In a recent Tanks & Terminals article, Ted Huck discusses a tank cathodic protection retrofit project in the Middle East utilizing horizontal directional drilling technology.
Originally constructed in 1995, the original design of this critical service ethylene storage tank included a cathodic protection system to protect the external tank bottom in contact with the ground. Over time, the system stopped providing enough current to meet NACE criteria for the control of corrosion.
Discrete Anodes Along the Tank Perimeter Not Satisfactory
The first retrofit cathodic protection system consisted of installing discreet anodes around the perimeter of the tank. While relatively easy to install, this method of retrofit installation often struggles to drive current to the full tank bottom. The results were not satisfactory so another method was needed.
Linear Anodes Installed Using Horizontal Directional Drilling
MATCOR had proposed an alternate approach, successfully being performed in the US but not tried previously in the Middle East. It involves the installation of multiple strings of linear anodes directly below the tank using horizontal directional drilling (HDD) technology. By drilling under the tank, it is possible to install anodes spanning the entire length of the tank. This method also allows for a testing device to measure the effectiveness of the cathodic protection system.
Click below to read the full article regarding this tank retrofit cathodic protection system, installed successfully in December 2019.
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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
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.
Last month, MATCOR successfully completed the first ever HDD tank cathodic protection system installation in the Middle East, utilizing a replaceable anode system.
Background—Initial Recommendation for HDD Cathodic Protection System
Equate Petrochemicals is one of the world’s largest producers of Ethylene Glycol. They initially contacted MATCOR in 2012 to discuss options for cathodic protection on a critical service Ethylene storage tank at their flagship Kuwait petrochemical facility. This tank was originally constructed in 1995, and the initial CP system installed with the tank was no longer providing sufficient current to achieve NACE Criteria. At the time, MATCOR suggested installing anodes directly under the tank using horizontal directional drilling technology. The plant’s engineering and operations team had significant reservations about this approach. The tank was critical to the plant’s operation and could not be taken out of service. Should the HDD operations result in damage to the structural integrity of the tank, the results would be catastrophic.
Perimeter Anodes—An (Unsuccessful) Alternative Approach
As a result of Equate’s concerns in 2012, they attempted an alternate approach, suggested by others, using perimeter anodes. Discreet anodes were installed offset around the perimeter of the tank—thus avoiding any possible risk to the tank during the anode installation. The use of perimeter anodes around larger diameter tanks is generally not a good idea. This is because it is very difficult to drive current to the center area of the tank, often resulting in adequate protection levels only for the outer edges of the tank bottom. For the Ethylene Storage Tank, the presence of heating pipes below the tank bottom only exacerbated the current distribution challenges. Ultimately, the results were not satisfactory.
In 2018, the plant engineering team reached back out to MATCOR to discuss our HDD solutions.
Replaceable Anode System Solution
MATCOR provided the plant with a detailed proposal to design and install a complete cathodic protection system using MATCOR’s Replaceable Tank Anode system. The RTA system is based on installing MATCOR SPL linear anode assemblies in a series of parallel slotted PVC pipes that have coke backfill pneumatically blown into the PVC pipe as part of the anode system installation. In addition to the linear anode segments and coke backfill, the slotted PVC pipes have a venting system to allow gases produced during the cathodic protection reaction to vent. This prevents gas buildup and blockage inside the PVC anode pipe.
One of the key advantages of the RTA system is that once the PVC tubes are installed, it is possible to flush out the anode assemblies and coke backfill should the anode assemblies fail and/or they are at the end of their design life making this a replaceable anode system that will assure cathodic protection for the entire service life of the tank.
Additionally, a slotted Reference Cell Tube would be installed to allow for two calibrated fixed cathodic protection reference electrodes to be inserted for full polarized and non-polarized potential measurements across the entire tank bottom. This would allow for testing of the CP system with calibrated reference electrodes for the life of the tank.
Experienced HDD Installation—Assuring a Safe Installation
While the plant conceptually agreed with MATCOR’s solution from a technical perspective, there remained a significant concern within the plant’s operation and safety groups about drilling under this critical service tank and the possibility of a catastrophic event should the drill head drift up to the tank bottom. MATCOR put together a thorough installation procedure including detailed information on the sophisticated drill head tracking systems being utilized to assure that the drill head location was being continuously monitored throughout the bore. Utilizing an experienced local HDD drilling sub-contractor, MATCOR deputed its senior HDD installation drilling supervisor to Kuwait for the installation. Our Senior HDD Drilling Supervisor has completed hundreds of tank HDD installations in the United States and his on-site presence, along with the advanced electronic tracking package being used, assured that each bore went as planned.
Replaceable Anode System Installation Complete!
In December of 2019, MATCOR, working with our local Kuwaiti sub-contractor and the client’s engineering, construction and safety teams, successfully completed the installation of the replaceable anode system. The initial commissioning results showed that the anodes were installed properly. Each anode was distributing current as expected, and the polarization levels were meeting appropriate NACE criteria. The system has been left to operate and fully polarize. A subsequent visit by MATCOR’s technical team is scheduled in early 2020 to make final adjustments to the anode system current output and to confirm that the system continues to meet NACE criteria.
MATCOR’s successful installation in Kuwait of a horizontal directional bored CP system under an existing critical service tank is a first for the Middle East Region. The innovative MATCOR design, combined with the technical knowledge and operational expertise, makes this an interesting and viable option for other tank owner/operators worldwide to consider for their existing tanks with CP systems that are not performing properly.
To get in touch with our team of cathodic protection and AC mitigation 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.
Are Vapor Corrosion Inhibitors Magic Dust or a Viable Corrosion Prevention Tool?
This article is intended to provide a basic primer on vapor corrosion inhibitors for use in corrosion prevention for above ground storage tanks and address where this technology stands.
There has been a significant effort within the oil and gas world to either promote or repudiate the use of Vapor Corrosion Inhibitors (VCIs) for tank bottom plate corrosion control. As a leader in the above ground storage tank corrosion control industry, MATCOR has partnered with Zerust® Oil & Gas to make VCI options available to our customers that are interested in applying this technology as part of their corrosion mitigation approach.
How Vapor Corrosion Inhibitors Work – Video Courtesy Zerust® Oil & Gas
What are vapor corrosion inhibitors and how do they prevent corrosion?
VCIs are chemical compounds that are released into a confined space, such as the underside of a tank bottom, and diffused through the sand pad material to reach the metal surface. These compounds are adsorbed onto the metal surface forming a strong bond that promotes and maintains a passive oxide layer on the metal and blocks other contaminant molecules from reaching the surface.
Are VCIs a non-permanent solution?
Corrosion protection using VCIs requires sufficient chemical concentration to thoroughly diffuse across the entire tank bottom surface area. The VCI has a finite life, after which it ceases to remain active. When this occurs, further chemical is required to replenish the spent VCI. The frequency of VCI replacement will vary depending a range of factors:
The rate of leakage through the tank chime
The operating temperature of the tank
The sand properties
The amount of chemical initially applied
As VCI technology is still in the early phase of adoption, the typical replenishment frequency remains one of the big unknowns. A conservative estimate would be a minimum of 3-5 years’ service life before replenishment although a least one source has reported upwards of 15 years of effectiveness.
How is VCI applied initially for above ground storage tanks?
There are a variety of application technologies depending on the application and whether the tank is new construction, existing tank during inspection, a tank that is in-service or a double floor tank. Other considerations include the substrate material or concrete pad. The VCI chemical can be provided in a powder or liquid form. Whatever system is utilized to deploy the VCI, consideration should be given to how it will be replenished over the life of the tank.
Can vapor corrosion inhibitors be used in lieu of cathodic protection?
Practically speaking, most tank operators are not looking to replace cathodic protection but are considering VCI as a supplement to cathodic protection or as a short-term solution for inadequate or depleted CP systems until a replacement CP system can be installed.
Can VCI be used as a complement to cathodic protection?
This is where VCI provides an exciting opportunity to supplement cathodic protection. While cathodic protection has a proven track record in corrosion prevention for tank bottoms, there are limits to the effectiveness of cathodic protection. Cathodic protection only works when the tank bottom is in intimate contact with the sand bottom. Localized corrosion can occur wherever there are air gaps under the tank bottom. These can occur due to flexing of the tank bottom, imperfections in the plate steel, lapping of the plate steel, poor compaction of the sand bottom, presence of aggregate or non-conductive materials such as asphalt or oil, and at crevices in the tank ring wall. These are all areas where cathodic protection may not be effective and the proper application of VCI would be an excellent means of providing corrosion protection in these localized areas. Cathodic protection and vapor corrosion inhibitors are symbiotic. CP current distribution has been shown to improve in the presence of VCI.
How do I monitor that the VCI is working?
When applying VCI to a tank bottom, coupons, ER probes or UT probes installed under the tank are used to measure the effectiveness of the VCI and to alert the owner when the VCI requires replenishment. One of the concerns with using ER probes to measure corrosion rates under tanks is that ER probes provide an average corrosion rate and not localized pitting rates. It is understood that pitting corrosion is the dominant factor in tank bottom corrosion related failures and pitting rates can be significantly higher than average corrosion rates. There is a distinct correlation between average corrosion rates and pitting corrosion rates and the ER probes can be used to infer changes in the pitting rates.
Where do vapor corrosion inhibitors stand with industry standards and regulations?
According to API 651, there are several situations where CP is not recommended for specific tank foundation designs. In some of these designs, PHMSA recognizes that CP is not feasible. In these cases, VCI can be a viable option. API 2610, the Tanks and Terminals standard outlines the use of VCI for tank bottoms in section 12.5. API 651, the CP standard, is being updated currently and VCI is being included as an option in this document. The State of Florida has identified that VCI can be used in tandem with CP or a standalone solution, for more than 6 years. NACE is currently working on publishing a standard “NACE TG543”, which is a comprehensive document on the application of VCI under tank floors. PHMSA is currently reviewing Special Permit requests for the use of VCI without a functioning CP system. If a non-regulated tank’s CP system is not meeting criteria, or has depleted, but the tank is still a few years from its next inspection, VCI can be applied to protect the floor until CP system repairs can be economically accomplished.
What independent published studies exist supporting VCI?
A 2018 study published by PRCI provides the strongest validation of the effectiveness of VCI and concluded that:
VCIs were found to be effective in mitigating pitting of steel exposed to corrosive sand but was not as effective as CP for reducing pitting corrosion. The study confirmed the importance of using the manufacturer’s recommended concentrations, as low levels of VCI was found to be ineffective.
ER Probes can be used to monitor the efficacy of VCIs
VCIs are compatible with impressed current cathodic protection; however, VCIs change the native potential of the steel and this must be considered when selecting CP criteria in accordance with NACE SP0193
In conclusion, the application of VCI is a viable tool in our corrosion tool box that should be considered in conjunction with cathodic protection for critical service applications and as a standalone solution in some applications.
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.
Evan Savant, EnLink Midstream reached out to the MATCOR Technical Team asking about AST cathodic protection system tank isolation:
“Can you advise on the importance of isolation for a new AST connected to a Pipeline, and can you advise on the need to isolate the tank cathodic protection from the tank grounding?
MATCOR’s Director of Engineering, Kevin Groll PE, NACE CP4 responded:
I am unaware of any papers or technical documents on the subject, but I will summarize as follows:
Why can a lack of isolation hurt your cathodic protection? When trying to protect any type of structure from corrosion, cathodic current loss to nearby structures is always a concern. Losses can occur when the structure in question is directly bonded to other structures which may “steal” current. Offending metal structures that are close to the cathodic protection anode and structures with better resistance to earth (e.g., bare copper grounding, bare driven piles, etc.) will more likely take a significant amount of current.
How do you obtain isolation without losing overvoltage protection?
To prevent current loss, your target structure must be electrically isolated from the offending structures. However, once you isolate a structure, you will lose grounding (if it was purposefully grounded) and you will lose protection against overvoltage events, AC faults, and lightning strikes. Therefore, to obtain DC isolation but maintain AC continuity and overvoltage continuity, we use solid state decouplers (SSDs) and polarization cell replacements (PCRs). The primary difference between these devices is how much surge current they will carry.
Tank cathodic protection design considerations.
When we design an under-tank CP system with concentric rings, we assume that we will not have isolation from grounding and facility piping, and we also assume that most of the current will get to the tank bottom because of the proximity of the anodes. This is not always the case, as we saw in a recent project, but for the most part concentric ring systems can be powered high enough to overcome the lack of isolation.
Horizontal directional drilling installed linear systems show approximately 1.5 to 2 times as much current is required as a concentric ring system due to current losses. Again, we usually factor in enough current capacity to overcome these losses.
Deep anode systems and semi-deep anode systems suffer the worst losses. These systems will sometimes require isolation of the tanks to prevent critical current loss. If a system is already in place, testing can be performed to determine how much loss there is to existing structures by measuring the current returned on ground rods and pipes. This is accomplished by using clamp-on current meters around wires/rods and Swain meters around pipes.
It is important to note that tank terminal isolation and grounding are factors in these complex tank terminal applications that must be considered in the proper design of Cathodic Protection. MATCOR’s experienced team of engineers can evaluate your specific application and make the appropriate recommendations.
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.
The richest man in Africa, Aliko Dangote, undertakes $12B project including 153 ASTs
Chalfont, PA (April 2019) – MATCOR, Inc., the trusted full-service provider of proprietary cathodic protection products, systems, services and corrosion engineering solutions announced that it recently completed shipment of over 500,000 linear feet (150+ km) of its SPL™-FBR linear anode product along with other ancillary materials for 153 above ground storage tanks (ASTs) in Africa.
The anodes, which will prevent tank bottom corrosion, are part of an ambitious refinery project undertaken by Aliko Dangote, Africa’s richest man. The tanks are being erected on over 6,000 acres of swampland outside of Lagos, Nigeria.
MATCOR was selected for this project due to quick delivery and the company’s unique linear anode design, which does not require field splicing and saves significantly on installation time and costs.
“The ability to manufacture the large quantity of custom length linear anode segments in a very compressed time frame was key to meeting the tank contractor’s needs,” noted Ted Huck, Director of Manufacturing and Quality Assurance for MATCOR. “Our team handled a very complex order with a very tight delivery schedule while maintaining world class quality.”
Learn more about the project in our recent blog post.
This month MATCOR will ship the final tank anode system assemblies for Africa’s most audacious industrial project. The project is being undertaken by Aliko Dangote, Africa’s richest person, and when completed this $12 billion Dangote oil refinery could, according to a 2018 New York Times article, “transform Nigeria’s corrupt and underperforming petroleum industry. Planned as the world’s largest refinery…should process 650,000 barrels of crude oil daily.” With Nigeria poised to become the world’s third most populous nation by 2050 (surpassing the USA) and having Africa’s largest economy, this project is being touted as a milestone achievement in what many are dubbing the African Century.
As part of this ambitious refinery project, a total of 153 above ground storage tanks for crude oil, refined and intermediary products up to 92m in diameter (300 ft) are being erected on the 6,180 acres of swampland just outside of Lagos, Nigeria. MATCOR’s innovative tank anode system technology using linear anodes was selected by the project’s EPC contractor, Engineer’s India Limited, as the design basis for the cathodic protection for the project’s above ground storage tanks. Working closely with both the owner’s Indian based engineering team and the EPC contractor, MATCOR was successful in identifying the key tank contractors that would be bidding the tank erection and supplying the cathodic protection systems as part of their specifications.
Ultimately, MATCOR was successful in securing contracts to provide the linear anodes for each of the three tank packages, one from a Chinese contractor and two from a UAE based contractor.
In total, MATCOR has supplied over 500,000 linear feet (150+ km) of our SPL™-FBR linear anode product along with other ancillary materials for the under tank cathodic protection systems over an 18-month period. With all the anodes having been manufactured in MATCOR’s ISO 9001 certified Chalfont PA facility, MATCOR continues to be the global leader in the manufacturing of impressed current linear anodes for above ground storage tank cathodic protection.
Have questions about tank corrosion protection, or need a quote for services or our tank anode system? Contact us at the link below.
For new construction tank projects and for retrofits of existing tanks, it is common practice to install clean washed sand as the pad upon which the tank bottom is fabricated. The use of oil sand, crushed stone, asphalt, or other materials directly under the tank bottom should be avoided, as these hinder effective tank corrosion protection. This article provides guidelines to the specifications for the sand bedding materials. These guidelines are based on providing a low corrosivity environment compatible with cathodic protection to assure a long service life of the tank bottom.
For a typical 150-foot diameter tank using 12 inches of sand bedding the quantity of sand that is required is approximately 17,679 cubic feet of sand which is roughly 883 tons. The cost of the sand, including delivery, is a significant cost and far exceeds the cost of cathodic protection– especially if the sand specifications are quite stringent and require sourcing sand that requires significant transportation. Some consideration can be given to relaxing the sand recommendations, even if that warrants increasing the cathodic protection requirements, should there be a significant cost impact to complying with the sand specifications – consult your cathodic protection designer.
The best proxy for determining corrosivity of sand materials is the electrical resistivity of the sand. Clean washed sand typically has resistivity values more than 20,000 ohm-cm and in some cases can exceed 100,000 ohm-cm. The higher the resistivity of the sand, the lower the corrosivity of the sand; however, when designing cathodic protection, the higher the sand resistivity the greater the impact on the overall system resistance and the electrical power required for the cathodic protection system. API Recommended Practice 651 Cathodic Protection of Aboveground Petroleum Storage Tanks provides the following table classifying resistivity of soil/sand.
Measuring pH indicates the hydrogen ion content of a soil. Corrosion of steel is fairly independent of pH when it is in the range of 5.0 to 8.0. The rate of corrosion increases appreciably when pH is < 5.0 and decreases when pH is> 8.0. pH may be determined in accordance with ASTM G 51 or equivalent
Chlorides will affect the resistivity of soil, and act as a depolarizing agent which will increase the current requirement for cathodic protection of steel. Pitting corrosion on steel can begin at chloride levels of 10 ppm. Chloride content may be determined in accordance with ASTM D 512 or equivalent.
Sulfate levels >200 ppm frequently indicate high concentrations of organic matter. Sulfate content may be determined in accordance with ASTM D 516 or equivalent.
Sulfide levels > 0.10 ppm may indicate that sulfates have been reduced by bacteria. Sulfide content may be determined in accordance with EPA 0376.1 or equivalent.
The sand bedding material should be clean of rocks, clumps and other debris and for clean sand capable of passing through a #4 sieve, for washed river sand an alternative acceptable particle size is 100% pass through a 3/8” sieve.
Sand Depth of Cover
For new construction tanks, the typical design is to provide a minimum of 12 inches (30 cm) of sand cover; however, for tank bottom retrofits it is quite common to provide 6 inches of sand (15 cm) for double bottom installations or for applications where the existing bottom is removed, and the top layer of the underlying soil foundation is being removed.
Have questions about tank corrosion protection, or need a quote for services or cathodic protection design and materials? Contact us at the link below.