Hp tank & special permits

[Luis H]

It is also possible that they could also go to 3/2 of working pressure, similar to other cylinders.

For a 2640 psi x 3/2 = 3960 psi

For a 2475 psi x 3/2 = 3713 psi
For 2250 psi x 5/3 = 3750 psi

Both of this are close o the original hydro and with a little rounding up it would be the same.

I don't think the hydro test should be performed at a higher pressure. I think we would see too much permanent deformation; the stress will be beyond some of its yield strength.

 

[crpntr133]

Nope.
5/3rds 2400psi = 4000psi test
5/3rds 2640psi = 4400psi test

That's what I meant. I would think they would hydro it to the + rating pressure but there wouldn't be a plus rating just a raised normal working pressure. At either rate were all on the same page.
Can you imagine all the hassle that this will be with a fill monkey that doesn't know his butt from a hole in the ground? It can be hard enough to get a proper fill the way it is. Thank God that I have my own compressor.

 

[WD8CDH]

It is also possible that they could also go to 3/2 of working pressure, similar to other cylinders.

For a 2640 psi x 3/2 = 3960 psi

For a 2475 psi x 3/2 = 3713 psi
For 2250 psi x 5/3 = 3750 psi

Both of this are close o the original hydro and with a little rounding up it would be the same.

I don't think the hydro test should be performed at a higher pressure. I think we would see too much permanent deformation; the stress will be beyond some of its yield strength.

As you have shown, Luis, 5/3 of the working pressure is within 1% of 3/2 of the + pressure. That is why the SP tanks are hydro'd at 3/2 of the working pressure, basically the +10% overfill is built in.

 

[WD8CDH]

I recenctly purchased a HP80 tank and realized that this is a special permit tank. Does anyone know if this is an exception for an HP tank or are all HP tanks under a special permit?
Thanks for the help!

Not all HP tanks are Special Permit (but most are) and not all Special Permits are HP (but many are).

The 3AA standard specifies a specific strength of steel and the maximum stress that it could be under at working pressure. You could make a HP tank under the 3AA permit but the walls would be thicker than either a 3AA LP tank or a SP HP tank.

A Special Permit defines a specific material strength, stress etc. for a tank or series of tanks for a particular manufacturer. Typically for SCUBA, a Special Permit defines use of a steel that is stronger than the steel used in a 3AA tank. A DOT permit, Special or otherwise, defines the material strength, the percentage of maximum strength that the tank is operating under at working pressure, etc.. Since a Special Permit for a tank usually defines a stronger steel, the tank can either have thinner walls for the same pressure or a higher working pressure for a similar wall thickness with the same safety factor as a 3AA tank at it's 10% overfill.

There is some talk about DOT defining a new regular permit that is similar to the many Special Permits that are based on a higher grade of steel than what is defined in the 3AA permit. Logically, it would be a 3AAA permit following the progression of 3A and then 3AA permits, but it would be a federal Standard, so who knows what they will call it.

By the way, there are some new manufacture Special Permit Aluminum tanks that are made with stronger alloy than the ones used for current 3AL tanks. A similar Special Permit could allow for aluminum SCUBA tanks with a working pressure of 4500 psi and the buoyancy characteristics of an HP steel tank.

 

[DA Aquamaster]

By the way, there are some new manufacture Special Permit Aluminum tanks that are made with stronger alloy than the ones used for current 3AL tanks. A similar Special Permit could allow for aluminum SCUBA tanks with a working pressure of 4500 psi and the buoyancy characteristics of an HP steel tank.

That would be interesting. Getting shops to fill tanks to 4500 psi would be a pain, and given the SLC issues with 6351-T6 alloy, I suspect both shop and consumer confidence of AL tanks would be low.

So I am not sure I see a scuba market for them if all they really offer is a tank with the buoyancy traits of a steel HP tank and a lot of problems getting a full fill. Maybe it could have SCBA or aviation applications - but the tendency for fire drpartments to slam fill tanks may not bode well for it and getting FAA certification for anything new is a bit of a pain.

 

[WD8CDH]

That would be interesting. Getting shops to fill tanks to 4500 psi would be a pain, and given the SLC issues with 6351-T6 alloy, I suspect both shop and consumer confidence of AL tanks would be low.

So I am not sure I see a scuba market for them if all they really offer is a tank with the buoyancy traits of a steel HP tank and a lot of problems getting a full fill. Maybe it could have SCBA or aviation applications - but the tendency for fire drpartments to slam fill tanks may not bode well for it and getting FAA certification for anything new is a bit of a pain.

More likely, they would start with aluminum tanks made with this stronger alloy that were neutral when empty and have a fill pressure of 3500psi.

I don't see 4500psi as a show stopper. With the number of fire departments using 4500psi, the compressor market for 4500-5000psi is quite mature. At least every place that I have checked, more than 75% of the SCUBA shops could already fill to 4500psi and did a lot of business with the paintball market at that pressure.

I have been diving with these stronger aluminum tanks for a number of years and really like them. Currently, they are only available in a few sizes at 3000psi but they have the wall thickness of a 2200 psi 3AL aluminum tank. I use them on my rebreathers and on my quad 20 and my triple 35 OC SCUBA rigs. While they do require a little extra lead, the total weight of the tanks plus the additional lead is still less than conventional tanks. If they had thicker walls and a higher service pressure, the total weigh/size would be even more advantageous.

 

[D_B]

Luxfer had that carbon fiber wrapped 4300psi tank, but it was big enough that it had way too much positive buoyancy

I wonder what PSI does a HP80 need to hold 130cf ?

 

[DA Aquamaster]

You'd need 5600 psi to get 130 cu ft in an HP 80

4500 psi would get you 104 cu ft

 

[D_B]

You'd need 5600 psi to get 130 cu ft in an HP 80

4500 psi would get you 104 cu ft

Cool! 4500psi it is
... now just where did I put my stamps?

 

[Leadking]

I did not know this. The 3442s are special permits? Learn something new everyday.

Are the special permits separate for Canada?

YES look for TC XXXXX Transport Canada approval number. One manufacturer introduced tanks that were not TC approved. If using or transporting to Canada make sure they are approved.