The plant sells cold gas, and the box is a thermos

An air separation unit does not make oxygen or nitrogen. It sorts air by boiling point, at temperatures where carbon steel is brittle. What stands on site is a tall grey box on a steel frame: inside it a column, a few exchangers and a lot of stainless pipe, wrapped in insulation inside a carbon steel casing.

The piping decides the delivery date: every cold line is stainless, every joint is welded, and none of it can be inspected once the box is closed.

Air is separated across about 13 K

Dry air is 78.1 per cent nitrogen, 20.95 per cent oxygen and 0.93 per cent argon. At atmospheric pressure nitrogen boils at -196 C and oxygen at -183 C. That gap is why the column is tall: the stages needed for a given purity are set by a small difference in volatility.

The reboiler-condenser is the tightest part of the design. Oxygen in the sump boils at about -180 C. Nitrogen has to condense on the other side of the same surface, which means compressing it to roughly 5 to 6 bar. A couple of kelvin makes the exchanger large and sensitive to fouling.

A 216 K drop moves every line in the box

Ambient to liquid nitrogen is 216 K. Austenitic stainless contracts about 3 mm per metre over that range, roughly 0.3 per cent. A 30 m riser loses 90 mm between a warm start and steady operation, and a pipe anchored at both ends puts that load into the column.

Cold boxes are piped for the movement rather than against it. Long runs carry offsets and loops sized on the calculated contraction, supports allow axial sliding, and the line leaving the top of a column is often hung rather than stood on the base. Nozzle loads come from the column supplier and are checked by the piping designer; 90 mm restrained at a 6 mm wall cracks the shell before it bends the pipe.

Supports are stainless for the same reason. A carbon steel shoe on a cold line conducts heat into the insulation, and a support rigid in the wrong direction is an anchor.

The piping is 304L because it cannot be annealed afterwards

304/304L stainless pipe covers most of the cold section: 3 to 2,200 mm outside diameter, 0.08 to 170 mm wall, seamless or welded, to A312 or A213. The L suffix is the point. A field weld cannot be followed by a solution anneal, so the low carbon grade keeps chromium in solution and avoids sensitisation at the weld.

304L also has the toughness. With no ductile to brittle transition, a Charpy test at -196 C is a formality for a sound weld and a real test for a bad one. 316L, supplied as pipe or 316L coil, appears where the cooling water side or a client standard asks for molybdenum; the dry process gas does not need it, and the difference across a few hundred metres is not small.

Where the header goes past the standard range, the large-diameter stainless pipe range reaches 2,500 mm OD with 150 mm walls. A cold box rarely needs that; the warm-end air line often does.

Oxygen changes the cleaning, not the grade

Downstream of the point where oxygen concentration rises, the line is oxygen service, and oxygen service is a cleanliness specification before it is a material one. ASTM G93 sets the cleaning and the hydrocarbon limit, and a line that passes a hydrostatic test can still fail a wipe test. Oil, grease and coolant are fuel in a line carrying pure oxygen at pressure.

Materials follow the same logic. Monel 400 is the usual choice for valve trim and small high-pressure fittings, and it resists both the gas and the seawater that cools the plant. Carbon steel is not an option in the cold section, and copper alloys arrive as valves and instruments rather than as pipe.

Aluminium is a special case. Brazed aluminium plate-fin exchangers are standard in these plants, bought from a specialist rather than fabricated on site. A buyer who asks for an aluminium cold box is buying a proprietary exchanger, not a steel package.

Perlite, purge and the ice plug

Cold boxes are insulated with expanded perlite poured into the space around the equipment and kept dry by a nitrogen purge at slight positive pressure. The purge is not decoration. A casing leak pulls in moist air, the moisture freezes at the first cold surface, and the ice plug forms in a line that cannot be reached without cutting the box open.

Upstream, molecular sieve beds in the prepurifier take out water and carbon dioxide before the air is cooled, so neither freezes in the exchanger. A plant tripping on differential pressure across the bed has a saturated bed, not a bad exchanger.

Welding is the only inspection that matters

Cold box piping is TIG welded with ER308L filler, argon purged, with ferrite held at 5 to 10 FN so the weld does not crack as it solidifies. Butt welds are radiographed and the box is helium leak tested before the perlite goes in. After that, a leak shows up as a rising impurity in the product gas.

Hydrostatic testing needs water with a chloride limit and immediate drying. Residual water in a stainless line is a pitting cell waiting for an oxygen-rich shutdown. Pickling and passivation after welding removes heat tint and restores the film, and on a cold box it is done line by line before assembly rather than on the finished unit.

The warm end is a different world. Compressor discharge and the feed line to the prepurifier can be carbon steel: A106 seamless pressure pipe for the main run, A53 pipe for utility connections, both with the usual certificate. The change from carbon steel to stainless happens at a flange or a transition joint, and the specification has to say which side of it the insulation begins.

The casing, the frame and what the order looks like

The casing is Q345 plate, or A36 where the design is American, 3 to 6 mm thick, stiffened with rolled sections and welded to a frame that carries the box and its platforms. Cutting and bending the panels is ordinary plate work, and welding has to be weather tight rather than pressure tight. The frame is structural steel, often rolled H-beams, with square stainless tube for the small brackets inside the cold space.

Outside, the box is painted, while the handrails and cable trays are usually Z275 galvanized rather than painted, because they are the parts that get knocked.

The stainless order is short: grade, standard, size, wall, length and tests. A 30 m run in 6 m lengths carries four butt welds; in 12 m lengths it carries two, and on a cold box the weld count is the cost. Impact testing at -196 C, intergranular corrosion to A262 practice E, product analysis on each heat and a 3.1 certificate are the normal package. The delivery date is set by the pipe.