User avatar
DaveMann
Lung Diver
Posts: 61
Joined: Mon Aug 15, 2016 11:12 am
First Name: Dave
Location: Fort Myers, Fla., USA

Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 4:44 pm

I've searched for the answer to this question here and on google and just can't find what I'm looking for. I'm probably not using the right search terms.

Let's assume an empty aluminum 80 that can be filled to 3000 psi, and a full steel 72 with 2250 psi. If you were able to magically transport the air from the 72 into the empty 80, what pressure would the 80 show?

Or in other words what percentage of an aluminum 80 @ 3000 psi is a steel 72 @2250?


Dave

jconley
Skin Diver
Posts: 25
Joined: Fri Jun 17, 2016 1:24 am
First Name: Jeff
Location: Apex NC

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 5:30 pm

Dave,
First what people refer to as a 72cuft steel cylinder is really 71.2cuft at 2475psi. The 71.2 was 71.2cuft at a 10% over fill. Which was only during the first five years. After the first hydro the cylinder is only suppose to be filled to 2250psi.

So take 71.2 / 2475 = approximately 0.0288 cuft / psi. Then take 0.0288 x 2250 = approximately 64.8 cuft. So a 72 steel is really a 64.8cuft. cylinder (with some rounding error). Now take a 80cuft cylinder. 80/3000 = approximately 0.0267 cuft / psi.

This shows why I contend that if you are calculating air consumption it should be done in cuft/min. This number can be used across different cylinder sizes. Where psi/min is only valid for the same size cylinder.


Back to the problem.
64.8/80 = .81 So a 71.2 steel filled to 2250 is 81% of the volume of a 80cuft cylinder. Now we take 0.81 x 3000psi = 2430psi

:D

rx7diver
Lung Diver
Posts: 61
Joined: Fri Aug 28, 2015 11:22 am
First Name: Ronald
Location: Columbia MO

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 7:11 pm

jconley wrote:Back to the problem.
64.8/80 = .81 So a 71.2 steel filled to 2250 is 81% of the volume of a 80cuft cylinder. Now we take 0.81 x 3000psi = 2430psi

:D
An Al 80 nominally holds 77.4 cu ft at its working pressure of 3,000 psig.

EDIT. To find the pressure the Al 80 must be at to hold 71.2 cu ft of air, simply solve (3,000 / 77.4) = (x / 71.2) for x. Your answer will be in psig.

Now, to find the amount of air in a 72 that's "filled" to only 2,250 psig, solve (71.2 / 2,475) = (y / 2,250) for y. Your answer will be in cu ft.

So, if you want to know the pressure the Al 80 must be at to hold y amount of air, simply substitute this number (y) for 71.2 in the first equation, and solve for x. You'l get 2,509 psig. (Note: keep all your numbers in your calculator until the very end--that is, don't write down the intermediate calculations--and then "round" when you write the final answer.)

Safe Diving,

rx7diver

User avatar
DaveMann
Lung Diver
Posts: 61
Joined: Mon Aug 15, 2016 11:12 am
First Name: Dave
Location: Fort Myers, Fla., USA

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 8:36 pm

Thank you both. That is exactly what I was looking for.


Dave

jconley
Skin Diver
Posts: 25
Joined: Fri Jun 17, 2016 1:24 am
First Name: Jeff
Location: Apex NC

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 9:53 pm

RX7Diver,
Thanks for the info that an 80cuft tank only holds 77.4cuft.

User avatar
luis
VDH Moderator
Posts: 1747
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 10:00 pm

jconley wrote:Dave,
First what people refer to as a 72cuft steel cylinder is really 71.2cuft at 2475psi. The 71.2 was 71.2cuft at a 10% over fill. Which was only during the first five years. After the first hydro the cylinder is only suppose to be filled to 2250psi.

:D
You can get the 10% over fill on any DOT 3AA steel cylinder that received the plus (+) stamp on the most recent hydro test. It doesn't matter if some "+" stamps were skipped.

All my steel 72 are "+" stamped. My steel 72 all get filled to 2500 psi (or a bit more).

Take a look at this thread:
viewtopic.php?f=12&t=4737

BTW, I have accurately measure the volume of all my steel 72, (quantity 18 cylinders) using the weight of the water capacity, and the volume varies from 70.1 cu ft to 72.7 cu ft. None of them are actually 71.2 cu ft. The average of all 18 cylinders is about 70.9. About half of my cylinders were made by PST and the other half are Norris. Considering the variation, I normally use 71 cu ft as the rough average.
Luis

Buceador con escafandra autónoma clásica.

User avatar
georgeaustin
Master Diver
Posts: 274
Joined: Mon Nov 04, 2013 8:41 am
First Name: george
Location: Los Angeles CA

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 10:14 pm

How does continuous and regular overfilling of steel tanks affect material from a structural POV? I've got a set of LP 38's that failed hydro for over expansion and I suspect cause is likely related to thousands of overfills through the course of many decades

User avatar
luis
VDH Moderator
Posts: 1747
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 10:44 pm

georgeaustin wrote:How does continuous and regular overfilling of steel tanks affect material from a structural POV? I've got a set of LP 38's that failed hydro for over expansion and I suspect cause is likely related to thousands of overfills through the course of many decades
From another recent post of mine:

Even rental tanks that are filled daily (or even a few times a day) don't experience enough cycles to make a dent into fatigue life curves. The fatigue life curves are normally plotted on a logarithmic scale.

Fatigue failure is a big issue with the number of cycles experienced in rotating machinery or a vibrating object, not so much for a recreational scuba cylinder.

Also most fatigue life curves are based on reversing stresses. In other words stresses that cycle in both direction (positive and negative cycles), like bending a beam back a forth. A pressure vessel only sees positive tensile stresses, which is less of a fatigue issue.


But to more directly answer your question about over fills.

A lot will depend on the level of overfill. If it did not exceed the hydro test pressure, then it should not had exceed its elastic limit and a relative few cycles should not be harmful.

If the cylinder was galvanized, I bet they did not follow the pre-test expansion procedure.

Then some steel cylinders do occasionally fail due to a variety of other material issues or wall thickness imperfections,
Luis

Buceador con escafandra autónoma clásica.

rx7diver
Lung Diver
Posts: 61
Joined: Fri Aug 28, 2015 11:22 am
First Name: Ronald
Location: Columbia MO

Re: Tank Volume and Pressure Equivalencies

Tue Aug 23, 2016 11:42 pm

georgeaustin wrote:How does continuous and regular overfilling of steel tanks affect material from a structural POV? I've got a set of LP 38's that failed hydro for over expansion and I suspect cause is likely related to thousands of overfills through the course of many decades
I was taught a long time ago that if a scuba cylinder were stored at maximum pressure (i.e., at its working pressure) for long periods of time, then it is more likely to fail its hydrostatic test. (Luis, can you speak to this?) georgeaustin, any chance your LP 38's have a history of being routinely stored at maximum pressure for long periods of time?

Safe Diving,

rx7diver

User avatar
georgeaustin
Master Diver
Posts: 274
Joined: Mon Nov 04, 2013 8:41 am
First Name: george
Location: Los Angeles CA

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 6:38 am

rx7diver wrote:
georgeaustin wrote:How does continuous and regular overfilling of steel tanks affect material from a structural POV? I've got a set of LP 38's that failed hydro for over expansion and I suspect cause is likely related to thousands of overfills through the course of many decades
I was taught a long time ago that if a scuba cylinder were stored at maximum pressure (i.e., at its working pressure) for long periods of time, then it is more likely to fail its hydrostatic test. (Luis, can you speak to this?) georgeaustin, any chance your LP 38's have a history of being routinely stored at maximum pressure for long periods of time?

Safe Diving,

rx7diver
Probably - hard to really know for sure as written documentation is non existent

User avatar
captain
Plank Owner
Posts: 1440
Joined: Mon Jul 04, 2005 11:32 am
Location: LaPlace, LA

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 9:20 am

This thinking that a tank goes bad if full for a long period of time reflects the myth that springs in firearm magazines go bad if the magazine is loaded fora long period of time. It doesn't happen.
Captain

User avatar
luis
VDH Moderator
Posts: 1747
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 7:04 pm

rx7diver wrote:
I was taught a long time ago that if a scuba cylinder were stored at maximum pressure (i.e., at its working pressure) for long periods of time, then it is more likely to fail its hydrostatic test.

Safe Diving,

rx7diver
You may be thinking of “sustain load cracking” on the older aluminum alloy cylinders (AL- 6351). That is kind of a unique phenomena that occurs to the old aluminum alloy cylinders that are left pressurized for extended periods of time.

Steel structures should not experience degradation for any constant stress, as long as the stress is not close to the material yield strength and corrosion doesn't degrade the structure. Extreme heat can also degrade the material strength.

Many structures are design to operate at around 60% of its yield strength (if fatigue cyclic loading is not a big concern). The 60% of yield is normally considered enough of a safety margin when the expected loads are well understood. The safety margin is supposed to account for unexpected loads (like handling or other accidental loads) and for some geometry imperfections.

The two things that normally kill a steel cylinder is rust or excessive heat (like the heat from a fire). Exposure to heat can ruin the heat treatment on the alloy, but in the case of steel cylinders we are talking temperature somewhere above 800 degrees F (I am not a metallurgist and my memory is not the most reliable so don’t rely on that number).

A tank that was exposed to a fire (even just to the radiant heat of a fire) needs to be hydro tested before it is used again, even if it has a current hydro stamp.

BTW, aluminum cylinders will start losing their heat treatment (start annealing) at very low temperatures, as low as 300 to 350 degrees F. That is why aluminum cylinders cannot be painted with any process that involves any kind of heat.

Some dive shops will not fill any aluminum cylinder that has a paint job that doesn’t look like it came from the factory, like custom paint jobs, etc

I remember two accidents in Florida were two freshly painted cylinders blew up in the same day (I think it was in the 70’s). The owner of the cylinders used a baked on paint or some hot flame sprayed paint (I can’t remember the details). The first one blew up without killing anyone. He just got pissed and went to a second dive shop to fill his second tank. The second ne was not so lucky for the shop owner. At least, that is what I remember.
Luis

Buceador con escafandra autónoma clásica.

rx7diver
Lung Diver
Posts: 61
Joined: Fri Aug 28, 2015 11:22 am
First Name: Ronald
Location: Columbia MO

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 8:48 pm

luis wrote:
rx7diver wrote:I was taught a long time ago that if a scuba cylinder were stored at maximum pressure (i.e., at its working pressure) for long periods of time, then it is more likely to fail its hydrostatic test. (Luis, can you speak to this?)
You may be thinking of “sustain load cracking” on the older aluminum alloy cylinders (AL- 6351). That is kind of a unique phenomena that occurs to the old aluminum alloy cylinders that are left pressurized for extended periods of time.

Steel structures should not experience degradation for any constant stress, as long as the stress is not close to the material yield strength and corrosion doesn't degrade the structure. Extreme heat can also degrade the material strength...
Thanks, Luis, for the detailed explanation.

Now I want back all of the $$ I dutifully dumped, over the years, at the end of each diving season--especially when I was diving manifolded HP 120's! Actually, I probably will continue to drain my tanks down to ~300 psig at the end of each diving season. I dive only small singles now. And it's only money, right??!!

Safe Diving,

rx7diver

User avatar
luis
VDH Moderator
Posts: 1747
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 10:11 pm

I always leave my tanks full. I would never waste good compressed air.

I don’t fill my cylinders right away after a dive because I use the residual air for regulator work, but I have enough steel 72 that I can rotate them and always have at least 1/2 dozen cylinders full of air.


One important point is that if your tank is full, you should not store your cylinders in the winter in a cold garage or outdoors.

The compressed breathing air is supposed to be very dry, with a dew point of about -65 degrees F, but that dew point is for the air at ambient pressure. When the air is compressed the dew point goes up to about +10F to +15F depending on the fill pressure. My garage normally doesn’t get that cold in the winter, but it gets close.

If the cylinder gets that cold you can get condensation inside.
Last edited by luis on Thu Aug 25, 2016 12:24 pm, edited 1 time in total.
Reason: corrected the wrong word. It was late last night.
Luis

Buceador con escafandra autónoma clásica.

rx7diver
Lung Diver
Posts: 61
Joined: Fri Aug 28, 2015 11:22 am
First Name: Ronald
Location: Columbia MO

Re: Tank Volume and Pressure Equivalencies

Wed Aug 24, 2016 10:56 pm

luis wrote:One important point is that if your tank is full, you should not store your cylinders in the winter in a cold garage or outdoors.

The compressed breathing air is supposed to be very dry, with a dew point of about -65 degrees F, but that dew point is for the air at ambient temperature. When the air is compressed the dew point goes up to about +10F to +15F depending on the fill pressure. My garage normally doesn’t get that cold in the winter, but it gets close.

If the cylinder gets that cold you can get condensation inside.
Thank you for this. I'm embarrassed to admit I've never considered this. Now that I no longer have a basement, I store my tanks in my garage. So, this is extremely germane.

Return to “Tanks and Valves”

Who is online

Users browsing this forum: No registered users and 32 guests