Anode Current Ratings and Soil Resistivity

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Ted Huck

VP, Technical Sales

Jeffrey L. Didas Elected NACE International President

Chalfont, PA (April 2018) – MATCOR, Inc., the trusted full-service provider of proprietary cathodic protection products, systems, services and corrosion engineering solutions announces that senior engineer and pipeline practice lead Jeffrey L. Didas has been elected to the position of president for NACE International (NACE), the Worldwide Corrosion Authority. His term as president is one year commencing at the close of the NACE Corrosion Conference & Expo 2018, taking place April 15-19 in Phoenix, Arizona.

Jeffrey Didas, Practice Lead - Pipelines, MATCOR, Inc.
Jeffrey L. Didas will serve a one-year term as NACE president commencing at the close of the NACE Corrosion Conference & Expo 2018

As NACE president, Didas will advise, govern, oversee policy and direction, and assist with the leadership and promotion of NACE International to support the organization’s mission. He will also serve as chairman of the executive committee and an officer of the association. His responsibilities will include presiding at all official functions of the board of directors and executive committee, including the annual membership meeting of the association and the annual NACE banquet. This position is part of Didas’ five-year commitment to NACE following previous roles as vice president elect and vice president. Following his term as president he will serve one year as past president and one year on the nominating committee.

“I look forward to serving NACE as president over the next year,” said Didas. “My focus will be on member engagement, retention and benefits, moving forward with the strategic plan and our vision for NACE 2030, and promoting the groundbreaking corrosion industry IMPACT Study.”

Didas, an industry expert sought worldwide and active NACE member since 1975, has 44 years of diverse corrosion experience working for pipeline and energy company owner-operators and most recently for MATCOR.

Prior to his executive leadership roles, Didas held a variety of national NACE positions including:

  • Treasurer of NACE International, the NACE Foundation and the NACE Institute
  • Director of the Member Activities Committee – MAC
  • Committee chair for several technical exchange groups (TEGs), including the Corrosion Control Coordinating Committee (TEG 022X), Pipeline Crossings: Steel-Cased, Thrust-Bored, and HDD TEG 208X) and Steel-Cased Pipelines (TG 012)
  • Technology coordinator for technology management group TMG C1 – Corrosion Prevention and Control for Concrete, Land Transportation and Coating Technology
  • Vice chair of the NACE Institute Policy & Practices Committee
  • Member of the Technical Practices Committee – TPC/Technical Coordination Committee – TCC since 1978

He has also served as chair, vice-chair, and general member of several administrative committees over the past 44 years.

Didas received the NACE Brannon Award in 2014 and the NACE Distinguished Service Award in 2001 for his many contributions to the organization. He also received the Appalachian Underground Corrosion Short Course (AUCSC) Colonel Cox award in 2010.

Didas holds the highest level of NACE certification as a Corrosion Specialist and a number of other NACE certifications, including Cathodic Protection Specialist, Coatings Specialist, Chemical Treatment Specialist, Senior Corrosion Technologist, Corrosion Technologist, Corrosion Technician and Level 3 Certified Corrosion Inspector. In addition he is a SSPC (Society for Protective Coatings) certified Protective Coatings Specialist.

Didas graduated from Thomas A. Edison State University in Trenton, NJ, with a BSET in Electrical Engineering. He acquired his ASEE in Electronics Technology from Springfield Technical Community College in Springfield MA.

About NACE

NACE International, The Worldwide Corrosion Authority, serves nearly 36,000+ members in 130 countries and is recognized globally as the premier authority for corrosion control solutions. The organization offers technical training and certification programs, conferences, industry standards, reports, publications, technical journals, government relations activities and more. NACE International is headquartered in Houston, Texas, with offices in San Diego, California; Kuala Lumpur, Malaysia; Shanghai, China, Sao Paulo, Brazil and Al-Khobar, Saudi Arabia.

Visit nace.org for more information.

About MATCOR

Quick Ship Cathodic Protection for Tanks

This program applies to replacement cathodic protection systems for above ground storage tank (AST) bottoms.

cathodic protection for tanks
Tank Emergency? Contact Us About Our Quick Ship Cathodic Protection for Tanks Program

With existing ASTs, you may not always have the luxury of a planned tank bottom cathodic protection system replacement. After taking a storage tank out of service for inspection, you are often required to make an immediate decision as to the integrity of the existing floor. In some cases, this means a new floor has to be quickly planned and installed to minimize the time that the tank is out of service.

MATCOR Quick Ship Cathodic Protection for Tanks Program

MATCOR is pleased to announce our stock tank bottom anode system to meet your replacement needs with very short notice.

For your tank bottom replacement applications where a very fast delivery is required, MATCOR will now be maintaining stock of our Tank Ring Anode System.

  • Up to 200 ft diameter SPL-FBR tank ring anodes
  • Pre-assembled and ready to ship from our Chalfont PA facility
  • Two day turnaround
  • Set up in concentric rings with five foot spacing
  • Requires a minimum of just 6 inches of sand cover from the new tank bottom
  • Designed for 25 mA/ft output which is generally sufficient for 50+ year anode life based on a nominal current density of 2 ma/ft2 of surface area.

For more information please contact your MATCOR sales representative or contact us at the link below.


Have questions or need a quote for a replacement tank bottom cathodic protection system? Contact us at the link below.

CONTACT A CORROSION EXPERT

AC Mitigation: 4 Approaches

AC mitigation is the process of designing and applying pipeline grounding systems to:

  • Prevent voltage spikes during fault conditions
  • Reduce AC current density to protect against AC induced corrosion
  • Maintain AC step and touch potentials below 15 Vac to protect personnel from shock hazards
AC mitigation prevents voltage spikes and corrosion and protect workers
AC mitigation prevents voltage spikes, protects pipelines from corrosion and protects workers in areas where the pipeline parallels high voltage transmission lines.

Pipelines that parallel overhead high voltage AC transmission power systems are subject to AC interference. AC interference has several potential adverse impacts on the safety of personnel and pipeline integrity. Assuming that these conditions exist, there are several measures that can be taken to mitigate the AC interference present on a pipeline. These AC mitigation strategies are detailed in various international standards including NACE SP0177-2014 Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems.

There are four basic approaches to mitigating AC Interference. These mitigation strategies are:

1. Fault Shielding

One of the primary concerns with high voltage AC transmission systems parallel to buried pipelines is the risk that a fault condition at a transmission tower could result in the rapid discharge of fault current near the pipeline. This could lead to direct current arcing in soil – rare but very damaging. More common is the rapid ground potential rise that subjects the pipeline coating to large voltage gradients that result in coating damage. Fault shielding is a suitably designed grounding system that is installed between the tower footing and the pipeline that acts to shield the pipeline and shunt harmful currents away from the pipeline by providing a low resistance path to earth. This typically takes the form of a parallel shielding wire, either copper or zinc, connected to the pipeline.

2. Gradient Control Mats

When high levels of AC voltage are present on a pipeline, either during a fault condition or as the result of an inductive coupling during normal steady-state operations, personnel in close proximity to and/or touching any above ground or exposed appurtenance are at risk for electrical shock step or touch hazards. Installing gradient control mat, which is a system of buried bare conductors, typically galvanized steel, copper or zinc, connected to the structure, provides localized touch and step voltage protection by creating an equipotential area around the appurtenance.

3. Lumped Grounding Systems

Lumped pipeline grounding systems consist of shallow or deep localized grounding conductors that are connected to the structure at strategic locations to reduce the AC voltage level along the pipeline. This provides protection to the structure during steady-state or fault conditions from nearby electric transmission.

4. Gradient Control Wire

Gradient control wire grounding systems function the same as the lumped grounding system. With this type of system, long continuous grounding conductor(s) are installed horizontally and parallel to the pipeline. They are strategically located and sized to reduce the AC induced voltage along the pipeline during steady-state or fault conditions from nearby electric transmission.

For mitigating high levels of AC induced voltage along a pipeline, gradient control wires are the most common form of AC mitigation. Hybrid systems that combine lumped grounding systems with gradient control wires are also common. Regardless of the type of pipeline grounding system used, all of these AC mitigation approaches involve installing a grounding device to the affected structure to allow AC induced current and fault current to be quickly discharged off of the pipeline.

AC Modeling

Prior to installing an AC mitigation system, it is common to use a complex AC modeling software to evaluate the impact of fault currents and estimate the steady state induced currents that can be expected along the pipeline. This information is used to determine the quantity and location of mitigation required based on numerous factors, including the resistivity of the soil, the physical characteristics of the pipeline, the operating parameters of the HVAC transmission system and the spatial distances between them.

Engineered AC Mitigation Systems

Based on a thorough assessment of the pipeline and high voltage AC transmission system interaction, including modeling results when available, an AC mitigation system is designed by experienced engineers familiar with the mitigation strategies detailed above. This engineered AC mitigation system would detail the quantity and location of grounding installations required for a specific application. MATCOR’s MITIGATOR is an example of this type of AC mitigation system.

Other features of an engineered AC Mitigation system include:

Special Backfill

It is quite common to install the grounding conductor in a special backfill material. The purpose of the backfill can vary depending on the conductor material chosen and the type of backfill used. The benefits of various types of AC mitigation backfill include:

  • Enhanced surface area – conductive backfills such as carbon or conductive concrete are used to effectively increase the surface area of the grounding conductor reducing the overall resistance to earth.
  • Corrosion/Passivation Protection – some backfills are designed to protect the grounding conductor from corrosion or passivation of the conductor that could adversely affect the life or impede the performance of the grounding conductor.
  • Hydroscopicity – some hygroscopic backfills readily attract and retain water from the environment, helping to maintain a low uniform resistance around the grounding conductor.

Solid State Decouplers

These devices are almost always used in conjunction with AC mitigation systems and are usually installed wherever the grounding system is connected to the pipeline. These devices are designed to allow AC current to flow off the pipeline during steady-state or fault conditions while blocking all DC current. This effectively isolates the pipeline’s cathodic protection (CP) system from the AC mitigation system, preventing the mitigation system’s grounding conductors from taking CP current from the pipeline.

MATCOR provides complete AC Mitigation solutions including design, supply of materials, turnkey installations and comprehensive testing services.


Have questions or need a quote for an AC mitigation system or services? Contact us at the link below.

CONTACT A CORROSION EXPERT