Plus ratings and hydros'
- Thread starter phillybob
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How about some steak?
Actually, your method could be used to prove the cylinder will handle more pressure. If the DOT would allow the technician to test the cylinder at the higher pressure and use your actual EE is less than the REE life would be much simpler and users would be happier. I am all for that. See my signature line.
Scared Silly
Contributor
OT - Today I picked up two of my old LP72. Had a nice chat with hydro testers as one of my cylinders failed. They did a pretest at 80% of the 5/3 and got a 21% permanent expansion. Got the same thing when they did the full 5/3. The other cylinder had a permanent expansion of 7%. It passed but they wanted me to know. Of course being old PST they said they not really surprised. However, I was especially surprised because the horror stories I have heard have been with their HP cylinders. The other this was the first hydro these cylinders had as they had been unused for the past 30 years.
SparticleBrane
Contributor
Wow that is crazy for LP72s to fail. Perhaps they were overfilled a lot during their previous lives?
captain
Contributor
Sounds a bit suspicious but without knowing the history fo the tanks you can't be certain. What method did they use and was it a scuba shop or commercil cylinder tester.
I have had 8 72's ranging in age from 52 to 35 years old, a mix of Norris and PST hydroed in the last two years and they all passed with less than 3.7% expansion.
Scared Silly
Contributor
It was a commercial cylinder tester. I have used them for many cylinders in the past. About the only thing I could think of was they hydroed them to the wrong pressure. Both cylinders were from the same batch and though 35 years this was their first post mfg hydro test. So the cylinders sat in storage for the past for the past 30 years. So anything could have happened while in storage.
Tomorrow I am going to go back ask for the hydro test data. I would have asked for it when I picked them up but I was in a hurry for an appointment.
Tomorrow I am going to go back ask for the hydro test data. I would have asked for it when I picked them up but I was in a hurry for an appointment.
captain
Contributor
Other than stored with water in them which would probably have failed them with just a visual the only other cause would be for them to have been exposed to fire.
The test pressure should be on the record but that doesn't necessarly mean they didn't go above it during the test.
Two of mine had 3800 on the record as the test pressure rather than 3750 and they passed. They passed so I didn't question them about it.
Scared Silly
Contributor
That was my first worry, so when I got the cylinders I did a visual inspection (first I had to put on my fancy hat that I got from PSI) The insides were spotless. I figured the hydro was a no brainer after that.
This morning I got the report. They did several qual tests before my cylinders and many cylinders after mine. So no issues with equipment.
The pressures were 3023 psi and 3768 psi for the pre test and test pressures, respectively. So well within the norms of 3000 psi and 3750 psi, respectively. The other cylinder which passed had test pressures within 3 psi but had only 3.4% permanent expansion (the one that failed was 21.7%).
What is bafflingly is that one passed the other failed. These two cylinders were doubled from birth and their serial numbers differ by 161. So for all practical purposes what one saw the other saw.
If exposed to fire I would have expected the plastic back pack to be melted. These cylinders looked they had been dove for a few years, basically emptied, and put away for 30 years.
Rather odd. Oh well. Such as life ...
This morning I got the report. They did several qual tests before my cylinders and many cylinders after mine. So no issues with equipment.
The pressures were 3023 psi and 3768 psi for the pre test and test pressures, respectively. So well within the norms of 3000 psi and 3750 psi, respectively. The other cylinder which passed had test pressures within 3 psi but had only 3.4% permanent expansion (the one that failed was 21.7%).
What is bafflingly is that one passed the other failed. These two cylinders were doubled from birth and their serial numbers differ by 161. So for all practical purposes what one saw the other saw.
If exposed to fire I would have expected the plastic back pack to be melted. These cylinders looked they had been dove for a few years, basically emptied, and put away for 30 years.
Rather odd. Oh well. Such as life ...
ctcdg
Registered
Greetings to all,
Please allow me to introduce myself. I am Darrell Garton of CTC Seminars. I wrote the comments to 49 CFR that were referenced by Captain in post #18 on March 22. I have worked on various drafts of the CFR, CGA pamphlets, and with Bill High of PSI (and now the new owner, Mark Gresham). I have been teaching cylinder requalification for over 20 years, and CTC is the only training approved for both Transport Canada and US DOT.
I generally don't get involved in these types of discussions, but I was asked to comment by one of your contributors who saw my article referenced.
The comments in this thread range from partially correct to dangerously wrong. I cannot possibly address everything stated here, but I would like to correct some wrong thinking. For those of you interested enough to plod through this, you will find the areas in which you may have been mistaken. (Don't think you're exempt! I found errors in virtually every post except the one that said, "Nothing is foolproof to a sufficiently talented fool") Just because I correct one of your errors here, please don't think I'm singling you out for abuse. In some cases the reason I am correcting someone's error is because I see that you have the technical understanding, and simply got one small point wrong. The errors that go off the chart, I'm not even bothering to address.
First let me describe fatigue failure: BANG!!!
It is likely to be the last sound you ever hear. It doesn't give much warning, and may not even be detected by the hydrostatic test. The hydrostatic test is looking for yield, not fatigue. Imagine a paperclip; if you put enough force against it, you will bend it - that's called yield. The hydrostatic test is looking to see if the cylinder yields at test pressure. Now imagine bending a paperclip back and forth multiple times; eventually it will snap due to fatigue. Overfilling cylinders to the outrageous pressures described by some posters is likely to cause fatigue.
Now let's talk about overfilling cylinders. The plus rating is based on technical analysis from 1942, during WWII. Because of the conservative design of our 3A cylinder specification (3AA didn't come along until 1948 but is basically the same design, using high strength allow instead of plain carbon steel), it was found that a 10% increase in fill pressure was justified and technically sound, so long as there has not been significant reduction in wall thickness. The elastic expansion can be used as a means to determine average wall stress, as expressed by the Bach and Clavarino formulas that have been discussed here. The Bach formula establishes the limiting wall stress based on wall thickness and test pressure, while the Clavarino formula expresses the inverse relationship of wall thickness to elastic expansion, whereby an REE value can be determined. The REE values that are established for the increased filling limits of 173.302a(b) are based on roughly a 5% increase in wall stress from that of the manufacturing limits of section 178.36 and 178.37 (3A and 3AA cylinder specifications). The use of elastic expansion to determine wall stress is a means of determining the average wall stress, not the maximum. The maximum wall stress can only be determined by an actual wall thickness measurement, such as by using an ultrasonic wall thickness gauge. The regulations do not say you must calculate both the average "and" the maximum, they say "either/or", "Either the average wall stress or the maximum wall stress does not exceed the wall stress limitation shown in the following table:" [173.302a(b)(3)]. This section goes on to say in [173.302a(b)(3)(iii)], "Compliance with average wall stress limitation may be determined by computing the elastic expansion rejection limit in accordance with CGA C-5, by reference to data tabulated in CGA C-5, or by the manufacturer's marked elastic expansion rejection limit (REE) on the cylinder." The regs say "cannot exceed", they do not say "must be less than". So, when you put it all together, one of the primary criteria for putting the plus mark on the cylinder it that the elastic expansion does not exceed the REE. It's all about the wall thickness, and if the elastic expansion exceeds the REE, it means we have lost wall thickness.
It is ONLY the 3A, 3AX, 3AA, 3AAX, and 3T specifications that are permitted the plus mark. Marking of a special permit or exemption cylinder with a plus is not permitted, unless specifically allowed in the applicable special permit.
In 1993 Transport Canada adopted a new metric specification that incorporates the higher fill pressure of the plus into the specified service pressure (thereby creating a 1.5x test pressure, instead of 5/3). Should one of these new (post 1993) TC metric cylinders exceed its REE, it must be stamped with a "K" following the service pressure marking (see B339 24.6.3.4), and the fill pressure is then reduced by 10% (see B340 5.2.3.1).
This isn't just some arbitrary "I'm comfortable with it" mumbo-jumbo. Asking your fill operator to fill a cylinder over it's rated pressure is asking him to risk his life for your desire to have a little more air. And don't think if you fill it yourself, you're only risking your own life. What about everyone else on the boat, and the rescue team that has to come looking for you??
Our 3A and 3AA cylinder designs are based on many factors, among which cycle life, yield point, and burst pressure data pertain to this discussion. (I'm not going to list those limits, because some !@#$ will improperly interpret them and think he can just do it until he needs glasses. Those of you with the technical know-how can go look it up.) At their rated service pressure, these cylinders have an unlimited service life, meaning they can be cycled indefinitely, because they are not experiencing yield or fatigue. However, at test pressure there is a limit to the number of cycles a cylinder can withstand, and at their burst pressure, there is ONLY ONE cycle! If you overpressurize cylinders enough, you are eating into that cycle life, and just like bending the paperclip back and forth, someday it will fail.
While it is true that cylinder designs are based on test pressure, one cannot simply say if it passes a higher test pressure, it's OK to use at a higher service pressure. (Although, the poster who said he could see doing it by using EE numbers with the Bach and Clavarino formulas to evaluate wall stress was correct. But reverse engineering a container that happens to be built stronger than the design criteria isn't going to gain anyone's approval for remarking it to a higher service pressure!). 3A and 3AA designs can withstand pressures well over their minimum specified test pressure before experiencing a failure. However, that's not to say that the cylinder won't be damaged if continued cycling occurs. Also, testing a cylinder at a higher test pressure INVALIDATES a subsequent test at a lower test pressure. The poster who said he would be willing to test the 3AA 2250 at 4000, but would then have to test it according to its proper DOT criteria doesn't understand permanent expansion. Once stretched to 4000psi, there will be no permanent expansion at 3750psi. And to answer the question as to whether 4000psi would damage the cylinder; repeat testing in the event of an equipment malfunction does occur, and it is performed at an increase of 10% or 100psi, whichever is less. CGA C-1 tells us that we should never go over 110% of the test pressure when performing a repeated test. So, for your 3AA2250 with a 3750psi minimum specified test pressure, you should be able to test that cylinder to 4125psi and still pass the test. HOWEVER, that does NOT mean that you could justify a higher fill pressure.
The guys who have said overfilling is OK for short periods as well as the ones who said that they have left them at pressure for extended periods and the cylinder is OK don't understand cycle life. The guys filling the cylinder to its test pressure don't understand that there is limit to how many times a cylinder can go to test pressure. The guy who was filling to 6500psi shouldn't have been allowed to leave his padded cell. That is over the design minimum burst pressure. The guys filling to 3600 and think its OK might want to buy some earplugs. You are approaching the pressure at which cycle fatigue occurs, and one day you might experience that BANG! I mentioned earlier.
If you have trudged your way through this diatribe thus far, you might be interested to know that the reason Direct Expansion testing is not permitted for marking cylinders with the plus is due to the error induced by the calculations for the compressibility of water. Errors in that calculation will directly effect the elastic expansion, and that is what the entire wall stress analysis is based on. So, in the U.S., the water jacket test must be used if you are going to mark the plus.
While I can imagine that I have offended just about every poster to this thread, that was not my intent. Hopefully, each of you can take out of this some useful information that might just save somebody's life.
I realize there are many questions that I left unanswered, and those of you who have been to my training know that this is a very simplified, shortened version of the subject. However, for those of you who might be overfilling cylinders with the mistaken impression that it is OK; STOP! You are playing Russian Roulette.
Best regards to all, sorry if I have offended,
Darrell Garton
CTC Seminars
visit us at ctcseminars.com
Please allow me to introduce myself. I am Darrell Garton of CTC Seminars. I wrote the comments to 49 CFR that were referenced by Captain in post #18 on March 22. I have worked on various drafts of the CFR, CGA pamphlets, and with Bill High of PSI (and now the new owner, Mark Gresham). I have been teaching cylinder requalification for over 20 years, and CTC is the only training approved for both Transport Canada and US DOT.
I generally don't get involved in these types of discussions, but I was asked to comment by one of your contributors who saw my article referenced.
The comments in this thread range from partially correct to dangerously wrong. I cannot possibly address everything stated here, but I would like to correct some wrong thinking. For those of you interested enough to plod through this, you will find the areas in which you may have been mistaken. (Don't think you're exempt! I found errors in virtually every post except the one that said, "Nothing is foolproof to a sufficiently talented fool") Just because I correct one of your errors here, please don't think I'm singling you out for abuse. In some cases the reason I am correcting someone's error is because I see that you have the technical understanding, and simply got one small point wrong. The errors that go off the chart, I'm not even bothering to address.
First let me describe fatigue failure: BANG!!!
It is likely to be the last sound you ever hear. It doesn't give much warning, and may not even be detected by the hydrostatic test. The hydrostatic test is looking for yield, not fatigue. Imagine a paperclip; if you put enough force against it, you will bend it - that's called yield. The hydrostatic test is looking to see if the cylinder yields at test pressure. Now imagine bending a paperclip back and forth multiple times; eventually it will snap due to fatigue. Overfilling cylinders to the outrageous pressures described by some posters is likely to cause fatigue.
Now let's talk about overfilling cylinders. The plus rating is based on technical analysis from 1942, during WWII. Because of the conservative design of our 3A cylinder specification (3AA didn't come along until 1948 but is basically the same design, using high strength allow instead of plain carbon steel), it was found that a 10% increase in fill pressure was justified and technically sound, so long as there has not been significant reduction in wall thickness. The elastic expansion can be used as a means to determine average wall stress, as expressed by the Bach and Clavarino formulas that have been discussed here. The Bach formula establishes the limiting wall stress based on wall thickness and test pressure, while the Clavarino formula expresses the inverse relationship of wall thickness to elastic expansion, whereby an REE value can be determined. The REE values that are established for the increased filling limits of 173.302a(b) are based on roughly a 5% increase in wall stress from that of the manufacturing limits of section 178.36 and 178.37 (3A and 3AA cylinder specifications). The use of elastic expansion to determine wall stress is a means of determining the average wall stress, not the maximum. The maximum wall stress can only be determined by an actual wall thickness measurement, such as by using an ultrasonic wall thickness gauge. The regulations do not say you must calculate both the average "and" the maximum, they say "either/or", "Either the average wall stress or the maximum wall stress does not exceed the wall stress limitation shown in the following table:" [173.302a(b)(3)]. This section goes on to say in [173.302a(b)(3)(iii)], "Compliance with average wall stress limitation may be determined by computing the elastic expansion rejection limit in accordance with CGA C-5, by reference to data tabulated in CGA C-5, or by the manufacturer's marked elastic expansion rejection limit (REE) on the cylinder." The regs say "cannot exceed", they do not say "must be less than". So, when you put it all together, one of the primary criteria for putting the plus mark on the cylinder it that the elastic expansion does not exceed the REE. It's all about the wall thickness, and if the elastic expansion exceeds the REE, it means we have lost wall thickness.
It is ONLY the 3A, 3AX, 3AA, 3AAX, and 3T specifications that are permitted the plus mark. Marking of a special permit or exemption cylinder with a plus is not permitted, unless specifically allowed in the applicable special permit.
In 1993 Transport Canada adopted a new metric specification that incorporates the higher fill pressure of the plus into the specified service pressure (thereby creating a 1.5x test pressure, instead of 5/3). Should one of these new (post 1993) TC metric cylinders exceed its REE, it must be stamped with a "K" following the service pressure marking (see B339 24.6.3.4), and the fill pressure is then reduced by 10% (see B340 5.2.3.1).
This isn't just some arbitrary "I'm comfortable with it" mumbo-jumbo. Asking your fill operator to fill a cylinder over it's rated pressure is asking him to risk his life for your desire to have a little more air. And don't think if you fill it yourself, you're only risking your own life. What about everyone else on the boat, and the rescue team that has to come looking for you??
Our 3A and 3AA cylinder designs are based on many factors, among which cycle life, yield point, and burst pressure data pertain to this discussion. (I'm not going to list those limits, because some !@#$ will improperly interpret them and think he can just do it until he needs glasses. Those of you with the technical know-how can go look it up.) At their rated service pressure, these cylinders have an unlimited service life, meaning they can be cycled indefinitely, because they are not experiencing yield or fatigue. However, at test pressure there is a limit to the number of cycles a cylinder can withstand, and at their burst pressure, there is ONLY ONE cycle! If you overpressurize cylinders enough, you are eating into that cycle life, and just like bending the paperclip back and forth, someday it will fail.
While it is true that cylinder designs are based on test pressure, one cannot simply say if it passes a higher test pressure, it's OK to use at a higher service pressure. (Although, the poster who said he could see doing it by using EE numbers with the Bach and Clavarino formulas to evaluate wall stress was correct. But reverse engineering a container that happens to be built stronger than the design criteria isn't going to gain anyone's approval for remarking it to a higher service pressure!). 3A and 3AA designs can withstand pressures well over their minimum specified test pressure before experiencing a failure. However, that's not to say that the cylinder won't be damaged if continued cycling occurs. Also, testing a cylinder at a higher test pressure INVALIDATES a subsequent test at a lower test pressure. The poster who said he would be willing to test the 3AA 2250 at 4000, but would then have to test it according to its proper DOT criteria doesn't understand permanent expansion. Once stretched to 4000psi, there will be no permanent expansion at 3750psi. And to answer the question as to whether 4000psi would damage the cylinder; repeat testing in the event of an equipment malfunction does occur, and it is performed at an increase of 10% or 100psi, whichever is less. CGA C-1 tells us that we should never go over 110% of the test pressure when performing a repeated test. So, for your 3AA2250 with a 3750psi minimum specified test pressure, you should be able to test that cylinder to 4125psi and still pass the test. HOWEVER, that does NOT mean that you could justify a higher fill pressure.
The guys who have said overfilling is OK for short periods as well as the ones who said that they have left them at pressure for extended periods and the cylinder is OK don't understand cycle life. The guys filling the cylinder to its test pressure don't understand that there is limit to how many times a cylinder can go to test pressure. The guy who was filling to 6500psi shouldn't have been allowed to leave his padded cell. That is over the design minimum burst pressure. The guys filling to 3600 and think its OK might want to buy some earplugs. You are approaching the pressure at which cycle fatigue occurs, and one day you might experience that BANG! I mentioned earlier.
If you have trudged your way through this diatribe thus far, you might be interested to know that the reason Direct Expansion testing is not permitted for marking cylinders with the plus is due to the error induced by the calculations for the compressibility of water. Errors in that calculation will directly effect the elastic expansion, and that is what the entire wall stress analysis is based on. So, in the U.S., the water jacket test must be used if you are going to mark the plus.
While I can imagine that I have offended just about every poster to this thread, that was not my intent. Hopefully, each of you can take out of this some useful information that might just save somebody's life.
I realize there are many questions that I left unanswered, and those of you who have been to my training know that this is a very simplified, shortened version of the subject. However, for those of you who might be overfilling cylinders with the mistaken impression that it is OK; STOP! You are playing Russian Roulette.
Best regards to all, sorry if I have offended,
Darrell Garton
CTC Seminars
visit us at ctcseminars.com
Darrell, thanks for taking the time and dropping in.
Thanks for saving me a bunch of time. I can drop the Maximum wall stress calculations from my day!
Thanks for saving me a bunch of time. I can drop the Maximum wall stress calculations from my day!
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