Hydrostatic testing...how much can the tank expand and still pass?

[Luis H]

Interesting enough, the VIP's of the SCUBA tanks are not a DOT regulations but just a SCUBA industry recommendation. If you are not transporting your tanks across states lines, legally, you dont even need a hydro on your scuba tank. Try getting them filled at a LDS without the hydro or VIP is another matter though.

Actually you are only partially correct.

VIP are required during re-qualifications per CFR49, every 5 years.
I think CGA has followed the scuba industry standard for yearly VIP on scuba tanks, which in turns it almost (or basically) makes it very close to a DOT requirement. Don't quote me on this last one, I am not positive.

And, yes the federal DOT is for interstate commerce, but my understanding is that some states DOT do invoke federal regulations even if you are just transporting your cylinders in your own personal vehicle. No one will ever check unless you are involved in a automobile accident.

You are right, in that it is all kind of academic if you rely on a LDS for air.

 

[Luis H]

So Luis,

Keep in mind I said "may" not "will", nor "should". And that I did not say "condemn", but "unfit to return to service" which could also be "remove from service" or more appropriate "rejected". Both differ from "condemnation" with regard to what options are still open to both the facility and the owner of the tank.

Tank characteristics very from design to design. Some of the PST E series are notorious for having been incorrectly condemned. PST's solution was that you use an allowed "leak check" to pre-stress the tank to 85% of hydro pressure. And still, even after that, many of them will test in the 9%+ PE range. If it was one of these that we are talking about I could or can agree.

However, if we're talking about your average AL80, the norm is to see 0% PE. The idea is to identify tanks that are loosing their elasticity "before" they fail catastrophicly. If a tank is identified as being that close to the pass/fail point with regard to PE and has other characteristics that are also indicative of a tank that is at or nearing it's retirement point, it would be remiss of the tester to not bring it to the attention of any and all involved. To simply stamp the tank as "A O.K." and send it out the door without notation could result in a catastrophy or at the very least a continuation of procedures that are destructive to a tanks longevity.

If you read my posts under “how thick are the walls of a tank” you should understand better why most aluminum tanks don’t tend to hit the yield level during hydro test.

Do not assumed that a steel tank that shows a 9% residual expansion is anywhere near end of life situation. If you keep track of that tank you may easily see that during the next hydro it could show a very low residual expansion.

The codes are specific about the requirements for “rejecting” and “condemning” a cylinder. I am sure you know where to find the definition.

Just keep in mind that the pass/ fail criterias have a conservative factor already built into them. A tech adding its own conservative judgement on a quantifiable test is actually irresponsible and does not meet the codes.

The behavior of an aluminum and a steel tank during testing is very understandably different based on mechanics of material and deformable solids. A small amount of yielding on a chrome-molly steel tank is of no concern to me. Which is what is intended to be measured as residual expansion, but often what you are measuring is the cylinder returning to its true round geometry. That is the reason why all steel cylinders should go thru the round out procedure recommended by PST (actually it is a manufacturer requirement on their cylinders).

A steel tank that has a high permanent expansion is not necessarily loosing its elasticity. It could have been just very slightly out of round. That is the primary reason for the pretest procedure.

The number of hydro tests a tank will experience during my life time is minimal when talking about fatigue life. Therefore, reaching the yield stress during hydro testing is not an issue; they are design with that in mind.

What makes a material ductile is the separation between the yield stress and the ultimate stress level. In a ductile material (like the chrome-molly steels used in pressure cylinders) the ultimate stress level is substantially higher than the yield stress. A catastrophic / rupture type of failure should never occur on a cylinder even if it just barely failed hydro test.

Most steel tanks don’t fail a properly performed hydro test, unless they have been on a fire or their structural integrity has been heavily compromised by corrosion. Rust is what makes a steel tank fail during a visual inspection.

 

[northen diver]

Actually you are only partially correct.

VIP are required during re-qualifications per CFR49, every 5 years.
I think CGA has followed the scuba industry standard for yearly VIP on scuba tanks, which in turns it almost (or basically) makes it very close to a DOT requirement. Don't quote me on this last one, I am not positive.

And, yes the federal DOT is for interstate commerce, but my understanding is that some states DOT do invoke federal regulations even if you are just transporting your cylinders in your own personal vehicle. No one will ever check unless you are involved in a automobile accident.

You are right, in that it is all kind of academic if you rely on a LDS for air.

The VIP is part of the hydrostatic testing of the tanks every five years. There is nothing in the DOT regulations stating a VIP "only" every year, or the VIP plus on the SCUBA tanks every year. See this is what I am talking about. The company that hydrostatic my tank does a VIP. When the LDS gets the tank back from hydro testing, they then proceed to do another VIP so it gets a decal. The customer gets charged twice for a VIP.

CGA will fill a tank without a VIP decal. The problem with the gas companies is that they deal with medical grade O2, industrail grade O2 and some aviation O2. Not so much in compressed air. But again, the CGA is not DOT and they are a self regulatory organization also. It still isnt a law or regulation from what they "suggest" to be done on gas cylinders.

 

[spectrum]

See this is what I am talking about. The company that hydrostatic my tank does a VIP. When the LDS gets the tank back from hydro testing, they then proceed to do another VIP so it gets a decal. The customer gets charged twice for a VIP.

That's a sloppy part of the subcontracting system and why a dive shop with a hydro station can cut costs. Given the work involved in doing the hydro process the VIP done there really amounts to little more than observations and has little to do with the hydro cost. The cylinder is open, they need to dry it and they need to look inside, not a big deal. If they see something ugly the check it with a pick. If they can clean it they can charge you.

When the shop sends the cylinder to the hydro contractor it goes without the valve and boot, just the raw cylinder. As Luis mentioned the contractor did a visual as part of the procedure.

He also had to dry the cylinder and who knows what sort of moisture the cylinder was subjected to before it got put back together. In other words the visual done at the hydro facility cannot be indicative of the cylinder condition at the time when it it gets re valved and pressurized with dry air. The dive shop may even throw a new tank neck O-ring on there at the same time as part of the service.

This is really no different than the dive shop insisting on doing a VIP when you bring in a brand new mail order cylinder that is at zero PSI. Lacking positive pressure anything is possible.

Pete

 

[fweber]

The VIP is part of the hydrostatic testing of the tanks every five years. There is nothing in the DOT regulations stating a VIP "only" every year, or the VIP plus on the SCUBA tanks every year. See this is what I am talking about. The company that hydrostatic my tank does a VIP. When the LDS gets the tank back from hydro testing, they then proceed to do another VIP so it gets a decal. The customer gets charged twice for a VIP.

CGA will fill a tank without a VIP decal. The problem with the gas companies is that they deal with medical grade O2, industrail grade O2 and some aviation O2. Not so much in compressed air. But again, the CGA is not DOT and they are a self regulatory organization also. It still isnt a law or regulation from what they "suggest" to be done on gas cylinders.

First the visual that your tank receives at the time of hydro is primarily concerned with the structural integrity of that tank and not so much it's cleanliness or fitness for your intended use. Could it, or should it, be different than this? Probably. But, that's the reality. The hydro tester(we should technically be calling this re-qualification) is concerned with the structural aspects of your tank. After passsing that, the tank can then be put into any number of services that may or may not require further service. Like oxygen use, which would then require cleaning to that standard. The tank may even be sent to and returned from the test(re-qualification, because Luis is watching) center without it's valve.

CGA will not fill a tank! The CGA is not a fill station. The CGA is an NGO(representing the compressed gas industry) that was asked by the government to determine standards for the industry. Their standards are so much law, in that they are repeatedly referenced by the DOT as the standard to which things must be done. I have some amount of experience at this and have copies of the CGA pamphets, CFR 49 parts 100-185(all 1300 pages), British BS standards, and a few others behind me on a shelf, including the recommendations of most of the manufactures, Asahi, Lux, Cat, PST, Faber, etc.

Suffice to say. It sounds like you're planning to take a Visual course. Great. You'll then be able to put your own, "this tank was viz'd by me" stickers on your tanks. And after you purchase the compressor for your psd team you'll be able to fill them.

 

[fweber]

Spectrum types faster than I do.

 

[knotical]

There's a description of the hydrostatic test here:
Hydrostatic

 

[fweber]

... sorry, I tried to split a quote up a failed

 

[Luis H]

First the visual that your tank receives at the time of hydro is primarily concerned with the structural integrity of that tank and not so much it's cleanliness or fitness for your intended use. Could it, or should it, be different than this? Probably. But, that's the reality. The hydro tester(we should technically be calling this re-qualification) is concerned with the structural aspects of your tank. After passsing that, the tank can then be put into any number of services that may or may not require further service. Like oxygen use, which would then require cleaning to that standard. The tank may even be sent to and returned from the test(re-qualification, because Luis is watching) center without it's valve.

CGA will not fill a tank! The CGA is not a fill station. The CGA is an NGO(representing the compressed gas industry) that was asked by the government to determine standards for the industry. Their standards are so much law, in that they are repeatedly referenced by the DOT as the standard to which things must be done. I have some amount of experience at this and have copies of the CGA pamphets, CFR 49 parts 100-185(all 1300 pages), British BS standards, and a few others behind me on a shelf, including the recommendations of most of the manufactures, Asahi, Lux, Cat, PST, Faber, etc.

Suffice to say. It sounds like you're planning to take a Visual course. Great. You'll then be able to put your own, "this tank was viz'd by me" stickers on your tanks. And after you purchase the compressor for your psd team you'll be able to fill them.

Very good...
smart as_ ...

 

[Luis H]

There's a description of the hydrostatic test here:
Hydrostatic

Thanks

I was looking for that web site. It has a good description and good pictures.