Changes in a High-Pressure Drum to Eliminate Recurrence of Cracks Due to Corrosion Fatigue

Abstract

Abstract This paper gives the results of an investigation which was undertaken for the purpose of determining the cause of the cracks found in the No. 5 boiler at the Edgar Station of the Boston Edison Company. It also discusses the cause of the cracks found in the water columns and in certain of the superheater fittings and headers of this same boiler. The boiler in question is one of the first high-pressure units installed in this country and, therefore, it was quite essential that the cause of the cracks be determined and suitable steps be taken to prevent their recurrence. The study sets forth the changes which were made. As a result of the investigation it was not felt that any of the difficulty could be attributed to faulty metal. The material was of the proper chemical composition and had suitable physical properties. The primary cause of the cracks was believed due to corrosion fatigue resulting from stresses set up by changes in temperature on the surface of the metal in combination of a relatively low pH value of the water which was at times in contact with the metal that developed cracks. As a result of the removal of the cracked metal and the changes in design which would prevent fluctuating temperatures on the surface of the drum at the boiler feedwater inlet, as well as other design changes which would raise the pH value, particularly in the water columns, it is not anticipated that further cracking will take place.