DuraShield™

DuraShield™ is a high temperature, corrosion resistant coating with a high level of lubricity, which maintains the flexibility, durability, hardness and temperature resistance needed in the harsh environments found in an airheater. This innovative technology, patented by Paragon, will minimize deposition and corrosion in the heat transfer surfaces (basketed element) of the airheater, thus improving performance, decreasing maintenance costs, and extending the life of the airheater. DuraShield™ combines the attributes of epoxy cross linked with specific silicone/silicate and polymeric compounds to produce a surface finish that is lubric (slippery), yet capable of operating temperatures up to 1000? F. The DuraShield™ coating is non-brittle and flexible. DuraShield™ typically remains intact under the normal stresses encountered in an airheater, even if it is impacted with enough force to bend the underlying metal. This patented coating was designed not to crack or separate from the base metal when subject to thermal shock, exit gas temperatures, temperature cycling and under boiler upset conditions.

Overcoming ABS Deposits

The effects the ABS deposits have on airheater operation include the increase of pressure differential across the airheater, an increase in air to gas leakage within the airheater, a reduction in the thermal performance of the airheater and an increase in the corrosion rates of heating elements, cold end structures and down stream equipment. Ammonium bisulfate (ABS) formation in the airheater is a result of the operation of an ammonia based NOx reduction system in a flue gas environment containing sulfur tri-oxide (SO3) and water vapor (H2O). It has been well established that ABS forms a sticky deposit on the surface of the heat exchange elements in an airheater. These deposits generally build up on the airheater surface in the areas where the metal temperature is between 300? F and 420? F. This range is primarily found in the intermediate heat exchange layer of most airheaters. Two major problems caused by ABS: • Rapid airheater plugging which is not easily cleaned with sootblowers. The sticky nature of ABS rapidly attracts and collects flyash which plugs the interior portions of the airheater that are the most difficult to clean. • The corrosive properties of ABS combined with condensing sulfuric acid result in a high rate of airheater element corrosion. This corrosion not only effects the loss of element mass, it also results in a rough surface finish or oxidized layer on the steel which further promotes ABS deposition.

Do Coatings Help Solve ABS Problems?

Research on SCR systems and airheaters over the past 15 years has proven that porcelain enamel coatings drastically reduce the rate of plugging and corrosion in airheaters installed on SCR equipped boilers. Unfortunately, porcelain enamel coatings, being comprised of relatively brittle glass particles have demonstrated an inability to last for extended cycles in certain actual full scale installations. Once compromised by cracking, the roughened enameled surfaces rapidly collect ABS and provide a pathway for the corrosive materials reach the underlying steel, which is very susceptible to rapid corrosion. The overall benefits of using a corrosion resistant and lubric coating in the ABS zone are illustrated in the following graphs. NOTE: In recent years, some steel material has been described as “LACR” or “Low Alloy Corrosion Resistant.” These designations are essentially meaningless as they do not fall under any ASTM standard nor the standards of any other government agency. Metallurgists consider these terms to be both ambiguous and irrelevant as there is no minimum alloy requirement or corrosion resistance standard associated with these descriptions. Steels which conform to the ASTM A606-04 standard must meet specific chemistry requirements and must have an established Corrosion Index established by the G101-04 standard. Such steels are correctly identified in the steel industry as High Strength Low Alloy steels with Enhanced Corrosion Resistance. It is clearly illustrated by the above results that a coating with the proper lubricity and corrosion resistant characteristics, and with the ability to resist cracking and deterioration, is of substantial benefit in eliminating problems caused by ABS. Unlike the porcelain enamel coatings of the past, which are basically glass compounds fused over a metal surface at high temperature, DuraShield™ does not contain any glass or other brittle compounds which can crack and create rough surfaces for ABS to accumulate.

Coating Procedure

Panels of the desired substrate material and size are selected and spray coated with the DuraShield™ Coating. The coating thickness is specified by the particular plant environment and application. The panels are cured in a hot air circulating oven pre-set and at the prescribed cure temperature for the DuraShield™ coating. Sprayed panels are placed in the oven chamber on hangers and exposed to the prescribed cure temperature for the prescribed cure time. Following cure, the coated panels are removed from the oven chamber and cooled to ambient temperature. Once cooled, the cured DuraShield™ is evaluated for conformance to relevant cured product property requirements by the frequent testing of representative samples.

DuraShield™ Tested

Heat Test A hot air circulating oven or a furnace (when higher temperatures are required) is used to expose cured test panels to the desired temperature for the time at temperature specified for the particular test. The test temperature is chosen based on the application service requirements and is generally higher than the expected service temperatures by a pre-determined margin. The oven or furnace is pre-set and at the test temperature before test specimens are introduced. Test panels are placed in the oven or furnace on hangers. After the elapsed test specimen exposure time the test panels are removed from the oven or furnace and cooled to ambient temperature. Once cooled, the condition and coating integrity are evaluated by the methods prescribed in the particular protocol. Sulfuric Acid Test A cylindrical heavy-wall flat bottom reaction flask sized to accept several test panels is equipped for heating (heating mantle) and fitted for reflux. A quantity of acid solution is placed in the flask so that the test panels rest in a near vertical position with about half the lower portion below the level of the liquid solution. [The acid test solution is simply made by adding the appropriate amount of acid to water to obtain the desired percent (% v/v) of acid in the solution.] The apparatus is closed and the acid solution is heated to a gentle boil and maintained at a gentle boil for the duration of the test period. At the conclusion of the test period the heating mantle is removed and the contents cooled to near ambient temperature prior to opening the reaction flask and observing the general condition of the test panels in the flask. Then the test panels are removed from the flask, residual liquid from the panels is rinsed away with pure water and the panels are air dried at ambient temperature. The condition of the dry panels and coating integrity, are then evaluated by the methods prescribed in the particular protocol.

DuraShield™ Benefits

1. High Lubricity – DuraShield™ coating provides a highly lubric surface which is difficult for ABS or any other particulate matter to adhere to. An added advantage of this lubricity is the deposits that accumulate within the element profiles can be easily cleaned with normal soot blowing. 2. Flexibility – Unlike “enamel” type coatings, DuraShield™ is highly flexible and will maintain its integrity, adherence and durability even when coated sheets are repeatedly bent and flexed as occurs to the element in a horizontal airheater application during each and every airheater rotation. Enamel type coatings are primarily glass and generally crack even with slight bending, thereby creating paths for ABS and sulfuric acid to reach the substrate element material and shorten the life of the element. DuraShield ™ is so flexible that a coated sheet can be bent into a 3/8 inch radius without damage. 3. Durability and Integrity - Enamel coatings must be cured at extremely high temperatures (1500? F or above) in order to melt the glass particles contained in the coating. As a result, enameling must be applied over special “enameling steels” which can corrode very rapidly because they contain almost no metalloids (alloys) or copper. Thus, even the slightest imperfections or cracks in the enamel provide pathways to the base metal which typically result in rapid corrosion and separation of the enamel from the element. DuraShield ™, however, can be, and is, applied over alloy steels including A606-04 type 2 and 4 and ASTM A606-04 1008. For greater integrity and durability, Paragon offers the coated element over A606-04 material which is High Strength Low Allow steel with Enhanced Corrosion Resistance (often referred to by the trademark name CorTen). Paragon testing has shown when, compared to enameling steel, A606-04 element substrate has over 30 times the corrosion resistance. 4. Hardness – Despite its lubricity and flexibility, DuraShield ™ is a hard surface resistant to erosion from ash and soot blowing. 5. Temperature Resistance - DuraShield ™ maintains its integrity throughout the entire temperature range of the airheater, and is, in fact, designed for service temperatures of 1000? F. 6. Corrosion Resistance - DuraShield ™ has been tested under the most severe combinations of sulfuric acid combined with heat. DuraShield ™ coated element has demonstrated lifecycles 25 times that of uncoated corrosion resistant steels in boiling sulfuric acid.

Satisfied Customers . . . . A Paragon Tradition

Paragon Airheater Technologies is dedicated to total customer satisfaction. We use quality mill-certified raw materials and craftsmanship in every aspect of our manufacturing process. Our research staff and engineering group can custom design solutions for any specific application and our technical services group is available for consultation and installation assistance.

Performance Guarantee Contact your Paragon Representative for details on the Product Performance Guarantee

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