Neil Wilds, Global Product Director – CUI, and Dr. Jeffrey David Rogozinski, Global Product Director – Fusion-Bonded Epoxy/Pipe, Sherwin-Williams Protective & Marine, USA, explain how considering different coatings can help you make the most of pipeline infrastructure repairs.
Many energy pipeline infrastructure assets face harsh operating conditions related to temperature extremes. For example, buried and aboveground pipelines pumping heated contents face extreme cold in northern climates and sometimes drastic temperature differentials where pipes transition from buried to exposed. Compressor stations also experience dramatic temperature fluctuations at compression points, leading to condensation forming on affected assets.
These temperature differentials create conditions that can encourage corrosion. In fact, they’re partly responsible for the repairs that are eventually required on many pipeline and compression station assets.
Condensation and other moisture on the exteriors of affected equipment can deteriorate protective coatings as it works its way down to the metal substrate, where pitting corrosion can develop. This moisture can also drip onto surrounding equipment and promote corrosion there. In addition, moisture can become trapped under the traditional insulation systems that are applied to assets for thermal protection. There, it can create the potentially dangerous condition of corrosion under insulation (CUI). In addition, thermal stresses and the associated material fatigue can deteriorate protective coatings.
While corrosion is certainly expected on coated assets over time, it does not have to happen rapidly. When it does, pipeline operators should take the time to investigate alternatives so they can avoid having to perform premature maintenance in the future. For pipelines, that could mean choosing different materials or adding layers of protection. On compressor station assets, it may mean removing insulation in favour of coatings that offer a similar level of thermal retention to retain process heat and mitigate condensation potential, while also eliminating CUI. Each decision offers the opportunity to extend asset service lives and thereby decrease long-term operational and environmental costs in the process.
Enhancing pipeline corrosion resistance
Pipeline repair needs typically become evident via line monitoring and various inspection methods. Technicians will likely observe cathodic protection (CP) systems for early warning signs of corrosion in buried pipelines. In addition, they’ll perform periodic testing using ultrasonic devices, magnetic particle inspections and/or internal inspections with remote-operated devices. Direct visual inspection of buried pipes is not possible, but technicians can observe the exterior condition of non-insulated exposed pipes.
When such inspections reveal corrosion or the potential for it to develop, pipeline operators have a unique opportunity to improve future performance. The field history of how the coatings performed in service can serve as a baseline for decision making. If that baseline falls short of performance expectations for an asset and its operating environment, operators may want to explore other coating options.
Evidence of abrasions that cut through the fusion-bonded epoxy (FBE) coatings typically applied to pipe exteriors is a helpful example for considering alternative coatings. Such FBEs are designed to mitigate corrosion. But when the continuous coating layer is breached, exposing the metal substrate below, corrosion can run rampant – especially considering how the elevated operating temperatures of pipelines contribute to increased corrosion potential. In this case, the pipeline operator could add additional layers of protection to provide a stratification of properties from the outermost layer to the metal substrate below. For example, in especially rocky soil conditions, the pipeline owner could add a durable abrasion-resistant overcoat (ARO) on top of the FBE layer that’s applied directly to the pipe surface. The resulting dual-layer FBE coating system features the base anti-corrosion FBE layer to prevent pipeline corrosion and the top abrasion-resistant layer that protects the base layer from wear.
Perhaps the environment the pipeline is buried in is wetter than originally anticipated and is causing premature corrosion: in this case, a moisture-resistant overcoat (MRO) could be a suitable alternative to the original single-layer FBE system used. Such MROs reduce the rate of moisture ingress into the coatings, minimising the potential for moisture to work its way down to the pipe substrate and increase the potential for coating delamination due to fundamental adhesion loss and/or cathodic disbondment.
Sometimes operators can realise both abrasion and moisture resistance in the same coating system. As an example, Pipeclad® HOT 150 Flex MRO Abrasion-Resistant Overcoat provides excellent protection against moisture, as well as corrosive and abrasive elements in the environment. It’s also rated for use on assets running at high operating temperatures.
Eliminating CUI by default
Corrosion can also be a difficult issue on numerous assets associated with compressor stations. Atmospheric corrosion is one animal to corral, but CUI is a completely different one to manage due to it being hidden below insulation systems – with potentially no visible signs of its existence.
CUI is common, and nearly inevitable, on pipeline infrastructure assets that are covered by traditional insulation systems consisting of thick mineral-based insulation wrapped around assets with metal cladding installed on top. The systems protect personnel from burns, while also helping materials inside pipes, valves, vessels and other equipment maintain elevated temperatures. They’re also used to minimise the temperature differential between the inside and outside of heated assets to mitigate the development of condensation, which could become a source of exacerbated corrosion potential.
While highly effective at these tasks, traditional physical insulation systems are particularly prone to troublesome CUI because moisture and contaminants very often infiltrate cladded insulation systems. Once there, the moisture will never dry out, leaving it and electrolytes in constant contact with the insulated asset’s metal substrate. With the elevated heat from the covered asset present in this moist, warm environment, corrosion potential is very high. However, asset owners can help to minimise the potential for CUI by using an effective CUI-mitigation primer, such as Heat-Flex® ACE or other similar coatings with superior proven performance.
When CUI – or the conditions for it – is present, pipeline operators have a somewhat surprising option to eliminate it. They can instead opt for using a thermal insulative coating (TIC) system featuring an effective CUI-mitigation primer underneath the TIC. Such a system can deliver comparable levels of thermal performance as traditional insulation systems, with just the coating acting as a thermal barrier. With this high level of insulating capacity possible in coatings, pipeline operators can, in many cases, remove physical insulation systems in favour of applying a few layers of the coating material. In doing so, they can maintain the necessary thermal performance while also eliminating CUI.
The potential for CUI to disappear when using TICs is realised because the coatings take the place of physical exterior insulation systems. With no physical system mounted on top of a valve, for example, there is nothing under which corrosion could form. The corrosion zone that would otherwise exist between the insulation and the substrate is gone, and CUI potential is simply eliminated by default. The only thing left to worry about is atmospheric corrosion, which is much slower to form and easier to identify compared to corrosion hidden underneath insulation systems.
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