Extreme service conditions and a tight fit challenge designers

Conical bottom of tank loaded onto trailer for transport to Oshawa.

When Norsk Hydro Canada Inc., of Bécancour Québec, contracted Fabricated Plastics Limited (Fabco) of Maple, Ontario, to design and fabricate a large diameter FRP (Fiberglass Reinforced Plastics) storage tank, several challenges had to be dealt with. This tank was required to control the downstream processing in Norsk Hydro's magnesium refinery. As this vessel was to be 5,700 mm in diameter and 7,122 mm high, it was too large to transport in one piece to the customer's facility.

In addition to the transportation challenges, Fabco had to address the design of the tank for the service conditions, which included a solution having a specific gravity of 1.31 and a design temperature of 100 C. The tank also had to be designed for positive and negative pressures of +12 Kpag and -12 Kpag respectively.

The tank's configuration incorporated a conical bottom and flat top (due to the limited head room at the tank's final location). The fabrication of the flat top incorporated the use of steel support beams laminated to the top and encapsulated with FRP laminate, to handle the positive and negative pressure requirements. The FRP laminate applied to the steel reinforcing members would protect them from the corrosive nature of the tank's contents, in case of accidental splashing, or spillage.

In order to maintain the tank's contents at a constant temperature, a 50 mm thick layer of polyurethane foam insulation was applied to the exterior of the tank shell, top and conical bottom. A 4 mm thick FRP protective outer skin then covered this insulation.

The tank's conical bottom required some form of support, to keep the bottom drain nozzle a predetermined distance from floor level. It was decided that Fabco would also design and supply a steel support structure, into which the tank would be installed, once it arrived at the Norsk Hydro plant.

The tank was to be installed inside Norsk Hydro's existing facility in a location that had not originally been designed to accommodate such a large structure. The area in which the tank was to fit was extremely tight. The final location exceeded the tank's proportions by only inches or fractions of an inch. Dimensional accuracy was, therefore, crucial as to whether the vessel would be able to occupy the space allowed. The fact that the tank was being made in sections that would be assembled outside of the controlled environment of Fabco's plant further complicated this matter.

The drain nozzle at the base of the conical bottom was designed to support a 500 lb. steel pinch valve, as well as to be used as a lug to facilitate the moving of the tank from the horizontal to the vertical position, during the installation.

Assembled tank being loaded onto a ship at the Port of Oshawa.

Once the design had been established, the tank sections were produced in Fabco's plant, under strict quality control conditions to ensure that all of the requirements were met. The tank was produced in two sections; a top section and the conical bottom section, both of which included a portion of the tank shell. Since both sections were over 20' wide, special shipping arrangements had to be made to transport them from Fabco's plant to the Port of Oshawa.

The assembly commenced in Oshawa with the erection of the steel support structure into which the conical bottom section was secured. The top section was then lifted and placed onto the bottom section and, while a crane held the top in position, the two pieces were joined by applying FRP laminate over the seam area. Care had to be taken to make certain that the joint laminate cured properly. When necessary, heaters were used to maintain the temperature in the joint area at around 20šC. Fabco's strict quality control resulted in the successful final assembly at the Port.

Norsk Hydro then arranged to have a ship pick up the completed tank to transport it to their facility in Bécancour. Upon its arrival at the Port of Bécancour, the tank was offloaded and placed onto a transport trailer, which moved the tank to the Norsk Hydro plant, stopping beside the facility into which it was to be installed.

The critical task of positioning the tank inside the building commenced with the placement of the steel support structure into the proper position. Pushing the tank through an opening that had been enlarged to accommodate the vessel's dimensions then followed this step. Norsk Hydro's Project Engineer created a wheeled dolly that would support the tank in the horizontal position. Steel angles were secured to the floor creating a track into which the wheels of the dolly would fit, ensuring that the dolly and tank would stay on course when being pushed into position inside the plant. When all was ready, the tank was guided towards the steel support.

Once in position for lifting, the tank was slowly raised using lifting lugs imbedded in the top. A spreader bar distributed the load on each of the top lugs. A temporary plate bolted to the face of the drain nozzle accommodated a cable, enabling the installers to lift and hold the bottom while the tank was moved into the vertical position and finally into the support steel.

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