ABSTRACT
Water wall tubes from boiler operating since 14 years were subject to repeated failures ranging from pinholes to cracks and ruptures. A failure analysis on received tube was carried out including destructive and non-destructive testing. Feedwater, drum water and live steam qualities were evaluated based on the data provided by the plant authority. The results obtained through this study indicate a mechanism of caustic gouging inducing hydrogen embrittlement. The root causes determined were improper carryover control and waterline creation by low flow and/or high blow down rate. Therefore, it was recommended to conduct a total inspection of the boiler to determine the extent of the damage. As preventive maintenance, a more strict control of the carryover is necessary through the monitoring of sodium/phosphate ratio, reduction of oxygen up to the permissible level, fixation of the chemical boiler treatment dosing problems.
INTRODUCTION
Steam-power cogeneration plants represent an important utility, which provide energy (thermal and electrical) to drive thermal desalination processes such as multi stage flash (MSF) plants. Due to the increasing fresh water demand, MSF plants should offer high level of reliability with very limited forced shut down periods. Despite efficient operation and maintenance (O&M) practices, saline water conversion corporation (SWCC1) is still facing steam/power cogeneration plant failures especially in boiler tubes. During the last 35 years of O&M experience, different boiler tube failures were reported and studied within the SWCC facilities.1-5 It is well established that boiler tube corrosion failures are caused mainly by waterside deposit/corrosion processes that affect the protective magnetite layer present on the internal surface of tube metal. These waterside processes combined with external fireside conditions induce different forms of tube failures as discussed by many authors.6-8
Among these different failure mechanisms, hydrogen embrittlement (HE) shows a limited occurrence within SWCC steam/power cogeneration plants. HE is defined simply as degradation of mechanical properties of metals which results in decrease of fracture resistance and subcritical cracking in the presence of dissolved hydrogen.9 The sources of the hydrogen, the paths it takes to enter the material and the embrittlement mechanisms are extremely diverse. These factors must therefore be established in each particular case. For boiler tubes, the common source of this dissolved hydrogen is a corrosion process on the magnetite layer that could be acidic or alkaline. The former is generally due to the so called phosphate attack and the latter corresponds to the caustic corrosion. The mechanisms of these processes are well described elsewhere.10,11 The susceptibility of metallic alloys to this type of environmental cracking depends on the complex interaction between stress, material and environmental parameters. Typical locations of this phenomenon include areas with high heat flux, thermal-hydraulic flow disruption, and localized overheating.12
