Filling LP tanks to high pressure

[Luis H]

Thanks Halocline.



Over filling a tank cuts into the safety factor (see note 1 at the bottom). By definition it is cutting into the margin of safety and increasing risk. That should be easy to understand...

Lower safety factor decreases the cylinder ability to tolerate accidental loads, rust (see note 2), or other defects.

The fact that no incident has occurred only proves that the condition for an incident has not been met yet. You could try to say that the probably of the conditions may be low, based on the previous experience, but that is all you can say.


One of the scariest part of some of the statements above is that some people do not recognize the increase risk. If you don't recognize the increase risk, you are not going to try to mitigate the risk and that will increase the probability of an incident.


Note1: One of the most common definitions for safety factor is the ration of applied stress to the material strength. The lower the stress (relative to the strength) the higher the safety factor and increase safety due to ability to handle extraneous loads.

Note 2: Test at the University of Rhode Island (and personal experience) has shown that (under the right conditions) a new steel tank with a new hydro and perfect VIP can rust to a critical level in less than six months.


BTW: the difference in risk between a 3500 psi fill and a 4000 psi is huge (I am specifically referring to a 2400 psi cylinder with this numbers.)

 

[halocline]

Ughhhh.

Do I need to explain basic hypothesis testing?

No, you don't need to explain anything to me, maybe other than how you got so smart.

 

[Jax]

That wasn't a burst disk it was an entire tank. The explosion was caused by a scuba tank that ruptured.
I've been standing next to a tank when the burst disk popped, it's startling but not destructive. A piece did embed itself in the wall, glad I wasn't in front of it!


This isn't the same tank that wrecked the truck but it probably looked something like this after the explosion.
View attachment 174872

That's an aluminum tank.

 

[AfterDark]

Ughhhh.

Do I need to explain basic hypothesis testing?

In order for you to make your assertion YOU have to have data to Test support your claim empirically.

The data as you admit, DOES NOT support your claim.

This makes it an invalid hypothesis. Therefore:

Practically (meaning in the practical experience of the use of the custom) there is no greater danger.

Scientifically (meaning no data on record or in the applied physics of the manufacture of the structure, in the conditions described) there is no greater danger.

And

statistically (meaning an examination of the historical record of incidents involving such conditions) there is no greater danger.

What I said is factual. There is no "partial" to it.

halocline, this is why I haven't and will not reply. Why bother?

---------- Post added January 13th, 2014 at 08:02 PM ----------

That's an aluminum tank.

You too? So what? Are Al tanks less or more powerful?

---------- Post added January 13th, 2014 at 08:17 PM ----------

Thanks Halocline.



Over filling a tank cuts into the safety factor (see note 1 at the bottom). By definition it is cutting into the margin of safety and increasing risk. That should be easy to understand...

Lower safety factor decreases the cylinder ability to tolerate accidental loads, rust (see note 2), or other defects.

The fact that no incident has occurred only proves that the condition for an incident has not been met yet. You could try to say that the probably of the conditions may be low, based on the previous experience, but that is all you can say.


One of the scariest part of some of the statements above is that some people do not recognize the increase risk. If you don't recognize the increase risk, you are not going to try to mitigate the risk and that will increase the probability of an incident.


Note1: One of the most common definitions for safety factor is the ration of applied stress to the material strength. The lower the stress (relative to the strength) the higher the safety factor and increase safety due to ability to handle extraneous loads.

Note 2: Test at the University of Rhode Island (and personal experience) has shown that (under the right conditions) a new steel tank with a new hydro and perfect VIP can rust to a critical level in less than six months.


BTW: the difference risk between a 3500 psi fill and a 4000 psi is huge.

Common sense doesn't work here Luis where's your facts!

 

[pocky21]

Luis,

I want to thank you for being not only informative, but objective as well.

Something I'm not 100% sure on after reading some technical data - is hydro test pressure equivalent to, or below a cylinder's specified minimum yield stress?

 

[Luis H]

Luis,

I want to thank you for being not only informative, but objective as well.

Something I'm not 100% sure on after reading some technical data - is hydro test pressure equivalent to, or below a cylinder's specified minimum yield stress?

Thanks.

The hydro test pressure is intended to be at the lowest limit of yield strength.

Material properties that you will find published on data sheets actually have some variation. Many times the only thing published is the minimum yield strength and the minimum ultimate strength. I have worked on some designs were we have to measure material properties and/or get material certification sheets for that particular material lot.

The simplified description of yield strength is the stress level where the material starts to transition flow elastic deformation to plastic deformation.

Here is some data from my steel 72's:
Actual average volume inside a steel 72 is about 11,960 cc (730 cu inches, or 0.42 cu ft) measured by water weight.
The elastic expansion is normally around 55 cc and
The permanent residual (plastic) expansion often runs less than 1 cc, but the worst case was 4 cc.
For a few of my cylinders the permanent expansion was 0%. Their expansion was 100% elastic.
So you can see that the hydro test pressure is the very low end of the yield stress level, but it is right at the starting region.


I have measure the average wall thickness of all my steel 72's (using ultra sound equipment). I have also measure actual internal volumes and other data to precisely calculate the REE numbers of each individual tank.

---------- Post added January 13th, 2014 at 10:00 PM ----------

Common sense doesn't work here Luis where's your facts!

Whenever possible, I try to deal with facts... sometimes, when designing a structure we have to make assumptions, since we can't always predict the future. But even the assumptions have to be justified and based on the best factual information we have.

Every time I have designed a pressure vessel or a structure, I have to calculate a safety factor.

Previous history or probability of incidental loads plays only a part into estimating the risk factor. The amount of risk will play a part on the level of safety factor (see note). When calculating risk there are two inputs into a risk matrix: probability of incident, and consequences from an incident. Estimating the probability or the consequences is never a precise science.


I have given some advice on how to mitigate some of the increase risk. I just hope to keep the probability of an incident way down.


Note: depending on the type of design, as engineers we often have to follow codes or specifications. I have worked on a variety of different industries and different applications have to follow different requirements.

 

[lv2dive]

Facts. Kinda like the FACT that in this entire thread nobody has been able to reference one single incident resulting from cave fills on well-maintained steel tanks? 😊

 

[Luis H]

Facts. Kinda like the FACT that in this entire thread nobody has been able to reference one single incident resulting from cave fills on well-maintained steel tanks? 

And if you are all careful I hope it never happens.

This is the only argument that comes up. And all it proves is that no documented case has happened yet.


I can give hundreds of examples of incidents that haven't happen yet, but if it is a possible event, we try to design and plan for it. Even when we try, new incidents occur all the time.

None of my structural designs have had a failure... yet. Those that mean that I can get complacent?


Again, I am not suggesting to stop doing anything... just don't be ignorant of the risks, and be careful.
The risk is a well documented fact.







I don't know the total number of active cave divers, but total population of scuba divers is very small. The total population of active cave divers is very small. When you are in a group it may seem large, but it really isn't.

Does anyone knows an actual number of how many active cave divers are in Florida.


The other part of this statistic is how many cycle a these cylinders have seen. I know some consider 20 or 30years to be a very long time, but it really isn't.

Lets assume 30 years as a large number. Are this cylinders being filled 100 times per year (roughly two times most every weekend). That is total of 3000 cycles. As I said before, that is less cycles than the cylinders in a compressor see in less than a minute. Yes there are many design difference, but this is just to give you some perspective of the number of cycles.

The point is that these cylinders have not seen much of a fatigue life yet. And most of them will never see that many cycles in their entire life.

Note: Rental cylinders in the Caribbean get about 360 fills a year max. Maybe some of you guys dive that much, but I have to work.


I will try to touch on the potential for highway accidents later.

 

[victorzamora]

Luis, as an Engineer I totally get where you're coming from and I love your explanations. When I heard about Hydro testing, I assumed it was checking for plastic deformation....it's good to know I was right.

One thing I'd like to mention, however, is on the topic of safety factors. Some tank manufacturers are producing tanks rated for low pressure now that are identical to older HP tanks. Reportedly, they're identical in material, build, and design. As far as I'm aware, the reason for the change in rating was to allow for the 3/4" neck threads instead of the smaller (weird, right?) 7/8" neck threads. Also, as far as pressure vs safety....I think that a pressurized cylinder, regardless of the pressure rating, gets more dangerous with increased pressure. Obviously, the only way to ensure a tank doesn't explode is to keep it at ambient. The point that I'm making is that, without actually knowing the dimensions (wall thickness, diameter, etc) of the tank and the material of the tank, we don't know what the actual design stress is as rating systems are so arbitrary. As you said, it's easy to calculate....but I just don't have the dimensions needed for it.

As far as cycles go, your numbers make a ton of sense. 30 years of 100 cycles per year, 3000 cycles, that's SUCH a low cycle count. Especially at the rate of the cycles we're discussing, it's not really enough to be cyclic loading. I think tanks will rust and be forgotten about long before cyclic loading becomes an issue.

As for highway accidents, isn't gauge pressure the main concern? Like, not gauge vs rated pressure....but, just gauge pressure in the tank.

 

[flots am]

And if you are all careful I hope it never happens.

This is the only argument that comes up. And all it proves is that no documented case has happened yet.

There's a nuclear reactor in Japan that shaved the safety margins and lost.

The designer for another reactor refused to compromise and saved the country from a second huge disaster.

My life (or the life of the shop's tank fill guy) isn't worth taking a 2640-rated tank and pumping it to 3400+. Nothing would probably happen if I did, but it's a "probably" with huge consequences if I was wrong.

flots.