Innovative refurbishment helps prolong pulp digester lifetime

Paul Boughton

Celulose Nipo-Brasileira (Cenibra) is the fourth largest pulp producer in Brazil and wholly owned by Japanese interests. The company produces 900000t/y of eucalyptus-based elemental chlorine free (ECF) pulp, mostly for the Japanese and European markets.

Located in the state of Minas Gerais,
200km northeast of Belo Horizonte, Cenibra employs about 5300 people and indirectly affects the lives of more than 27000 others n the region.
In 2002, mill staff discovered that they had an unwanted guest in their ECF pulp digester -- corrosion.
All of a sudden said Cenibra's Aroldo Parreiras dos Santos, we found during the first planned shutdown for overhaul and repairs that the digester shell had decreased in thickness by 4 mm. When we started to investigate the causewe then found that the digester shell had corroded another 3mm. Obviouslysomething had to be done. Of course we were surprisedsince up until that time the digester had run like clockwork."
The company's first action was to use anodic protectionbut the corrosion was so heavy that another solution was needed.
"There were various methods available such as using metal spray or lining the digester with stainless steel platesbut we felt the overlaying technique would be the best for us as it would restore the shell to the right thickness added Parreiras dos Santos.
According to Cenibra, Kvaerner Pulping and its partner Uddcomb Engineering were the natural choice of supplier to correct the problem because of their past experience with the company and also because the they would be on site during the planned shutdown last September.

The digester

The continuous digester was originally installed in 1977 and over the years a number of plant modifications and upgrades had been carried out by the two companies.
For example, in 1995 Cenibra invested in a second continuous fibreline from Kvaerner Pulping. In 2000 the company was again selected to jointly develop a new cooking method -- the Kobudo Mari process for improved cooking yield -- and to supply a pressure diffuser and an oxygen delignification stage for washing. The results were way above expectations, with a significant increase in production, higher yield and fewer rejects eing recorded.
The next upgrade package came in 2001, with a Compact Press to improve washing after oxygen delignification and a new Duald bleaching stage to enable Cenibra to produce ECF pulp.
For its latest upgrade, Cenibra opted for the Digester Refurbishing Kit from Kvaerner Pulping and Uddcomb Engineering.
The Digester Refurbishing Kit is one of 12 Rebuild kits currently offered by Kvaerner Pulping. They are part of the company's MORE servicing concept, which consists of mill study and analysis, overhaul and spare parts, rebuild, and education. It uses the trademarked Uddcomb Method to protect against further corrosion and erosion as well as to ensure quick application.
According to Uddcomb, no overlay welding using this method has had to be repeated in any sulphate continuous digester anywhere since its introduction in 1986.
The overlay material most often used is known in the world of welding as AWS312, a material that the Swedish Corrosion Institute found to be the best of ten tested materials for this application. Using 312 wire ensures high chromium and nickel content and a weld micro structure which corresponds to Duplex stainless steel.
The stainless steel layer of weld metal becomes 100percent homogeneous, giving the digester added wall thickness and excellent corrosion/erosion resistance. Dilution of the base metal and distortion of the vessel is very low.
The total time required for refurbishing the digester and the shutdown was 12 days. Kvaerner Pulping's and Uddcomb's experts in welding stainless steel overlay needed 172 hours to complete the job inside the digester.
We have a very good relationship with Kvaerner Pulping staff. I feel that those on site work with us as partners noted mechanical maintenance co-ordinator Fernando Barbosa.
There are normally a number of reasons why corrosion and erosion occur in different types of vessels explained Torgny Svensson, Uddcomb Engineering supervisor. On the other handit is almost impossible to pinpoint one single reason that causes the problems."
He went on: "As the aggressive phase moved downwardsthe digester walls started to corrode at a high rate. When a design wall thickness of 31 mm is subjected to 6--7mm of corrosion in one yearthe situation is critical and something must be done. Our methodwhich involves overlaying the inside of the digester shell with stainless steelis well suited to a case like this."
"We enter through existing manholessetting up platforms inside the digester where there is corrosion. Then we mount our welding robots on rails around the digesterafter which we commence the overlay welding. "
At Cenibra he used five machines with two torches on eachwith the welding being done vertically downwards in shifts of 3.9metres and at a width of 50mm. Six strings of overlay covered a width of 300mm.
All five machines work independently of one another and covered an area of five square metres every 24hours to a thickness of 3mm. One operator controlled each machine.
"No one really knows how long it will lastbut from our experience so far we know that digesters repaired with this technique from 1986 onwards are still unaffected by corrosion. So Cenibra can certainly look forward to a long period of operation without corrosion problems concluded Svensson.

Boiler contracts too

In a separate development, Aker Kvaerner subsidiary Kvaerner Power has won E60 million worth of contracts to supply power boilers for UPM's paper mills in northwestern France and Wales.
Both mills produce newsprint paper based on recycled fibre. Instead of landfilling, the unusable part of recycled paper will be burned with an advanced combustion technology and used for renewable energy production for the mills.
UPM is one of the world's leading biofuel users, with approximately 60percent of its fuel being biofuels. The new investments will further increase the biofuel usage.
Kvaerner Power will deliver power boilers using bubbling fluidised bed technology to both mills. The boilers have steam capacities of 82megawatt(MW) each. The two new power boiler orders from UPM follow only a month after an previous order to for UPM's Rauma paper mill in Finland. This is for the supply of a power boiler with 107MW steam capacity.

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