LP and HP

[TMHeimer]

Okay, this is something I should know, so I asked it here. What is the actual difference between LP and HP tanks? I don't mean like which can be over filled more or which is better for a certain type of diving. I mean, what makes a tank LP or HP? What are the physics involved? Thanks.

 

[Cave Diver]

Low pressure tanks are generally steel and their rated capacity is normally achieved at ~2400 psi.
Many of them have a + stamping meaning they can be overfilled by 10%

For example my LP112's are rated at 2400 psi with a 10% over fill. So the actual working pressure of the tank is 2640 psi and contains 112 cft of gas at that pressure.

HP tanks are usually rated at 3000 psi or more. An AL80 contains approx 80 cft of gas at 3000 psi. Some of the new HP steel tanks have slightly higher working pressure ratings.

There are a lot of factors to take into account when deciding if you want to go with steel or aluminum or HP or LP tanks. Cave divers tend to favor LP steels. A lot of open ocean divers tend to prefer AL tanks. Just depends on what you're wanting.

 

[halocline]

AL80s have 77 cft of gas at 3000 PSI. HP (high pressure) tanks are generally rated at either 3500 PSI or 3442 PSI. A 3000 PSI tank is not generally considered "HP."

The terms HP and LP are usually in reference to steel cylinders. LP (low pressure) tanks typically have the marking 3AA while HP tanks usually have a special permit marking with an E followed by several numbers. (you might already know this)

As far as what the actual difference is in the alloy and design, that's a really good question. I suspect the answer is a combination of engineering and bureaucracy. The 3AA standard has been around for decades and has a great track record of tanks passing hydro for an extended time. I would be very surprised if the relatively low pressure of these tanks did not have something to do with their uses and the type of compressors they're associated with. Scuba applications are a tiny fraction of the 3AA tanks in existence.

 

[pollywogg]

I feel that this could be an interesting and informative thread, but I don't want it to go off-topic.
This is just my take on this post: The OP knows the difference in uses for HP and LP tanks, and service pressures.
I believe the question being asked is about the physical differences, i.e. sidewall thickness, alloy, what physically makes an LP an LP and an HP an HP?

 

[GrumpyOldGuy]

I was corrected on this before, learned the hard way. The TYPE of steel used for LP and HP has different characteristics in terms of tensile strength. They look similar, even share common dimensions, but the fill rating of the HP is greater due the increased strength of the steel used.

Steel tanks in general withstand overfilling better then AL due to material characteristics, but there are still limits, practical and for liability issues.

 

[TMHeimer]

Good answers guys. From what I gather, because steel is stronger, they are mostly HP vs. AL which are mostly LP? Are there exceptions (LP steels & HP ALs)?

 

[Cave Diver]

Good answers guys. From what I gather, because steel is stronger, they are mostly HP vs. AL which are mostly LP? Are there exceptions (LP steels & HP ALs)?

LP and HP tanks are mostly all (to my knowledge) steel. AL tanks are some of the mid range pressures.

LP ~ 2400 psi
Mid ~ 3000 psi
HP ~ 3400 psi

Aluminum tanks have to have thicker walls to handle the pressure and as such, tend to be heavier than a steel tank of equivalent pressure. I have LP steel tanks of an equivalent weight to an AL80 but with a much greater capacity.

I'm not sure of the alloys or dimensional difference between HP and LP tanks. If I recall, Tobin at DSS answered a similar question on another board. I'll see if I can get him to weigh in on the differences. DA Aquamaster likely knows as well.

 

[adm3745]

You could make a HP Aluminum tank, just with much thicker walls. Compare the size of a LP 77 or a HP 100 to an Al 80. About the same internal volume. I'll look up some numbers for you but I recall Al 0's wall thickness right around .5 where LP worthingtons were around .15-.19 .

The HP steel tanks use a higher tensile and less ductile steel in the tanks. I believe the safety margin is a little lower on these tanks as well. People have said they do not lend themselves well to overfilling like Aluminum.

 

[adm3745]

What are the physics involved? Thanks.

Very basically the tanks are designed so at the fill pressure, the stresses at the wall do not exceed a certain value or percentage of the yield strength of the material being used. They also test the material at the specified stresses over and over again to determine its cyclic fatigue. The higher you fill a tank, the more stresses at the wall, and the less amount of cycles you can fill that tank. Kind of like bending a paper clip alot). It has been said that LP steel tanks are designed to withstand 10000 cycles at Hydro pressure (4000 psi). You can look up the CFR49 codes from D.O.T to see the design criteria. They will say that Hydro pressure is a certain percentage of the fill pressure. Hydro pressure is supposed to be right around the yiled strength of the material. If you exceed the yield strength you get permanant deformation.

If you want to learn how they do it yourself, start by looking up hoop stress. You can also look up the yield strength for the molybdenum steel that worthington uses and find the wall thickness online.

http://www.epi-eng.com/images/MechBasics/BAS-Fatigue1.gif

 

[SkimFisher]

http://www.scubaboard.com/forums/4984103-post4.html