Bridge Height Limits Transport of Cl02 Storage Tanks
Located in Northern Ontario on the Trans Canada Highway is a bridge that enables pedestrians and vehicles to cross the Spanish River. The bridge itself holds no place in the annals of Canadian history. It is in fact, quite unremarkable as a typical bridge found in the area.
However, the Spanish River Bridge represented a major problem for an engineering project rquired by a local Bleach Pulp Mill. Part of the bleaching process requires the use of Cl02 - Chlorine Dioxide. This is produced in the plant on an as needed basis for two reasons. First, the chlorine is essential to the process and is needed in the continuous bleaching process to produce Pulp. It is stored in tanks and there is always at least on full tank with Chlorine Dioxide - even when maintenance work has to be performed.
Second, the operators of the mill have earned a great reputation as a company that is environmentally responsible, has an enviable safety record, and frequently incorporates legally required safety /environmental laws before they are enacted.
Cl02 as a hazardous substance is thus treated with great care. Therefore, having more than one tank ensures that there will always be an adequate supply of Cl02 and allows more time during maintenance shutdowns without any interruption of supply. The regular maintenance pays off both economically and in overall safety.
In the planning stages several problems immediately became apparent.
The size and proportions of the two tanks made fabrication, installation and shipping extremely difficult. Furthermore the tanks were to be installed into an area where space was limited due to existing plant structures.
The size of the tank was governed by the required capacity of 12,700 cubic feet (360 cubic meters) or 79,100 Imp. Gal. (95,000 US Gal.).
Available space indicated an unusually tall and slender tank - 15'-8' ID X 66'-0' (4.77 m x 20.12 m) high straight shell. Estimated weight was 48,000 lbs empty; 842,000 lbs full.
Other design conditions included:
Pressure: +/- 20' WC at top
Temperature: 32-82 degrees F.
Liquid Specific gravity 1.0,
Roof live load 50 psf
Seismic data: Za = 1; Zv = 0 V = 0.05
The Engineering approach was based on the customer's extreme environmental consciousness and safety considerations for handling the chemical. The original plan was for tanks with a larger ID. However, the transportation route inspections showed the smallest dimension to the Spanish River Bridge at 16'-2'. That measurement had to include any peripheral material or equipment on the outside of the tank.
The mill had insisted on having one piece, seamless tanks with no joints. Largely this was for safety, given the corrosive and highly toxic nature of Cl02. To fabricate such tanks, meant that the work would have to be done at Fabricated Plastics' plant. Working at the plant offers tighter control and efficient production, the detriment was the shipping . The height restriction of the local bridge reduced the diameter down accordingly so that the tank had to be made much taller. The one piece construction necessarily meant a standard mould approach could not be used, as this would render tanks with joints. Therefore, the tanks were made with a specially designed slip mould. Fabricated Plastics laminated the whole corrosion liner using an integral but moveable mould, additionally 1/8' layer of hand laid up structure was added, the same procedure was followed for the integrally moulded bottom, the slip mould was moved, allowing an integral lay up of the entire length of the tank shell without a seam, then the slip mould was removed. The head for the tank was then attached to the shell and bottom assembly. The assembly of the top, bottom and shell unit was sufficiently strong to allow the whole of the shell to be filament wound in one piece, thus ending with a seamless integral product.
The bottom of the tank was designed with an ingenious sizing that allowed the projection on the diameter to be minimized and to use a continuous filament wound ring with site supplied structural steel clips to anchor the huge tanks down - an improved method to accommodate the larger seismic loads created by the greater height.
Premium grade, corrosion resistant and fire retardant Derakane 510C 350 Vinyl Ester Resin was used throughout, with 5% Antimony Trioxide fire retardant added to the structural layers. To accommodate a superior corrosion resistance requirement the interior double corrosion liner incorporated a layer of Halar Veil, and 3 layers of 1.5 oz /ft2 Mat.
The corrosion liner thickness was excluded in the design calculations, as is the usual custom in such applications. The design complied with ASME RTP-1e-1999, using computerized lamination analysis to determine properties of the Type X structural laminate to satisfy the high seismic axial loads.
Exterior surfaces incorporated a layer of C-glass veil, applied over the final mat ply of the structural laminate, using a white resin topcoat with UV inhibitors. Once the fabrication of the two tanks was finished, the slow and arduous journey to their destination in Northern Ontario began. The tanks were transported on special low profile trombone trailers to the Spanish River Bridge, where at the bridge the tanks were carefully re-loaded with the use of two mobile cranes onto a special minimal road clearance transport float to clear the bridge and continue the final 0.5 km to the site. The Spanish River Bridge which had caused so much of the design difficulty was successfully crossed with literally millimeters to spare.
The hardest project becomes simple if you identify the criteria that will govern every aspect of the design, fabricaiton and installation.
These well designed and quality fabricated tanks have served the customer trouble free since 2006.