Difference between LP and HP tanks

[Arcaporale]

What is the difference between a LP and a HP steel tank? Please don't give me the obvious answer that the working pressure is higher with HP tanks. I want to know if there is a difference in the actual construction of the tank. What is different about a HP tank that gives it a working pressure of 3500? This stems from another thread about overfilling LP tanks to 3500psi. Someone suggested it's just the burst disk, but this doesn't seem reasonable. Durring my VIP class we looked at the wall thicknesses of theese tanks and they are very similar, if not the same, and I'm pretty sure they are made with the same steel.

 

[Jimmer]

I'm an engineer for a company that makes steam boilers and various pressure vessels for steam applications, and I cannot for the life of me understand why a company would sell the same cylinder as two different pressure ratings. It would just add so much uncessary material to the LP tanks. I suppose it is possible that some European made tanks were actually HP rated, but rated at lower pressures for the North American market. But I would be stunned to learn that PST or Worthington or Faber were currently making tanks designed for 3442 and selling them for use at 2400+.

 

[Arcaporale]

I'm an engineer for a company that makes steam boilers and various pressure vessels for steam applications, and I cannot for the life of me understand why a company would sell the same cylinder as two different pressure ratings. It would just add so much uncessary material to the LP tanks. I suppose it is possible that some European made tanks were actually HP rated, but rated at lower pressures for the North American market. But I would be stunned to learn that PST or Worthington or Faber were currently making tanks designed for 3442 and selling them for use at 2400+.

Well I know that some European tanks are rated to 275 bar or 4000psi, but when the same tank is sold in North America it is rated to 2400+ because we have stricter laws. But then what makes a HP tank legal in the US? Is this some type of elaborate marketing scheme?

The only reasonable answer I can come up with is that LP tanks are designed to hold a certain volume at a lower pressure for people who don't have access to compressors the pump up to 3500, so they make a bigger tank and stamp a lower pressure on it and all of a sudden they got an LP tank.

 

[divad]

How about, there never was a difference, that they just got a variance on the standards by putting a higher rated bust disc in the regular LP tanks? ( I know, conspiracy theory)

 

[Jimmer]

Well European standards may vary, my guess would be that North American law/standards (ie. DOT/TC or ASME) pressure vessel codes require higher factors of safety, which is why a tank rate for 4000 PSI at say a 2.5 FOS, would be rated for 2500 with a 4 FOS. Now this is speculation, but I would expect this is why a European tank would be filled higher over there. Now any North American manufacturer such as Worthington or PST would be designing by N.A. standards to begin with, and a 3442 PSI tank would be designed for 3442, and a 2400+ tank would be designed for 2400+.

 

[Arcaporale]

Also I know that that PST HP tanks were only rated to 3500 because they had an exemption to the law which is why they say EJ..... somthing or another on them. The E stamped on the tank signifies it's exempt status and the number after it is the exemption code. The repair tech at the dive shop I used to work at told me that in a few years this exemption will expire and that PST won't try and renew it because the company no longer makes tanks.

Does anyone know if the new Worthington HP tanks fall under the same category? Perhaps when you buy a HP tank you just paying extra for the exemption, rather than the supposed additional engineering required to make the tank HP rated.

 

[Jimmer]

The exemption question is a good one, maybe Lee from Sea Pearls can answer that for us, if he sees this thread.

Also I know that that PST HP tanks were only rated to 3500 because they had an exemption to the law which is why they say EJ..... somthing or another on them. The E stamped on the tank signifies it's exempt status and the number after it is the exemption code. The repair tech at the dive shop I used to work at told me that in a few years this exemption will expire and that PST won't try and renew it because the company no longer makes tanks.

Does anyone know if the new Worthington HP tanks fall under the same category? Perhaps when you buy a HP tank you just paying extra for the exemption, rather than the supposed additional engineering required to make the tank HP rated.

 

[Arcaporale]

This thread could probably be broken down into about two or three seperate ones, and still not get the info I'm looking for.

 

[Leadking]

The exemption question is a good one, maybe Lee from Sea Pearls can answer that for us, if he sees this thread.

As my name is mentioned, I will reply;

U.S.tensile strength (105,000-125,000 psi for 3AA cylinders) is not the same as European tensile strength (135,000-155,000 psi as required by EN 1964 part 1 and ISO 9809 part 1) and I have test data that shows Faber cylinders delivered to us show a tensile strength of 115,000-123,000 psi for their 3AA cylinders (as required by DOT)

As an example, all of the E cylinders, Exempt or soon to be SP, Special Permit cylinders are made from a harder steel and cannot exceed 159,000 psi yield which exceeds 3AA tensile yield by as much as 50%. As you can see the E/SP cylinders being produced more closely follow the European metallurgy which allows for higher pressures.

view all the posts at
http://www.scubaboard.com/showthread.php?t=119180&page=2&highlight=tensile

 

[Leadking]

Well I know that some European tanks are rated to 275 bar or 4000psi, but when the same tank is sold in North America it is rated to 2400+ because we have stricter laws. But then what makes a HP tank legal in the US? Is this some type of elaborate marketing scheme?

The only reasonable answer I can come up with is that LP tanks are designed to hold a certain volume at a lower pressure for people who don't have access to compressors the pump up to 3500, so they make a bigger tank and stamp a lower pressure on it and all of a sudden they got an LP tank.

A tank sold in the U.S. as a 2400+ cylinder which will be a DOT 3AA cylinder cannot have a tensile yeild greater than 125,000 psi and therefore would fall far short to qualify for a European HP cylinder. The confusion is that there are different steels used in construction. Manufacturers use the identical tooling to manufacure LP and HP cylinders, the difference is in the metallurgy not the dimensions of the cylinder.