Creep rupture failures in superheater tubes of high pressure boilers

Abstract

The influence of biaxial stress at elevated temperature on the behaviour of superheaters located in flue gas atmosphere has been studied. The superheaters are alloy steel tubes carrying superheated steam (40° to 540°C) at high pressures (about 100 atmospheres) and the external surface is subjected to convention and radiant heating with temperature in the range 700-900°C. There is almost a constant heat transfer from outside to the steam flowing within the tube. In certain cases due to lack of vigilance on the part of boiler operators a high input of heat or lack of sufficient steam flow in the tube caused relatively rapid creep deformation due to the biaxial stress (hoop stress and longitudinal stress). In the initial stages, the material bulged and gradually thinned down due to creep. Further, the presence of stress at high temperature accelerated the building up of thick oxide layers at the external surface. This resulted in poor heat transfer from external surface to the steam. The tube got overheated subsequently. At a later stage longitudinal microfissures were heated to about 900°C due to operational errors and the sudden bursting of the tube and subsequent quenching by escaping steam resulted in the formation of martensite plus bainite structure at the over-heated zones. (Shri S. Arunagiri, Assistant Engineer and Dr. G. Venkataraman, Assistant Superintendent of Central Laboratory, Quality Control Department, Bharat Heavy Electricals Limited, Tiruchirapalli)