management System, we:
Document procedures for what we do
Train our team on the proper processes
Hold ourselves and our suppliers to high quality standards
Self-audit to ensure we are doing what we say we will do
Measure our performance daily through KPIs (key performance indicators)
Strive to continuously improve
Collect and act on YOUR feedback, comments and complaints
We’d love to hear from you about our manufacturing quality, please comment or contact us at the link below.
To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.
This article provides a brief overview of the important role of cathodic protection remote monitoring systems in today’s pipeline operations. We will cover the CP equipment and features that can be monitored and how data is transmitted.
Modern pipeline operations face increasing pressures to incorporate advanced technologies to:
Drive down operating costs
Improve system reliability
Comply with regulatory requirements
Monitor the health of their pipeline networks
Monitor the critical systems that are integral to pipeline integrity
The use of advanced cathodic protection remote monitoring systems has become a critical component in the pipeline operator’s toolbox to meet these challenges.
CP remote monitoring (and control) has proven to be a reliable and cost-effective means to oversee the proper functioning of cathodic protection systems and AC Mitigation systems that are critical to assuring pipeline integrity and the proper protection against pipeline corrosion. Where operators in the past would have to send technicians out to remote pipeline locations to collect snapshot data on a frequent basis, the smart deployment of cathodic protection remote monitoring systems can provide continuous real time data that can be accessed from any cloud connected handheld or desktop device. Additionally, a remote monitoring unit for cathodic protection is well-insulated; this construction affords them excellent protection against lightning strikes. The financial, environmental and safety impact of eliminating hundreds of thousands of windshield hours is staggering across the vast pipeline industry.
Cathodic Protection Remote Monitoring – What can you monitor?
Cathodic Protection Rectifiers – the installation of RMUs with built in interruption capabilities should be standard on all new pipeline installations and retrofitting older units can provide significant cost savings and improve CP system reliability.
DC Cathodic Protection Test Stations – with today’s continuing advances in remote monitoring technology and costs, it is quickly becoming very cost effective to install remote monitoring units on all test stations. When combined with the ability to easily interrupt all of the influencing current sources on a pipeline, regularly scheduled testing of the CP system can be performed quickly and at virtually no cost.
AC and DC Coupon Test Stations – the latest NACE guidelines for AC Mitigation (SP21424-2018*) emphasize that the localized DC current density has a significant impact on AC corrosion and gathering data on both AC and DC current densities at areas of interest/risk is critical to a successful AC Mitigation strategy. Effectively doing so requires the ability to monitor these values over time as AC loads vary during the day and seasonally.
Critical Bonds – monitoring the effectiveness of critical bonds is necessary (and in many cases required by local regulatory bodies) to assure pipeline integrity.
NACE SP21424-2018 “Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation, and Monitoring”
How does a CP remote monitoring system transmit data?
Today’s operators have a range of options to assure that remote monitoring systems can regularly communicate data to their host data collection systems. The availability of conventional cellular networks combined with various commercial satellite systems assures pipeline operators of the ability to communicate with devices in even the remotest of locations. Your monitoring system provider can work with you to select the appropriate communications technology for each cathodic protection remote monitoring unit (CP RMU) location.
In addition to choosing how the communication is to occur, another key factor to consider is whether the communications are to be one way (monitoring only) or two-way (monitoring and control). For test station applications where data collection is the goal, one way transmission of the monitoring unit’s data is all that is required. For rectifier units, the ability to control the system output and/or the ability to initiate an interruption cycle for close interval surveys or test station polling purposes necessitates the ability of the remote monitoring unit to receive and act on communications as well as to transmit data.
Software Interfaces – Installing the appropriate CP RMU hardware is just one step in implementing a successful remote monitoring (and control) program. The data must be collected, stored, and accessible for the operator. Sophisticated cloud-based interfaces have been developed that incorporate critical features including firewall-friendly, password protected internet browser access. These systems allow for multiple client user accounts with configurable permission levels and automated alarm and status information including email and text alerts for designated alarm conditions.
In summary, the use of remote monitoring technology is a key component to the successful operation of any modern pipeline integrity management program. While MATCOR has extensive experience with all of the major RMU manufacturers, we have recently teamed up with Mobiltex, a leader in the field of remote monitoring, to bring state of the art technology to the pipeline and cathodic protection industry. Mobiltex’s CorTalk® line of CP RMU units combined with their CorView interface offers all the features necessary to implement a comprehensive, cost-effective, and highly robust cathodic protection remote monitoring program.
Please contact us at the link below if you have questions about cathodic protection remote monitoring, or if you need a quote for services or materials.
Pipeline cathodic protection design for new pipelines may appear to be a rather easy task for anyone with a basic understanding of cathodic protection. However, as with all design efforts there are a wide number of factors that need to be considered for a sound design that meets generally accepted industry practices.
This article highlights 12 things that the pipeline cathodic protection system designer needs to consider when developing a CP system design. This is not intended to be a comprehensive list as every project has its own unique challenges, but these 12 items would all typically have to be addressed during the design phase. It is assumed that the basic pipeline information is already available to the CP designer including pipeline length, pipeline routing and pipeline characteristics (material, wall thickness, coating type, operating temperature, etc.). Armed with this basic information the CP designer should also consider the following in their design efforts.
12 Things to Consider for New Pipeline Cathodic Protection Design
Soil Resistivity is a factor in many of the design calculations and assumptions (e.g. current requirement, anode resistance, attenuation, AC interference, etc…) Actual soil resistivity data should be collected along the proposed route. Learn about soil resistivity testing.
Design current requirement is selected based on the soil type(s) using some accepted industry guidelines taking into consideration the coating manufacturer’s recommended coating efficiency or other industry accepted guidelines. Additional current requirements for mitigating interference currents should be considered based on the designer’s experience.
Distribution of CP System Stations should take into consideration the total current required, the pipeline attenuation characteristics, the availability of power for impressed current cathodic protection systems, varying soil regimes, isolation valves and other factors to determine how many, what size and where each CP System will be located.
Foreign pipelines and other DC interference sources should be evaluated as part of the CP system design efforts and generally warrant immediate mitigation measures or testing and monitoring provisions for observation and assessment.
AC Interference assessment should be performed to determine if there are one or more high risk categories for AC Interference. Should the initial assessment confirm that there is potential for AC Interference an experienced AC Interference and Mitigation specialist would typically use sophisticated AC modeling to assess the risk and propose appropriate mitigation. From a CP perspective, there is a relationship between DC current density and AC induced corrosion risks where too much cathodic protection accelerates the AC induced corrosion rate so care must be exercised by the CP designer to avoid high DC current densities in AC risk areas.
CP Station design includes the type of anode configuration, anode selection, installation methodology, etc… The CP designer will typically provide detailed Bill of Materials as well as CP System issued for construction drawings and construction details showing the location of equipment and providing installation instructions.
Isolation of MLVs and Stations is a key design criterion that impacts the pipeline cathodic protection system design. Some owners are strongly in favor of isolation of MLVs and Stations from their main pipeline while other owners prefer not to isolate and have to maintain isolation and instead require the that CP system be sized to account for losses to current drains.
Power supply type, sizing and selection is another of the decisions that is determined by the CP designer with consideration given to the pipeline owners specifications and preferences. For most pipeline applications, impressed current systems are typical and these require a DC power source. Electrical AC to DC power supplies (“rectifiers”) are the most common power supply but for remote areas with limited AC power availability, alternate power supplies such as solar, wind, fuel cells, thermo-electric generators or other sources may be required.
Terminal piping is often associated with a new pipeline construction project and the pipeline CP system designer must often provide a supplemental design specifically for the terminal or station piping, or account for these in the primary pipeline CP system design
Use of temporary CP systems is often recommended when permanent power may not be available for some time. These typically involve the installation of galvanic anodes strategically along the pipeline.
Provisions for testing and monitoring are critical components to any successful pipeline CP system design. This often includes the use of remote monitoring systems for all of the system power supplies, specialized test coupons for AC and DC Interference, and numerous cathodic protection test stations placed at the appropriate strategic locations to be able to properly test and monitor the CP system performance.
As noted earlier, this is far from a comprehensive list of all of the factors for a specific pipeline CP System design. Every project may have its own unique challenges; however, the 12 items listed above represent a great starting point for any new pipeline cathodic protection system design challenge.
Please contact us at the link below if you have questions about pipeline cathodic protection design, or if you need a quote for services or materials.
This article discusses the most common soil resistivity testing method and provides some guidelines for properly collecting sufficient data for the cathodic protection system designer.
One of the most important design parameters when considering the application of cathodic protection for buried structures is the resistivity of the soil. Soil resistivity testing is an important consideration for assessing the corrosivity of the environment to buried structures. It also has a tremendous impact on the selection of anode type, quantity, and configuration. Thus, it is critical that the CP designer have accurate data on the soil conditions at both the structure and at any proposed anode system locations. The lack of sufficient soil resistivity data can render a cathodic protection system (CP system) design ineffective and can result in costly remediation efforts during commissioning.
Soil resistivity is the principal diagnostic factor used to evaluate soil corrosivity. When performing soil resistivity testing, there are numerous factors that can be assessed, including soil composition, moisture content, pH, chloride and sulfate ion concentrations, and redox potential. These are all common components of a lab or in-situ soil testing program and all have an impact on soil resistivity. While a comprehensive soil testing program may be warranted, especially when performing failure analysis, for most environments the soil resistivity testing data provides an outstanding basis for assessing soil corrosivity. Below is a typical chart correlating soil resistivity with soil corrosivity.
While there are several methods for measuring soil resistivity, the most common field testing method is the Wenner four-pin method (ASTM G57). This test uses four metal probes, driven into the ground and spaced equidistant from each other. The outer pins are connected to a current source (I) and the inner pins are connected to a volt meter (V) as shown in Figure 1.
When a known current is injected in the soil through the outer probes, the inner probes can be used to measure voltage drop due to resistance of the soil path as current passes between the outer probes. That resistance value R can then be converted into a soil resistivity value with the formula: ρ=2×π×a×R where “ρ” is measured in ohm-cm and “a” is the spacing of the pins in cm. This value represents the average soil resistivity at the depth equivalent to the spacing of the probes so if the probes are spaced 5 foot apart, the value derived would be equivalent to the average soil resistivity at 5 foot depth.
For cathodic protection system design, it is common to take multiple soil resistivity measurements using this methodology with various probe spacings. For shallow anode placement, it is usually sufficient to take reading readings at 2.5 ft, 5 ft, 10 ft, 20 ft, 25 ft. For deep anode applications, soil resistivity measurements may be recommended at much deeper depths corresponding with the anticipated depth of the deep anode system.
It is important to note that the soil resistivity values generated from the four pin testing represent the average soil resistivity from the earth surface down to the depth, and each subsequent probe spacing includes all of the shallow resistance readings above it. For cathodic protection design purposes, it is often necessary to determine the resistance of the soil at the anode depth by “subtracting” the top layers from the deep readings. This process of “subtracting” the top layers requires some form of computational adjustment. One popular approach is called the Barnes method which assumes soil layers of uniform thickness with boundaries parallel to the surface of the earth. If the measured data indicates decreasing resistance with increasing electrode spacing, this method can be used to estimate the layer resistivities.
The resistance data (R) values should be laid out in a tabular format and then converted to conductance which is simply the reciprocal of the resistance value. The change in conductance is then calculated for each subsequent spacing. That value is then converted back to a layer resistance value by taking the reciprocal of the change in conductance. Finally, the layer resistivity is calculated using ρ=2×π×a×R.
For the Barnes analysis below, the data shows that a low resistance zone exists between 60m depth and 100m depth.
Spacing a (m)
Conductance 1/R (Siemens)
Change in Conductance (Siemens)
Layer Resistance (ohms)
Layer Resistivity (Ohm-m)
Soil Resistivity Testing Equipment Considerations
Electrically speaking, the earth can be a rather noisy environment with overhead power lines, electric substations, railroad tracks, and many other sources that contribute to signal noise. This can distort readings, potentially resulting in significant errors. For this reason, specialized soil meter equipment that includes sophisticated electronic packages capable of filtering out the noise is critical when taking soil resistivity data.
There are two basic types of soil resistivity meters: high-frequency and low-frequency meters.
High-frequency Soil Resistivity Meters
High-frequency meters operate at frequencies well above 60 hz and should be limited to data collection of about 100 feet in depth. This is because they lack sufficient voltage to handle long traverses and they induce noise voltage in the potential leads which cannot be filtered out as the soil resistivity decreases and the probe spacing increases. These are less expensive than their Low-Frequency counter parts and are by far the most common meter used for soil resistivity testing. For CP design purposes, these are frequently used to assess soil corrosivity and for designing shallow anode applications.
Low-frequency Soil Resistivity Meters
Low-frequency meters generate pulses in the 0.5 to 2.0 hz range and are the preferred equipment for deeper soil resistivity readings as they can take readings with extremely large probe spacings. Some models can operate with spacings many thousands of feet in distance. These models typically include more sophisticated electronics filtering packages that are superior to those found in high-frequency models. For CP designs involving deep anode installations, a low-frequency meter is the preferred equipment to provide accurate data at depths below 100 ft.
Field Data Considerations
When collecting accurate soil resistivity data for cathodic protection system design, it is important that the following best practices are taken into consideration to avoid erroneous readings:
Suitability of the testing location.The use of the Wenner four pin testing method requires sufficient open area to properly space the pins to collect data to the depths necessary. For deep anode cathodic protection systems this would require a minimum of three times the anticipated anode system depth.
Avoidance of buried piping and other metallic objects. The presence of any buried metallic structures (piping, conduit, reinforced concrete structures, grounding systems, etc…) provides low current paths that could cause a short-cutting effect that would distort the resistance readings and yield an erroneous soil resistivity reading.
Depth of the probes. It is important that the probes are properly inserted into the earth. For shallow resistivity readings, probes that are driven too deep can impact the shallow readings. Ideally, the pins should be no deeper that 1/20th of the spacing between the pins and no more than 10 cm (4 inches) deep.
Avoid areas of high electrical noise. Soil testing should not be performed directly under high voltage transmission systems or near other outside sources of current in the soil such as DC light rail systems.
Accurately record the test location and conditions. It is important that the location of the testing is accurately recorded along with the soil conditions and temperature at the time of testing. Testing should not be performed in frozen soil, or during periods of extreme drought or abnormally wet conditions.
Soil resistivity testing with accurate collection of data is the best indicator of the corrosivity of the soil for buried metallic structures and has a significant impact on the design of cathodic protection systems. The most common test methodology for field collection of soil data is the Wenner four pin method. When properly collected, and using appropriate analytical techniques, the soil resistance field data can provide an accurate assessment of soil resistivity values for use in designing an appropriate cathodic protection system.
We appreciate the question: “How does soil resistivity impact current rating.” The short answer is that resistance has nothing to do with anode rating. Here is a more detailed response:
Anode current rating – all anodes have a current rating based on how long they can be expected to operate at a given current rating. All anodes have some defined expected life based on current output and time – so many Amp-Hours of service life. For example a magnesium anode may have an expected consumption rate of 17 lb/Amp-year (7.8 kg/amp) so if a 17 lb anode is operated at 0.1 amps it would have a life of 10 years. For MMO anodes, they too have an expected life. For our linear anode rated at 51 mA/m it is important to know that that rating is actually 51 mA/m for 25 years. So a 100m anode segment with this rating would have an expected life of 127.5 Amp-years. If this anode were operated at 5.1 amps (full rated capacity) it would be expected to operate for 25 years. IF it were operated at 2.55 amps (50% of rated capacity) it should last 50 years. The anode life is generally linear. Please note that resistance has nothing to do with the anode current rating – the anode current rating merely calculates the life of the anode as a function of how many amps for how long of time.
Actual current output – just because you install an anode rated for 5.1 amps for 25 years (our 100m segment of 51 mA/m SPL-FBR) does not mean that the anode will output this amount of current. It just means that at that current rating you can expect 25 years of life. The anode is merely one component of the overall cathodic protection circuit. The actual output of the anode is function of Ohms Law ( Voltage = Current * Resistance). It would make sense to note that if the system Voltage were zero (the rectifier were turned off or disconnected) then the anode would not have any current output. Likewise if the 100m anode segment were installed in a very low resistance environment and driven by a powerful rectifier, the current could be much higher than 5.1 amps which would result in a much shorter life.
Why anode rating is important to the CP designer – the CP designer is tasked with protecting a specific structure for a given period of time (protect this pipeline for 30 years.) The CP designer then calculates, based on actual testing or established guidelines, the amount of current that should be sufficient to achieve appropriate CP levels to protect the structure. This results in an answer of some number X of amps required. If the requirements are to protect the structure for Y number of years, then the anode life required is X * Y (# of amps times # of years). This defines the minimum amount of anode life that is needed.
The next question the CP designer must address, once it is determined how much current is needed, is how to design a system that will generate that amount of current. Since Ohms Law dictates that Voltage = Current * Resistance (V=IR) then if we know that the Current = Voltage/Resistance (I=V/R.) Thus the CP designer must understand how to calculate system resistance (R) and must provide sufficient driving force (V) Several factors affect system resistance (R) including anode geometry – the longer an anode, the lower its resistance – which in many applications is a big benefit to the linear anode. One of the great benefits of the linear anode is that because of its length, in most applications the soil resistivity plays a lesser role since the anode resistance to earth is generally low for a wide range of soil resistivities due to its length. For extremely high resistance environments, linear anodes may be the best option since short anodes will not have a low enough resistance.
There are other factors that go into CP design including current distribution and making sure sufficient current is being applied across the entire structure.
CP Design can be very complicated. I hope that the above explanation is helpful, but if there is a specific application to evaluate, please contact us with the details. We are also available, for a reasonable engineering fee, to develop and/or review CP system designs.
Marine environments can be some of the harshest environments on the planet for corrosion of steel structures. Indeed, the earliest application of cathodic protection can be traced back to Sir Humphrey Davy and the British Navy’s investigation into corrosion on copper sheathed wooden vessels. This video demonstrates MATCOR’s impressed current sled anodes that are successfully being used to protect steel piles for jetties, docks and other similar steel structures in marine environments.
At 1:03 in the video, we demonstrate how the marine anode sled operates with a trade show model.
At 4:05 you see a MATCOR Sea-Bottom Marine Anode Sled being lowered into the water as part of the cathodic protection system protecting a steel jetty structure in Indonesia. The jetty is constructed with four interior rows of concrete piles and an exterior row of 247 bare metallic piles. The operator initially considered galvanic anodes to protect the jetty from corrosion – until they compared the cost, time and effort to install the required 374 aluminum anodes each weighing 200 each. Instead they opted for six marine anode sleds, taking only three days to install.
For assistance with near shore marine anode systems, please CONTACT US.
Whether designing a few above ground storage tanks or performing tank farm design for an entire facility, proper consideration should be given to the adverse impact of corrosion that can occur on the tank bottoms. When addressing the issue of tank bottom corrosion, consider the environment, the tank size and design, and the type of tank foundation to be employed. There are definite advantages in certain materials based on the size and requirements of an above ground storage tank (AST) foundation. By carefully assessing the tank farm surroundings and long-term requirements, costly and potentially dangerous corrosion related tank failures can be avoided. Whether you are relying on a reputable company in the industry or taking on your own front-end engineering and design, there are across-the-board tank farm design recommendations to consider when it comes to corrosion prevention:
In terms of corrosion prevention for under ground storage tank (AST) foundations, is cathodic protection (CP) effective?
For tanks erected on compacted soil or sand foundations, with or without a concrete ring wall, cathodic protection is considered a “good engineering practice” and has been proven as an effective means of addressing tank bottom corrosion concerns. When you compare various methods of corrosion prevention for above ground storage tank bottoms, CP is shown to prevail over asphalt or concrete unless your project involves smaller diameter tanks. The corrosion failure rate is greater for tanks built on asphalt or concrete compared to tanks where a concentric ring cathodic protection system is installed.
In terms of corrosion, when is asphalt or oil/sand acceptable for above ground storage tank (AST) foundations?
Asphalt foundations are not common in the United States, as the mechanical integrity of asphalt can be an issue depending on the AST environment. As well, the use of oil/sand layer designs has been phased out by most tank owners in the United States due to the adverse impact that these oil/sand layers have on tank bottom cathodic protection systems. While historically prevalent in the Middle East and Asia, most larger national oil companies have abandoned this approach because it causes shielding of cathodic protection (CP) current, allowing corrosion to occur. Kuwait Oil, Aramco, and others now prefer clean sand combined with CP as the base material of choice. This is standard in the United States and has been for several decades.
What is a Concentric Ring Cathodic Protection System for above ground storage tanks (AST)?
A. Designed for long-term storage, an AST cathodic protection ring system offers a factory-assembled design whereby the anode rings are ready to install with cable leads that extend past ring wall penetration. Concentric rings sizes are made to order, requiring no onsite welding, cutting, or splicing. The anode locations are marked, rings are laid out, and cabling is placed using a proven labeling system for future monitoring. A mixed metal oxide (MMO) anode is centered among a low-oxygen-generating coke backfill to eliminate depolarization.
Are there some cases where concrete foundations are advantageous for tank farm corrosion prevention?
During installation of above-ground storage tanks, there are some advantages to concrete foundations for tanks when it comes to corrosion—the high pH of the concrete acts to passivate the steel, unless you have an above ground storage tank (AST) liner pad or something that is between the concrete and the tank bottom. If you can effectively seal the chime from the ingress of water and oxygen, the corrosion rates are generally quite small. Unfortunately concrete foundations for larger diameter tanks are not typically practical and can be quite expensive to properly install. Concrete foundations with appropriate AST liners are best for smaller diameter tanks.
In tank farm design for corrosion prevention, what are the best recommendations for above ground storage tank (AST) liners?
Plastic secondary containment liners are largely phased out in the United States and have been replaced by geotextile membranes that serve the same secondary containment purpose as plastic—they are conductive to allow cathodic protection (CP). The general standard in the United States is to have a CP system directly under the tank in order to minimize stray current or current losses due to earthing systems around the tank. Since the tank bottom is a large bare structure and the anodes are closely coupled to the tank bottom, there is usually very little current drain to other structures; the system if properly designed can accommodate modest current drain. While a plastic liner provides isolation from other nearby structures, when a problem arises with the CP system or if the CP system reaches the end of its projected service life, there is no way to install a new CP system without replacing the tank bottom.
Tank farm corrosion prevention is more manageable now than ever before. The MATCOR Concentric Ring Cathodic Protection System™ is just one of many excellent options for protecting your above ground storage tank (AST) from damaging corrosion.
For assistance with tank farm design, our Concentric Ring AST Cathodic Protection System™, project management or installation, please CONTACT US.
This presentation explores current tank cathodic protection trends, specifically for above ground storage tanks.
Statistics show owners of above ground tanks often experience external corrosion issues because of limited or poor installation methods. Typical above ground storage tank (AST) methods of the past involve a ring wall foundation that is generally formed with a sand or soil base, or even concrete for smaller tanks. It has previously been acceptable to use a galvanic ribbon anode system (generally magnesium), but this system often fails prematurely due to unstable sand-based foundations and poor connections. For this reason, the industry is moving away from the galvanic anode system and to newer concentric ring tank cathodic protection systems for above ground storage tanks.
Good Engineering Practices
While there are newer designs for AST cathodic protection systems, your first consideration should always be good engineering practices. The proper installation of a high-end tank cathodic protection system begins with known design specifications based on the tank size and diameter. This presentation compares traditional grid anode systems with newer linear anode concentric ring systems for the cathodic protection of above ground storage tank bottoms. In addition, congested terminal environments often lead to interference and less current at the tank bottom.
Grid Anode vs. Concentric Ring Tank Cathodic Protection Systems
While the field-fabricated and field installed grid anode system has been in use for over 20 years, some faults have been discovered. Field installation presents welding challenges for the contractor because the system must first be secured, and it cannot be installed directly over sheet liner. The ribbon anode and titanium conductor bars have to be field cut to the appropriate lengths. At the conductor bar to anode ribbon intersections, a weld is applied. The field assembled grid system is subject to weld failures, the spot welds can be damaged easily during subsequent sand installation, and care must be taken to hold the system in place so that it does not short to the tank bottom. All of these installation challenges can adversely impact the system performance. Additionally, bare MMO in sand is an oxygen generator when used for cathodic protection. Oxygen is a depolarizer and in some instances this can lead to issues with maintaining polarization criteria.
Advantages of the Concentric Ring System
In comparison, newer concentric ring systems for above ground storage tanks include factory assembled anode rings that come equipped with the appropriate cable leads to extend past the ring wall penetration. No onsite field assembly is required. The system is pre-assembled in concentric ring sizes designed for your tank and requires no cutting, splicing, or welding, and the MMO wire is backfilled within a braided fabric sleeve with coke breeze. Anode locations are simply marked, each ring is laid out at the proper diameter, and cabling is extended toward the ring wall. The anode cables are labeled for ease of identification and to allow for monitoring of current to each anode ring. Unlike the grid system, the MMO anode is centered in a coke backfill – this coke environment inhibits the generation of oxygen eliminating the issues with depolarization.
The concentric ring tank cathodic protection system is designed for longevity. A typical under-tank ring system using MMO anodes exceeds a 30-year life, however can be designed to extend life beyond 100 years.
Additional Considerations for Tank CP
Some tank operators opt for a “replaceable” anode system, however time and manpower are required to extract and replace the anodes and backfill and the design life is only 30 years.
Volatile corrosion inhibitors (VCI) are often used in conjunction with cathodic protection systems where CP cannot be installed or may be ineffective, such as ring wall crevices, poor bottom-to-sand contact, and more. It can be pumped under tanks via shielding high-density polyethylene (HDPE) containment liners.
Today, tank owners have more effective choices than traditional grid anode systems for tank cathodic protection. The MATCOR Tank Ring Anode™ System is trending as a high-end solution for above ground storage (AST) tanks.