What's in a hydro?

[Rred]

"However, if a tank that fails hydro is tested again, it might "pass" the next hydro.."
And how might a tank fail one hydro and then pass the next?
Rashly assuming that in all cases, the same hydro process was done to the same standards.

 

[guruboy]

"However, if a tank that fails hydro is tested again, it might "pass" the next hydro.."
And how might a tank fail one hydro and then pass the next?
Rashly assuming that in all cases, the same hydro process was done to the same standards.

Good question! Unfortunately, I'm not a metal expert or a hydro tester.

I have seen a tank that "failed" a hydro with 14% PE be re-tested and "pass" the re-test with 1.4% PE.

 

[tbone1004]

"However, if a tank that fails hydro is tested again, it might "pass" the next hydro.."
And how might a tank fail one hydro and then pass the next?
Rashly assuming that in all cases, the same hydro process was done to the same standards.

if it was performed properly the only way it would is if it was right on the borderline and it could have been a calibration/gauge issue

 

[Rred]

Well, like tbone said. Although one can argue that calibration and such ARE critical to hydro testing, and in that case the tank never actually failed the first hydro--that was operator error.

My friend and I had three lightly used Alu80's "fail" eddy flux testing back when all that nonsense was new. Well...no, the poorly trained newb using the equipment didn't know how to recognize normal thread tappings. Yes, the shop bought us three brand spanking new tanks shortly afterwards.

 

[Luis H]

If a galvanized cylinder was not tested using the proper per-test procedure, it can fail the test. I would be very surprised at a very high permanent expansion number. If the cylinder is tested again, it can pass with great data because the first test behaved as the pre-test procedure.

This is a function of the by-metallic composite material properties of the bonded zinc layer (the hot dip galvanized layer) on the steel cylinder.

I am not a metallurgist (just a mechanical and structural engineer). Even do I understand the mechanics of what is happening, I am not going to be good at trying to explain it. The video posted above will explain it much better than I can.

This video was posted above. Please take a look at it again… and again… and if needed view one more time.



BTW: it doesn't matter if the galvanized cylinder is a special permit steel cylinder or a 3AA cylinder. The 3AA is a high alloy chrome-moly steel.

 

[Luis H]

I posted this in another thread and it I feel like it is also relevant here.

I have been putting off replying to this until I talked to someone that actually known’s something about contract laws and regulations. After talking to our legal counsel at work I wrote the following email to a friend:


"All galvanized cylinders should go through the pretest expansion procedure. This applies to all hot dipped galvanized cylinders (special permits and 3AA codes). Many call the pre-test procedure, the "round-out" procedure, but it turns out that that is not accurate.

The galvanizing coating is a bonded zinc layer. The zinc layer is very thin. It is so thin that it is not a structural component, but it still behaves as a by-metallic composite structure. The zinc layer is very thin, but it still affects the rate of elastic expansion return of the overall composite structure.

The pre-test requirements just involve pre-stretching the cylinder by pumping it to 90% of the test pressure. Holding it for about a minute and then releasing the pressure. Then performing the hydro test.

This requirement is not from the DOT. It is from the manufacturer and it doesn't conflict with any DOT regulation or code (this is very important).

Some have argued (in ScubaBoard) that because it is not a DOT requirement the facility is not required to perform it. Well that is not exactly accurate either. The DOT may not enforce it, but you can still take it to a small claims court for violating a manufacturers requirement and ruining your personal property.

I asked this to our legal counsel (at work) and she agreed that the manufacturers requirement is still holds if a shop agrees to perform a service on that equipment. I am not a lawyer (I am engineer), but my argument to her was that if I take my car to a certified auto-mechanic to perform a required service. The manufacturer of my car very specifically provides a required procedure on how to perform that service. The mechanic decides to do it his own way for whatever reason and he ruins my car. He is liable for the damage he caused by not following the manufacturers required procedure.

That should be the end of the story, but as always, very few things are that easy. According to our legal counsel (again I am not a lawyer) the judge in a small claims court may have a lot of latitude to decide. She agreed that I have a strong case, but normally nothing is that simple... "



I have personally have had tree cylinders replaced by a hydro tester that erroneously condemned them. It never went as far as even needing to threaten with small claims court or even reporting to DOT. I just politely requested the test data and when I saw it I notice several discrepancies… It didn’t take much before he realized that he screwed up.



If anyone else has actual relevant legal knowledge please share with us.

 

[Luis H]

Note: I should add that my answer above is also a bit of a simplified answer. The hydro test pressure is designed to take the cylinder to the lower limit of the yield strength value. The means than a small amount of yielding (reflected in the permanent expansion) is allowed.

Some yielding is steel can cause work hardening, but not necessarily bad enough to consider the material brittle.

The hydro test is specifically checking if a material is brittle (as opposed to ductile). A brittle material will fracture (like a fragmentation grenade) as opposed to a ductile material which will first yield and at some pressure it will eventually develop a fracture, but will not fragment into smaller pieces. Most of it will yield and deform.

Some amount of yielding is commonly used to form parts. This forming process can be done without excessive work hardening and making the parts brittle.

Think about forming a metal coat hanger. You can bend it a number of times, but at some point you bend it enough to make it brittle and it fractures.



Steel cylinders are actually formed by yielding a flat plate into the shape required, but it is important to realize that after the forming process the metal is heat treated to relive the stresses generated during the forming process.

 

[JamesBon92007]

Let's not forget that if you own your own compressor, I don't think it's illegal to fill them without a hydro.

My understanding, from what I was told in the 60s, is that it's not illegal to fill a tank that is out of hydro, it's illegal to transport it. That might mean taking it on a public highway or it might mean to pick it up and move it over a few feet. I'm not real clear on that point

 

[guruboy]

Thanks, Luis. Great information as usual.

I found it interesting that they showed a new failure criteria for one of the SP tanks. Instead of failing for 10% PE, they are failed if the EE exceeds the REE value.

 

[BRT]

My understanding, from what I was told in the 60s, is that it's not illegal to fill a tank that is out of hydro, it's illegal to transport it. That might mean taking it on a public highway or it might mean to pick it up and move it over a few feet. I'm not real clear on that point

In interstate commerce unless your state has a law including DOT laws into theirs