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Fabrication of pressure vessels of duplex steel for pulp mills

Pori Works fabricated the first duplex 2205 pressure vessels in 1992. Introducing a new material into the workshop went relatively well because of the long experience in fabricating pressure vessels and machines of different austenitic stainless steels. So far 118 pressure vessels for pulp mills have been fabricated of solid 2205 duplex steel at our workshop: 61 vessels for South-East Asia, 48 vessels for Europe and 9 vessels for North America.

The pressure vessel codes used so far for fabrication have been:

  • - SFS (Finnish requirements), based on German code
  • - ASME Code
  • - TKN (Swedish code)
  • - ADM (AD Merkblatt, German code)

Plate materials used – depending on requirements – have been:

  • - W. 1.4462 AD W2 + VdTÜV Wb 418 – ASME SA 240 S31803
  • - Avesta Sheffield 2205 PKVP-03.155 Rev. 2

Edge preparation for shell plates is made by machining. Double-V or double-U grooves are used depending on the material thickness. Careful edge preparation helps keep repair costs low.

Semi-spherical and elliptical heads delivered in segments are ordered as cold formed without annealing. The sub-supplier makes edge preparation and pre fit-up. Material thickness of vessel shells has been 4 to 40mm. This makes submerged arc welding (SAW) an attractive process for most longitudinal and circumferential weld seams of shell and major nozzles. Other pressure-retaining parts are welded using the shielded metal arc process (SMAW), see figure 4. For some non-pressure-retaining parts flux-cored arc welding (FCAW) is used.

Welding instructions (WPS) shall be based on welding procedure tests. For 2205 material the control of heat input during welding is necessary to avoid reduction of corrosion resistance and toughness of the welded joint. The allowable range depends on the case but normally it is 1.0 to 2.5kJ/mm. It is relatively easy to keep within that range while most of the welding is flat position. Documented spot checking helps the welders to keep in mind that it is important to follow the welding parameters given in WPS. The number of necessary welding production tests depends on the requirements of each code. So far we have not met any problems in fulfilling the test requirements.

The qualifications of welders and plate workers for 2205 work shall always contain a theoretical part even if they are already familiar with austenitic stainless steel work. One can pass the welding test but still make expensive mistakes in actual work. Qualification tests should be done using 2205 base and filler material even if the qualification standard allows using austenitic material.

The quality control of a 2205 pressure vessel is based on a quality plan made soon after the order. The NDE methods based on the code used are visual (VT), dye penetrant (PT) and radiographic examination (RT). All the welds are examined visually 100%. PT can be 100% for all nozzle to shell welds (ASME) or 25% of shell weld seams and 10% of nozzle and fillet welds (ADM). Typical examination RT is 100% for longitudinal welds and 25% (ADM) or spot (ASME) for circumferential welds. The percentage of rejected X-ray films is used for measuring welding quality. This figure has been less than 2% during the past few years, which has been considered acceptable.

Conclusions

SAF 2205 with its high strength and corrosion/erosion properties is becoming more popular. It can often be justified on a cost basis. Wall thickness reductions of 50% on carbon steel and 35% on clad steel can be achieved. A kraft pulping process includes a large number of pressurized reactors and atmospheric large tanks and towers, as well as various main equipment. Total savings achieved through favorable characteristics of duplex materials are huge, considering that a total cost of an average fiber line equipment delivery can be up to MUSD 100

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