Four environments inside one vessel

A flue gas desulphurisation unit is not one specification. The gas arrives hot and dry, meets a spray of limestone slurry, leaves the absorber saturated and cold, and then travels through ducting that never dries out again. Each of those four stretches asks a different question of the metal, and quoting a single alloy across the whole unit is how these projects end up either over-ordered or rebuilt. Three pages put it on the record: the 2507 listing names flue gas scrubbing equipment directly in its application list, the 904L page names combustion gas desulphurisation plants, bleaching tanks and sulphuric and phosphoric acid service, and the 825 plate page carries stock thicknesses from 4.8 mm through 50.8 mm for air pollution control. The ladder is not guesswork; it is written on those pages.

The inlet duct and the line where droplets dry

The most aggressive few hundred millimetres of the plant are usually at the start, where hot gas meets the first sprays and any droplet reaching hot metal evaporates and leaves its dissolved solids behind. What concentrates there is chloride, carried in with the coal and the make-up water and cycled rather than discharged — so the loop finds its own level, and that level belongs on the enquiry rather than in a standard. General thinning is not the failure; local attack at the wet-dry line is. Three candidates sit at the top of the catalogue for exactly this reason. C-276 is described on its own page as one of few materials to withstand wet chlorine gas, hypochlorite and chlorine dioxide, with exceptional resistance to strong solutions of oxidising salts such as ferric and cupric chlorides. 254 SMO is published for high chloride environments at 100 ksi minimum tensile and 45 ksi minimum yield in sheet and strip. Where even that is marginal, the titanium round bar page puts corrosion in seawater below 0.005 mm per year, which is why quench zones and damper blades are often titanium while the duct around them is not.

The absorber shell: where lining economics decide

The shell itself rarely needs to be solid alloy. The slurry runs at a pH set by the limestone addition, and the vessel is large enough that buying it in stainless is rarely the cheapest answer; the usual route is a carbon plate shell with a lining, or a thin alloy backing rather than a thick pressure wall. Either way it starts from plate: our Q235, Q345 and A36 hot rolled plate programmes cover 0.3 to 200 mm, and those plates go through cutting, bending, punching, shearing and welding before leaving. Where the specification does call for solid alloy — agitators, nozzle collars, recirculation spools — the molybdenum-bearing austenitics take over: 317LMN is rated superior to 316L and 317L with nitrogen added for strength and low carbon so it can be used as-welded, and it travels as A240 sheet, tube and plate and coil. Note what the 904L page says about joining it: higher-alloyed filler can be necessary because the weld metal is a cast structure.

Outlet duct, dampers and the stack liner

Cleaned gas leaves saturated and cool, carrying far less sulphur dioxide but no less water, and everything downstream stays wet for the rest of its life. This is where duplex grades earn their place, because the duty needs strength and chloride resistance at once and the wall can therefore be thinner. Super duplex 2507 has 25 per cent chromium, 4 per cent molybdenum and 7 per cent nickel, with the page adding that a light gauge often achieves the design strength of a thicker nickel alloy and listing the wire family as AWS ER2594, E2594-16 and E2553T-1. Use below 600 degrees Fahrenheit or 316 Celsius is its stated limit, which is not the constraint here but tells you the window is real. S32760 comes with PREN 40, listing in NACE MR 01-75 for sour service, Charpy figures of 80 J at ambient and 45 J average with 35 J minimum at minus 46 Celsius, and 3.1 certification standard with 3.2 at surcharge. Further downstream, where temperature and chloride have both dropped, LDX 2101 publishes 21.5 per cent chromium, 1.5 per cent nickel, corrosion superior to 304L and comparable to 316L, and 65 ksi minimum yield that lets the section get thinner.

Everything outside the gas path

The mistake that costs the most money is letting one alloy decision run past the plant boundary. Nothing about the access steel, the duct supports or the tower frame touches the slurry, and there is no reason to buy it in stainless. The steel structure page sets the parts out plainly: main beams, columns and trusses in welded Q235B and Q355B H sections and Q235B C and Z purlins, with Q355B either painted with alkyd or hot dip galvanized. Hot dip galvanized H-beam carries the zinc-iron alloy layer description, and anything built from sheet follows the galvanized processing route: laser cutting, metal bending, welding and polishing, zinc coating, then assembly and packing. The utility lines around the unit are equally ordinary — A106 for steam and condensate, A53 for service and structural duties.

Fabrication and the numbers behind the drawing

Duplex changes the workshop as well as the order, and that belongs on the enquiry before the plate is cut. The 2205 data recommends forming below 600 degrees Fahrenheit wherever possible and, where hot forming is needed, working uniformly between 1750 and 2250 degrees Fahrenheit, with solution annealing afterwards in the 1040 to 1080 Celsius band. Availability is explicit too: the duplex coil programme runs 0.2 to 60 mm thick and 3.2 to 2000 mm wide in three to ten tonne coils with a one tonne minimum, quoted at seven to fifteen working days after deposit, and the 2507 coil listing sits inside the same family. Where the job needs pipe rather than plate, the stainless pipe range, the square section programme and large diameter thick wall pipe cover outlet and slurry lines, with finishing through surface treatment.

Six lines that map the unit before you buy

Write the enquiry as zones, not as one vessel: metal temperature and whether the surface is ever dry, in each one. Give the chloride figure the loop is designed against and whether solids settle anywhere, then the pH range across start-up and normal running. State which items are lined and which are bare, because the failure modes are different. Mark every seal, lap and bolted joint, which are crevices by construction and set the localized requirement rather than the general one. And settle the certificate: 3.1 as standard, 3.2 where the client or insurer insists, with the surcharge agreed rather than discovered. Answer those and the ladder reads itself, from titanium at the quench zone through duplex in the outlet duct to galvanized sections holding it all up.