PST LP 95 Failed first hydro-Rare?

[oxyhacker]

All the more so because a hydro shop isn't really in any way required to do the exercise cycle if they don't want to. It's not part of the official DOT-mandated hydrotest procedure but is just something PST cooked up, to try and give their tanks a little extra help without actually doing anything illegal.

So if a shop doesn't commit to doing it, and your tank fails because it doesn't do it, you don't really have any legal grounds for complaint.

Anyone bringing in a hot dipped galvanized steel tank for hydro should also bring a copy of the test procedure and be absolutely sure the tester will follow it.

 

[pescador775]

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.

 

[roakey]

I found a newer version (2003) of the pressed steel hydro procedure on the web:

http://www.underctek.com/open/boards/pstd100.pdf

Roak

 

[drbill]

Don't think I've posted here, but too lazy to review the thread. My PST HP120 failed its second hydro for the same reason (over expansion and failure to return to normal spec). I never over-filled the tank, but it probably received hot fills at one of our local LDS' fill station. Even their former manager said that was probably the reason.

I wrote PST's president who ignored my letter.

 

[skynscuba]

All the more so because a hydro shop isn't really in any way required to do the exercise cycle if they don't want to. It's not part of the official DOT-mandated hydrotest procedure but is just something PST cooked up, to try and give their tanks a little extra help without actually doing anything illegal.

So if a shop doesn't commit to doing it, and your tank fails because it doesn't do it, you don't really have any legal grounds for complaint.

Does Worthington specify such a procedure for their tanks? They seem fairly similar to the PST tanks in finish. I guess their 3442psi tanks haven't been around long enough to need a hydro, other than at the factory.

Gary

 

[wreckchick]

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.

This thread is old, but I'm bored...

If you give me your PSI Inspector number, I'll happily say that the claims you make to know better than actual inspectors might have some validity. Since my guess is you have no idea what you're talking about, I'll just say that not only does SLC exist, I can just about go down a line of tanks and tell you which ones will show neck cracks before I even take the valves off. It's a known phenomenon that pretty much everyone knows about and agrees exists... well, except for you.

R

 

[Stu S.]

Well divers, I am a metallurgist, and worked in the pressure vessel business. Things metallurgical do not happen due to temperatures your tank will see. And the best way I know to give a tank a "set" is to super-pressurize it close to the burst pressure.

Think of the steel in a tank as a spring. A force pulling on a coil spring, for instance, will expand it a certain amount. If the spring gets weak, it will expand too much. Hydro testing expands a scuba tank with pressurized water from within. We then measure the amount of water displaced in a larger tank holding that scuba tank. If the steel of the tank becomes weak, it will expand too much from the pressure within, and we have to fail the tank.

Every time the tank is filled, the steel will get weaker by a minute amount. Overpressurizing would typically speed up the process of making a tank weak. How much would require experiments. Take heart. The tanks I am familiar with are designed to take something like 10,000 pressure cycles. If you cut the useful life of that tank in half, there would still be plenty of diving in one.

As for that failed 95, you may have gotten on that was weak from the start, or it was really used.

As for me, I specify my LP's to be filled to 3200 psi. That's all I need. They are filled and ready for a trip to Pompano Beach, tomrrow.

Merry Christmas!

 

[Bob3]

... the tanks never actually return "to spec"--the metal is in fact permanently stretched ...

I always get the paperwork back on my tanks so I can see the results. One old tank, a USD from 1956 always comes back with 0 permanent deformation. Out of a dozen bottles there were only a couple that did that though, so the permanent stretch is the norm.

 

[divemaster_jim]

I've always made it a point to never store a tank over-filled and to use a over-filled tank right away, well at least in a couple of days anyway, not yet had a tank fail a Hydro.

 

[Saint_Thomas]

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

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.