Because the pressure rating is calculated from those two dimensions, and a nominal size does not contain that information.

Ordinary pipe is described by a nominal bore and a schedule, which is a convention rather than a direct measurement. High-pressure tubing is described by exactly what it is: an outside diameter and a wall thickness. From those, plus the material's allowable stress at temperature, the pressure rating follows directly.

The consequence is that ordering by size alone specifies nothing useful. Two tubes of the same OD with different walls have different bores, different weights, different ratings and different fitting requirements.

It also means the bore varies with the wall, so flow calculations must use the actual internal diameter rather than the nominal figure. Always quote OD and wall together, and confirm both on delivery against the mill certificate rather than assuming.

Because drawing gives dimensional control and added strength, and because a weld line has no place in a pressure boundary at these levels.

Cold drawing pulls the tube through a die over an internal mandrel. That controls the outside and inside diameters simultaneously and to close tolerance, produces a smooth surface inside and out, and work-hardens the material so the finished tube is stronger than the annealed stock it came from. Where an even higher rating is needed from the same size, the tube is supplied in a more heavily cold-worked temper.

Seamless matters because a longitudinally welded tube carries a weld running the entire length of the pressure boundary. That weld is a metallurgical discontinuity, it is the most likely initiation site for a fatigue crack, and it cannot be inspected along its full length once the tube is installed.

For low-pressure service welded tube is perfectly adequate and cheaper. At these pressures it is not used.

Tighter than ordinary tubing, because the tube's surfaces are working surfaces rather than just containment.

The outside diameter must be round and within a close tolerance, since the coned sealing surface is machined into it and any ovality carries straight through into the cone. An out-of-round tube produces a cone that seats on part of its circumference.

The outer surface must be free of scoring, draw marks and handling damage in the region that will be coned and gripped. A longitudinal scratch running through a sealing area is a leak path that no amount of tightening will close.

The bore matters for a different reason: at high pressure, particulate carried into the system damages valve seats and instruments, and a contaminated bore is difficult to clean once the system is built.

Inspect tube on delivery for surface damage, and reject lengths with scoring rather than cutting around it and hoping.

Capped, supported, and protected from anything that can score or dent it - the damage that matters is usually done before installation.

Tube is supplied with end caps to keep the bore clean. Leave them on until the moment the tube is cut, and re-cap any part-used length. Once swarf, grit or moisture is inside a long tube it is genuinely difficult to remove.

Store straight lengths supported along their length rather than allowed to sag, and off the ground. Do not drag tube across concrete or across other tube - a longitudinal score is exactly the defect that ruins a sealing surface, and it is easy to inflict while carrying.

Keep stainless away from carbon steel storage racks, grinding dust and carbon steel tooling. Embedded iron particles rust on the surface and initiate pitting on material that would otherwise be immune.

Segregate materials and grades in the store. Mixed stock that has lost its markings has lost its traceability.

Material certification traceable to the heat it was made from, and more where the service demands it.

A mill certificate states the material specification, the heat or cast number, the chemical composition and the mechanical test results for that batch. At these pressures the certificate is part of what you are buying, not paperwork - it is the evidence that the tube is what the pressure calculation assumed.

The heat number should be traceable from the certificate to the marking on the tube itself, so a length in the rack can be tied back to its documentation. That traceability is routinely required on process, oil and gas and pressure-equipment work, and it is lost the moment tube is stored unmarked.

Sour service requires certification against the applicable materials standard, with hardness limits and heat treatment condition stated. Some applications also require positive material identification on receipt.

File the certificates against the installation, not just the purchase order.