The causes of early cracking in petrochemical flanges
By: Brigette Federico
Submitted: 2010-05-05 01:48:47 | Word Count: 383
There have been tremendous advances in the design of pipe flanges used in petrochemical environments. However, despite improvements in stainless steel alloys and flange manufacture, early cracking is still widely observed.
Cracking of flanges in high pressure, high temperature environments – such as you get at petrochemical plants – are extremely hazardous, as well as costly. NACE International (previously known as the National Association of Corrosion Engineers) has conducted numerous studies into this. One such study involved pipes at a petrochemical plant carrying super-heated butane at high pressure. The alloys were chosen for their extreme toughness and durability. Yet extreme early cracking had occurred at the pipe flanges. One of these was examined in detail.
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NACE used a variety of microstructural techniques to examine the damage, including optical microscopy, grain boundary chemistry and X-Ray analysis. Through these, they discovered various molecular and tensile changes. These included carbon precipitation, grain sliding and brittle granulation resulting in a “rock candy” appearance. However, these alone did not explain how the cracks themselves had appeared – or why they were so deep.
However, the study also used a technique called finite elemental analysis (FEA). This gave a detailed analysis of all the stresses the pipe fittings were subjected to, i.e. internal pressure, external loading due to thermal movement, and bolt loading stress. When this was done, it was discovered that a combination of all these factors led to an unacceptably high degree of stress at the flange site. A general stress analysis program would only have taken pressure and thermal stress into account, and given the incorrect diagnosis that stress was within allowable limits.
The alloys that we at Chemipetro provide are the strongest and most corrosion-resistant in their class. Super Duplex, for example, is resistant to pressure, corrosion and thermal stresses. However, you can further improve on this by ensuring pipe fittings have a fully supportive external architecture, ensuring no undue stresses are being introduced when they’re fitted.
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