PST LP 95 Failed first hydro-Rare?

[in_cavediver]

Do you mean that you fill the 2400 psi ( 2640 w/+ )
to 3200 ?

For some tanks, around 3600. Not all LP tanks are good candidates and I will not try to name them all. I do know from first hand expiereince 85's, 95's, 104's, 112's, and 121's are done.

I was thinking about buying a LP 121cf.

By the way, I'm glad I bought my compressor before my first dive.

VIP and hydro are only required for commercial operations, not for individuals.

For the hydro/vis. Although technically not required, I would strongly recommend them anyway. Take a class on VIP, get the oxyhacker book and do it yourself. Then you can do it whenever its appropriate. Skipping hydro's make no sense since they are around $13 a tank where I am at and its once every 5 years. Darn cheap.

 

[PhilEllis]

The "hot fill" myth got started a few years ago when some old aluminum tanks exploded down in Florida. Luxfer's lawyers threw up a bunch of mud to see what would stick, including the hot fill claim. This crap, like so called "sustained load cracking", immediately began to circulate in the diving community as gospel. It wasn't long before the mud started sticking to the steel tank product. Years ago, hydro testers and shop owners claimed that storing a full steel tank would cause fatigue related failure, now it's hot fills, or whatever. It is all nonsense originating from opinions given by dumb cluck state investigators and sleazy lawyers. Heck, even the glib claims about sustained load cracking of old aluminum tanks had to be modified when an aluminum cylinder which had been inspected and tested exploded immediately after the valve had been reinstalled and the tank was being filled, by the same guy that had VIPed the tank! They said that that particular tank was an example of something else, "catastrophic" failure. However, if they had mentioned good, old fashioned metal fatigue they would have been on the right track. I'm telling you, none of those people and none of the dive community really know much if anything about these matters. I don't claim to have the answers either but if "hot fills" were a problem with steel cylinders we would have heard about it from PST, Faber, or others. I'm darn sure not going to base my opinions on what some hydro tester or dive shop says, or certifying group for that matter. These people are all repeating junk that they've heard from know nothings, and people who are trying to cover their butt. I'm not a metallurgist but it seems to me that steel tanks take a "set", particularly the newer types. The type of tank manufactured in the US has a fairly low tensile strength. This means that they are "stretchier" than a tank which is formed with harder walls. It also means that this softer alloy is capable of flexing many, many times before it becomes weak. It means that this type of alloy can experience blows and temperature shocks without failure. However, it could also mean that a fair hydro test can only be conducted after a tank of this type has been exercised at higher pressure than the very conservative working pressure specified for normal use.

Well, I am not an engineeer, I am not a metallurgist, and I am not a cylinder designer, nor am I a hydro operator.........but I do have some knowledge about this process and have had the benefit of a number of conversations with Worthington Cylinder applicaion engineers. Here are my thoughts on the "pre-hydro" specification recommended by PST for their cylinders.

The hydrostatic examination to which we subject our cylinders is designed to answer one question and one question only.........does the subject vessel retain its design ability to undergo the metal stretch present during normal filling operations and will it return to its previous size (with some tolerance specified) when that pressure is relieved?

A cylinder flexs and expands with every fill cycle and typically returns to its original size and shape as the pressure is reduced. They are clearly designed to do this over and over and over. However, there comes a point when the "work hardening" is such that the cylinder is stretched one time too many and it will no longer return to its original shape and size.

It has been my experience that you can take a failed cylinder from a hydro inspection and subject it to a second immediate hydro and it will pass the second time! Is this an indication that the cylinder is now good? Abolutely not! It could just as likely indicate that the first test finally stretched the metal beyond its ability to return, leaving no "stretch" left for the second test. The second test would then appear to be "normal" when measured by the amount of displaced water. Last spring, I specifically asked a Worthington engineer why they didn't recommend a "pre-hydro" stretch procedure for their cylinders. I said very simply and very plainly, "I would rather they would fail". He went further to say that "it would be silly to introduce a procedure that might do more to fool the hydro test than to improve its accuracy".

I think the PST "pre-hydro" stretching procedure is designed to maximize the number of "pass" results from a hydro procedure, by taking some of the stretch out of the metal just before the official test so it is more likely to stretch less in the "official" test, thereby improving the pass rate. This simply doesn't seem right to me. I know that there are some here who think the whole idea of inspection and hydro is simply a game. I personally think those tests are in place to assure safety to the degree that such can be assured. I wouldn't want anything done that would affect the validity of the hydro test, and the "pre-hydro" procedure sure seems like such a scheme.

Anyway, just my opinion. Thanks.

Phil Ellis

 

[in_cavediver]

Phil,

No offense intended but it is foolish not to take the manufacturers guidlines for testing. With all due respect to Worthington and thier reps, PST did apply for an get a special permit from the DOT for manufacture and thier procedure, while not required, is also not illegal (according to Vance). I don't know if there is something different with the PST and Worthington tanks to warrant this difference or if its just that Worthington didn't want to take the time that PST did with the testing evaluation.

Well, I am not an engineeer, I am not a metallurgist, and I am not a cylinder designer, nor am I a hydro operator.........but I do have some knowledge about this process and have had the benefit of a number of conversations with Worthington Cylinder applicaion engineers. Here are my thoughts on the "pre-hydro" specification recommended by PST for their cylinders.

The hydrostatic examination to which we subject our cylinders is designed to answer one question and one question only.........does the subject vessel retain its design ability to undergo the metal stretch present during normal filling operations and will it return to its previous size (with some tolerance specified) when that pressure is relieved?

A cylinder flexs and expands with every fill cycle and typically returns to its original size and shape as the pressure is reduced. They are clearly designed to do this over and over and over. However, there comes a point when the "work hardening" is such that the cylinder is stretched one time too many and it will no longer return to its original shape and size.

It has been my experience that you can take a failed cylinder from a hydro inspection and subject it to a second immediate hydro and it will pass the second time! Is this an indication that the cylinder is now good? Abolutely not! It could just as likely indicate that the first test finally stretched the metal beyond its ability to return, leaving no "stretch" left for the second test. The second test would then appear to be "normal" when measured by the amount of displaced water. Last spring, I specifically asked a Worthington engineer why they didn't recommend a "pre-hydro" stretch procedure for their cylinders. I said very simply and very plainly, "I would rather they would fail". He went further to say that "it would be silly to introduce a procedure that might do more to fool the hydro test than to improve its accuracy".

I think the PST "pre-hydro" stretching procedure is designed to maximize the number of "pass" results from a hydro procedure, by taking some of the stretch out of the metal just before the official test so it is more likely to stretch less in the "official" test, thereby improving the pass rate. This simply doesn't seem right to me. I know that there are some here who think the whole idea of inspection and hydro is simply a game. I personally think those tests are in place to assure safety to the degree that such can be assured. I wouldn't want anything done that would affect the validity of the hydro test, and the "pre-hydro" procedure sure seems like such a scheme.

Anyway, just my opinion. Thanks.

Phil Ellis

 

[oxyhacker]

I agree that there is something about the PST-recommended exercise cycle that feels a bit sketchy, but PST tanks do not seem to be bursting in any measureable quantities, and the procedure has been around so long that the DOT has had plenty of time to discourage it if the DOT felt it was unsafe. So I am not going to waste time worrying about it. One can argue that the standard hydro process is just a poor match for the expansion characteristics of some exemption tanks, and needs some tweaking, and from a consumer point of view one might even applaud PST for finding a way to improve the odds that tank owners will get the full use out of the tanks they paid for.

Note that the procedure only applies to hot-dipped galvanized tanks (though since it has no official standing there is no reason why it can't be done to non-galvanized tanks as well). This is to me the really interesting part, since it suggests that galvanization is changing the temper or strength of the tank. I know the Worthington tanks were orginally going to come painted, it'll be interesting to see how their galvanized ones do at hydro once they have been around for awhile.

I think the PST "pre-hydro" stretching procedure is designed to maximize the number of "pass" results from a hydro procedure, by taking some of the stretch out of the metal just before the official test so it is more likely to stretch less in the "official" test, thereby improving the pass rate. This simply doesn't seem right to me. I know that there are some here who think the whole idea of inspection and hydro is simply a game. I personally think those tests are in place to assure safety to the degree that such can be assured. I wouldn't want anything done that would affect the validity of the hydro test, and the "pre-hydro" procedure sure seems like such a scheme.

 

[PhilEllis]

Phil,No offense intended but it is foolish not to take the manufacturers guidlines for testing.

Not trying to be snarly here, but it might well be very foolish to "take a manufacturers guildines for testing", especially if the intent of their guidelines are designed to affect the result of an established test. The manufacturer doesn't get to establish how the federal test is performed, nor do they get to state the conditions under which the test is performed. It is supposed to be done the same way on EVERY pressure vessel, regardless of the manufacturer, and regardless of the application. That is why the proscribed PST procedure is an "informational" document, not binding on any hydro test facility.

With all due respect to Worthington and thier reps, PST did apply for an get a special permit from the DOT for manufacture and thier procedure, while not required, is also not illegal (according to Vance).

I agree that the "informational bulletin" published by PST is not illegal. The hydro facility does not care what is done to the cylinder prior to testing. They test them as they get them. PST did get a special permit for the manufacture of their cylinders, as does every manufacturer. That special permit does not, however, mandate the use of any PST recommendation for pre-testing prep of the cylinders.

I don't know if there is something different with the PST and Worthington tanks to warrant this difference or if its just that Worthington didn't want to take the time that PST did with the testing evaluation.

I think Worthington (at least according to their engineers) has no interest in any procedure that might cause a cylinder that should fail the hydro to otherwise pass. I would guess (but I certainly don't know) that they would consider a "pre-test stretching of the cylinder to 90% of its expected stretch during hydro" to be a scheme designed to acheive some objective that might not be in the best interest of the owner and user.

I think a periodic hydro inspection of a pressure vessel to be a good thing. As long as the hydro test is well validated (as is the current federal procedure for hydro inspection for pressure vessels), I think it is a good think when a cylinder that should fail actually does fail. I realize that in non-destructive testing there are false positives and false negatives in all test results. When a cylinder fails a hydro test, we can never know if the result is a true one or if it is a false negative associated with the test method. Nor do we know the opposite. However, I personally think it is better not to do anything that might effect the outcome of the test.

Phil Ellis

 

[PhilEllis]

Note that the procedure only applies to hot-dipped galvanized tanks (though since it has no official standing there is no reason why it can't be done to non-galvanized tanks as well). This is to me the really interesting part, since it suggests that galvanization is changing the temper or strength of the tank. I know the Worthington tanks were orginally going to come painted, it'll be interesting to see how their galvanized ones do at hydro once they have been around for awhile.

Vance, I wouldn't think that the galvanizing of the cylinders would have an effect on the properties of the cylinders. I know that the galvanizing bath is quite hot, though I would expect well below the threshold necessary to effect the positioning and layering of the molecules or to effect the grain structure of the metal. I do know that in aluminum, there are a number of "sub-threshold" temperature treatments designed to have impact on certain properties. The same might be possible in steel. If I remember to do so, I will ask the Worthington engineers if galvanizing has an effect during our next conversation. Thanks.

Phil Ellis

 

[Rick Murchison]

It is my impression that the PST special procedure has more to do with zinc than with steel; that if the tank were stripped of its zinc coating there would be no need for the special procedure. In lay terms, zinc ain't as springy as steel.
Rick

 

[in_cavediver]

Phil,

We might have to agree to disagree on this one. As a consumer, and owner of several PST tanks, it is in my best interest to ensure that all appropriate methods, as recommended by the tank manufacturer, are used when hydro's come about.

As for whether its appropriate or not, my simple test is whether the hydro procedure specifically prohibits it. In this is case is does not. As for proper procedures, the CGA and the DOT work together to establish the proper procedures for testing cylinders (DOT ultimately sets the rules). After all, it is in the best interest of everybody to have safe tanks, of all types, out there. In this case, I will defer to the experts on the procedure and when hydro's come about next may, send a copy of the PST bulletin with the tanks.

Not trying to be snarly here, but it might well be very foolish to "take a manufacturers guildines for testing", especially if the intent of their guidelines are designed to affect the result of an established test. The manufacturer doesn't get to establish how the federal test is performed, nor do they get to state the conditions under which the test is performed. It is supposed to be done the same way on EVERY pressure vessel, regardless of the manufacturer, and regardless of the application. That is why the proscribed PST procedure is an "informational" document, not binding on any hydro test facility.
<snip>
Phil Ellis

 

[Puffer Fish]

Maybe I missed something, but the OP said he has a *LP95*, not a HP PST tank.

Cave fills are still very common here in cave country. The difference is that we fill them SLOWLY so the tank doesn't heat cycle much. When you overfill, and you do it quickly, the tank gets very hot. This is bad for the tank. As you have found out.

Be VERY careful when purchasing used tanks. Or purchase them with the assumption that they will fail their next hydro.

Overfilling is overfilling. Doing it slowly does not make any major difference, as long as the maximum recommended fill rate is not exceeded. And steel tanks tend to fail more than aluminum.

But it is fairly common for testers not to know the correct proceedure for a given tank.

 

[Puffer Fish]

It is my impression that the PST special procedure has more to do with zinc than with steel; that if the tank were stripped of its zinc coating there would be no need for the special procedure. In lay terms, zinc ain't as springy as steel.
Rick

Rick, the issue is a subject called hydrogen embrittlement, and has nothing to do with the zinc. Almost all plating processes can cause this (like zinc plating).