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Increased usage of duplex materials-Typical applications

A typical pulping process consists of various process steps. In the first step, wood chips are cooked at a high temperature and pressure to release the lignin which glues the wood fibers together. This is followed by pulp washing, where the cooking liquor is removed, after which impurities are screened out of the pulp. The two final main process steps consist of bleaching the pulp in high brightness, then pulp drying and baling.

The use of duplex material first began in the fabrication of pressurized pulp digesters a few years ago. Figure 2 shows roughly the extent to which duplex material is used today in the pulping process.

Pulp digesting

In sulfite pulping, where the chips are cooked in acidic conditions (pH 2-4) digesters have been manufactured from 316L though the tendency is to higher alloys. Corrosion resistance has been studied in real conditions. Table 2 shows the superior resistance to pitting of SAF 2205, based on coupon testing in a sulfite digester for eight months (ref. 1). In kraft pulping substantial savings can be achieved when using duplex 2205 material, taking into consideration the different codes (in this case the ASME Code and the Swedish Pressure Vessel Code (TKN)). As can be seen, a saving of 50% in material thickness is achieved compared to using solid carbon steel and a slightly lower saving as regards the austenitic grades.

Figure 3 shows the material savings in batch digester fabrication for kraft pulping. According to the Avesta survey, September ’98, altogether 114 digesters, including both continuous and batch digesters, have been fabricated of duplex material (table 3). As a conclusion, the following differences in digester wall thickness can be achieved:

Duplex digester wall

  • 76% of solid 304 wall (ASME)
  • 54% of solid 304 wall (TKN)
  • 79% of clad steel wall (ASME)
  • 68% of clad steel wall (TKN)

The biggest batch digesters made from duplex 2205 are 400m 3 which today is our standard size for new digesters. Figure 4 shows a 400m 3 batch digester under fabrication (Metso Pori Works, Finland). Pressurized vessels in general, manufactured by our company, vary from 400 to 1100m 3 and the wall thickness varies between 4 and 30mm.

Washing and screening

Single pieces of process equipment, such as pulp washers and presses, have traditionally been manufactured of various stainless steel grades, but today duplex materials are increasingly used even for fabrication of parts of those. In kraft pulping, water quality and the degree of process closure are the major influences on the environment in brown stock washing. Corrosion rates of more than 5mm on carbon steel have been reported. Depending on pH control, duplex steels are proving to be the most suitable. Duplex materials are typically used today also in the fabrication of filter drums and pulp conveyor screws. In big modern pulping lines wash presses are, however, increasingly being used instead of traditional drum filters. The TwinRoll press (figure 5) is a typical washing apparatus.

Earlier press vats were typically made of duplex material, today even all parts of certain press types are fabricated of duplex. Just recently we have also started to fabricate traditional drum washers from duplex. Figure 6 shows a duplex vacuum washer drum in the workshop.

Oxygen delignification

The majority of O 2 stages operate at medium consistency and 316L has generally been used as the fabrication material. SAF 2205 is constantly gaining ground at the expense of other grades. Today almost all O 2 reactors made by our company are of duplex 2205 as well as all the batch digesters. According to the Avesta survey of 1998, various pulping equipment suppliers in the world have so far fabricated totally 107 oxygen reactors of duplex material. Figure 7.

Bleaching

Besides a similar type of reactors and washers as in the preceding process stages, typical for a pulp bleaching process are large pulp storage towers. Such storage tanks and towers have previously been constructed from such materials as solid concrete with tile lining, carbon steel with 2mm thick stainless steel cladding, or solid stainless steel austenitic material. In the case of vessels manufactured from solid austenitic materials, the costs are high due to the relatively low yield strength giving around 40% higher wall thickness than when using duplex 2205 material. However, in the case of very large storage towers, the wall thickness of the bottom part is so high that it is still economical to make that part of carbon steel with SS lining (figure 8).

One example of a storage tower is 16 meters in diameter and has a height of 33 meters, specified in 316L. The superior strength of SAF 2205 made it possible to design the tanks in lighter gauge material making their cost lower than their 316L counterparts. For this reason the specification was changed to SAF 2205. Due to the practicality of the construction the lowest thickness used has been 6mm. Also in the case of bleach plants, duplex 2205 materials are used for some of the bleach filters, depending on their position in the bleach sequence. In this case too, the high strength and corrosion resistance is the deciding factor.

Our prediction is that duplex materials will eventually replace most austenitic materials in the pulp and paper industry with a few exceptions in the bleaching process.

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