By Paul G. Horbaly, Business Development Manager, Power Generation & Transmission, Sherwin-Williams Protective & Marine
Coal-fired power plant conveyors are intricate systems that transfer coal from rail cars or barges to furnaces in the heart of the plant. Up to several hundred feet in length and as large as 12 feet in diameter, the systems consist of welded steel plate and rolled tubes that enclose a complex system of rollers, stands, a conveyor belt and maintenance walkway. Flexible composite belts are supported by a series of heavy duty rollers and structural steel channel and frames. The entire system is borne by structural steel I-beams, channel and cross braces that extend 100 to 200 feet off the ground and have a steep incline.
Two types of coal conveyors see use today: truss and tubular designs. The truss style requires a lower initial investment than the tubular design. The truss design is an “open” style with an aluminum roof or shroud over the belt and rollers, and the walkway is typically not covered. The tubular design is fabricated of rolled steel plate and fully encloses the conveyor belt, rollers and walkway.
Owners expect conveyor systems to have a 40-year service life. Today, many conveyors in service are more than 25 years old. However, with proper maintenance and parts replacement, asset life may be extended to provide 50 to 60 years of service. One critical component is the coating system, which prevents corrosion for the life of the mechanical system.
Coating systems must be resistant to impact, abrasion and chemicals and ultraviolet degradation. A conveyor system is one of the more visible pieces of equipment at a facility, so a proper coating system needs to retain color and gloss, providing a clean appearance as well as preventing corrosion.
A conveyor system’s steel components are exposed to a range of environmental conditions, from “moderate” to “severe heavy industrial exposure,” according to National Association of Corrosion Engineers International (NACE) terminology. Conveyors are also exposed to local climate conditions, such as coastal salt air, snow and ice or constant humidity. Coal dust deposits will build up on the conveyor’s surfaces, which may trap moisture. Combined with the elements, the coal dust mixture will accelerate corrosion on exposed surfaces.
Specifying the coating system for new conveyors is best achieved by thoroughly understanding the plant environment, prior protective coating history and economic considerations based on investment over the asset’s total service life. Coating selection, specification and application should be designed in accordance with NACE, the Society for Protective Coatings (SSPC) and manufacturers’ specifications. However, the most important aspect of the project is the compliance of the coating contractor with the owner’s specifications.
Employees at many coatings manufacturers are also NACE-certified coatings inspectors with extensive experience in field coatings. They may be called on to take the lead in establishing overall project specifications. Input from the owner’s maintenance and engineering staff, equipment fabricator, coating applicator, inspector and project manager is vital to this specification process. Local service conditions must also be carefully accounted for in the process.
Coating systems should stand up to abrasion and impact that can result from shipping and handling this large equipment. Fabrication shop application and potential jobsite conditions if touch-up is required are other important factors. Low volatile organic compounds (VOCs), high solids or waterborne coatings should receive careful consideration as power plants are held to high levels of environmental responsibility by regulators and industry watchdog groups.
New technology protective coatings exist that deliver benefits such as rapid cure, ease of application and overall performance. These new technologies deliver several different advantages, but it is critical to consult with a manufacturer’s representative and corrosion specification specialist to understand the full benefits of the various products available.
Shop Coatings Application
New coal conveyor systems are typically coated in the fabricator’s shop under optimal humidity and temperature conditions to assure superior adhesion, specified coating thickness and appearance.
Coating a conveyor on-site, with associated rigging extending more than 100 feet in the air, could cost two to three times as much as shop coating.
A properly executed quality assurance program will reduce the need to make maintenance field touchups. Some coatings manufacturers provide an extended warranty, provided that a third-party inspector is present during all phases of the coating project.
Due to the large surface area being painted, coatings are often spray-applied to ensure uniform thickness as well as excellent visual appearance, provided site conditions are appropriate.
Before coating, the steel is typically prepared by blasting the surface with an abrasive media to achieve the specified surface cleanliness, free of dirt, mill scale and rust, and establish the proper profile. The surface will have a consistent light gray appearance and an angular profile (approximately 2 mil.). The angular profile provides an increased surface area and promotes coating adhesion.
A typical coating system begins with an inorganic primer, applied only in shop environments due to inherent characteristics of the coating. Over the past decade, zinc-rich epoxy has gained greater acceptance. Should the finish or topcoat be damaged or compromised, the zinc acts as a “sacrificial” component; corrosion will attack the zinc before reacting with the carbon steel substrate. An intermediate chemically resistant epoxy coat is then applied, followed by a polyurethane finish coat designed to resist weathering and enhance overall appearance.
Whether the coating system is applied in the fabricator’s facility or as a maintenance recoating project, inspectors play a vital role in certifying that the owners receive the protective coatings system originally specified.
Recoating Existing Conveyors
Field coating specification should include the coatings manufacturer, plant maintenance and engineering staff, applicator and inspector. A thorough maintenance history of the existing conveyor system should be made available to the team to assist in making coatings recommendations.
Field recoating must take several key factors into consideration. If a conveyor is in a remote area, spray application may be possible, but the conveyor is often located where overspray would raise concerns and the brush-and-roll method is used. One drawback of the brush-and-roll method is that it applies half the thickness of spray, thus nearly doubling labor costs.
Typical maintenance recoating options include organic zinc primers, zinc rich epoxy, high performance epoxy, polyurethane, acrylic and polysiloxane or moisture-cure urethane topcoats. Inorganic zinc is reserved for shop coating applications due to special application considerations.
One recent example of a field recoating system involved a working conveyor built in 1953 and originally coated with red lead primer. While lead primer is no longer used, the conveyor may be coated over with two coats of acrylic to extend the coating system’s life, reasonably expected to last another 12 to 15 years.
Moisture-cure urethane coatings deserve serious consideration if local weather conditions are expected to be cool and humid during application. A moisture-cure urethane zinc primer is often used in conjunction with a single-component, moisture-cured urethane topcoat. This type of system is typically used in projects that take place in the spring or autumn, when humidity varies. Air moisture actually works with these coatings to enhance rapid curing. This coating system also offers the benefit of faster return to service than other options and eliminates the risk associated with weather-related work interruptions.
Polysiloxane coatings are also being considered in power plant applications. They offer exceptional performance but are one of the higher-priced options. Polysiloxanes, however, eliminate an intermediate coating step and reduce the project’s labor costs.
Each conveyor coating project has unique performance requirements, field and application considerations. Every coating option should receive thorough consideration before the final specification is sent out for bids.
Owners typically consider a maintenance painting sequence after about 20 years. For power plants, “maintenance painting” means spot touchup and repair when the amount of corrosion is limited or found in discrete areas, and may include spot prime and full coat. Full repaint may be required, which involves complete coating removal and replacement.
The protective coating process is an integral and critical step to assure that the full service life of assets is achieved and return on investment is maximized. Shop- and field-applied coatings have been enhanced over the past several years, but surface preparation and proper application are still the most important steps in the protective coatings process. The expertise provided by coatings manufacturers, their corrosion specification specialists and the protective coatings contractor should be involved in the process as early as possible to achieve maximum efficiencies and return on investment for the power plant owner.
NACE International publishes a guide to protective coatings, “Expected Service Life and Cost Considerations for Maintenance and New Construction Protective Coating Work,” by Jason L. Helsel, Michael F. Melampy and Kirk Wissmar. The guide served as a reference for this article.