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

[RayfromTX]

From the linked thread from 2007, this seems to be the most important and relevant post. The thread is worth a read.

I spoke with our engineers today and found that DOT 3AA cylinders have a typical tensile strength of 105,000-120,000 psi and Worthington's tensile strength on our X-Series (E-14157) is 135,000-155,000 psi with nominal at 145,000 psi which they tell me they hit "pretty well on the money" every time.

This demonstrates that a high pressure E cylinders is at least 30% higher tensile than 3AA cylinders.

Please pass this information on and do everything you can to stop the rumor that HP and LP cylinders are the same except for the markings.

Dive safe

 

[2airishuman]

So, different alloys or manufacturing treatments was the first possibility that occurred to me. This is the other possibility that occurred to me. That is, two tanks could be physically identical, and one could be rated "LP" and the other "HP" for business decision reasons that have nothing to do with the physical characteristics. I can see how this theory is appealing, since we hear of LP tanks routinely being filled to high pressures in cave country and are said to pass hydro again and again.

There is an actual difference in the steel.

I have some notes somewhere about the details which I can't find right at the moment. But, by memory:

The 3AA specification used for LP cylinders allows any of several alloys to be used, but substantially all cylinders are made using 4130X. However, 4130X is itself a broad classification and many steels with varying properties can be made that fit within the 4130X specification. In any case, 3AA limits the maximum design strength of the steel to 70,000 PSI (at hydro test pressure), and has some requirements on the relationship of tensile to yield strength that are in the specification to limit the brittleness of the steel so that, in the event of a rupture, no fragments are produced.

49 CFR 178.37 - Specification 3AA and 3AAX seamless steel cylinders.

In contrast, the various special permits -- the PST, Worthington and Faber ones are almost identical -- define the composition of the steel much more narrowly. They also define a heat treating and quenching regimen in greater detail. They allow a somewhat higher maximum design strength, and specify a number of tests to limit the brittleness of the steel in place of the limit on the ratio of tensile to yield. These tests include a fold-over test where a cylinder wall must not break when folded over an anvil of relatively small diameter (about 1/4" if I recall correctly), and a test where a cylinder weaked with partial cuts is overpressurized and burst, and must remain in one piece. There is also a test of actual yield strength, and this and some of the other tests must be performed with every lot of cylinders manufacturered. Any of the special permits have these details and are available on the phmsa web site.

The result of all this is that the steel in modern HP cylinder made under the special permits is stronger, so a higher pressure can be used for a given wall thickness, with the same margins of safety.

As an aside, there are some older HP cylinder made under 3AA, that have thicker walls and are much more negatively buoyant. The evolution of design pressures is overall mainly based on the desire to have cylinders be more or less neutral. The higher the design pressure for a given material, the more negative the cylinder will be. This is why, every time the wall composition has changed, we have had a working pressure change, 2640 for most 3AA steels, 3000 for 3AL, and 3442 or 3500 for the special permit steels, and 4000+ for the cabon fiber cylinders.

 

[Lorenzoid]

Thanks, @2airishuman (and others). Goes to show that resurrecting an old topic can shed new light when we have new voices.

 

[BRT]

So far, @Charles2 's observations come closest to what I'm looking for.


IF the two tanks are in fact made of alloys possessing different strength properties, then I think that would answer the question. But are they?

As for DOT or other government specifications, the tank itself isn't aware of specifications. A tank has its physical/structural properties and that's all, as far as any of us end-users are concerned. If, assuming for the sake of argument, the interior and exterior dimensions and the alloy material are nearly identical, why would the tanks be subject to different government specifications? I could be wrong, but assuming there is no government regulation preventing it, I would think that labeling a tank "LP" would not be as effective a marketing tool as labeling a tank "HP." If "HP sells better than "LP" why would all manufacturers not label all their tanks "HP"?

LP tanks always pass hydro. HP's may not.

 

[2airishuman]

LP tanks always pass hydro. HP's may not.

There is some truth to this.

Galvanized PST cylinders may fail hydro because of distortion of the geometry. This can be avoided by pressurizing the cylinder to 90% prior to running the test, but not all hydro facilities are willing to do this. Worthington cylinders do not have this problem because the special permit was amended to eliminate the problematic portion of the test. As for Faber, time will tell, check back in 5 years.

I have had 3AA cylinders fail hydro. Some galvanized 3AA cylinders exhibit the same geometry problem. Some fail because they have corroded to the point where they are no longer safe -- and those should fail, which is why we have the tests.

 

[oldschoolto]

There is some truth to this.

Galvanized PST cylinders may fail hydro because of distortion of the geometry. This can be avoided by pressurizing the cylinder to 90% prior to running the test, but not all hydro facilities are willing to do this. Worthington cylinders do not have this problem because the special permit was amended to eliminate the problematic portion of the test. As for Faber, time will tell, check back in 5 years.

I have had 3AA cylinders fail hydro. Some galvanized 3AA cylinders exhibit the same geometry problem. Some fail because they have corroded to the point where they are no longer safe -- and those should fail, which is why we have the tests.

Galvanized tanks need to be hydro'd by a shop that understands that they need to be done differently than plain steel tanks. I had to point it out to the guy at the shop..

Jim...

 

[tbone1004]

Galvanized tanks need to be hydro'd by a shop that understands that they need to be done differently than plain steel tanks. I had to point it out to the guy at the shop..

Jim...

I have a good hydro shop. They actually round out all galvanized steel bottles prior to hydro regardless of lp vs. HP. Says it rarely is an issue, but better to be safe than sorry.
If you are worried about it, bring it to the shop full of air, they usually drain right before going into the tank so sometimes that is enough to help it through, but better to get a new hydro shop.

FWIW my hydro shop records and prints the results from the round-out procedure on my slip when it comes back so I can see both the round out results and the actual hydro results

 

[Magnus Lundstedt]

Adding a very small amount of a Element can have major changes in the property of the steel... Most alloys are very closely held secrets... Also the forming and heat treatment...

Think about two wrenches that look the same , But... One is super strong and the other is junk...

Jim...

Also can two cylinders with the same outer form and the same weight made of the same material be different.

If one have better equality off wall thicknes it makes it stronger than the one with more uneaven wall thicknes.

It depends of manufacturing tollerance.

As example from other manufacturing, I can mention computer processors, where the manufacturer make a test run of every chip out of the production line, and depending on what temperature they mesure during the test run they stamp it with different clock frequencies.


And then it can be as simple at one manufacturer only certified this model for the US market for one pressure, but for other markets for different pressure, but certifying costs money.




In Europe we usualy have 200/230/232 or 300 bar. (232 bar is 3 365 psi 300 bar is 4 351 psi)

Our 300 bars cylinders are often the same shape and weight as US HP cylinders.

So then the questions:
Can you fill an US HP to 300Bar? Probably.
Is it safe? Don't know.
Would I try? Hell no!

I fill my 12l 230 bar to 250 hot, it is often ~230 in the water.

 

[Blasto]

As example from other manufacturing, I can mention computer processors, where the manufacturer make a test run of every chip out of the production line, and depending on what temperature they mesure during the test run they stamp it with different clock frequencies.

For the most part, chip binning is a marketing decision - most lower-bin SKU are perfectly functional at higher-bin SKU specs. An example is Core i5 vs i7 - the i5 chips are just as good when they come off the line, then they turn off HT and some cache.

If one have better equality off wall thicknes it makes it stronger than the one with more uneaven wall thicknes.
It depends of manufacturing tollerance.

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.

What could be a real physical difference, OTOH, is heat treatment. Lower yield steels are generally specced at greater elongation to break, so you may actually need a different treatment to meet the specs. Pure speculation as to whether it's done here.

 

[67macoshark]

The working pressures on cylinders make sense if you do the math...

DOT Wall Stress Calculations: S = P(1.3D^2 + 0.4d^2)/(D^2 - d^2) This is the 3AA spec

S = maximum wall stress (psi)
P = Test pressure (psi)
D = Outside diameter (inch)
d = Inside diameter (inch)

This is with my bank cylinders at home.

S = 10,000[1.3(9.283^2) + 0.4(8.147^2)]
(9.283^2) - (8.147^2)

S = 69,987 psi

3AA limits the maximum design strength of the steel to 70,000 PSI (at hydro test pressure)

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