why the "LP" versus "HP" service pressure rating in the US?

[Charles2]

An interesting point is these cylinders should be stamped with a 6,000 psi working pressure (5/3 of 10,000 psi test pressure), but are stamped 5,000+ with a REE value of 199cc.

Curious. Do your tanks have the test pressure stamped on them as 10,000 psi and the service pressure of 5,000 psi?

Also, I am guessing that you meant 3/5, not 5/3.

 

[67macoshark]

Curious. Do your tanks have the test pressure stamped on them as 10,000 psi and the service pressure of 5,000 psi?

No test pressure stamp. I got the model number from the serial number stamped on the cylinder so I could get the REE value from Norris. It was manufactured to the 6000 psi specs, but stamped with a 5,000+. It was originally built for the USA military, so they may have different standards than the DOT.

Also, I am guessing that you meant 3/5, not 5/3.

I did...it is corrected. Thank You!

 

[Magnus Lundstedt]

The above would then apply to tanks just as well. Quality management considerations make it practical to manufacture at much tighter tolerances than required, so you'd see 99.8% of the tanks pass both standards, 0.15% only the LP standard, 0.05% none. At that point it's easier to scrap the 0.15% than deal with binning. It's extremely unlikely that a manufacturer would have two tanks differing only in tolerance.

One manufacturer would probably not do that when it is simple things like tanks. (but in some other cases manufacturers do.)

In this case the comparison is between two different manufacturers.

 

[Clark Fletcher]

There is an actual difference in the steel.

Thanks @2airishuman for that explanation.

I've been following this thread with very keen interest since it started. I started comparing different tank specs and the only logical explanation to me was different steels. However, I could find nothing definitive in writing to support that. In fact, information on both Faber and Worthington HP and LP tanks said they all are chrome moly tanks.

But, I also know from my own experience working with pressure vessels that even slight changes in steel alloy content can make a huge difference in that particular alloys strength and ductility. And, even when two alloys have the exact same alloy content, processes like heat treatment and cold hardening can change strength and ductility dramatically.

Closer to home for us, if you look strictly at the specs on the Worthington LP85 and HP100 (without knowing they were HP or LP), the outer diameters are identical. They are both Chrome Moly cylinders. And, the weight per inch on the "85" tank is slightly higher than the "100" tank. From this we could conclude that "85" has a slightly higher wall thickness than the "100". I say that because generally speaking slight differences in alloy content can't account for the weight difference. This might erroneously lead us to believe that the 85 is the HP tank and the 100 is the LP tank.

So, the difference is either in the alloy content or whether the steel was subjected to some hardening process before that tanks were manufactured. Again, there's nothing I could find in writing anywhere that specifically states what the difference is.

Sorry about the geek out moment. Brittle fracture and cold overpressure failure of pressure vessels is one of the subjects I used to teach. I couldn't help myself.

 

[2airishuman]

You can read about the alloy composition on page four of the special permit:

https://www.phmsa.dot.gov/staticfiles/PHMSA/SPA_App/OfferDocuments/SP14157_2014110787.pdf

... and the heat treatment on page seven.

 

[Clark Fletcher]

Perfect. Thanks @2airishuman.

 

[Steveed]

You can read about the alloy composition on page four of the special permit:

https://www.phmsa.dot.gov/staticfiles/PHMSA/SPA_App/OfferDocuments/SP14157_2014110787.pdf

... and the heat treatment on page seven.

Machinist here - nice read, thank you.

 

[oldschoolto]

I was just ordering a 19 cu pony and low and behold... Just like I said.. Luxfer cylinders are made from a proprietor 6061-T6.. alloy.. called L6X..

Jim....

 

[Shotmaster]

If you know the history of Aluminum cylinders, Catalina didn't have issues with failures since they used a quality virgin alloy. My uncle is a metallurgist who ran a large aluminum smelting/remelting facility. They delivered custom alloys to automotive and industrial customers, based upon their tolerance for different contaminants like lead and copper. Of course remelted alloys are cheaper, but the unknown aspect can cause problems overtime like it did for Luxfer and others. I prefer steel, but do have 6 Catalina's for my daughter. From my personal experience I would find out more about "L6X" before buying.

 

[oldschoolto]

I do believe that "ALL" pressure vessels are made from high grade virgin alloys.. remelt is not even used for 6061 truck rims.. The other thing is what is the base metals used in the furnace.. using high grade scrap turnings and stamping waste will give you a high grade end metal..

I have never seen a high end manufacturer use anything but top grade base stock.. frying pans and Chinese tools use crap metals of Unknown alloys

Jim..