Global paper and dissolving wood pulp producer Sappi recently replaced the pulp dryer at its Ngodwana mill, in Mpumalanga, after its old dryer had reached the end of its economic life.The
new dryer was installed one week after the old 35-m-long, 19-m-high and 12-m-wide pulp dryer was removed last month.
“To be cost effective, the old dryer had to be demolished and the new dryer had to be installed in its place inside the existing building. “Under favourable circumstances, this operation would take a minimum of 12 to 18 weeks to complete, but the project team faced a deadline of six weeks for the complete installation, inclusive of modifications to the existing pulp machine.
“This called for extraordinary measures and, after analysing all the options, the team came up with two possible solutions,” says Sappi project director Andrea Rossi, adding that the project included the construction of a dissolving wood pulp plant, thereby modernising and partially converting the mill.
“The first option was to remove the roof of the existing building, cut the old dryer into reasonably sized pieces and remove them through the roof with a crane. “The new dryer would then have to be preconstructed in sizable pieces outside the building and, following the removal of the old dryer, the new dryer modules could be lifted in place and assembled,” he explains.
Rossi adds that this method, although applied successfully elsewhere in the world, takes considerably longer and the confined space to reassemble the dryer into position is difficult. Further, the removal of the roof panels would have required the strengthening of the support structure, which could only be executed once the old dryer was decommissioned.
He states that the company discussed a different option with the supplier of the dryer to accelerate project time.
“The second option the team investigated was to completely construct the new dryer on its steel platform outside the building and use a multiwheeled mobile trolley to manoeuvre it into place.
“The same trolley could also be used to remove the old dryer in one piece to demolish it outside the building,” Rossi points out.
He adds that this would require the end wall of the existing building to be removed, but the roof to remain intact.
“The concept was discussed with international crane and rigging company specialising in project logistics Sarens, which proposed the use of a self-propelled modular trailer (SPMT).
“These trailers comprise multiple hydraulic lift units on steerable wheels, which are interconnected to form an SPMT. Sarens has successfully used this system worldwide to move abnormally large pieces of equipment and the company was confident that this would be the best way to proceed,” Rossi explains.
He adds that, after considering the merits of the proposal, Sappi agreed and the engineering teams of Sarens and dryer manufacturer Andritz started to work out the details.Rossi states that Andritz had to redesign the dryer frame, as the standard dryer is a relatively lightweight structure, which is not designed to be moved once it has been constructed.
“The Andritz and Sarens mechanical engineers collaborated to determine the maximum lateral and transversal forces that could be expected during a move with an SPMT and then strengthened the dryer structure sufficiently to overcome these forces,” Rossi says, adding that, based on the outcome of this study, the construction method was changed and the project team was able to start with the necessary preparations.“The area outside the old dryer building had to be raised, compacted and levelled to enable the smooth movement of the SPMT.
The new pulp dryer is an element of the drying line which is used to dry the dissolving wood pulp (specialised cellulose) in sheet form, ready for cutting and baling.
“The pulp sheet traverses the length of the dryer 22 times before it finally leaves the dryer as a continuous sheet. It is then cut and baled into saleable product. At any given time, there will be 770 m of semidry pulp inside the dryer,” Rossi says.The dissolving wood pulp will be produced in a newly constructed batch digester plant and bleached in a new bleach plant,” explains Rossi.
He states that the new dryer is highly efficient and weighs 550 t, compared with the previous 700 t dryer.
“The new dryer is also more environment friendly, as it uses less electricity than its predecessor and has a life span of more than 30 years.
“Construction of the new dryer started last year in November and was completed in February 2013,” he notes.
Rossi adds that the new dryer was erected complete with all platforms, walkways and handrails.
“All fans, motors and control instrumentation were installed, ready for cable connection. The old dryer was demolished and removed by a specialist demolition team from producer of recycled ferrous and nonferrous metal products in Southern Africa The Reclamation Com-pany,” he says.