Plant & Works Engineering
Protective coating extends lifetime of flue gas duct
Published:  02 June, 2016

Conditions in a flue gas duct are extreme. Heightened exposure to heat and aggressive chemicals can cause the steel structure to crack or fracture resulting in leakage. In the worst case, this can lead to plant shutdowns and expensive repairs.

As a manufacturer itself, Henkel says it fully understands the problem and has chosen to take a proactive approach by treating the flue gas duct at its own power plant in Düsseldorf with LOCTITE PC 7255. This protective coating is a sprayable, ceramic reinforced epoxy that has proven its ability to extend the lifetime of a wide variety of plant from tanks and chutes to heat exchangers, centrifuges and impellers.

Henkel’s Düsseldorf power plant supplies energy to the entire manufacturing site. One of its five steam boiler units is coal-fired whose pollutants are filtered out downstream in a flue gas desulphurisation unit. During the process hot and humid waste gases condense, often forming droplets of water containing sulphuric acid which can attack the steel walls of the flue gas duct.

Once damaged, either a section or even the entire duct has to be replaced, incurring not only the expenditure associated with the repair but also the high cost of downtime.

Before opting to treat the flue gas duct with LOCTITE PC 7255, the structure had been protected with glass fibre composite panels. Another common alternative is bromobutyl rubber. In both cases, the material has to be cut to fit the geometry of the duct exactly in order that it can provide effective protection; a task that is laborious, difficult and often wasteful.

By comparison, the application of LOCTITE PC 7255 is quick and effective. This low friction, ceramic-reinforced epoxy is sprayed on, easily coating large surfaces and complex structures, creating an ultra-smooth and high gloss finish. It is resistant to acids and alkalis and also protects against abrasion, corrosion and especially chemical attack.

Having cleaned the surface of the flue gas duct with a high-pressure water jet, Henkel maintenance engineers then sandblasted the steel walls to achieve a cleanliness level of SIS SA 2.5. This also had the effect of roughening the surface to peak-to-valley height of at least 75 micrometres.

As the surface area of the structure at the Düsseldorf plant is fairly large, around 250 square metres, it was then treated with LOCTITE SF 7515 as temporary corrosion protection, to prevent flash rusting between the preparatory work and the application of LOCTITE PC 7255.

Finally, the sprayable ceramic coating was applied in two coats, each with a thickness of at least 250 micrometres. After just 24 hours, the treated surface could bear mechanical loading and was ready to withstand its hostile environment.

By comparison with conventional corrosion protection, the LOCTITE PC 7255 process is estimated to achieve overall cost savings of around 17%; the sum of the product, application method and shorter downtime. Another plus point is that the product contains no solvents.

For Henkel, the benefits were clear. LOCTITE PC 7255 offered the potential to significantly extend the service life of the flue gas duct, virtually eliminating the need to consider the costly replacement of the 25 metre long duct. Not only would such a replacement cost between 80,000 and 90,000 Euros, its installation would also necessitate shutting down the unit for at least four weeks.

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