Piping pressure ratings vs source tank pressures

[PagoMitch]

I have been designing hospital med gas and lab/commercial gases for decades. In few cases has the pressure rating of the gas piping (O2, N20, He, N2, Ar, etc) downstream of the tank(s) regulator been equal to or greater than the unregulated tank pressures.
For instance, for Medical Grade O2, a tank pressure of 2400 psig is regulated to 60-65 psig, transported thru the O2 piping for a designed 5 psi dp, and delivered to Patient Rooms outlets at 55-60 psig. Type L cleaned and capped 1" copper is rated at a nominal 420 psig. It has always been assumed that the pressure regulator does its job; and if it fails, the pressure relief valve does its job. I have never heard of a hospital or facility where both these systems failed simultaneously, discharging 2400 psi O2 into the hospital piping. If this were to happen, it would probably blow out the sealing rings on the O2 outlets, discharging 2400 psi O2 into Patient Rooms. If this had happened, everyone in the Healthcare world would have heard about it.

So.
We have a Lab project that requires delivery of mid to higher pressure gases to user provided equipment; 200 psig for Argon/Nitrogen, and 1200 psig O2.
The local branch of a "major national piping products" supplier has been tasked by the owner to provide the manifolds, regulator, PRV, etc. I am only providing the connecting piping.
In the process of doing their calcs, they have advised me that our selection of piping is inadequate, because the piping is not rated for the unregulated tank pressure.
For O2, tank pressure is 2400 psig, and our 1"/0.012" wall copper tubing is rated at 1500 psig - with a 4:1 safety factor.
For Argon and Nitrogen, tank pressures are 2000 psig, and our 3/4"/0.065" wall Type K copper tubing is rated at 724 psig - with a 4:1 safety factor.
In none of these systems is the piping rated to take unregulated tank pressure.

I have had multiple discussions with the company; our rep, and the factory engineers are adamant that our design is inadequate.

So either 1) there has been a major shift in gas systems design that I am unaware of (?), or 2) the players at the "major national piping products supplier" are in over their heads.
I sincerely hope it is not the former (although I will admit it is a possibility...), while it is difficult to believe it could be the latter. These guys have one job; piping systems. How could they get this so wrong?

I am reminded of a quote attributed to Charles Darwin - "Ignorance more frequently begets confidence than does knowledge".

TIA.

 

[LittleInch]

Well thicker pipe cost more and make the supplier more money....

But also risk vs cost.

If you can eliminate the risk ( fully rated pipes) for relatively low amounts of money compared to the alternative then why not do it?

1500 psi rated to 2400 isn't that big a jump compared to 60 psi vs 2400.

Different design and risk philosophies.

No one answer.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

"The local branch of a "major national piping products" supplier has been tasked by the owner to provide the manifolds, regulator, PRV, etc. I am only providing the connecting piping.
In the process of doing their calcs, they have advised me that our selection of piping is inadequate, because the piping is not rated for the unregulated tank pressure."

Two key points in bold above indicate that the owner has decided to give the pipe supplier design responsibility. They have apparently determined what PRV settings shall be. If your pipe will not sustain the PRV setting, or whatever they say the max system design is, and it is indeed their responsability, it must be changed.

As for your design ...
How did you manage to select the piping without knowing the PRV setting and maximum system design pressure? Perhaps it was the owner that specified max pipe pressure should be the tank pressure.
If you did not know that, you selected your pipe prematurely and probably should have made a conservative preliminary selection that would sustain full tank pressure.

I can see quite a bit of risk with laboratory work, where worker skills are focused toward the medical aspects of their job, rather then on PRV settings and allowable tubing pressures. It's full pressure for me, unless a smaller number can be agreed by all parties.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Can you ask the for or post the P&ID they use?
Have they undertaken a design and safety review or HAZOP / HAZID?
Do they even have relief valves in the system?

It's a bit of a problem if one part of the system (the piping) has not been included in the design and safety review process.

But at the end of the day can you do it?
Present the cost impact to the owner and then stand back.

It might be your cost impact is low for only a small amount of piping compared to the cost of relief valves, vents and the reduction (even if small) of risk of overpressure. If your piping is fully rated, then that risk falls to essentially zero for burst other than material failure or installation failure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PagoMitch]

Thanks for the replies gents.
Before I reply individually, I must reiterate that the SOP for every EVERY tank-based Hospital Med Gas system is designed in this manner. 2000-2400 psi cylinders, regulator to 65 psi or so, prv, conveyed thru 420 psi (or so, depending on the size) copper tubing, with brazed or silver solder joints rated at around 700-1000 psi.

LittleInch - "Why not do it?" Other than the reasons indicated above (SOP), finding a material that is code compliant, with reasonable joints for the O2 1500PSI is difficult. While this is not in a hospital, NFPA 30 requires that hospital Med Gases be Copper or "listed corrugated metal tubing". I have used SS in the past, but per another topic in this forum discussed at length (impingement curves, etc), copper is the way to go at these pressures.
That said, I could increase the copper pipe size from 1"/0.12" wall to 1.5/0.25" wall; this is readily available. However, this is only available hard drawn, and fitting companies require annealed pipe for their fitting to "bite". As indicated earlier, soldering/brazing does not have the pressure rating, which would only leave welding as a joining option. I have never seen welded copper piping, but a google search indicates that it does appear that it can be done, using a helium based TIG. Then I would have to deal with speccing filler material with no impurities, and the annealing at each joint, qualifications of coper welders, and...it goes on and on solving a problem that I am not even sure exists.

1503-44 - The client provided the flow rates, and then we selected pipe sizing and materials. Then calls to gas suppliers provided tank pressures, and we determined regulator pressures. Our direction was that the client (via the gas supplier) would provide manifolds, regulator, and prv. But the gas supplier needed more info, so we provided our info to them. We shared our calcs with the clients preferred "piping products company", where they confirmed our pressure drop calcs, and stated that the piping did not meet tank pressures... and here we are.

LittleInch again - We are still working on our piping drawings, and this is a rather proprietary project for the client. Hence, I have used few identifiers. But the client is...large, and does have an established safety review program. Could we do it? At the end of the day... yes, for a cost. I think. The type K copper for Ar and N2 piping could be changed to 316L seamless for not a lot of additional $ - although bronze fittings (for copper) are about 1/3 the cost of 316L fittings. However, the 1500 psi O2 piping would be a different story. Per earlier, we cannot go to stainless. Which would direct us to a Monel 400 series; it does have the same exemption pressure (per EIGA 13/02/E) as copper at 3000 psi, so that's good. While 1"/0.12" wall copper is about $45/ft, 400 Monel is about $70/ft. For the nominal 600 ft we have, $15k is (I guess) not that much in the grand scheme of things.
But...again, I am ...still... not sure if this is a real problem. Guess we can kick this up the chain and see what the client's safety guys want.

 

[Prometheus21]

Hello,

That is interesting. I've been on the other end of this process, I've designed the equipment used for filling cylinders, high pressure hoses, pressure regulators, bundle manifolds, 12, 16, 32 & 64 cylinder bundle filling (distribution systems) ect. That being said every piece of equipment in said system is designed so that the connected piping doesn't have to be rated to take unregulated tank pressure on the end users part. In other words the end-user only have to worry about the regulated pressure.

"It has always been assumed that the pressure regulator does its job; and if it fails, the pressure relief valve does its job" - correct. Now some end-users still equip additional safety measures in their system to avoid a high pressure discharging into the main system, but it's not common.

"As indicated earlier, soldering/brazing does not have the pressure rating" - That depends on the design. Filling gap, tolerances, materials and production methods plays a major role in the strength of the joint. For reference; we make brazed silver joints with (mostly) brass, that withstand leak testing at 5000 psi, and a hydrostatic pressure at 1600+ bar (23000 psi) (both SS tubing + brass/ brass - brass combinations). Now with copper you will have a reduced pressure rating for sure, but a properly brazed joint will in many cases be as strong or stronger than the base metals being joined. As long as the tubing can handle the pressure, the brazed joint can as well.

NB! Since brazing doesn't involve melting the base metals they retain most of their physical properties. The low temperature involved in the brazing process minimizes the danger of metal distortion or warping. Now this doesn't reduce the need for extensive cleaning afterwards to make the system approved for oxygen service.

As for pricing: we usually use Monel 400 or Cu-Ni 90-10 alloy if we can. But the pricing is higher and some customers then demand the use of standard copper instead.

 

[LittleInch]

Pagomitch,

That's a good well argued response which you need to write doyen and detail with cost and schedule impact and send it to the relevant party, be that the owner or whoever is employing you to do the work.

On the SOP issue I get you, but now you're doing something different. It's not 60psi O2, it's 1200. Oxygen is a fluid I've managed to avoid so far but I know it has its own special issues. In my field this is like coming into a terminal with a high pressure pipeline. We don't full rate the terminal pipework and fitting because it would cost a fortune compared to regulating pressure and installing relief valve. But there are some occasions where eliminating an overpressure risk is worth it. That's clearly what the other party think, whether they know it or not.

So my recommendation is present the two options to whoever can make a decision is a costed fashion and then accept their ruling and build it. IMHO.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.