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Ballun, P. Since the s, two types of epoxy coatings have been commonly specified and used for iron valves in the waterworks industry: fusion-bonded epoxy and liquid epoxy. Liquid epoxy is a two-component mixed material that is applied by spray, brush or other methods and chemically cures after application. The purpose of this article is to explain the typical requirements that apply to these coatings and compare some of their properties.
The coating is also tested for impact resistance by dropping a weight onto the surface in accordance with ASTM D In production, coatings are visually examined for defects and randomly tested to verify coating thickness.
The result has been the production of valves with highly reliable epoxy coatings for over 40 years. Over the last decade, there has been significant debate among valve users and producers about setting standard requirements for epoxy coating thickness and holiday testing. Because of the intricate geometry of gate valves, providing uniform coating thicknesses and holiday testing on a production basis can be costly.
Therefore, the associated valve standards require only a minimum coating thickness of 6 mils for gate valves and 8 mils for quarter-turn valves. Additional requirements were initiated by the U.
Environmental Protection Agency, which in the s began regulating drinking water additives including treatment chemicals and water system components including valves and coatings. Various state water authorities slowly adopted the NSF 61 requirements resulting in an entire industry of testing authorities with labs dedicated to certifying products to this new standard.
From the start, the waterworks valve industry found the NSF 61 approval process to be burdensome and costly. Even though all the materials in a valve, including the coating, could be independently tested to verify compliance with NSF 61, the standard required that actual production valves be tested in independent labs on a frequent interval.
Normalization factors were developed in the standard to take this into account resulting in coating manufacturers certifying various coatings for either pipe or valve service. Valve manufacturers typically employ a near-white grit blasting operation to meet these requirements and provide a good surface profile for the coating.
Care is taken to prevent oxidation of the blasted surfaces before coating by starting the coating process within the same work shift as blasting. Liquid epoxy is furnished as a two-part kit that is thoroughly mixed and applied to the valve surfaces by spray, brush or roller, taking care to vent the vapors to promote the removal of the solvents from the coating.
Because of the solvents involved, there is typically a limitation to the thickness of a single coat, such as 16 mils. If a greater thickness is required, additional coats are applied within a prescribed coating window. The mixture also has a finite pot life of hours depending on temperature and humidity conditions. Dry time for handling is typically 7 to 10 hours, but water immersion may require days of additional cure time to assure full dispersion of the solvents.
The parts are either moved or conveyed to a spray booth where they are attached to an electrical source to achieve an electrostatic charge. The powder coating is then sprayed over the part and the parts are returned to the oven for post curing in the oven for minutes. In some factories the heating and spraying process are controlled with a conveyor system. Once removed from the oven, the parts are allowed to cool before installation and water immersion see headline photo.
Fusion-bonded epoxy coatings do not require an additional days of cure time as with liquid epoxy since no solvents are used. Figure 2. Typical high-voltage holiday test equipmentAfter either coating process, all parts are visually examined to ensure adequate coverage and the dry film thickness is measured in random locations.
When required by the purchaser, a holiday test is conducted to identify any voids in the coating in accordance with ASTM G In a holiday test, a voltage is applied over the coated surfaces and any continuity between the test wand and substrate surface will be indicated on the detector unit Figure 2. Epoxy coatings less than 20 mils in thickness can be checked using a low-voltage i. When voids are identified, the coating is repaired and retested. Their resistance to damage due to handling can be compared by reviewing their direct impact resistance in accordance with ASTM D and adhesion strength in accordance with ASTM D AWWA C requires a minimum impact strength of 20 in-lbs, which can be met by liquid epoxies, but is not typically reported quantitatively.
Fusion-bonded epoxy coatings typically have twice the impact strength and can be as high as in-lbs. The adhesion strength is also rarely published for liquid epoxies but has been measured to be in the to psi range when good surface preparation practices are followed.
In general, fusion-bonded epoxy coatings exhibit twice the adhesion strength in the range of to psi Val-Matic, These coatings prevent corrosion, tuberculation and wear in valves and promote efficient flow of fluids though piping systems.