ABSTRACT
A simple set-up and methodology was developed to semi-quantitatively measure water transport (liquid and vapor) through intact and damaged insulation when a thermal gradient across the insulation is maintained. The main objective was to contribute to a better understanding of water transport mechanism in different insulation materials which can be related to corrosion under insulation (CUI). Most tests were conducted using closed cell polyurethane (PU) insulation. For comparison purposes, tests were conducted with mineral wool insulation as well as with insulation externally covered with water vapor permeable or impermeable layers. Due to size limitations, only PU results will be presented.
Results from this study indicated different time scales for the following main processes affecting the moisture content at the metal/insulation interfaced) Water transport from the environment to the metal surface through intact insulation. 2) Water transport from the environment to the metal surface through damaged insulation. 3) Water transport out of the metal surface through the insulation. A potentially effective approach for CUI mitigation based on this understanding is illustrated.
INTRODUCTION
Corrosion under Insulation (CUI) is an important integrity threat and a significant contributor to leaks in many industries (Oil and Gas: Production, Transport (pipelines) and Refining; Chemical and Petrochemical Plants; etc.)1-3. This type of corrosion can cause failures in areas that are not normally of a primary concern to an inspection program. The failures can be catastrophic because the corrosion typically occurs over a sizeable surface area. The rate of CUI depends on several factors, such as the amount and composition (e.g. chloride content) of water present beneath the insulation, availability of oxygen, and temperature.
It has been recognized that a critical factor controlling CUI is the presence of moisture (liquid water) at the steel surface. However, the minimum amount of moisture required to sustain CUI has not been quantified for different insulating materials. For warm / hot piping, there is a thermodynamic driving force to move the water away from the pipe surface and hence reduce likelihood of corrosion unless the overall system is unable to remove the moisture causing corrosion.
