HDPE 80 mil Floating Covers - CCD Tank

LOCATION: Key Lake, Saskatchewan

TIMEFRAME: Fall 2011 – Spring 2012

SCOPE OF WORK: Design of Floating Cover over CCD Tank

PROJECT PARTNERS Cameco Corp MDH Engineering Saskachewan Red Engineering Edmonton North West Hydraulic Consultants (NHC)

Challenge

The overall objective was to design a floating cover over the surface of a counter-current decantation (CCD) tank operating in outdoor conditions at Key Lake in Saskatchewan. During winter months this location can observe temperatures as low as -40C. The client spends significant amount of capital and resources each year to maintain the liquid temperature in the tanks by injecting steam into the liquid. To keep the costs down they proposed insulating the walls and place a cover to improve the efficiency of heating. Spray foam was applied to the walls of the tanks to obtain an R-24 insulation value. For the covers, hexagonal HDPE sheets also know as “lily pads” were considered and installed on a few tanks as shown in figure 1, lily pads float on the surface of the liquid, automatically arranging themselves into a close packed formation to completely cover the liquid surface. The biggest issue with this type of cover system is wind uplift. Higher than average winds speeds will push the lily pads towards the tank walls and in some cases will stack one over another.

There were several challenges we came across since the inception of this project. The first and foremost challenge was to design a cover with the following design constraints:
• Be resistant to liquid with a pH of 1.
• Design Temperatures:
o +28 °C to -45 °C (Ambient)
o +40 °C to 5 °C (Liquid Temperature)
• Minimum R-27 insulation value using a minimum 150 mm thick foam
• UV resistant, capable of a 15 year service life.

The other challenges included fabrication of the panels. Some of the challenges faced during fabrication included:
• Folding the HDPE 80 mil sheet due to stiffness
• Panels required to be fabricated with tight tolerances due to the deployment geometry
• Encasing the 8” ethafoam by 80 mil thick HDPE sheet
• Seaming the corners of the pie shaped panel.
• Testing the entire panel for leakage
• Optimize size and weight of panel to aid in shipping
• Pull strength of lifting tabs on the panels due to surface tension of liquid and in event of
• Custom designed lifting frame to deploy panels over the fluid

Solution

Layfield offered a turnkey solution by providing design, fabrication and installation of floating cover over the CCD tanks. Materials were identified based on design constrains as the liquid in the tank was significantly acidic. HDPE80 mil was chosen to meet the performance objectives of the project. Layfield offered 200 mm thick polyethylene foam (Ethafoam) to meet the insulation and thickness requirements of the specification. It was estimated that it will require 4 inner panels and 28 outer pie shaped panels to cover the top of the CCD tank. It was not possible to build these panels on site due to complexity of design and detail work. Fabrication of the panels was carried out in a controlled shop environment. HDPE 80 mil due to its crystalline nature is stiff and difficult to fold. To fold the HDPE80 and to get the desired shape of the panel, Layfield built a custom frame and created the folds. Foam was encased and the panels were extrusion welded along the overlap and on the corners to create a water tight structure. Testing of corner seams with conventional methods was difficult due to thickness of extrusion over the seams. Layfield came up with a technique to qualify the welds, air was filled into the panels and seams were sprayed with soapy water to detect any leakage. Layfield also ensured that tight tolerances were maintained during fabrication and the panel sizes met the dimensional criteria set in the specification. Lastly, the strength of the pull tabs were tested with a custom built frame that was attached to a fork lift. Each and every panel was tested to ensure pull strength of the attachment of the tab to the liner shell. The pull tab was extrusion welded on to the HDPE sheet. The panels were also tested by NHC consultants for pullout strength taking into account the forces acting on the bottom of the cover due to surface tension. The final design was stamped by a professional engineer from Saskatchewan.

Results

The result was a custom designed floating cover made with tight tolerances in a shop environment. All the design constrains were taken into account during the design and the fabrication of the floating cover. It took around 20 drawing revisions before the final design was approved. The installation of panels is scheduled in June 2012.

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