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

[Shotmaster]

I can tell you with great confidence as I watched tractor trailer size hot storage, transport molten aluminum daily from Birmingham to the Saturn Plant, and none of it was virgin alloy. I read the alloy print out from the AtomicEmission Spectrography , which is very precise. There are others that I can't mention. Like I said, the users provided a metal specification that set limits for contaminants based upon application. Price is a big player, and the difference is large when you are talking about millions of engine blocks, heads, and manifolds per year. Just because ingots show up at a plant all nice and shiny doesn't mean they are virgin alloy. If I remember correctly the failure of aluminum diving cylinders (pressure vessels) was from lead contamination.

 

[oldschoolto]

You can't call a "CAST" Saturn intake manifold or low compression cylinder head as a high grade , high stress product.. We are talking about high pressure vessels that are "FORGED" to finalized products.. Big big difference.. Even the Keth Black Hemi is made from a forged billet ....

Jim...

 

[Shotmaster]

Perhaps this might clear up what I was saying.

https://www.researchgate.net/public...rs_A_review_of_causes_and_protection_measures

Specifically "recycled scrap is used in the alloy".

Alloy 6351 allows up to 400 ppm lead contamination within the specification.

I'm not poking you in the eye. Just saying that there is more to the story.

 

[oldschoolto]

On page two middle of the first paragraph States " nor was the alloy made from scrap metal" and "Was made from scrap" but it's a nice read... I do understand that they are talking about two different cylinders having cracking in the neck using 6351alloy and a odd ball tank of 6061 that had a crack in the neck.. But, To my knowledge no other "new" alloy cylinders have had cracking..

But it's still fun to talk about stuff like this.. at least I do....

Jim..

 

[Shotmaster]

I went through all this years ago when the problem developed, they didn't know what caused the cracking and attributed to the lead. I don't think we need to dig all this up. Especially since it only involved like 17 cylinders worldwide.

 

[NAM001]

At the risk of getting a little bit out there and just for fun, let’s calculate the new service pressure of a 3AA-2400 when built to SP13488 specifications.

• First, the wall thicknesses are calculated based on test pressures, not service pressures – so we will first have to determine the test pressure of the 3AA-2400.
  • Test pressure specification for 3AA is equal to 5/3 times the service pressure.
    
  • 2400 psi times 5 divided by 3 equals 4000 psi – Test pressure
• Next, the test pressure can be raised by the ratio of the different allowable alloy strengths in 3AA vs SP13488.
  • 3AA max strength allowable = 70,000 psi
    
  • SP13488 max strength allowable = 90,500 psi
    
  • New test pressure = 4000 psi times 90,500 divided by 70,000 = 5171 psi
• We can now calculate the new service pressure
  • Service pressure specification for SP13488 is 2/3 times the test pressure
  • 5171 psi times 2 divided by 3 equals 3448 psi

3448 psi – Hmmm! Sound like any other number that we use in SCUBA? So is it any wonder that there could be two different specification tanks that could possibly have similar dimensions and weights? Could a manufacturer use the same tooling but with different alloys to make two very different tanks?

A big caution remains – and that is that the two specifications are very, very different – not only in strength of the materials, but also in the factor of safety. You cannot overfill the low pressure cylinder to the high pressure ratings without assuming an unknown risk that could be very hazardous to your health. Do not overfill cylinders.

Safety factor is an omportant aspect. the US uses a 4:1 safety factor and other nations will use a 2.5 or 3 as I have been told. so the same tank with say a 1k rupture point in the us will be stamped as 2500 working pressure and another country will stamp it as 3300 because of the safety factor for that country.