Safe: PPE for the team included respirators, goggles, suits and gloves. Surface fans ensured fresh air circulation |
Inspection
"Sistema de Aguas de la Ciudad de México," the public office in charge of the city’s sewer system, arranged for a team of engineers to inspect the 35-year-old sewer. Due to its large size, 196,000ft (approx) long and 21.3ft diameter, the personnel carrying out the inspection used a boat to go through the pipe. The inspection involved evaluating the condition of the tunnel and the damage suffered by the concrete walls. Once the water in the tunnel was drained, the engineers were able to use an all-terrain car named "unimog". Based on the engineers’ findings, the authorities decided to coat the concrete pipe.
Problem
According to the engineers’ report, the upper dome of the sewer had been attacked by acidic vapors generated by the decomposition of organic matter in the water. The decomposition of sewer water produces sulfuric acid, high amounts of methane, and hydrochloric acid, all of which accelerate the rate of chemical attack on the pipe. Along with a pH decrease, a consequent risk of passivation is driven on the reinforcing steel. This effect is described as the deterioration of the protective film covering the steel in the concrete. Furthermore, the transportation of all sorts of debris caused abrasion on the concrete walls, leading to cracks and exposed steel supports which could eventually compromise the structural integrity, if not promptly treated.
Solution
The Mexican authorities, in conjunction with the Engineering Department of Mexico University, the office in charge of technical support for this project, agreed to use the Belzona 5811 (Immersion Grade) to protect 129,166.9 ft2 of concrete wall.
This is a two-component, high performance epoxy barrier coating designed to protect metallic and non-metallic surfaces against attack from aqueous solutions. One feature of the product is its capability of being spray applied under the "wet" conditions of the substrate and the environment. The Belzona 5811 (Immersion Grade) is suitable to operate under immersion conditions up to 122°F and is recommended to be applied in two coats of 10 mils each, for a total dry film thickness of 20 mils. The product consists of a base and a solidifier which are mixed in proportions of three parts of base per one part of solidifier. For this specific application, the immersion grade used was presented in two colors: beige and grey.
Equipment, personnel and surface separation
The equipment used for this application was a Graco pump equipped with a SuperCat plural component proportioner, which mixed the Belzona product as it was being used. The base and solidifier of the Belzona 5811 (Immersion Grade) were separately placed into two hoppers located at both sides of the pump. Each hopper was equipped with a silicon rubber heating band to increase the temperature of both the base and solidifier up to 149°F.
A customised cart was designed to transport the spray equipment, product, crew, and scaffolding equipment, which was erected on the cart for the spraying personnel to reach the top of the pipe. Since the main floor was not level, the cart was constructed with wheels that rolled along the walls of the pipe.
All personnel in charge of the application were required to wear safety harnesses and Personal Protective Equipment (PPE) including respirators, goggles, suits, and gloves. The applicators were trained to perform in confined spaces. Gas detectors were used to constantly monitor the air inside of the pipe. Fans located at the surface were also used to ensure that fresh air was constantly recirculated into the pipe.
In order to access the pipe, located 70 to 80 meters underground, the crew members took an elevator. A safety meeting was conducted every morning before accessing the tunnel. Each applicator was required to report to a safety supervisor whenever accessing or exiting the tunnel.
Surface preparation was performed by a qualified company contracted for this purpose. The surface was pressure washed using 5,000 psi high-pressure water jetting to clean the substrate and achieve a minimum 3 mil surface profile as per NACE No. 5/SSPC-SP 1210. Environmental parameters such as temperature of the surface, dew point, and relative humidity were measured and logged on a daily basis.
Application
The application was carried out by local company, Ivresse International.
"Sistema de Aguas de la Ciudad de México" required a 28-day interval for the application to be completed. The total area of deteriorated concrete to be coated was 129,166.9 ft2. The job was carried out during the dry season when the level of the water in the system was the lowest.