Finite elemental analysis (FEA) was one of several techniques used by NACE (an international association of corrosion engineers) to investigate failure in petrochemical flanges in 2001. The study involved cracking around welded high-temperature pipe fittings. Made from high-strength austenite stainless steel alloys, these should not have failed.

The study was carried out on 24” diameter pipes leading from charge heater units. These pipes carried i-butane at an operating temperature of 650oC, under high pressure, to reactor vessels. The pipe was welded to 24″ flanges made of the same steel alloy. This was the site at which cracking occurred, despite the steel being selected for its ability to withstand temperatures in excess of 650oC.

Tests were carried out on a piece of failed pipe welded to a flange. The pipe, which had three distinct cracks of 5 – 20 cm long, was first subjected to a dye penetration test. The mechanism of damage, changes in microstructure and alterations in grain boundary chemistry were then analysed using a variety of metallurgical techniques. These included optical microscopy; scanning electron microscopy; X-Ray fluorescence, tensile tests and FEA.

Although the flanges and pipe fittings were made of the same alloy, there were marked differences between the grains of the flange and pipe metal. Cracks near the weld went deep into the flange, and carbide precipitation, grain sliding, and brittle granular precipitation could be seen.

Most telling were the FEA results. These confirmed that pipe flanges undergo severe thermal, internal pressure and bolt-loading stresses which, when combined, can lead to catastrophic failure. We at Chemipetro supply super duplex flanges, which are heat, pressure and chemical resistant. However, the importance of correct fitting must never be underestimated.