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March 21 - March 27, 2011

How does rapid depressurization affect lining performance in vessels and line pipe, and how can you identify any failure that might result?

Selected Answers

From Simon Hope of BIS Salamis (M&I) on April 4, 2011:
     Explosive decompression has been the bane of the offshore process coatings manufacturers for many years now. The first major failures occurred in spray-applied glass flake vinyl and polyester linings in systems operating at high pressure (100s Bar) and temperature (around 100 C min). Research found that small (organic) molecules were migrating as liquids at the operating criteria into the interstitial spaces that were created during spray application due to the random orientation of the flakes. When the process is depressurised in an emergency, the molecules which are liquid during operation, volatilise creating huge pressure stress within the coating that may manifest itself as either a blister or an area of cohesive failure within the coating. The problem can be improved by using trowel-applied coatings with larger flakes, which are 'organised' due to the way the material is trowelled and rolled, thus reducing the interstices. Care must also be taken to ensure that the material is used within its Tg (glass transition temperature), as this exacerbates the failure rate due to the weakening of the coating film. Cold-wall effects play a large part in coating failures where stresses are introduced along with migration to the substrate.

From James McDonald of Accelerated Services on March 30, 2011:
     Rapid depressurization of lined vessels can cause catastrophic failure to the lining system. This problem is most evident in vessels or pipelines with CO2 in the vapor space above the cargo. The vapor permeates the surface of the coating, and when a rapid depressurization event occurs, that vapor expands inside the coating and typically causes cohesive failure . The West Texas Oil & Gas Market has struggled with this problem for years due to the high CO2 levels used in tertiary recovery.

From Chris Faulstich of SALC, Inc. on March 25, 2011:
     I would think that rapid depressurization would lead to cavitation, causing loss of coating(s) and even metal in the area of cavitation.

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