hp vs lp

[zimmo]

There's a problem with your numbers. You can't have 2 steel tanks that have the same internal volume and diameter, and have masses ~20% different. Steel alloys don't have densities that different. See:

Density of Steel

The HP tank would have to be significantly taller.

 

[Slamfire]

They're not my numbers. I just copied and pasted them from Faber's site :Faber Industrie.

If internal volume and outside diameter are the same, wall thickness should be the same. The only other places left for additional mass would be the shoulders and the bottom.

 

[zimmo]

If internal volume and outside diameter are fixed, wall thickness can change. EG: double the wall thickness (which reduces ID), but increase height of tank in order maintain the same internal volume.

I'm trying to get the more detailed specs from faber, but I'm waiting on their registration website (looks like it's from 1995).

 

[Leadking]

It is these STUPID posts I have been replying to for years. The wall thickness on a lp109 and hp130(3442) are virually identical because they are made from the SAME TOOLING. The walls of the 130 are not more robust. They are made from a different steels. The HP130 uses a higher tensile strength steel.

Sorry if I come off too irate, but posters who do not inderstand cylinder contruction confuse the diving public and remove the safety factor built into the product.

For arguments' sake a LP109 is roughly the same dimension as a HP130. I believe that the HP130 weighs more empty because te walls of the tank are more robust. I've heard of folks replacing (doubling-up) the burst discs on the LP109 and cramming 4000+psi into it - at which point it carries more air than the HP130. Probably not applicable unless you're expedition diving, and certainly not the safest method - but it's one reason I've heard folks picking LP tanks.

I personally own 8 HP tanks because we do lots of shore diving and the smaller format and lighter weight are beneficial (also diving in cold water with a drysuit the negative properties get lead off the waist).

 

[OldNSalty]

It is these STUPID posts I have been replying to for years. The wall thickness on a lp109 and hp130(3442) are virually identical because they are made from the SAME TOOLING. The walls of the 130 are not more robust. They are made from a different steels. The HP130 uses a higher tensile strength steel.

Sorry if I come off too irate, but posters who do not inderstand cylinder contruction confuse the diving public and remove the safety factor built into the product.

Educate me. If people over-fill these lp tanks made from steel of a lesser tensile strength...sometimes as much as 50%...how much more could the HP tanks hold (as a percentage)?

Or does it not work like that?

Disclaimer-I have no intent on trying to overfill an hp...just curious.

 

[Leadking]

Educate me. If people over-fill these lp tanks made from steel of a lesser tensile strength...sometimes as much as 50%...how much more could the HP tanks hold (as a percentage)?

Or does it not work like that?
Disclaimer-I have no intent on trying to overfill an hp...just curious.

The rule of thumb is to keep filling until it blows and then back off a little!

 

[cbrich]

I too have posted on this subject many times also. Just look at the Faber specs (I am partial to Faber over Worthys for several reasons, which I will not get into), and do your own comparison, it's not that hard. I own both, and I do not have preference.

When comparing these factors, talk to the Dive Shop that will do the filling. They will have rules about filling. My LDS will not fill a LP cylinder beyond 2640 psig (it has to have the + rating), and will not fill a HP cylinder beyond 3,450 psig. If you have a out-of-city VIP sticker, they question very hard; If you have a generic VIP sticker, you will be told to go down the street. The first thing you should do is contact your LDS.

However if you have your own compressor, then why are you asking, buy whatever you want, and fill it to whatever you want. The mythological scuba police might come after you, when you blow up, but that is a very big MIGHT.

 

[cbrich]

Educate me. If people over-fill these lp tanks made from steel of a lesser tensile strength...sometimes as much as 50%...how much more could the HP tanks hold (as a percentage)?

Or does it not work like that?

Disclaimer-I have no intent on trying to overfill an hp...just curious.

This question can only be made by a Materials Engineer after he has some destructive testing on that particular cylinder. Or you get a haskel that is rated to 25,000 psig, and do your own destructive testing. So in short, nobody will give you a straight answer.

 

[Slamfire]

Going back to the question of whether 3AA cert has a larger safety margin than an exempt cert. I'm not a materials engineer but I think I found the 3AA spec here: 178.37 Specification 3AA and 3AAX seamless steel cylinders..

Paragraph f2 says:

(2) For cylinders with service pressure of 900 psig or more the minimum wall must be such that the wall stress at the minimum specified test pressure may not exceed 67 percent of the minimum tensile strength of the steel as determined from the physical tests required in paragraphs (k) and (l) of this section and must be not over 70,000 psi.

This is in regards to the Faber cylinder example I posted earlier. Will the exempt cert still require that the wall stress not exceed 67% of the tensile stress? If the 67% transforms to a higher number on the exempt HP spec, then you are indeed using a less conservative standard, aren't you?

Irrespective of standards or certs I guess it boils down to the non-negotiable laws of physics. Is the tensile strenght of HP steel relative to any given fill pressure higher or lower than the tensile strenght of an LP cylinder relative to your fill pressure of choice. It would be nice to have the tensile strenght figures for HP and LP cylinders...

 

[Leadking]

Irrespective of standards or certs I guess it boils down to the non-negotiable laws of physics. Is the tensile strenght of HP steel relative to any given fill pressure higher or lower than the tensile strenght of an LP cylinder relative to your fill pressure of choice. It would be nice to have the tensile strenght figures for HP and LP cylinders...

Your answer is in the DOT permit filing SP14157

(5) Hardness Measurement. The tensile strength equivalent of the hardness number obtained may not be more than 159,000 psi; (HRC 34(Brinell 334)). When the results of a hardness test exceed the maximum permitted, two or more retests may be made: however, the hardness number obtained in each retest may not exceed the maximum permitted.

So is 159,000 a bigger number than 70,000?