Aligning External Floating Roof Systems and Internal Tank Linings

Maintaining the integrity of aboveground petroleum storage tanks (API 650 tanks) is crucial – from top to bottom and from the interior to the exterior. All surfaces typically require protective coatings to prevent the steel from corroding and diminishing the tanks’ operational lives. To preserve the structure and function of these tanks, those coatings and the tanks themselves need to be part of set inspection and maintenance protocols.

API 650 tanks are inspected in accordance with the API 653 Tank Inspection Standard. Developed by the American Petroleum Institute, API 653 governs the inspection, repair, alteration and reconstruction of aboveground storage tanks (ASTs). Inspectors will review the existing condition of protective coating systems across all areas of the tank. That includes evaluating internal linings, vapor spaces and external floating roof (EFR) coating systems, most often simultaneously upon a standard 10-year inspection cycle.

If coatings maintenance or damage repair is required following inspection, repairs will be conducted in accordance with the API 653 standard, API 652 Recommended Practice – Linings of Aboveground Petroleum Storage Tank Bottoms, and the owner’s coating specifications that govern the asset.

With advancements in coatings technology and abundant field documentation demonstrating proven performance, tank linings performing for 20 years or more is becoming a new norm – and a new minimum of performance. That time frame represents a maintenance efficiency benchmark that welcomes the opportunity to align the service life of exterior EFR coatings with the 20-plus-year tank lining performance target. In fact, doubling EFR coating service lives to 20 years would be a welcome target.

This article will explore the rationale and solutions behind aligning EFR coating system performance to 20-year tank lining performance cycles to reduce costs and enhance operational efficiency.

Maintaining Exterior Roof Coatings

EFRs are subject to harsh environmental conditions from the exterior and challenging corrosive conditions on the interior that can accelerate wear and tear. Exterior exposures include rain, snow and ice, ultraviolet (UV) rays, chemicals and a variety of atmospheric contaminants – all of which contribute to corrosion potential. The tank’s location and environment also play a substantial role in coatings performance as referenced in ISO 12944 – from a C2 (Mild) environment to a C5-M (Seacoast Heavy Industrial) environment.

External exposures can wreak havoc on coatings. Moisture, chlorides and organic material can all be present on the EFRs of API 650 tanks, with water ponding common on their reasonably flat roofs. In addition, solar energy and vapors from stored contents contribute to tank expansion and contraction, which flexes the steel roof structure and applies multi-directional stress to the coatings.

Certain coatings may crack under the various stresses encountered, particularly in high-risk failure areas like weld seams, allowing moisture and contaminants to work their way inside and underneath the coatings where they can accelerate corrosion. Left unchecked, such pitting corrosion could compromise the EFR structure, leading to water intrusion inside the tank.

Planned inspections to review the exterior condition of EFRs at 10-year intervals are required per API 653 guidelines. Any exterior signs of corrosion are easy to spot via visual inspections, with blistering, peeling and rust sure indicators that coatings are failing. Inspectors may also perform ultrasonic or radiographic testing to determine any areas of wear or damage to coatings or the steel substrate.

Maintaining Interior Tank Linings

Internal tank linings within API 650 tanks encounter different and additional aggressive exposures compared to exterior surfaces. Temperature fluctuations, water intrusion, microbiologically influenced corrosion (MIC), vapor space seal scenarios and contact with volatile substances are common exposures that can lead to the deterioration of interior linings. Still, tank linings have historically held up better than exterior EFR coatings, primarily due to the additional movement- and atmospheric-related stresses the exterior coating system encounters.

The better, longer-term performance of tank linings has led to longer inspection cycles of 10 and even 20 years for interior linings. The norm is 10 years, but advanced single-coat ultra-high-solids epoxy amine linings, novolac resin-based, flake-reinforced coatings and other technologies are easily allowing for 20-plus-year performance cycles in many tanks. In fact, numerous such linings that have recently undergone their second 10-year inspections are in such good shape at 20 years that they’re getting additional full 10-year inspection renewals, bringing their operational lives to 30 years or more. This proves the opportunity for inspection cycles to be extended without sacrificing the protective integrity of tanks.

Built to a dry film thickness (DFT) of 20-25 mils in a single coat, tank linings effectively prevent tank contents, moisture and other contaminants from coming into contact with the tank’s steel substrate. The thick coatings effectively resist chemical attacks from corrosive stored contents, and they resist abrasions that could otherwise occur when filling, cleaning and maintaining tanks. The coatings even perform well on the underside of cone roof structures, with high edge retention properties that ensure a high film build remains on the sharp angles, corners, nuts, bolts, rivets and welded seams that can be found on cone roof structural beams.

Enabling Aligned Inspection Cycles

Knowing that the 20-year mark is becoming a new norm for tank lining performance, tank operators have an opportunity to align API 653 inspections with similar activities for EFR coating systems. Synchronizing EFR coating system performance with internal tank lining performance at that two-decade benchmark offers a strategic advantage for operational efficiency and cost savings.

To achieve this goal, EFR coatings need to perform much better than they have. That means looking to new technologies beyond the traditional thin-film two-coat systems that have historically been used. Those systems were typically immersion-grade epoxies applied at 5-8 mils DFT per coat, with a stripe coat applied to all welds for additional protection around those rough surfaces. With UV degradation causing epoxies to lose thickness, the resulting 10- to 16-mil DFT film tends to diminish and offer less protection over time. Operators consider themselves lucky if they can reach 10 years out of traditional thin-film EFR coating systems.

Newer coating options for EFRs offer significantly greater flexibility than traditional ones, allowing the materials to flex with the roofs and minimize the cracking that can lead to exacerbated corrosion and premature repair needs. Such two-coat systems begin with a zinc-rich moisture-curing urethane primer, such as Corothane® I GalvaPac 1K from Sherwin-Williams, applied at 3-4 mils DFT to the steel tank substrate following surface preparation. This flexible zinc layer provides corrosion protection and resists abrasions and chemicals. When used with a recommended topcoat, the primer is also rated for immersion service, which is particularly important with EFRs to mitigate the effects of water ponding on tank roofs.

Up next in this newer recommended EFR coating system is a two-component, ultra-high-solids epoxy amine known as Dura-Plate® UHS from Sherwin-Williams. The epoxy coating is also engineered specifically for immersion service to mitigate the ponding water effect. In fact, it is also used as a tank lining and has the distinction of being the first lining certified for 30 years of service, within an API 650 tank used for dedicated diesel fuel service.

Dura-Plate UHS has approximately 6% flexibility, which is more flexible than steel, making it an ideal solution to handle the movement and flexions of an EFR. It also has greater edge-retentive properties (more than 75%) compared to conventional epoxies, providing a higher film build on weld seams, sharp corners, edges and vent openings, which are all numerous on EFRs. While typically applied at 15- to 20-mils DFT for EFRs, the coating can be built to a 50- to 60-mil DFT coat in just one pass. At any thickness, the single-coat solution therefore saves labor costs and enables faster return-to-service times.

Melding Inspection Cycles for Cost Savings

Achieving a 20-year life from external EFR coatings requires materials that perform in difficult environmental conditions. With that performance benchmark addressed by using a zinc-rich urethane primer and flexible ultra-high-solids epoxy topcoat system in place of traditional epoxies, tank owners now have the opportunity to align tank coatings performance inside and out – conceivably at the 20-year service mark, which would also align with a 20-year lining inspection interval.

The interior lining should make it that far without requiring any major maintenance – unless mechanical repair is required – but periodic maintenance may be required on the more challenging external EFR structure. That exterior maintenance has a far greater likelihood of being limited to spot repairs before the 20-year mark rather than full rehabilitations due to the newer coating systems being used on EFRs.

Switching to longer inspection cycles and reducing the frequency of inspections offers a number of economic advantages. Naturally, performing fewer inspections saves tank owners the labor, equipment and administrative costs associated with repeated examinations. It also minimizes disruptions to normal operations. In addition, owners will need to make fewer coatings repairs and steel replacements over the life of the tank, reducing their maintenance costs and their total cost of tank ownership.

Streamlined scheduling and budgeting are other advantages of aligning tank inspection cycles. Asset management can be more predictable and manageable by improving operational planning and reducing the risk of unexpected costs stemming from unplanned downtime or emergency repairs. Owners can also budget for both internal and external coating inspections and maintenance on the same schedule for better planning and less financial unpredictability. Most importantly, tank operators can consolidate scheduled downtime and reduce operational interruptions with aligned interior and exterior inspection schedules.

Optimizing Tank Maintenance

As new coatings developments arise that deliver longer-term service on EFR exteriors, tank operators now have the opportunity to extend their roof coating system inspection cycles and optimize maintenance schedules by aligning those reviews with the 20-year inspection cycles that are becoming more common for tank linings. Doing so offers a significant cost-saving opportunity by reducing the frequency of inspections, minimizing downtime and lowering long-term maintenance costs. It also offers a pathway to a more sustainable and efficient approach to tank maintenance.

Tank owner/operators have already begun experimenting with extended life EFR coating systems on existing tanks. The industry has taken note of these developments and is taking steps to review and potentially formalize recommended best practices for such extended life systems. These recommendations will take time to develop and publish while coating systems are properly specified, validated and documented to become part of an API Recommended Practice that is recognized globally by the industry.

ABOUT THE AUTHORS

Michael Harrison, Global Product Director – Linings, has more than 30 years in the development and support of linings in all protective segments with a specific focus on the oil and gas, mining, power generation, and water and wastewater markets. Contact: Michael.Harrison@sherwin.com

Justin Hair is an Energy Account Manager for Sherwin-Williams Protective & Marine based in Tulsa, Oklahoma. With approximately 30 years of service dedicated to the oil and gas coatings industry, he has specialized in multiple subject matters related to aboveground petroleum storage tank industry challenges. Contact: Justin.M.Hair@sherwin.com

Brian Buchta is an Energy Account Manager for Sherwin-Williams Protective & Marine. He has served the protective and marine market since 1998 and has been with The Sherwin-Williams Company for 23 years. Contact: Brian.r.Buchta@sherwin.com

Calvin Sumner is an Energy Account Manager for Sherwin-Williams Protective & Marine. He has worked for the company for 19 years, including nine years as a dedicated oil and gas representative specializing in the midstream market. Contact: Calvin.L.Sumner@sherwin.com

Figure 1. External floating roofs (EFRs) have traditionally faced inspection cycles of three to five years to ensure coatings and the roof structures themselves are in suitable condition. Advanced coating systems are enabling longer performance, offering the possibility of aligning EFR and tank lining inspections at the 20-year mark.

Figure 2. Aligning interior and exterior inspections on 20-year cycles would reduce the frequency of inspections, minimize downtime and lower long-term maintenance costs for significant cost-saving opportunities.

Figure 3. Today’s high-build tank linings effectively resist chemical attacks from corrosive contents and abrasions to stave off corrosion for 20 years or longer, allowing operators to avoid frequent shutdowns and keep tanks in service.