FITNESS-FOR-SERVICE ASSESSMENT OF A STEAM PIPELINE OPERATING IN THE CREEP RANGE AFTER 230000 HOURS OF SERVICE

High energy pipes, carrying steam at high pressure and temperature, are very common in the industry. In this paper; a Fitness-For-Service assessment was performed for a pipeline connecting the boiler of a paper mill to the cogeneration turbine and the process headers. A pressure of 6.4 MPa and a temperature of 475 degrees C were the operating conditions for 230000 hours of continued service, thus arising concern about metal loss due to erosion/corrosion and high temperature creep damage. Original pipeline design information was unknown, hence, an extensive NDE program was carried out to obtain the required data for the FFS analysis. Because of the elevated number of in-service hours, an API 579-1 Level 3 assessment was required and a FE shell model of the line was set up to evaluate plastic strain accumulation due to creep through a time-dependant inelastic analysis. The predicted critical components were then investigated experimentally taking metallographic replicas in situ and the actual damage was finally, qualitatively, compared, with that calculated with the API 579-1 procedures, with good agreement. The results of the assessment led to an estimate of 70000 hours of residual life for the pipeline and could drive the planning for future NDE according to a RBI approach.