If professional engineers were asked to define a perfect world, their answer would no doubt include projects that were blessed with a clear and precise description of the requirements, a viable schedule, availability of materials, and a reasonable amount of design freedom. Of course there is no perfect world, so engineers must work with restrictions that vary from difficult to almost impossible. Typically, a project is a series of compromises that are made intelligently and creatively in order to achieve the optimum result.
A contract won by Fabricated Plastics Limited of Maple, Ontario just north of Toronto provides a case in point that admirably illustrates that an engineering project is not simply design, fabrication and installation. It is an ongoing work in progress that can change several times in the interval between design and completed project.
The contract was given to Fabricated Plastics by a mining consortium and involved an Acid Regeneration Plant in the Province of Quebec. The purpose of the plant was to extract Titanium Dioxide from limenite ore. This process was performed using a separator /absorber unit which extracted the titanium oxide and regenerated the hydrochloric acid.
Most commonly, titanium oxide is a white powder and is used as an ingredient in sun screen lotions, mirrors and eyeglasses. In this case the product was a black powder and used as an abrasive in various applications. Basically the process involved placing the ore in the separator which provided a cyclonic action in a hydrochloric acid environment at high temperatures.
The original vessels included a separator section with a tangential inlet that was subjected to both erosion and corrosion, and an absorber section that was subjected to corrosion only as the majority of the abrasive content had been removed in the separator.
The separator had a 3.5 inch (90 mil) glass backed Tefzel thermoplastic liner, a conductive carbon veil, 2 layers of Nexus veil, and 2 layers of 1.5 oz mat. The absorber section had an FRP corrosion liner comprised of 2 layers of Nexus veil and 2 layers of 1.5 oz. mat. Derakane 441-440 was the resin used throughout. Tefzel liner was also used in the separator internals which included packing support beams, the chimney, a conical chimney tray, and 3 sets of deflector baffles in the lower cone section.
While the Tefzel liner was adequate for resisting the corrosive nature of the process, it proved to be less so for the combination of the high temperature and abrasive particulate that was a component of the gas stream entering the Separator through the tangential gas inlet. Shortly after their startup, the original vessels had to be taken out of service to allow repairs to the liner to take place, a procedure which had to be repeated throughout the units' short lifespan. The packing support beams also showed evidence of damage that necessitated using a set of safety beams when entering the separator at maintenance time. Additionally, the thermoplastic liner posed extreme challenges in the proper design and fabrication of the internals. This was particularly true at the attachment points.
Fabricated Plastics presented the client with a solution that entailed an all FRP construction using a silica carbide liner rather than the Tefzel liner previously used.
The replacement vessels were designed, fabricated and inspected in strict accordance with ASME RTP-1 protocol. The interior corrosion liner was made with 2 layers of 3 oz /yd2 ECTFE veil, 5 layers of 1.5 oz ECR mat for the separator section and 3 layers of ECR mat for the absorber section. Also the tangential inlet, now moulded as an integral part of the whole, was afforded an additional 2 layers Nexus veil and 4 layers of 1.5 oz mat, both using silica carbide filled resin. To facilitate easy visual inspection of the abrasion resistant liner an extra 2 layers of Nexus veil with red pigment was applied between the abrasion resistant liner and the main corrosion liner. Derakane 470-HT was the resin used throughout due to its superior corrosion resistance at high temperatures.
Design pressure: -40 kPag (-5.8 psig)
Design temperature: Separator 240 degrees F. (115 degrees C.); Absorber: 200 degrees F. (95 degrees C.)
Total empty weight: 75,000 lbs (34,019 kg) including internal support beams and grating.
Total design weight: 371,000 lbs (168,283 kg) including fouled internals and inlet elbow.
The whole assembly was to be vacuum tested after field assembly.
The custom designed and fabricated FRP packing support grid together with the HLU fabrication was supplied in 24 inch (610 mm) wide sections to fit through the 30 inch (76 cm) manway.
As with all engineering projects a schedule was in place to accommodate the needs of the client. In this case the effort was to provide a smooth transition between dismantling the old equipment and erecting the new, so that a minimum of production time would be lost. Unfortunately the company that had supplied the previous vessels was no longer in business. This meant that the client had to provide the design loads for the internals for possible upset conditions based on their own experience and without the input of the previous process designer. In fact, due to the tight delivery schedule, fabrication was well underway while these details were still being finalized. When the client began dismantling the failed equipment it was discovered that the fouled packing was considerably greater than had been estimated. 30 metric tonnes (30,000 kg) rather than 18.5 metric tonnes (18,500 kg). This would require a complete review of the design calculations for affected components together with recalculation of seismic loads.
Exacerbating these circumstances was the fact that the old unit was out of service and the delay that would be caused by redesigning and re-fabricating was not an option. Seemingly the only available option was to revise the maintenance procedures in a way that would ensure the foulded packing weight never exceeded the 18.5 metric tonnes (18,500 kg) design load.
The apparent stalemate was solved by Fabricated Plastics engineers by redesigning the safety beams under the packing to accommodate the higher load factor and having them permanently in position, as opposed to being positioned only at maintenance time. But the client had to accept somewhat lower design safety factors in the remaining components of the packing support system.
Despite the difficulties encountered during this work, Fabricated Plastics delivered a finished product that fully satisfied the Client.
As one of Fabricated Plastics engineers commented 'Ingenuity and creativity are the indispensable qualities that make the difference between success and failure in an engineering project'.