The plant is concrete; the steel is at the edges

A water or wastewater works is basins, walls, slabs and buried pipe, and almost none of the tonnage is stainless. The stainless appears where something has to move, seal, slide or be cleaned: screens, weirs, penstocks and gate guides, flocculator rotors, dosing spools, odour ducting and handrails, none of it a large order. All of it is specified by one question asked at a different point in the process: what is dissolved in this water, and at what temperature.

Chloride usually answers it. Chlorine answers the rest, and motion decides whether strength enters at all.

Raw water sets the floor, and a brackish intake moves it

River water at a few tens of milligrams of chloride per litre is 304 or 316L territory. An estuarine intake is a different job, because the chloride climbs into the thousands and falls back after rain, so the metal sees a cycling concentration rather than a steady one. The 254 SMO page puts itself in high chloride environments such as brackish water, carrying 6.0 to 6.5 per cent molybdenum alongside nitrogen at 0.18 to 0.20. 317LMN is the step below, described as superior to 316L and 317L especially in acidic chloride service, with low carbon that allows as-welded use.

A 304 screen in brackish water pits at the water line within a couple of years, and the replacement is a 316L that pits more slowly. 904L, at 24 to 26 per cent nickel and 4 to 5 per cent molybdenum, is the grade that stops the cycle, and its low carbon keeps intergranular resistance when welded.

Ferric chloride is a chloride salt, not a chemical curiosity

The coagulant dosed at the inlet is usually ferric chloride, a concentrated chloride salt, and the dosing area is where a water plant's stainless bill goes wrong. The C-276 data sheet claims exceptional resistance to concentrated oxidising salt solutions, naming ferric and cupric chlorides, precisely the duty. Liquid ferric chloride at 40 per cent will pit 316L at the weld and at the tank floor; the nickel alloys are bought where a drip tray leak is not acceptable.

The same page carries the sentence that matters at the other end of the plant: C-276 is among the few alloys that hold up against wet chlorine gas, hypochlorite and chlorine dioxide.

Disinfection is the harshest point in the works

Sodium hypochlorite storage is a small tank with a large problem. The liquid attacks the wetted wall, and the headspace attacks the roof, the instruments and the vent, because hypochlorite decomposes and the gas concentrates where nobody looks. Chlorine dioxide is worse: made on site, into water that is already chlorinated.

The titanium rod page gives the reason titanium keeps appearing on dosing skids: a dioxide layer that re-forms after a scratch, resistance to dilute acid below 10 per cent, to 50 per cent NaOH, and to chlorides, at 4.51 g/cm³.

Aeration and mixing is where strength and corrosion meet

A flocculator paddle, an oxidation ditch rotor and a sludge mixer share a duty profile: a shaft in bending and torsion, rotating in chloride-bearing water, at a stress that reverses every revolution. That is corrosion fatigue, where a water plant buys an alloy for two properties at once. The 2205 duplex plate page says it directly — applications subject to both an aggressively corrosive environment and cyclic loading can benefit — and lists rotors, fans, shafts and press rolls among its uses.

The strength argument is arithmetic. 2205 yields at about twice an austenitic, so doubling the allowable stress lets shaft diameter fall by the cube root of two, about 21 per cent, and mass per metre by about 37 per cent. Both need checking against deflection, because a long ditch shaft is usually stiffness-governed and that is where the saving disappears. What does not disappear is the corrosion side: a PREN of at least 34, and a test in which 2205 passes boiling 42 per cent magnesium chloride while 316L and 317L fail.

2507 and S32760 at a PREN of 40 cover the warm, high-chloride end, and the duplex coil range covers the 0.2 to 60 mm sheet those parts are cut from.

Effluent and odour control: the scrubber is a chemistry problem

Odour control concentrates the plant's waste chemistry into one vessel. A scrubber on a sludge thickening area sees hydrogen sulphide on the gas side and hypochlorite or caustic on the liquid side, and its materials read like a chemical plant. The 2205 page lists effluent scrubbing systems by name; 825 covers the acid-and-alkali combination; C-276 covers the chlorine.

On the liquid side, 904L is named for sulphuric and phosphoric acid, and 317L remains the fallback, with a tensile of 515 MPa and a yield of 205 MPa, a step above 316L in a warm scrubber.

Pumps, valves and the intake structure

The intake is a pump station before it is anything else, and a pump station is valves, shafts and a wet well. Monel 400 lists valves and pumps, pump and propeller shafts, marine fixtures and fasteners, and fresh water tanks among its applications, a fair description of a coastal intake. S31803 and S32205 duplex pipe is stocked from one tonne upward in 12 m lengths, with a pressure test and third-party inspection available on request - the practical spare size.

304 and 304L pipe covers the washwater side from 3 mm to 2.2 m outside diameter in walls of 0.08 to 170 mm, and the large diameter thick wall programme covers the heavy penetrations through a basin wall, where a liner is welded to a pipe and inspected from both sides.

The rest of the site is galvanized and plate

The tonnage, as opposed to the alloy, is handrails, walkway frames, covers and plinths. Square tube in sections from 10 × 10 up to 100 × 100 mm, supplied at 1 to 12 m, is what those frames are built from; Z275 coil at 0.12 to 3 mm thickness with Z60, Z80 or Z275 coating is what covers and flashing are cut from; Q235 plate and A36 plate in the 0.3 to 200 mm range cover plinths and embedded items.

A wet plant is a corrosion test for anything galvanized below the handrail line, and the steel structure page is right that derusting then galvanizing or painting belongs in the specification, not on site. The galvanized processing chain — cut, bend, weld, polish, then coat, then assemble — keeps the coating intact until it is installed.

What the enquiry should carry

Six items. The point in the process, because a screen and a dosing skid differ. Chloride and temperature there, including the seasonal high. Whether the stream is chlorinated, which moves the answer from stainless to nickel alloy. Whether the part is static or rotating, because rotation brings fatigue into the grade choice. The connection and the cut list. And the test and certificate the client will witness.

Cutting, bending, punching, welding and surface treatment are quoted from the same drawing, which is the difference between a works package and a material list.