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Technical Discussions
bill king
bill king
07:52 Jan-15-2006
Gas cylinder UT inspection

I wanted to find out about non relevent indications on the bottom of gas cylinders. I was talking with an Eng. who was having problems with a DOT inspection his company's been doing using UT instead of hydro. At the bottom og the cylinder were the transition is sometimes they get nonrelevent indications which cause excesive regection rates on some expensive cylinders. At that transition though there is some potential for some corossion. Is it possible to tell the difference using an A scan between the two. Thanks.


    
 
 
Richard Paciej
Richard Paciej
07:49 Mar-11-2006
Re: Gas cylinder UT inspection
The part of the cylinder that you are referring to is the SBT region or sidewall to base transition. It is a high stress area. If a crack forms here, it could grow and cause a cylinder to fail. Cylinders manufactured along time ago have a shaper transition region and this gives rise to higher stresses. Newly manufactured cylinders also have this region but the transition is much smoother. This is the area where the wall thickness of the cylinder increases as it approaches the base of the cylinder.

The DOT requires the reference standard or the reference cylinder to have a reference notch for the SBT. Typically, the size of the notch is 10% of Tmin or 10% of the minimum wall requirements of the cylinder. The Tmin of cylinders range from 0.1 to over 0.250 inches and it depends on its diameter and service pressure. So the reference notch would range from 0.01 to 0.025 inches deep and is typically one inch long.

Corrosion in the SBT region is a potential problem. If it is purely wall thinning(and does not reach below the Tmin) and does not have any cracks, it should be okay. However, cracks can develop from corrosion pits.
Line corrosion crack develop at or near the SBT where water or other corrosive liquid and gas meet. In the SBT region, it would be difficult to distingush between just corrosion and corrosion with a crack(s). Also, since it is near the bottom of the cylinder, it is difficult to approach the defect from both sides using typical ultrasonic equipment used to test cylinders. You would have to do it using a hand held unit. This would be a secondary inspection and is not permitted by the DOT(yet).

Of course, if you can state that you only can either get cracks or general corrosion in this region, you could use an A-scan to distingush the two. An A-scan for a crack would give a response similar to the reference notch and most likely be sharp. Corrosion would give an A-scan that is more broad. However, there is always a chance that the two defects are there mixed together.

CGA has a standard that describes ultrasonic testing of cylinders. If you an not a CGA member, you will need to purchase it. The title of the standard is

C-20 Methods for Ultrasonic Examination of Metallic, DOT and TC 3-Series Gas Cylinders and Tubes.


I hope that this helps.

----------- Start Original Message -----------
: I wanted to find out about non relevent indications on the bottom of gas cylinders. I was talking with an Eng. who was having problems with a DOT inspection his company's been doing using UT instead of hydro. At the bottom og the cylinder were the transition is sometimes they get nonrelevent indications which cause excesive regection rates on some expensive cylinders. At that transition though there is some potential for some corossion. Is it possible to tell the difference using an A scan between the two. Thanks.
------------ End Original Message ------------




    
 
 

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