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Increased usage of duplex materials in manufacturing of pulping equipment

Authors: Alpo Tuomi

Introduction

In many industrial areas, chemical processes are developing in a direction which is setting ever-increasing demands also on the construction materials of the equipment. This same development has taken place in the pulp and paper industry. The development of pulp and paper processes has been closely linked to the development of stainless steels and both industrial sectors have strong connections in the Nordic countries. Due to demands from the early pioneers in the pulp and paper industry in the Nordic countries and the constant search for better acid-resistant materials, the stainless steel industry in the Nordic countries was born and has become a world leader in this field.

Experience in this field goes back many years and suppliers have been involved in replacing pulp digesters made of a riveted carbon steel construction and in the case of the sulfite industry, vessels lined with acid proof bricks. The next step was a process where old digesters were lined internally with approximately 3mm stainless steel sheets held by a vacuum system between the parent metal and the liner. In some cases the digesters were strip-lined with 150mm wide stainless steel strips. This was a mammoth task which, however, served its purpose since there was really no other way, and solid stainless steel vessels were out of the question in those days due to the high prices of heavy wall thicknesses.

Later on solid stainless steel vessels have replaced digesters manufactured from clad steel material. The introduction of the Avesta cold-stretching method made it possible to manufacture solid stainless steel vessels with 40-50% lower wall thickness thereby being price competitive with standard solid stainless steel vessels. However, cold stretching was not approved globally by the pressure vessel authorities and even this technology is now part of fabrication history since 2205 duplex material was introduced.

Today almost all pressure vessels used in the pulp and paper industry are manufactured of duplex materials, mainly digesters used for the cooking of chips in alkali conditions, as well as reactors in the oxygen delignification and bleaching processes, which work both in alkali and acid process conditions. Furthermore, duplex material is increasingly used also in the fabrication of non-pressurized vessels, as well as main process equipment.

Why duplex?

The obvious main reason for the increased use of duplex materials in the pulp and paper industry is economics. Material savings due to the favorable strength properties of duplex materials give clear benefits. On the other hand, special expertise is required in fabrication, which is discussed later in this presentation in more detail.

Cost comparison

While some alloys have higher pitting resistance than others, it is clear that the more highly alloyed steels are significantly more expensive than the lower alloyed steels. Table 1 shows the relative prices of different materials used for fabrication of pulping equipment.

It is interesting to note that the duplex steels offer very good value in terms of pitting resistance against price. As an example: though the PREs are equivalent, the price of SAF 2205 is about half that of 904L. This should, however, be used with caution, as we have found that impurities, acid concentrations, etc., can have an important effect on corrosion resistance.

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