Hello,
Late to the party, but better late than never I guess:
My company designs and manufactures equipment for high pressure oxygen service: up to approx.379 bar (5500 psi). We have been using a multitude of fittings, piping and valves for decades with no issues.
Up to 230 bar (approx 3300 psi) you can easily use seamless tubing made of 316L. I would not recommend 304 as this has shown to rust under certain conditions, creating an additional ignition hazard you would most likely avoid with 316L. Now for the velocity limits: do they exist? YES. Should you follow them? YES. Does the industry follow them? Mostly. We have customers who reach sonic velocities with 6x1 tubing, and have been doing so for the last 40 years, safely. I would go as far as to say it is common, especially in cylinder filling, bundle applications and areas involving flexible hoses with fluoropolymer lining.
Now, lately certain major gas companies have started phasing out their use of stainless steel in O2 tubing applications. To comply with relevant ignition testing (For Europe BAM or CTE with an ignition test usually inspired by ISO 15996) a transition to 90-10 copper -nickel has been shown to be highly successful. Tungum Alloy has been considered, but shown to be suffering in offshore conditions; stress corrosion cracking due to repeated filling cycles. Inconel MA754 is also a valid alternative, but expensive.
As for fittings, Swagelock is great. No complaints after many years of using their products. Just make sure their product is cleaned before assembly/service.
As for brazing fittings to tubing: it has been done for the last 70+ years in oxygen applications with operating pressures up to 379+ bar, 5500+ psi. We have personally tested several 6x1 tubings in 316L with CW614N/CW724R brass fittings brazed with silver as filling material up to a hydrostatic pressure of 1600+ bar (23000 psi). At that point the tubing started straightening out as it was coiled 450 degrees in a flexible coil application. Sadly, a good brazing technique nowadays is as common as good technical drawing, a rare sight indeed.
Now 1-1.5'' seems excessive, unless you have a high mass-flow requirement. If not I would recommend disregarding the velocity limit and reducing the tubing size considerably. Now, this is only possible if your customer signs off on it, and you increase the cleanliness requirement of your system by a minimum of 10. (Higher velocity - higher chance of impact ignition ect.) I cant stress this enough: do not disregard the velocity limit if you cant garanty the cleanliness of the system.
