NDTnet 1998 Aug, Vol.3 No.8

Abstract

Since 1990, the liquefaction propane gas professional observe a great request for the LPG underground tanks. These tanks are more safe for the environment and more aesthetic for the customers. However, classic NDT (XR, US) can not be applied to the underground tank. Indeed, the welds are not easily accessible by classic NDT method. A new NDT must be developed. We have chosen acoustic emission because it is a overall method, all the structure is test in the same time and you can test the non approachable weld. Besides, acoustic emission only detect growing defects therefore the more dangerous for the life time of the tank. Our paper describe the pressure tests realised on LPG Underground tanks with and without defects and the signal analysis.

The pressurization system allow an increase pressure to 24 bars with a average speed of 0,6 bar/mint the gas use for the pressurization is the process gas (propane).

At the pressure test, 21 bars, approximately 40% goods tanks are classified in class " good". Consequently, 60% are classified in the class "bad". After pressurization test, complementary investigations (100% of the weld) have been realized. These studies prove, there always is a link between AE classified in class "bad" and one default not allowed by French regulation, for example, lack of fusion. Moreover, we have detected by acoustic emission some bilstering in the coating epoxy. At the pressure test, 21 bars, approximately 80% of the bad tanks are classified in class "bad". Consequently 20% are classified in class "good" in spite of the defect in the weld. These defects are not growing defects, it is the reason for, we have not detected crack. After, AE test we have realised mechanical test to prove that the not growing defects are not dangerous for the in service tanks. The pressure vessels we cycled, a time equivalent to 40 years of time life. All the LPG underground tanks cycled were good, no leakage after the mechanical test. The mechanical test prove that the pressure vessel with no AE activity have a real resistance in service condition. Consequently, although they have a defect, they can stay on service without danger.

The evaluation and analysis of these data give us the opportunity to classify the LPG underground tanks according integrity and ability for further service. The detection of faulty tanks has been tremendously improved with a success yield of about 80%. Fatigue test proved that the remaining 20% not detected by AE were absolutely free of leakage and safe for service. Since the precision of the detection and the efficiency of the "feature extraction" improves with the number of tested LPG - Tanks, TUV and INSTITUT DE SOUDURE decided to create a data bank for small LPG -Tanks.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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