Lower Cost with Reduced Safety Risks
What is a pulp digester?
To convert trees to paper, one must chip the wood and then "cook" the chips in what appears to be a huge steam pressure cooker. The chips are mixed with chemicals that will dissolve out the lignin, turpentine-like components that bind the fibers together as wood. Once those binders are removed, the loose fibers can then be recombined in a flat product, paper.
The problem: excessive corrosion inside of the pulp digesters
The pressure cooker, called a digester, faces high temperatures, widely variable corrosion conditions, and highly abrasive conditions as the chips pass through the unit. Most digesters were built of steel, often with a stainless steel weld overlay lining. This construction works, but the maintenance cost of periodically relining is not cost-effective. There is also a risk of catastrophic failure of the steel shell, with great risk to the mill workers.
The solution: Outokumpu 2205 Code Plus Two duplex stainless steel
Outokumpu 2205 Code Plus Two duplex stainless steel is now the material of choice for construction of new digesters. Superior strength, corrosion resistance, and abrasion resistance allow this stainless steel to provide safe, cost effective service. The duplex stainless steel is used as "solid," meaning it replaces both the steel shell and the weld overlay liner.
What are duplex stainless steels?
Duplex stainless steels combine the microstructures of austenitic and ferritic stainless steels in rough equal proportions. The resulting stainless steels are corrosion resistant, tough, weldable, and about twice as strong as the austenitic stainless steels. The duplex stainless steels are highly resistant to chloride stress corrosion cracking, overcoming a major limitation of the common austenitic grades.
How is stainless steel less costly than carbon steel construction?
The duplex stainless steels are economical for initial construction because of their high strength. These grades have superior life cycle costs because they can operate many years without the expense of weld overlay relining that is required in the steel shell constructions. The outstanding performance of the duplex stainless steels reduces the required frequency of inspection, further reducing the costs of the stainless steel digester.