Abstract

Because of process design and construction, FGD installations normally have bypass ducts, which necessitates use of dampers. Due to corrosion from acid dew resulting from interaction of hot acidic flue gases and colder outside environments, carbon steel cannot be used as construction material under these specific conditions. In the past, commercial stainless steels have suffered by pitting and crevice corrosion and occasionally failed by stress corrosion cracking. Only high alloy specialty super-austenitic stainless steels with 6.5% Mo should be used and considered for this application. Experience in Germany and Europe has shown that with regard to safety and life cycle cost analysis as well as providing a long time warranty, a new specialty stainless steel, alloy 31 - UNS N08031 - (31 Ni, 27 Cr, 6.5 Mo, 02 N) has proven to be the best and most economical choice. Hundreds of tons in forms of sheet, rod and bar, m well as strip (for damper seals) have been used and installed in many FGD installations throughout Europe. Under extremely corrosive conditions, the new advanced Ni-Cr-Mo alloy 59- UNS N06059 - (59 Ni, 23 Cr, 16 Mo) should be used. This paper describes qualification and workability of these alloys as pertains to damper applications. Some case histories are also provided.

INTRODUCTION

Dampers in flue gas desulfurizing or scrubbing units of power stations and waste incineration plants serve to cut off, divert and control waste gas flows as required under normal process or emergency conditions. Most dampers are large-size components typically installed in inlet/outlet flue gas ducts and/or bypass ducts.

Experience in recent years has shown that dampers made from carbon steel suffer from severe corrosion under the service conditions involved in this application. The main factors lie in the FGD process itself as well as the plant parameter: and the large size of the dampers.

This mainly concerns FGD systems using a wet limestone scrubbing process which inevitably generates corrosion problems irrespective of whether overpressure or subatmospheric pressure is employed. In fact, subatrnospheric operating pressure will create a corrosive environment even in a “dry” FGD unit.

In the USA these problems have been the subject of detailed investigations, the results of which have been compiled in a comprehensive EPRI survey1.

One of the conclusions drawn by the survey was that FGD damper components made from commercial stainless steels invariably suffer pitting and crevice corrosion within a relatively short exposure period and are thus inadequate for this application. In other words, in view of the required operational safety and long service life without the need for premature maintenance measures, there is no alternative to nickel base alloys. In principle, this finding was originally reconfirmed in Germany. Meanwhile, however, another solution to the problem has been developed in the shape of the new alloy 312(UNS N0803 1), a special stainless steel with high Mo and Cr contents (27 Cr 6.5 Mo; 0.20 N) which fully meets the requirement profile both in terms of economy and corrosion resistance.

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