Fitness for Service and Creep Rupture Damage Assessment
We performed API 579/ ASME FFS-1 Fitness For Service on Overheated Boiler Tubes.
The boiler tubes in the client’s plant were exposed to overheating, which resulted from faulty tube connections, restricting the cooling circulation to near zero in the second pass. This resulted in temporary overheating of the SA 213-T2 tubes prior to the installation of jumper tubes.
The overheating, which began on December 19 1997, resulted in one tube failure after about 14 hours, a second tube failure after 36 hours, and a third tube failure on January, 1998 – after near 43 hours of operation at elevated temperature. The unit was operated for a total of 94 hours at elevated temperature.
We also performed a creep rupture damage assessment on the remaining uncracked tubes which experienced pressure at elevated temperature for 94 hours.
The first and second boiler pass circuit design temperature was 750 F and the design pressure was 4,300 – 4,500 psi. The measured hourly second pass boiler tube temperature was 725 F and the maximum average temperature was 732 F. The maximum hourly superheater outlet pressure was 3,832 psi and the maximum pressure was 4,096 psi. Percent creep rupture damage was calculated for the 94 hours of elevated temperature operation.
The SA 213-T2 ferritic material tubes have an ultimate strength of 60 ksi ultimate strength, and a minimum yield strength of 30 ksi. A number of the tubes exhibited at least 22 percent elongation.
Tubes under internal pressure operating at elevated temperature for extended periods of time can eventually split longitudinally due to creep tensile instability. Three of the boiler tubes experienced such failures. The remaining tubes which did not fail were subjected to creep rupture damage during the pressurized period at elevated temperature. Fitness For Service was evaluated on the damage and its effect on suitability for continued operation.
The evaluation was carried out using the minimum elevated temperature creep and creep rupture properties of the materials. The temperature at which creep tensile instability occurs under the known operating temperature was evaluated for the times of high temperature exposure.
We Perform Fitness for Service on Equipment including Vessels and Heat Exchangers.
– Tom O’Donnell, PE
– (PDF) “Weld Defects and Failures: Quantifying Fitness For Service – How Finite Element Mathematical Simulations can Mitigate Risk, Cut Costs and Improve Overall Safety”
– Publications: Heat Exchangers, Pressure Vessels, Welds and Other Applications, Fatigue, Elevated Temperature