Charging extra for VIP??
- Thread starter scuberd
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Jack Hammer
Contributor
To me it's pretty simple. I'd like to know the hydro didn't cause any cracks to start forming in the neck...
R
redacted
Guest
As long as you are going to ignore what is known about Sustained Load Cracking development and growth, would it not be wise to have such an inspection conducted before every fill?
Jack Hammer
Contributor
So you're telling me it's impossible for any type of crack to develop or begin to form in in the threads of a tank that has gone to hydro and every single tank that has gone to hydro was 100% properly inspected and tested and it is, again impossible for there to be any issue with that cylinder so it's an utter and absolute waste of my personal time to look at the threads of my tanks when they come back from hydro?
Gotta love the internet...
2airishuman
Contributor
There are objective standards for the visual inspection. It is more common (I've been told it's by a factor of 10:1) for cylinders to fail the visual portion than to fail the hydrostatic portion of the requalification. In most cases a cylinder failing visual would pass hydro. The tests are sensitive for different classes of defects.
The visual test looks for localized weakness. For steel cylinders, this would be caused by corrosion, inadvertent contact with welding or cutting equipment (common for some welding gases), or usually severe mechanical damage. For 3AL cylinders, it could also take the form of cracks. Because these weaknesses are localized, they do not affect the overall elasticity of the cylinder. The only way they show up during the hydrostatic portion of the test is if they are severe enough to cause an actual rupture to occur when the cylinder is brought up to test pressure. Ruptures and neck thread failures, I've been told, are the most common causes of actual failures of the hydrostatic portion of the test.
The comparison of elastic and inelastic expansion during the hydrostatic test evaluates the average wall strength. It is sensitive for problems of a metallurgical nature, such as manufacturing defects or loss of temper due to exposure to high heat. It is also sensitive for overall loss of wall thickness.
The re-qualification procedures for steel cylinders haven't changed in 100 years. They are not great tests, but they are easy and cheap to perform, and have a track record of keeping the packaged gas industry safe without creating excessive cost due to false positives. The newer ultrasound tests (mentioned upthread) are far more accurate predictors of a cylinder's safety.
2airishuman
Contributor
So you're telling me it's impossible for any type of crack to develop or begin to form in in the threads of a tank that has gone to hydro and every single tank that has gone to hydro was 100% properly inspected and tested and it is, again impossible for there to be any issue with that cylinder so it's an utter and absolute waste of my personal time to look at the threads of my tanks when they come back from hydro?
Gotta love the internet...
If you believe that your hydro shop might stamp their RIN on a cylinder that isn't safe, then why are you using them?
If you have a cylinder where you believe neck cracks can progress to a dangerous degree as a result of the hydrostatic test itself, then why are you using it when other cylinders made of more trustworthy materials are available?
R
redacted
Guest
So you're telling me it's impossible for any type of crack to develop or begin to form in in the threads of a tank that has gone to hydro and every single tank that has gone to hydro was 100% properly inspected and tested and it is, again impossible for there to be any issue with that cylinder so it's an utter and absolute waste of my personal time to look at the threads of my tanks when they come back from hydro?
Gotta love the internet...
I'm not sure how you could misread my post like that. I agree that cracking can start anytime the tank is containing pressure.
But: Sustained-load cracking (SLC) in ruptured scuba cylinder made from 6351 aluminum alloy
Jack Hammer
Contributor
Great and valid questions. In short, I don't think those things will happen and are extremely unlikely to occur, but not impossible. A few minutes of my time is worth peace of mind. People are reading way too deeply into a simple comment.
When I got my VIP inspector cert we were shown a few rare examples of cylinders that passed hydro yet failed their VIP afterwards and told a simple check is worth the time. IIRC, one had cracks in the threads showing that supposedly were not visible before the hydro. As my instructor has since passed I'm unable to clarify my recollections with him.
Not directed @2airishuman, What I find comical is people who believe any opinions that differs from theirs is wrong and start making ignorant comments that they think make them look smart. You'd think this was a topic on religion...
I thought it was a topic on the pricing of a hydro+vip rather than a topic on the technical details of hydrostatic/ultrasound testing. Go figure.
WarrenZ
The Ultimate Bad Example
There is a problem with the link I posted earlier. Check out this video.
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