High operating steam pressure and localized overheating of a primary superheater tube

Abstract

Common boiler sites which likely have tube failures due to short-term overheating are superheaters and reheaters. Severe short-term overheating caused by an unusual operating condition occurs when the tube temperature significantly rises above design limits for a brief period. Therefore, immediate investigation on unusual occurrences preceding failure may be extremely important in identifying the cause of failure. The efforts should be thoroughly addressed to the operating procedures and system design. This paper presents failure evaluation on a primary superheater tube of a power plant. In situ hardness measurements on the selected region of the failed primary superheater tube were carried out. The average hardness was then used for estimating the average operating temperature prior to failure. Creep analysis was conducted to confirm whether or not the creep damage contributed to the failure. Microstructures of the as-received tube were observed through microscopic examinations. Similar incidents occurred at the different units of the power plant in 2007 were reported by Purbolaksono et al. [1] and Rahman et al. [2]. The first failure [1] showed that a primary superheater tube failed with a wide open burst after running at around 28,194 h following the firing of the low quality coal causing heavy clinker formations. At around the same time, another failure [2] occurred due to the similar root cause but it was also in combination with the coal-ash corrosion attack. 2. Operational background On 26 February 2011, one of the units in the power plant was forced to shut down due to a rupture in one of the primary superheater SA213-T12 tubes. It was reported the superheater tube had been in service for around 48,000 h. According to the operational records, the operating steam temperature and pressure for primary superheater are maintained at around 430– 460 _C and 176.5 bar (17.65 MPa), respectively. The superheater tubes have outer diameter of 45 mm, the thickness of 4.6 mm and a standard hardness of 163 HV. Due to limited coal supply in the market during some periods prior to failure, it was reported that several types of coals with different ash fusion temperatures as listed in Table 1 were used in the last 45 days prior to failure. It can be seen from Table 1, there are three types of coal (Types A, D, and G) having lower ash fusion temperatures in comparison to the average furnace flame temperature of 1400 _C. Thus, it would likely trigger the formations of heavy clinker during the firing.