Abstract
In order to develop effective and economical measures against ESCC in chemical plants, failure case studies, field inspection and a simulation test were carried out. Fifty eight cases of ESCC in chemical plants were analysed on the following items; 1) features of ESCC under thermal insulation and in the absence of insulation, 2) effect of insulating materials on the ESCC frequency, 3) temperature range for ESCC and 4) detrimental ions and their sources. Our case study showed that the detrimental ion was soluble chloride in most ESCC cases underneath insulation, and its source was the atmosphere rather than the insulation itself.
Field inspection was planned in order to monitor the detrimental ion contained in the insulation, 48 pieces of insulation were taken off from 6 plants in two sites, one site is located in coastal area and the other inland. A new method called Hole Drilling made it possible to sample a plug of insulation from plants in service without removing the jacketing. Chemical analysis revealed that the plants in the coastal area were significantly contaminated with chloride and fluoride ions. On the other hand, most insulation in interior plants were kept under acceptable level of the ASTM-C795 specification.
Wick tests were carried out using C-ring and butt-welded pipes of AISI 304 stainless steel pipes surrounded with calcium silicate insulation. Laboratory tests clarified the following key parameters for ESCC, i.e., 1) surface temperature range, 2) threshold resultant stress and 3) potential detrimental ions and their threshold concentration.
Based on this series of studies, inspection procedure and countermeasures which are applicable to plants in service and under planning or construction are discussed.
