Special reinforcement was the key to building high-strength P&P tanks

The two filament wound tanks were built to withstand high wind loads and to achieve a "Zone 2" earthquake rating.

When Fabricated Plastics Limited of Maple, Ontario, was called on to provide two large fiberglass reinforced plastic tanks for the storage of chlorine dioxide at a Québec pulp and paper mill, the most important requirement was to build them with sufficient strength to withstand extreme wind loads, as well as to achieve a "Zone 2" earthquake rating.

"In addition to putting ribs on the wall, you can increase the hoop strength of a tank in one of two ways," says Don Sablinskas, President at FABCO, "either by laying up a very thick laminate, or by 'winding in hoop' (that is, winding the glass reinforcement onto the mandrel at a zero angle). Increasing the wall thickness, of course, increases both the weight and cost of a vessel. On the other hand, a tank wound in hoop only (with conventional glass filament) will have very little axial strength."

To achieve the additional axial strength required without sacrificing hoop strength, the company selected a type of reinforcement frequently used by molders in other areas of the composites industry (including boat builders) but not in the corrosion field.

"We developed a very efficient tank design by combining hoop winding with unidirectional glass roving (Liasil)," Sablinskas says. "The hoop winding gave us the necessary strength to contain the liquid, and the unidirectional roving provided the axial strength. It also enabled us to reduce the wall thickness - a cost-effective measure that cut the weight of the tanks substantially, which, in turn, made it easier to assemble them."

Although chlorine dioxide is a highly corrosive material used as a bleaching agent in the pulp and paper industry, it was no problem to select a suitable resin for the units, in this case, a premium corrosion resistant resin.

Both tanks are 5.5 m (18 feet) in diameter and 16.7 m (55 feet) high. Because of their size, each was produced in five separate parts - a top, a bottom and three middle shell sections - for shipment by road to the mill site. Each of the five sections is approximately 3.5 m (11.4 feet) high with an inside diameter of 5.5 m (18 feet). The ribs are hollow structures wound directly onto the tank wall.

It was very important to provide a rib close to the ends of each shell section to maintain its stiffness. When cylindrical parts are being lifted by crane, they have a tendency to go out of round which makes it extremely difficult to align the parts during assembly.

The domed top, molded by hand lay up, is fitted with two explosion hatches which release suddenly to relieve any build-up of pressure inside the vessel. The top also has molded-in supports to which the feet of a maintenance access walk-way are bolted.

The flat-bottomed base of each tank is fitted with a manway and has specially designed (steel) hold-down lugs to prevent overturn during any earthquake conditions. Before the individual tank components were shipped to the mill (where FABCO field crew later assembled and bonded them together), the tanks were "test assembled" at the Maple plant to ensure mating parts fitted together.

A domed top is lowered onto the cylindrical wall of one of the chlorine dioxide storage tanks.

Assembly Procedure

Special lifting lugs were attached to the top section so that a crane could raise it off the ground for the first shell section to be slid into position beneath it during assembly. The top was then lowered onto the shell and the two parts were strapped together (on both the inside and outside of the components).

After the first joint had cured, the joined sections were raised again for a second shell to be slid into place and joined to them - a procedure that was repeated until each tank was fully assembled. In the interest of safety, the work is always carried out at ground elevation. Each tank was put together a few feet from the concrete pad and lifted into place with the crane.

Once the customer had approved the installation, FABCO's engineers proceeded to confirm the integrity of their design by carrying out strain gauge tests after each tank had been filled with water and allowed to stabilize for a 12-hour period. "We also did the tests as a further quality control measure," Sablinskas said. "They will allow us to make comparison tests in the future to ensure the tanks are still sound and performing as they should."

Assembly of tank commences.

Tank Stats

The capacity of each tank is 341,000 litres (75,000 gallons).

Its weight, empty, is approximately 16,000 kg. (35,300 lbs.); when filled, it weighs 380,000 kg. (837,000 lbs.).

FABCO was able to complete the job within 12 weeks of order date including engineering, approval, manufacturing and erection.

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