A major determining factor as to whether or not a candidate coating will be successful in a wet FGD is its mechanical performance.  Specifically, its adhesion to the substrate, its balance of permeability and flexibility, its impact resistance, and its abrasion resistance are the most import mechanical factors.

Liquid flow rates in wet FGDs are so extreme that coatings are more correctly characterized as immersed rather than simply atmospheric coatings.  This gives rise to several forces which act on the coating which are negligible in atmospheric coatings.  Osmotic blistering and the cold-wall effect have the ability to put the most stress on the bondline between the substrate and the candidate coating.  Belzona coatings have some of the highest adhesion values among all field-applied coatings often exceeding 5,000 psi in direct tension adhesion when fully post cured.  This is a case where more is better provided manufacturers use the same unmodified test method.  ASTM D4541 is becoming very prevalent while ASTM D1002 is the old standby.

Balancing fluid permeability with flexibility is exceedingly difficult but critically important.  The extreme case of this is readily observed in trowel smoothed glass flake vinyl esters.  Their use of micaceous fillers can lend them an exceedingly low fluid permeability.  Unfortunately this also makes them very susceptible to embrittlement and cracking due to thermal cycling.  At the other end of the spectrum are elastomeric liners which retain excellent flexibility by sacrificing their fluid permeability.  Solvent-free epoxies adopt a “Goldilocks” approach by balancing very low fluid permeability and an acceptable 1-3% elongation at break.  Here we do not believe that more is always better.  One does not need a coating to have 10%+ elongation at break since this simply will not be occurring in a wet FGD unless it is in the process of collapsing!  Likewise, one does not require 0 fluid permeation so long as the actual fluid permeation rate is slow enough to hold back general corrosion for 20+ years.  Solvent-free epoxies have been doing just this for many decades.

Obviously not all zones in a wet FGD will require a coating with high impact resistance but those that do need a coating with moderate impact resistance.  In the whole of industry the impact potential present in a wet FGD is not extreme since limestone is soft and plugs/chunks are not too common.  Often the more rigid a coating is the less impact resistance it demonstrates due to a decreasing tolerance for any flexing.  This can be an issue when impact resistance and corrosion resistance are needed in the same zone such as at the spray level or within the recirculation pumps and piping system.  It is worth mentioning that solvented coatings tend to yield very poor impact resistance since they are only able to shrink in one direction as they cure.  This leads to significant internal stresses within the coating which are only released upon cracking.

Abrasion is hideously complex and no one test can hope to embody all the variables at play.  Angle of attack, the hardness of both coating fillers and the abrasive, temperature, coating heat distortion temperature, the presence or absence of a slurry, abrasive kinetic energy, and abrasive flow rate are the major variables at play in any one industrial situation.  The Taber abraser test which all coating manufacturers quote fails to capture most of these variables including notably the hardness of the abrasive and the angle of attack.  The abrasive in the wheels used for this test is quite hard and significantly harder than limestone which is very soft.  If the abrasive in the wheels is harder than that in the coating, a large volume loss will be measured.  This is inconsistent with the same coating being abraded by a softer media such as sand or limestone.  Additionally, the Taber test’s angle of attack is zero or parallel to the coating surface similar to laminar flow in a pipe while the angle of impingement at the spray level of a wet FGD is closer to 45 degrees.

Finally, many manufacturers report a mass loss rather than a volume loss.  Yet a customer buys 30 mils of a coating not 0.26 pounds per square foot.  Coating densities can vary greatly depending on the fillers used and to a lesser extent the resin system.  Datasheet abrasion test results can give an indication of real world performance but are by no means the final word on a coating’s performance in a given situation.

Many very good coatings can be undone by repeated thermal cycling and while some excellent aging tests exist, very few manufacturers test to them let alone place them on their data sheets.  Likewise a coating which performs well initially may change dramatically over time if it employs a plasticizer to lend toughness and flexibility which is leached out by the service it is installed in.

Belzona enjoys tremendous success in the offshore and onshore oil and gas market which operates some of the highest value assets on the planet in which downtime of critical path vessels is measured in millions of dollars per day.  This industry has learned that with proper coating selection, installation, and inspection that inspection cycles can be extended by 100%-300% in critical path vessels.  In this industry atlas cell testing or NACE TM0174 is the gold standard for qualifying a coating since it encapsulates so many variables simultaneously.  Datasheets which list alkali resistance at room temperature are not very helpful to a specifying engineer selecting a coating for a wet FGD.  Likewise coated bars immersed in a test tube are never exposed to a cold wall effect which can utterly destroy a coating in weeks depending on its severity.

There are two wet FGDs at Coal Creek Station.  Ten years ago the walls were coated with 5811 in order to halt the general corrosion of the walls.  This was done to preserve the corrosion allowance, and reduce scaling of the walls.  This site does not produce gypsum but if it did, this would also increase product purity as another benefit.

During a major outage GRE wanted to recoat the tanks since the expected end of life of the coating was reached.  Belzona suggested inspecting the coating rather than recoating.  Gloss and thickness measurements would indicate if any erosion of the coating had occurred due to the agitated lime slurry.  Pull-off adhesion measurements would indicate whether general corrosion was occurring behind the coating.

The original 20 mils was present throughout the tank.  High gloss (once cleaned) added confidence that the coating was simply not being abraded by the limestone slurry.  Pull-off adhesion values exceeded 3,000 psi indicating that in 10 years no appreciable corrosion had taken place beneath the coating.  Based on the inspection there was no need to refresh the coating.

Belzona offers a compelling package:  We have decades of history in harsh environments, local and global support 24/7, unrivaled application expertise and a multitude of products for tailored solutions.

How can we help you repair, protect and improve your component equipment?

Chris Lucas 

clucas@rumfordgroup.com
937-435-4650
RumfordGroup.com