Air is a wet gas, and the pipe is where the water stops

Every compressor takes in atmospheric air at whatever humidity the weather offers and squeezes it into a smaller volume. The water does not disappear; it goes from vapour to droplets as the air cools on the way out. An aftercooler and a receiver take most of it, a dryer takes more, and the line then runs eighty metres with nothing in it but pipe. Every low point collects what the dryer missed. An air line is not a water line with a different label, and the first thing to fix on a wet system is the drain leg, not the grade.

Carbon steel is the default because the standard allows it by name

A53 pipe says it in one sentence: it is intended for mechanical and pressure applications and is also acceptable for ordinary uses in steam, water, gas and air lines. That sentence is why most plant air is carbon steel. The specification gives the buyer something to hold: seamless, ERW and furnace-weld types, grades A and B, carbon capped at 0.25 and 0.30 per cent, manganese 0.95 to 1.20, minimum tensile of 48,000 and 60,000 psi with yield of 30,000 and 35,000, and a size range from half inch to 36 inch nominal bore through schedules 40, 80, 160, XS and XXS. It ships to EN 10204 3.1B with tension, bend and flattening tests, and the same page lists NACE MR0175, HIC and SSC testing as options.

Tolerance matters more on air than buyers expect, because a header is assembled from a cut list. The A53 structural listing quotes plus or minus 0.1 mm cold drawn and 0.05 mm cold rolled, in 3 to 18 m lengths with plain, bevelled or threaded ends: a cut-list tolerance, and the difference between a spool that fits and one that gets forced.

The seam is the part nobody asks about until it leaks

A53 covers three manufacturing routes and they are not equivalent. Type S is seamless, Type E is electric resistance welded and Type F is furnace butt welded, and the chemistry table gives each its own column because the weld is part of the product. On a dry header at ambient temperature the seam rarely matters. It matters on a line that cycles, and on anything carrying hydrogen, which is why the seamless versus ERW comparison belongs before the purchase order rather than after.

Where the line runs hot, A106 takes over: cold drawn in 15 to 100 mm OD at 2 to 10 mm wall, or hot rolled in 25 to 700 mm at 3 to 50 mm, in grades A, B and C with minimum tensile from 48,000 up to 70,000 psi. Every length gets a hydrostatic test unless the buyer accepts a full-body non-destructive electric test, and the specification still calls for a 90 degree cold bend over a mandrel. At 40 C that is more paper than the job needs. Downstream of a steam sparge it is the minimum.

Instrument air is a cleanliness specification wearing a pressure label

The line feeding a cylinder, a positioner or an analyser is not judged on burst pressure. It is judged on whether a flake of mill scale reaches a 0.5 mm port. A carbon steel header that was hydrotested and left wet will shed iron oxide for years, and no filter downstream of the regulator catches it indefinitely. This is where the material changes rather than the schedule. The 254 SMO page lists hydraulic and instrumentation tubing among its applications, and 317LMN tube covers the general case with molybdenum at 4.0 to 5.0 per cent and nitrogen at 0.10 to 0.20, with low carbon that holds up in the as-welded condition.

Above that, 304 and 304L pipe runs from 3 to 2,200 mm OD at 0.08 to 170 mm wall, covering a 6 mm impulse line as readily as a plant air main, and square and rectangular tube is what the instrument stands and tubing trays are actually built from. Where the run stays outdoors and nobody will paint it, stainless is bought once and carbon is bought every five years.

Nitrogen, oxygen and the lines that must not leak

Nitrogen does two jobs that look identical on a P and ID and are not. Purging and blanketing lines carry gas intermittently at low pressure, can be carbon steel, and are usually the ones forgotten in the pressure test. Analytical and sample lines carry a few hundred millilitres a minute where any leak is a false reading, so they are small bore, stainless, and compression-fitted where the analyser comes out for calibration. 904L is worth noting for a different reason: its page describes laser cutting with nitrogen and backhand welding under inert gas to keep the seam as corrosion resistant as the base metal, and that is the standard a sample line has to meet on the inside rather than the outside.

Where the utility carries chlorides as well as pressure, duplex takes over. S31803 and S32205 are offered from 6 to 1,174 mm OD at walls to 80 mm and lengths to 12,000 mm, with third-party inspection to TUV, SGS, BV, ABS or LR on request, from one tonne. 2205 pipe sits in the same family at 550 MPa yield against 800 MPa tensile, buying the same duty at a thinner wall.

A utility line is a skid and a bracket before it is a pipe

The pipe is rarely the whole order. A receiver sits on a frame, a dryer on a plinth, a header on brackets welded to a structure, and the assembly arrives on a skid somebody has to design. Q235 plate and A36 plate cover the 0.3 to 200 mm range those parts come from, and the steel structure page makes the point that applies to every outdoor skid: it will need derusting, galvanizing or painting, and that decision belongs in the quotation. For items that stay outside and will never be repainted, galvanized plate and Z275 coil cost less than a coating with a maintenance interval attached.

What the enquiry should carry

Six items settle most of it: the service and whether it is wet; design pressure and temperature, which pick A53 against A106 against stainless; the dew point the dryer actually achieves, which decides between a drain leg every twenty metres and a stainless run; the bore, with the pressure drop the compressor can afford; the end connection and whether the spool is cut to a list; and the test the client will witness, because a hydrostatic test on every length and a full-body NDT are different prices.

Add the fabrication and the order becomes deliverable. Cutting covers shear, laser and waterjet work on bracket profiles. Bending produces the saddles and clamps, punching the bolt holes, and plate welding joins the skid. A final pass through surface treatment and the package ships complete: a different quotation from a pipe list, and the one that arrives on site with nothing missing.