Looking for a better tank cathodic protection system?
Cathodic Protection of the external tank bottom for large diameter above ground storage tanks has been adopted as good engineering practice around the world.
Unfortunately, many existing grid anode systems have experienced premature failures, resulting in excessive tank bottom corrosion and costly replacement.
A recent MATCOR article published in Storage Terminals Magazine provides an overview of these grid CP systems and an alternative concentric ring linear anode system (link to the full article below). Here are just a few key points:
Grid Tank Anode Systems
Consist of field assembled MMO ribbon anodes and titanium conductor bars
Require flawless design and installation
Subject to poor welding and other concerns
Failures can be catastrophic
Concentric Ring Linear Anode System
Factory assembled—no field cutting or splicing required
Easy, fast and reliable installation
Coke backfilled sock protects the anode
Redundant—each ring segment has two feeds
Long life compared to the grid systems of the 1990s
Josh Johnston, MATCOR’s director of sales, wanted to share a recent conversion of a new customer to the growing list of satisfied Durammo Deep Anode System users. As Josh explained, “the Durammo is a salesman’s dream product. It offers our customers an innovative product that has an amazing track record. Its design makes it safer and easier to install because it comes preassembled ready to immediately lower down the hole.”
A complete description of the Durammo deep anode system is available here.
As Josh continued, “The hardest part about selling the Durammo, is that it is different than what they have used and what others are offering. There is a certain leap of faith that we ask customers to take when trying something different. I can explain to them that it is less expensive, has a longer operating life, is safer and easier to install, that several thousands of these are installed across the country and around the world, some with more than 20 years in service. It all sounds great, but it is different. That is my job, to convince people to do something different.”
One such company is Enerfin Resources Company, a midstream company operating natural gas and crude oil field services assets in Oklahoma, Texas and Louisiana. MATCOR met with them in the Fall of 2019 and explained to the Enerfin team the benefits of the Durammo deep anode system. As Josh noted, “Enerfin was willing to try this “new to them” technology, based on the value we offered.”
In March, MATCOR installed three of the Durammo systems for Enerfin. Tony Gustin, Project Development & Construction Manager noted, “the installation of these systems was very professional and the factory assembled system dropped in place as easy as advertised. We are sold on this product and look forward to using MATCOR and the Durammo system on many future projects.”
If you are ready to try something better, but different for your next deep anode system project, contact MATCOR and we would be happy to help you take the next step.
A client recently raised the concern about the cathodic protection reaction causing a drying out of the sand under a large diameter above ground storage tank. This is a very interesting question. We recently developed a stoichiometric analysis to assess the cathodic protection carbon footprint of a deep anode system by calculating the amount of carbon dioxide produced. The same methodology can be used to assess the risk of drying out of the tank bottom.
For this analysis, let’s assume a typical 150 ft diameter above ground storage tank with a bare tank bottom and a 1-foot sand bed resting atop a non-permeable liner. Based on a common design criteria of 2 mA/ft2 of bare surface area, this tank would nominally require a total of 17.7 amperes of current.
How much water does a cathodic protection system consume?
For every 2 electrons generated, one H2O molecule is required. One amp-year is equal to 3.1536 x 107 amp seconds or coulombs. One Faraday or 96.487 coulombs is equal to one mole of electrons therefore, one amp-year is equal to 326.84 moles of electrons. With the 2 to 1 ratio of electrons to H2O molecules that means that for every mole of electrons, 0.50 moles of H2O are generated. H2O has a molar mass of 18.0 g/mol so for each amp year a total mass of 2,941.6 grams of H2O is generated – that is approximately 0.78 gallons of water per amp year.
For our 17.7 ampere, 150 ft diameter tank anode system, that would mean 13.8 gallons of water is consumed as part of the cathodic protection reaction each year. Assuming that there is no new water being added into the tank foundation (a perfect chime seal and a completely non-permeable liner), then over a 30-year operating life the CP system would consume a little more than 400 gallons of water. While that might seem like a lot of water consumption, what is the percentage of drying out that is occurring with the sand over that time frame?
Will the Tank Bottom Dry Out?
Well, typical sand has a bulk density of approximately 100 lb/cubic foot and the typical moisture content for commercial sand is between 2% and 6%. For purposes of this exercise, let’s assume that the moisture content is on the low end at 2%. This means that there are approximately 2 lbs of sand per cubic foot. A 150 ft diameter tank has 17,671 cubic feet of sand bedding which equates to 35,342 lbs of water or about 4,241 gallons of water. So, if no new water is added over the thirty-year operating life, the typical CP system will consume about 10% of the sand moisture for very dry sand.
Given our assumptions and calculations, it does not appear that significant sand drying will occur due to water consumption.
Another Consideration: Electro-osmotic Drying
This analysis does not consider the effect known as electro-osmosis. Electro-osmotic drying is a process that is used in the civil engineering world to dewater sludges by creating a DC electrical flow – the flow of electrons pulls polar water molecules away from the anode. For CP applications, this is generally not considered to have a significant impact except where there are very high current densities at the anode – for example some deep anode systems operating at very high output rates in certain soil formations. For tanks, this is not considered an issue.
If you have other technical questions, or for information on MATCOR’s above ground storage tank cathodic protection solutions, please contact us at the link below.
This article explores the carbon footprint of cathodic protection deep anode systems and compares it to that of a typical passenger car.
Deep Anode Systems
Deep Anode Systems are commonly used throughout the CP industry as a cost-effective means to discharge significant amounts of current to protect pipelines over long distances or large structures in a small area. One of the common components in a deep anode system design is the vent pipe.
The deep anode system vent pipe serves two important related functions:
To prevent gas blockage that will impede the
operation of the anode system
Prevents the accumulation of chlorine
concentrations where chlorides are available
Both issues are directly related to the electro-chemical reactions that occur at the anode to coke backfill, and coke backfill to earth interfaces.
There are two basic types of anodes used in deep anode systems—conventional “massive” anodes, and dimensionally stable anodes.
The conventional “massive” anodes are those anodes that consume as part of the electro-chemical reaction and as such their mass is critical in determining the system’s performance life. The dimensionally stable anodes, typically Mixed Metal Oxide (MMO), are catalytic in nature and do not consume as part of the anodic reaction.
Cathodic Protection Reactions
The primary cathodic protection reactions all involve generating gas:
In a properly functioning deep anode system, the gases that occur from these reactions predominantly involve the coke backfill creating carbon monoxide and carbon dioxide. If chlorides are present, some percentage of chlorine gas will also be generated.
The Importance of Venting the Deep Anode System
The gases generated in the coke column typically do not rapidly diffuse into the earth around the coke column and thus will build up. These gases are not electrically conductive and once enough gas builds up around the anode, then the anode can no longer effectively discharge current—a phenomenon known as gas blockage. If Chlorides are present, the chlorine gas reacts with water to create hydrochloric and hypochlorous acids that can attack the cable insulation and cause permanent damage. This is why it is important to properly vent these gases that are a part of the electro-chemical reaction that must occur for CP to function.
What’s the Carbon Footprint?
Given that deep anode systems generate gas, an interesting, although not commonly asked question, is how much carbon dioxide a typical deep anode system generates—in other words, what is the carbon footprint of a deep anode cathodic protection system.
With a few worst-case assumptions and a little stoichiometric chemistry analysis we can answer this question. Assuming all the reactions are generating carbon dioxide and there is no oxygen generation, then for every 4 electrons generated, one CO2 molecule is generated.
One amp-year is equal to 3.1536 x 107 amp seconds or coulombs. One Faraday or 96.487 coulombs is equal to one mole of electrons, therefore, one amp-year is equal to 326.84 moles of electrons. With the 4 to 1 ratio of electrons to CO2, that means that for every mole of electrons, 0.25 moles of CO2 are generated. CO2 has a molar mass of 44.01 g/mol, so for each amp year a total mass of 3,596 grams of CO2 is generated.
For a nominal 50 amp anode system, that would mean a maximum generation of 180 kg of CO2 per year if CO2 was the only gas generated.
How much CO2 is 180 kg/year?
The EPA estimates that the typical passenger vehicle generates 4,600 kg of CO2 per year.
Therefore, your 50 amp deep anode system generates about 4%—or just 1/25th—of what a typical passenger car generates annually.
If you have other technical questions, or for information on MATCOR’s deep anode cathodic protection solutions, please contact us at the link below.