Cane does not wait for a purchase order

Cane is cut, loaded and crushed inside about a day, and the sucrose starts falling the moment the stalk is severed. A mill that stops for two days does not recover those tonnes later in the season; the growers have nowhere to hold the crop. That fact shapes the way a sugar mill buys steel. Nothing critical is replaced during the campaign, everything is on the shelf before the first truck arrives, and the order is placed against a start date that moves with the weather.

Grades are therefore chosen for what the plant will do to them across a season of 150 to 200 crushing days, not for what the drawing says on the day it is issued. A repair inside that window is measured in lost tonnes rather than labour hours.

The milling train asks for strength, not chemistry

Juice extraction is a mechanical problem. Rollers turn slowly under a load that would destroy a thin shell, and the shafts, housings and press rolls take a shock every time the cane blanket surges. The 2205 duplex plate page names that duty directly - rotors, fans, shafts and press rolls requiring combined strength and corrosion resistance - and the reason is in the chemistry: 22 per cent chromium with 3 per cent molybdenum and 5 to 6 per cent nickel, nitrogen added, at a yield strength about twice that of the austenitic grades.

Duplex is not a free upgrade. Plate runs from 4.8 mm up to 152.4 mm, the working window is -50 to +600 F, and anything outside it needs restrictions that fall hardest on welded structures. Cold forming takes higher forces and a larger springback allowance, and stress relief is not recommended - if it must be done, it is a full solution anneal at 1900 F minimum. Duplex coil covers the rolled shells and chutes around the train from 0.2 to 60 mm.

The evaporator concentrates its own chloride

Raw juice arrives at roughly 13 to 15 Brix and leaves the set as syrup at 65 Brix. Water is boiled off, so everything dissolved in it concentrates by the same factor, near enough 4.5 times, before anything is taken out in clarification. Juice carrying 200 ppm chloride leaves the last effect at about 900 ppm, in vessels that run at 60 to 70 C in the later effects and 110 to 120 C in the first.

That is where a plain 304 stops being the answer. Type 304 is listed for commercial food processing equipment and stays right for the cold end - juice screens, mixing tanks, pans and crystallisers - at 210 MPa yield. The hot, concentrated end belongs to molybdenum: 316L at 2 to 2.5 per cent, and 317LMN, which adds more molybdenum plus nitrogen, is superior to 316L and 317L in acidic chloride service and stays free of chromium carbide at the grain boundaries as welded - the point on a tube sheet that cannot be annealed after rolling.

Pipes follow the station rather than the grade: 304/304L pipe from 3 to 2,200 mm OD, 0.08 to 170 mm wall, for the tube side, and large diameter stainless pipe to 2,500 mm for vapour mains.

Clarification is the acidic end

Lime, sulphur dioxide and sometimes phosphoric acid for refractory juices: this is the part of the mill that eats ordinary stainless from inside a weld. 904L is the grade the sheet points at - 19 to 21 per cent chromium, 24 to 26 per cent nickel, 0.02 per cent carbon, with copper at 1.2 to 2 per cent and molybdenum at 4 to 5 per cent - and phosphoric and sulphuric acid on its application list. It needs no preheat or post-weld heat treatment, but with about a third of the thermal conductivity of unalloyed steel, distortion is expected and a jig does more for the joint than technique.

Where the duty is milder, 317L gives 3 to 4 per cent molybdenum against the 2 to 3 per cent of 316, at 30 ksi yield, and is the sensible step before 904L.

Molasses and the distillery next door

Molasses is the awkward stream: dark, viscous, high in chloride and sulphate, stored for months at ambient temperature in tanks cleaned less often than anyone intends. 316L coil is the workhorse, 0.2 to 60 mm, 7.93 g/cm3, with thickness held to plus or minus 1 per cent. The distillery attached to the mill adds fermentation and dehydration trains; 2205 lists biofuels plants among its applications, and alloy 825 covers food process equipment from 4.8 to 50.8 mm where chloride and acid arrive together.

The parts that are machined rather than welded

Every mill keeps a stores list of small parts that are turned, not fabricated: valve stems, pump shafts, studs, wearing plates on the heaters. 416HT is the free-machining answer, cutting at about 85 per cent of what a free-machining carbon steel allows, with resistance to natural food acids on its corrosion list. It is also the grade most often misapplied. The same sheet calls it unsuitable for marine or other chloride exposure and gives it poor weldability, so it belongs in the machine shop and never in the juice main.

The boiler house belongs in the same order

Bagasse burned in the boiler is what makes a mill self-sufficient in steam and power, and that side of the plant is carbon and low alloy steel. A106 covers the steam side: cold drawn from 15 to 100 mm OD in 2 to 10 mm wall, hot rolled from 25 up to 700 mm OD with 3 to 50 mm walls, in the three grades whose minimum tensile rises from 48,000 to 70,000 psi. Superheated steam goes to P22, chromium near 2 per cent and molybdenum near 1 (1.90 to 2.60 and 0.87 to 1.13 on the sheet), 415 MPa tensile, supplied up to 16,000 mm and heat treated to the condition ordered rather than to a default.

The shed around all of it

Mill buildings are wet. Q345 plate from 0.3 to 200 mm carries floors, chutes and hoppers; A29M H-sections at 5 to 34 mm in 6 to 12 m lengths carry the frames. The steel structure page is blunt about the finish - steel structures corrode, so they are derusted and then galvanized or painted - and a mill roof is the easiest place to decide: Z275 coil from 0.12 to 3 mm, or prepainted TDC51D from 0.12 to 2 mm.

What goes on the enquiry

The season start date, and the list of stations that cannot stop. For each item: the medium with its chloride and acid load, the temperature, whether the part is welded and whether it can be annealed afterwards. Then the shape - plate thickness and width, tube diameter and wall, section size and length - followed by the cutting, bending, welding and surface treatment the shop will do itself.

A sugar mill buys steel twice a year and runs it for twenty. The order that works is written in the off-season, station by station, with the chloride number attached to it.