In-situ tank replacement

A Costa Rican company has devised a novel way to replace old tanks with minimal disturbance.

EnerQuim SA, founded in 2008, is a small chemical engineering company located in Costa Rica, Central America, dedicated to the design and manufacture of composite products, mostly in glass fiber reinforced plastic. Its main focus is on composite tanks used for the storage of liquid substances, solids or granular dust. 

In Central America, there is a significant market for tanks used for the storage of liquid fuels by the automotive or transport industry. Government regulations establish a useful life of 20 years for tanks already existing in service stations, which are generally single-walled tanks. While most of these tanks are made of steel, EnerQuim developed a novel way of replacing these tanks in-situ with glass fiber reinforced composite. This process allows the use of the existing tank as a mold, and does not practically alter the environment where these tanks are underground. It also means that there is a very low loss of storage capacity compared to the existing tank; a bit less than 5%.

EnerQuim builds the tanks by impregnating a series of glass fiber mats inside the existing steel tanks using vacuum-assisted infusion, which are then cured using ultraviolet light radiation thus achieving a complete replacement. The new tank, made of glass fiber reinforced plastic, is resistant to corrosion or degradation.

Since its first installation, the company has installed 100 new tanks, with a recent contract awarded to build a tank to store diesel with a capacity of 27,205 l.

The tanks are manufactured under a range of standards including the regulation of the Hydrocarbon Storage and Marketing System (Decree 30131), INTE W30:2020, (Requirements for Fiberglass Reinforced Polyester (PRFV) tanks, Underwriters Laboratories Inc standards UL 58 and UL 1316, American Society for Testing and Materials regulations ASTM D 4021-92, ASTM D 4097, American Society of Mechanical Engineers standards ASME-RTP-1- Reinforced Thermoset Plastic Corrosion Resistant Equipment and the Fiberglass Reinforced Plastics Institute’s FRPI SP9100 Standard Practice for Laminate Certification. As well as this, EnerQuim operators have training that accredits them as authorized persons to carry out work in confined spaces.

The replacement process

When looking to replace a tank, EnerQuim first makes a characterization of the static loads and other considerations that such tanks could be exposed to through the application of finite element-assisted structural analyses. These include the comparison and interpretation of the results using an analytical method, with the use of specialized software for such analysis. The company also carries out quality control of the composite material obtained in each of the new tanks built using ASTM D638 Test method Standard for the tension properties of plastics, and ASTM D790 Standard test methods for the bending properties of reinforced and unreinforced plastics and electrical insulating materials. These are carried out in an external laboratory and make it possible to determine the thickness required by the parts, as well as meet or withstand the corresponding static mechanical loads when buried at a certain depth. Extreme situations such as a hydrostatic load by saturation of the soil where the tank is buried, are also taken into account.

With the implementation of this methodology, it has been possible to replace and/or build a series of tanks and liquid fuel tanks, without any need to extract existing tanks and carry out the civil and mechanical works that would entail such a replacement. This, in turn, can cut processing time, reduce downtime during the operation of service stations, and avoid any repercussions that these aspects have through modification. In EnerQuim’s case, it manages to avoid having to demolish sections of the concrete earthenware, when inputting parts for the infusion process, such as the vacuum line used for the infusion of the reinforcement mats. In some cases, it is necessary to make small perforations of diameters close to 12 inch in diameter to replace the pipes that need to be interconnected with the new structure.

When EnerQuim tested the vacuum-assisted liquid resin infusion process in the lab as well as at the site, the engineers did experience complications since the materials are not specifically designed for use in this kind of infusion methodology. General purpose glass fiber fabrics with high weights were used, and resins based on polyester or vinyl ester with higher viscosities than those generally used for infusion. Other parts used in the process, such as the distribution mesh, the distribution or primary irrigation channel, the sealant tape, and the film for obtaining the "bag" in which the vacuum will be made and accessories such as valves and joints, were actually obtained from the agricultural industry – and are normally used in the construction of greenhouses. (In Costa Rica, country agriculture is one of the most important economic activities taking place, so the parts were easy to access.) Taking all this into account, there were a series of challenges to achieve a correct impregnation of all the reinforcement mats in an adequate time. EnerQuim also had to calculate of the distance between the primary distribution channels of the liquid resin and the dry. Added to all this were potential variations in the parameters of viscosity and gel times of the resins supplied, as well the temperature of the sites where the tanks are located. EnerQuim was also limited by its lack of previous experience in infusion projects of this magnitude. However, by fine tuning the location of the liquid resin entry points and vacuum generation port, parameters over which the engineers had no direct control – such as viscosity, gel time, reinforcement permeability and porosity – became less of a problem. Thus they were able to achieve a significant decrease in the costs associated with the construction of the new glass fiber reinforced plastic tanks.