by Quilyam Casallas*
The phenomenon of corrosion is a continuous, destructive process and its attack speed is a function of many variables, such as physico-chemical characteristics of the medium, service conditions of the element under attack, the characteristics of the building material, the design conditions, the protection mechanisms installed, the temperature, among many others.
In a hydroelectric complex there are many elements susceptible to a corrosion process, such as conduction and load pipes, metal gates for flow management, metal structures, machinery and equipment, conduction towers and power transformers among others.
Diagnosis
The Caldas-CHEC Hydroelectric Power Plant in Colombia is no stranger to the action of the corrosion phenomenon and its important infrastructure, like that of many similar complexes, are threatened in critical elements such as conduction pipes made of carbon steel on both internal and external surfaces.
The destructive action is concentrated in the lower areas because there the attack is combined with an abrasion action by the dragging of solid particulate matter such as sand and sediments. In the same way, significant corrosion attacks are generated in the welded joints due to changes in the metallurgical structure of the steel in the affected area. In the same way, significant corrosion attacks are generated in the welded joints due to changes in the metallurgical structure of the steel in the affected area. by the heat during the welding process.
At the end of last year, the Caldas Hydroelectric Power Plant carried out an evaluation and diagnosis study of the mechanical state of the metal siphons (pipes of 2.9 meters in diameter) of Moravo and La Estrella located in the Morro Azul village of the municipality of Chinchiná in the department of Caldas (Colombia) belonging to the water pipeline of the Esmeralda power plant.
Coating scheme
For the above, it contracts the company Integral Ingenieros and it is decided to intervene this infrastructure by carrying out the following activities on the internal surfaces:
*Cleaning with abrasive jet to white metal grade according to PSC-SP5 standard
* Application of an epoxy coating 100% solids at a thickness of 18 thousand in dry film, covering an area of 23,090 meters.
On the external surfaces it is decided to intervene the pipe following the specifications of the integral company as follows:
*Surface preparation: cleaning with abrasive jet to white metal grade according to the indications of the SSPC-SP5 standard.
*Base coating: Epoxy self-primer of high solids Series 200 to a thickness of 4-5 mils in dry film to cover 23,903 meters.
*Finishing coating: Aliphatic Urethane Enamel Series 36 at a thickness of 4 mils. The audit of the project was carried out by the firm Integral Ingenieros.
Application
The maintenance project with coatings of the conduction pipe begins with the location of 5 work fronts strategically located along the line to perform cleaning with sandblasting and application of coatings on both internal and external surfaces.
During the application of abrasive jet on the internal surfaces, a robot-type machine was used. This has three arms that support an equal number of nozzles that rotate in a circular way and are animated with a longitudinal movement, inside the pipe, by the action of an electric motor of adjustable speed depending on the need for more or less exposure time of the abrasive jet on the surface to be treated.
Where the attack and wear due to corrosion was greater and there was the presence of abundant material generated by the corrosive action, it was necessary to slow down the robot's advance speed or else the specified degree of cleanliness would not be achieved.
The application of the coating was carried out with Airless equipment 68:1 pump ratio and 527 nozzle for the Series 300 product achieving a thickness of 18 mils in a single application. This considerably facilitated the logistics of progress for the development of the project because the applied film managed to dry for handling at 8 hours and in this way it could be transited on the treated surface allowing the movement of materials such as sand, scaffolding, sandblasting equipment and application.
In the realization of the project were treated:
In the internal surfaces 23090 m2 with a consumption of 3,748 gallons of Epoxy aminino-amide coating Series 300.
On the external surfaces, 21,233 m2 were treated and 1773 gallons of high solids epoxy coating Series 200 and 1773 gallons of Aliphatic Urethane Enamel Series 36 were used.
The quality assurance was carried out by the company Integral Ingenieros controlling the degree of cleanliness, roughness profile, environmental conditions, skill of abrasive jet applicators and coatings, wet and dry film thickness, adhesion by the Pull-of method according to ASTM 4541 standard, film continuity and visual inspection.
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