It is important to keep in mind that hydro test is basically testing one thing and one thing only, that is the average material elasticity of the complete cylinder. It therefore is indirectly checking that the hydro test pressure is still below the materials average yield strength, but that is it. It is not checking anything else.
A tank can very easily pass hydro and still have cracks, pits, and many other local flaws that are hopefully caught during a visual inspection if done carefully.
As an example, it fairly common for an old aluminum tank (alloy 6351) to pass hydro and show neck cracks right after the hydro is done.
I am also personally aware of two steel tanks that passed hydro (in the last year) and during inspection they both had cracks, one had cracks in the neck, the other was a spun tank and had cracks in the bottom.
My point is that passing a hydro test is not a guaranty of total structural integrity. In many ways a careful visual inspection is more important and will catch more defects than a hydro test.
Pressure cylinders are about the simplest structures I have ever worked on, but because it is a closed container with a small opening they are a bit difficult to inspect.
On the other hand the geometry is so simple that there is very little need for complex testing. We dont see anyone (or hardly anyone) doing dye penetrant testing, magnetic particles testing, ultra sound inspection, x-ray inspection, etc etc. on a pressure tanks there is no real need. A careful surface inspection (inside and out) and a material condition test (hydro test) is enough for a high certainty of prediction of its structural integrity.
Notice in the last sentence the terms high certainty of prediction that is why we also include a calculated safety factor. Now, if you want to cut into the safety factor you may just be taking unnecessary risk even if the risk is low.
A tank can very easily pass hydro and still have cracks, pits, and many other local flaws that are hopefully caught during a visual inspection if done carefully.
As an example, it fairly common for an old aluminum tank (alloy 6351) to pass hydro and show neck cracks right after the hydro is done.
I am also personally aware of two steel tanks that passed hydro (in the last year) and during inspection they both had cracks, one had cracks in the neck, the other was a spun tank and had cracks in the bottom.
My point is that passing a hydro test is not a guaranty of total structural integrity. In many ways a careful visual inspection is more important and will catch more defects than a hydro test.
Pressure cylinders are about the simplest structures I have ever worked on, but because it is a closed container with a small opening they are a bit difficult to inspect.
On the other hand the geometry is so simple that there is very little need for complex testing. We dont see anyone (or hardly anyone) doing dye penetrant testing, magnetic particles testing, ultra sound inspection, x-ray inspection, etc etc. on a pressure tanks there is no real need. A careful surface inspection (inside and out) and a material condition test (hydro test) is enough for a high certainty of prediction of its structural integrity.
Notice in the last sentence the terms high certainty of prediction that is why we also include a calculated safety factor. Now, if you want to cut into the safety factor you may just be taking unnecessary risk even if the risk is low.
