Amchem recently completed a major coating project for India’s largest power utility at its 2 X 500 MW Thermal Power Plant in Southern India. Approx. 660,000 sq. feet of concrete and steel surfaces subject to seawater immersion / splash in Cooling Tower No. 1 were coated with 2,000 Microns of 100% Solids Polyurethane Coating trademarked Purethane^® 386/9000.
The coal fired coastal thermal power plant will draw seawater for use in CW system as make – up water. Two Natural Draught Cooling Towers ( one for each boiler of 500 M W ) standing 550 feet tall ( largest in Asia ) will cool approx. 16 Million Gallons Per Hour of seawater flowing through them.

To prevent concrete degradation and re-bar corrosion in the highly corrosive sea water immersion / splash environment, a number of measures were taken in the design. Principally, all concrete and steel surfaces in contact with sea water / sea water spray i.e up to drift eliminator level (50 feet) were to be coated with 2,000 Microns of 100% Solids High Build Polyurethane Coating. The polyurethane coating will provide a strongly bonded, tough, impermeable membrane, which prevents transmission of Chlorides, Sulphates and greatly retard transmission of Oxygen and Water Vapour into the concrete and underlying reinforcing steel. Additional corrosion protection measures include use of Sulphate Resistant Cement with 5-8% C3A content & Marine Grade Corrosion Resistant Steel Re-bar. Above the drift eliminator (coating line), the shell structure uses an organic migratory corrosion inhibitor.

The client selected 100% Solids Elastomeric Polyurethane from several candidate coatings based on its resistance to seawater immersion; crack spanning capacity and ability to be sprayed into a pinhole free monolithic membrane. Exhaustive evaluation was made on basis of test data and track record supplied by several manufacturers and five were short-listed for bidding.  Amchem was the successful bidder.

The client prepared stringent performance specifications. Considering the importance of a quality application, a considerable portion was devoted to surface preparation and application standards. The buyer, construction contractor and the Amchem  jointly drew up a detailed quality plan.

Amchem was required to send samples drawn at the work site for laboratory testing to measure various properties in accordance with ASTM Methods. Tests included Tensile Strength, Recoverable Elongation, Water Vapour Transmission, Flexibility, Impact & Abrasion Resistance etc. Test for resistance to seawater was done by immersion for 30 days followed by Weight Gain (1% Max.), Change In Hardness (5% Max.) and Change In Tensile Strength (5% Max.).

Amchem was also required to conduct field tests for Film Thickness and Adhesion ( 1 test per 1000 sq. feet approx. ), Surface Hardness and Holidays ( Steel ). Third party inspection was conducted initially by a NACE certified coating inspector and subsequently by representatives from the Organic Coating & Polymers Division of India's premier Chemical Technology Institute (IICT). Discontinuities in the coating on concrete were visually inspected and repaired.

Coating Application

The internal structure to be coated was very complex. It included the Fill Support Structure, Racker Columns, Baffle Walls, Duct, Walkways, Pond Floor, Pond Wall etc. at heights up to 15 metres ( 50 feet ) . The fill support structure is a maze of vertical lower columns (> 400 No's), upper columns ( > 800 No's ), diagonal columns and beams ( > 3,000 No's ).In order to access the various parts of the structure Amchem deployed specialized high reach equipment from JLG such as articulating boom, scissor lifts, manlift etc. The constrained spaces made access to the surface difficult and the high reach equipment served the purpose very well. The entire concrete structure was sweep blast cleaned using sand, primed @ 125 Microns WFT using Purethane^® PIV, 100% Solids ,Damp Tolerant Polyurethane Penetrating Primer and coated with 2,000 Microns  Purethane^® 386/9000 , 100% Solids Polyurethane Coating.

Three Graco Hydra Cat plural component airless spray equipment were used for the coating. In addition several blast pots and air compressors were deployed for surface preparation. The Graco equipment performed reliably as always with minimal repairs and downtime. Highly skilled crew of around 45 people conducted the surface preparation and coating application.

More than 9,000 litres of Purethane^® PIV Primer and more than 135,000 Litres of Purethane^® 386/9000, 100% Solids Polyurethane Coating were used for the coating operations. Purethane^® PIV is a specialized primer which penetrates into the concrete, sealing and consolidating the surface for improved adhesion. Water tolerance allows it to be sprayed onto damp concrete with excellent bond between the primer and concrete as well as primer and coating.

Water from the condenser is brought into the Cooling Tower through 10.5 feet (3.2 m dia) steel pipes up to Cooling Tower Bottom, which is bifurcated into 2 pipes of 7.8 feet (2.4 m dia)  rising up to 50 feet level. The pipes were blast cleaned using granulated copper slag to SSPC SP-10 with an Anchor Profile of 100 Microns, primed and lined internally with 2,000 Microns Purethane^® 386/9000.

Pond wall and floor were coated after the fill support structure was completed.