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It seems to me that the 72s that fail tend to be the ones that have been sitting for a long time without pressure cylces. They show up covered in cobwebs and dust, decades out of hydro, and they go way, way over expansion limit. It's not all 72s by any means but I notice that ones that fail seem to fit a pattern.
I thought rust pits were the death of steel cylinders?
I don't see how a steel tank sitting unpressurized with spider webs has anything to do with passing or failing.
I've restored some real relics that the last hydro was 3O years ago and they sat in some widows garage for the entire time either low in air or empty. I clean them up, send them to hydro and they pass every time. I have never had a steel 72 fail and I've sent dozens through. I give most away to our new urchin divers just for whatever I'm in
Them. I don't make a profit.
 
I thought rust pits were the death of steel cylinders?
I don't see how a steel tank sitting unpressurized with spider webs has anything to do with passing or failing.
I've restored some real relics that the last hydro was 3O years ago and they sat in some widows garage for the entire time either low in air or empty. I clean them up, send them to hydro and they pass every time. I have never had a steel 72 fail and I've sent dozens through. I give most away to our new urchin divers just for whatever I'm in
Them. I don't make a profit.
Just an observation i have seen.... some steel tanks that have not been cycled seem to lose their elasticity. Many pass no problem but the 72s that fail seem to have been sitting unused (not empty) for a long time.

Maybe a metalurgist can shed some light?
 
Just an observation i have seen.... some steel tanks that have not been cycled seem to lose their elasticity. Many pass no problem but the 72s that fail seem to have been sitting unused (not empty) for a long time.

Maybe a metalurgist can shed some light?
???
IDK?
 
It seems to me that the 72s that fail tend to be the ones that have been sitting for a long time without pressure cylces. They show up covered in cobwebs and dust, decades out of hydro, and they go way, way over expansion limit. It's not all 72s by any means but I notice that ones that fail seem to fit a pattern.
interesting... I have some PST 100 HP's set-up as doubles that have been in my garage for 24 years that I'm thinking about splitting up into singles and using them again
Maybe a metalurgist can shed some light?
I'll email my brother-in-law and see if he has any thoughts... PhD in metallurgy.
 
interesting... I have some PST 100 HP's set-up as doubles that have been in my garage for 24 years that I'm thinking about splitting up into singles and using them again
Have not seen the same issue with HP steels. I would bet they pass no problem.
 
[...]some steel tanks that have not been cycled seem to lose their elasticity. Many pass no problem but the 72s that fail seem to have been sitting unused (not empty) for a long time.[...]
I’m no metallurgist by any means, but the idea that a steel cylinder sitting pressurized somewhere “loses its elasticity” over time is almost certainly a coincidence rather than a material property. What you’re describing, time‑dependent strain under constant load, is called creep, and in Chrome Molybdenum steels like 4130 at room temperature the creep rate is essentially zero.

Creep requires prolonged exposure to temperatures high enough to activate diffusion processes in the crystal lattice. For 4130 steel, you only start to see measurable creep at temperatures above 200°C, probably more at 400°C if one assumes the usual 0.3 of the melting temperature (1500°C). At ambient temperature of 20 °C, the lattice is “frozen”, so decades of pressurization shouldn't have any effect.

Are you certain that these cylinders failed due to the actual hydrostatic test results, or were other factors at play? Failure due to changes in elasticity is incredibly rare during a hydro test. I can probably count on one hand the number of cylinders I've had to fail explicitly due to them not passing the hydro.
More often than not a failure during the hydro is a mistake of the operator, not the cylinder itself. Visual inspections on the other hand can lead to failures rapidly.
 
Just an observation i have seen.... some steel tanks that have not been cycled seem to lose their elasticity. Many pass no problem but the 72s that fail seem to have been sitting unused (not empty) for a long time.

Maybe a metalurgist can shed some light?
it's why the roundout process is important for these older galvanized tanks.
 
To 2800 psi, nothing.
What happens when you take them to hydro pressure every year?
I'd have to go find it, but I think the faber tanks were rated to like 10k hydros. So fills of 2800 is not even to that level - I think you'd be hard pressed to outdive a steel 72 (I know some still fail once in a while, but I'd guess many of those missed a roundout process or other hydro test failures)
 
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