The Influence of the Manganese Content of Mild Steel on the Rate of Crack Formation in Geothermal Steam Pipes at the Reykjanes Power Plant in Iceland

Abstract

HS Orka has been operating the Svartsengi Geothermal Power plant since 1976, the Reykjanes Steam Field since 1981, and the 100 MW Reykjanes Power Plant since 2006. The fields produce brine in the temperature range from 235°C to about 300°C and with salinity from about two third (2/3) to full salinity of seawater as the reservoir fluid is of seawater origin. Up to 2006 cracking of mild steel steam pipes occurred occasionally, generally in the vicinity of welding seams. However, after nine months operation of the Reykjanes Power Plant the cracking problem increased dramatically. This paper describes decades of dealing with the mystery of steel pipe cracking at the Reykjanes and Svartsengi geothermal plants. When the Reykjanes Plant was built and commissioned, the material test certificates were systematically filed. When going through the documents it was discovered that the pipes with the highest Mn content were usually the first to crack. Also, mechanical factors and protective film formation probably play part in the cracking process. Thus, the frequency of cracking increases after the steam has been filtered in a mesh-type moisture separator. Not all steam pipes crack and pipes containing brine or two phase flow have also fared considerably better but yet they are not free from cracking. Several metallurgical studies have been conducted on the mechanism of crack formation, some of them indicating the possible effect of hydrogen formation due to the presence of sulfide and chloride in the steam. The most promising solution for the continued use of mild steel pipes for the Reykjanes Power Plant seems to be to utilize seamless pipes with less than 0,8% manganese content. Stress relieving of circumferential welds should also be considered in high risk areas.