Duplex Stainless Steel Usage in the Pulp and Paper Industry

Carbon steels and standard austenitic grades of stainless steel were the dominant metals for quite some time inside the pulp and paper industry. These materials still play an important role, but duplex 2205 and also other duplex stainless steels are increasingly used in the creation of new equipment. The overriding basis for the target on duplex stainless steel is because of cost benefits. The actual reason is due to numerous factors. First, environmentally friendly push that got going in the 1970s has led to more serious processing environments, which require materials with better corrosion properties than carbon steel. In addition, that is a is shifting outside the every six months maintenance shut downs that routinely existed previously. It is led to the requirement of materials that demonstrate to even lower quantities of corrosion as time passes than was permitted before. Finally, it must be noted that here is a huge learning curve related to duplex stainless steel. Its breakdown of the has become gradual, but duplex usage has steadily gained in momentum after a while.

Advantages of Duplex stainless steels:

Duplex stainless steels have some of benefits that complement closely to the requirements of modern pulp and paper mills. First, they have got excellent corrosion properties, particularly in terms of stress corrosion cracking, pitting, fatigue, and abrasion conditionsi. While austenitic steels have corrosion properties adequate to handle many conditions in modern mills, Duplex stainless steels typical offer comparable corrosion properties at the lower material cost. Furthermore, Duplex stainless steels have nearly twice the potency of austenitic steels. This can lead to significant material weight savings. Furthermore, it must be noted that even in less corrosive environments when carbon steel could still technically be utilized, Duplex stainless steels often win out when viewing overall life cycle costs. The carbon steel may have a lower initial material cost than duplex steel, but with time a good manageable amount of corrosion can bring about higher business economics for carbon steelii. Immediate and ongoing expenses should take into account initial material costs, maintenance costs, inspection costs, repair costs, and even more importantly, the charge that is incurred when production should be shut down to support maintenance, inspection, and repairs.

Paper Production:

While Duplex stainless steels have excellent properties that align well with all the pulp and paper, it is very important remember that modern mills include lots of environments. You'll find five major steps to paper productions, and all these steps possesses its own specific corrosion conditions that must be considered individually in choosing materials. First, wood chips are fed into a digester, under high temperature and pressure conditions, in order to separate your glue which supports the wood chips together. Next, the pulp is processed through a series of washes as a way to further remove impurities and recycle the cooking liquor, also called black liquor at this stage along the way. That is then a bleaching method that enhances the brightness with the pulp on the appropriate amount depending on the desired pulp or paper product. After bleaching, the pulp is processed into liquid stock that may be transferred to a paper mill. The liquid stock is then processed through a number of wet end and dry end equipment to suction your water and compress the fibers in order to make the ultimate paper product. The equipment, temperature, pressure, chemical types and concentrations all vary considerably for each of these operations.

Moreover, all kinds of other factors impact material selections. First, the volume of water recycling within each mill constitutes a difference in processing environments. The paper industry is not able to operate a river of water through their plants and after that dump that processed water into the environment without cleaning it first. Instead, pulp and paper mills now usually are closed loop systems in which water is re-used more often than not and after that cleaned before being releasediii. These closed loop systems lead to severe corrosion conditions as a result of increased concentrations of chemicals. Also, the size and quality of the wood fibers, the actual ways of achieving each one of the major steps, and of course, the specifications for the pulp or paper being made will make quite a alteration in processing environments at the same time.

Duplex 2205 in Digesters:

Duplex 2205 (UNS S31803/S32205) was initially introduced to the paper industry in 1982. Two solid duplex 2205 kraft digesters were set up in a brand new Zealand milli. Kraft digesters, which are widely-used to cook the incoming wood chips in an alkaline solution, are the most common equipment type used for the first task in paper productioni. Operating environments include high temperatures, widely variable corrosion conditions, and highly abrasive conditions. Next initial solid duplex installation, there was no additional duplex activity for time until data could possibly be collected. It clearly went well because nearly all new digesters are now created from solid duplex. Today, there are over 114 digesters in operationi.

Duplex Stainless Steel kept in storage Tanks:

Because of the large level of storage tanks, duplex stainless steels offer exceptional material weight savings when utilized for producing these tanks. In the past, storage tanks were made primarily from carbon steel which has a corrosion allowance, or from austenitic grades. According to both laboratory testing and mill tests, evidently even austenitic grades (304 and 316L) don't always offer enough corrosion resistance for certain liquors, especially those with higher solid contents. Thus, Duplex stainless steels offer not just material weight cost savings, and also corrosion benefits. Finland currently uses white liquor tanks that are made of duplex stainless steel grade Duplex 2304, and duplex 2205 storage tanks are offered also being made.

While duplex stainless steel has performed well within the Finland mill white liquor storage tanks, some liquors have become difficult even for duplex materials. In 2002, the top 50 % of a black liquor storage tank used in production was substituted with solid duplex 2205. The tank was originally made with 316L (UNS S31600) coupled with experienced ongoing problems as a result of severe cracking. Unfortunately, the brand new duplex 2205 tank began experiencing corrosion damage in 2008 red carpet years operating. The corrosion issues include pitting and corrosion under deposits, along with stress corrosion cracks in the welding metal. Further analysis indicated that the high chloride content from the liquor (.5 to 1Percent) almost certainly triggered the corrosion. Testing has become carried out to evaluate if the lean duplexes might perform better. Also, suggestions have been created to lower the chloride content of the black liquor in order to switch to Super Duplex 2507 (is UNS S32750, not UNS S32750) . 

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