Failure analysis and retrofitting of superheater tubes in utility boiler

• The superheater tube failed two times, in July 2006 and May 2009 at the same place.
• Superheater tubes suffer from long term overheating.
• Attemperator over spraying was identified as the main affecting factor in overheating.
• Some modifications were applied in the boiler superheater tubes.

The extreme spray water mass flow rate deviation was observed to occur in the middle temperature superheater of Sahand 2 × 325 MW Power Plant utility boiler, which severely affected its economic performance and safe operation. Boilers operating in these conditions led to failure in superheater tubes at the same place for two consecutive times in a three year span. Thus, the failure analysis of superheater tubes by investigating the visual inspection, chemical, scale and creep analysis was carried out. The brittle failure occurred in the superheater tubes after the fuel was changed from natural gas to heavy oil. Failure analysis showed that tubes were suffering from long term overheating which was instigated by high spray water flow rate. In order to rectify the boiler operating conditions, some modifications were applied in the boiler unit 1 and operating parameters on this boiler were compared with boiler unit 2. The results showed that the 8.33% reduction in heating surface area corresponds to 52.84 and 17.80% reduction in spray water mass flow rate for capacities equal to 300 and 260 MW, respectively.