The Role of NDE in Maximizing the Value of High Temperature Tubing

Continued operation of high temperature tubing in superheaters and reheaters presents operation and maintenance personnel with risk-based decisions. The design of SH/RH tubing and associated dissimilar metal welds is well understood, as are the life degradation mechanisms, which have been thoroughly documented by EPRI. The major problems associated with life prediction of SH/RH tubing are the non-uniform distribution of temperatures and coal ash corrosion, with bending stresses contributing to the failures of DMWs. Basing run/repair decisions on the average life consumption projection (which can be obtained from plant records and deterministic computer programs) will result in many inservice failures of the weakest (hottest) tubes in the section. Basing section replacement decisions on the first few failures that occur results in removal of the majority of tubes that have hundreds of thousands of hours of remaining life. A risk-based approach to failures in SH/RH sections consists of periodic tube evaluation and repair/replacement of those tubes expected to have a high probability of failure during the next period of operation, typically 18 – 36 months. This paper summarizes the role of NDE inspection technology in assisting plant personnel in making SH/RH run/repair replace decisions. Early developments in the use of ultrasonic, radiographic and replication for predicting the life of tubing and DMWs will be reviewed and recent advancements increasing the accuracy of UT testing will be presented. The use of probabilistically based life prediction computer codes will be introduced and several case histories will be reviewed. Finally, an economically based risk assessment model that incorporates plant operating data, inspection results and acceptable risk levels will be presented. Introduction When steam plant temperatures and pressures of 1000 – 1050F and 2400 – 3600 psi were introduced in the late 1940’s and early 1950’s, the decrease in heat rate more than offset the use of expensive alloy steels for SH/RH tubing. The conservative design philosophy of boiler manufacturers and the ASME Code permitted the tubing to last for 30 – 40 years, despite the high temperatures and stresses. In the regulated utility atmosphere, a few failures in a SH/RH section prompted replacement of the entire section, as the cost of maintaining a boiler was not as closely watched as it is today. In the 1980’s, with life extension becoming a major strategy for utilities anxious to avoid building new generating capacity, boiler tube failure mechanisms were thoroughly studied and two main culprits were identified; uneven temperature distributions across the boiler and coal ash corrosion for tubing and bending stresses for DMWs. Figure 1 shows typical temperature distributions for a wall-fired boiler (tangentially fired boilers tend to have the hot tubes along the walls, giving the temperature distribution a bathtub shape). Figure 1. Temperature Distribution Across a Wall-Fired Boiler. Coal-ash corrosion occurs when the ash fusion temperature is lower than the tube metal temperature, resulting in a liquid slag that oxidizes the metal at a rate many times the normal 0.001 – 0.002” per year. The tube metal temperature increases as the tube operates, due to the build-up of an internal oxide scale that causes the tube to run hotter to produce the same steam temperature. This explains how a boiler that runs for years may suddenly experience coal ash failures, even though the fuel source has been constant. Another major factor that influences tube life is the creep rupture strength of the material. Thousands of tests have been run on the T11 and T22 steels used for tubing and when the results are plotted, there is a significant difference between the average properties and lower bound properties. Figure 2 shows the difference graphically. Temperature Pattern Across Boiler 880 900 920 940 960 98