Do Tanks Have A 5 'hydros' Limit ?

[redacted]

Perhaps it is a Scubatoys employee who is misinformed.

I would have to run down the source of this and correct the error or take my business elsewhere.

 

[teknitroxdiver]

Hopefully @scubatoys can comment on this.

 

[Wookie]

Did anyone ask @scubatoys what the actual store policy is? His dive shop, his rules, but to publicly dun a store without giving the owner a chance to set the record straight and to correct his employee (or confirm that the store policy is, in fact, to only allow 5 hydros) seems a little FUDDD to me.

 

[scubafanatic]

OK, I opened up a new thread over at the Scubatoys forum, and we'll see what happens. (I tried, but abandoned the '@scubatoys' option as it only allows 420 character max messages and Scubatoys hasn't been on this SB site since 3/28 anyway.) I'm a Scubatoys supporter, it's their hydro vendor I may have issues with, and I didn't want to publicly question the employee's info, in the interest of diplomacy, so I thought I'd follow up behind-the-scenes, without naming names.

 

[guruboy]

There's only about 3 hydro shops in the Dallas area (not sure about FtW). Wouldn't really matter what dive shop you go to, good chance it's the same hydro facility.

 

[2airishuman]

I get my welding tanks on an exchange basis with my gas supplier and some of them have had a dozen hydro stamps. I've read the regulations, and there's no limit on the number. If there isn't enough space on the cylinder neck, the hydro facility can attach a sheet metal tag and put the additional stamps on that.

 

[Nettie-NZ]

This is definitely the stuff of metallurgical myths and legends.

There is no engineering/metallurgical reason for limiting a dive tank to 5 hydros - so long as you are following acceptable hydro procedures. Many of the very large industrial pressure vessels I've been involved with have been hydro'd almost annually for decades.

You could get into a whole discussion of hydros vs NDT but that's a completely different thread.

 

[DEdiveguy1]

I gotta believe this was simply a misunderstanding - most likely an employee in need of additional training. Can't believe the hydro facility or the dive shop would spout this type of mis-information.

 

[Rhone Man]

As noted above composite tanks have 3 or 5 year re-qualification requirements (depending on the design) and have a 15 year life limit. The re-qualification process includes both the hydro test and a visual inspection. IF, and it's a big if, the statement was made in reference to a hop wrapped composite cylinder with a 15 year life limit and a 3 year re-qualification interval, then the statement is more or less correct. Just more or less, as skipping a few years between hydro tests won't allow you to still have a total of 5 done - the tank will still age out of service at 15 calendar years.

----

Some tank manufacturers will provide a 30 year "life span" for aluminum tanks for comparison purposes with composite tanks, but there is no legal or engineering limit applied to 3AL, 3AA tanks and no life limits are applied to exempt, or special permit all aluminum and all steel tanks used in scuba diving.

3AL aluminum tanks do have a fatigue life, but they are tested to 100,000 cycles to test pressure (5000 psi for a 3000 psi 3AL tank) so the fatigue life is somewhere above that number, and that number will be even higher with the fill pressure limited to the 3000 psi services pressure. To put that in perspective, you'd have to fill a 3AL scuba tank twice a day for 136 years to get to 100,000 fills.

There are some dive shops that refuse to fill aluminum tanks older than 20 years, but that's pretty ignorant thinking. I think in many cases it is an outgrowth of the many shops that refuse to fill aluminum tanks made prior to 1990. That is done to avoid filling 6351-T6 aluminum tanks that are prone to SLC cracks, but that date itself was arbitrary as all Luxfer aluminum tanks made after May 1988 are made from 6061-T6 aluminum, and Catalina tanks were always made of 6061-T6 aluminum regardless of age. Some shops have just extended this overly conservative rule of thumb to an even more conservative and totally non-fact based determination of a 20 year "service life" - mostly I suspect to sell more tanks.

3AA steel tanks do not have a fatigue limit and provided they are properly cared for and are not allowed to develop pits from rust, they'll last what amounts to forever, for all practical purposes, with the practical limit really being the ability to find a place to put a new test stamp every 5 years. The oldest tank I've seen come in for hydro test was a welding tank made in 1911, and it was still eligible for a plus rating.

Great post.

My anecdotal experience is that whilst steel tanks last virtually forever if well cared for (and sometimes even if not - I have to 45 year old steels which haven't always had the love and care they deserved), aluminum tanks start to have a pretty high failure rate by the time they hit their 5th hydro - especially if they have been heavily used like dive shop tanks (ie. regularly pressurised and depressurised). Personal tanks which get used less often, and may spend decent periods stored empty seem to last a bit longer.

 

[cool_hardware52]

I gotta believe this was simply a misunderstanding - most likely an employee in need of additional training. Can't believe the hydro facility or the dive shop would spout this type of mis-information.

Me too, I suspect somebody hear the 15 year max, 3 year retest required for hoop wrapped tanks and incorrectly assumed that covered all tanks. Probably a case of a little info being dangerous.

The rules governing Scuba tanks ( and HP tanks in general) are not straight forward. Explain the capacity of LP tank with and without a "+" rating and watch eyes roll back.

Tobin