The ID oxide scale is produced by oxidation in the reheater and superheater. The scale build up occurs when the tubes have experienced high temperatures for extended periods of time. The formation of ID scale reduces heat transfer and results in a further increase of tube metal temperature. Higher temperatures promote further growth of ID scale. The result is the ID scale feeds on itself and increases in thickness as it continues to grow.
The increase in ID scale and the associated tube metal temperature promotes creep in the tube metal. Formation of creep results in a loss of strength at high temperature and therefore a loss in remaining life. The final outcome of excessive scale is a thick lipped, long term overheat failure.
The ID oxide scale is generally a concern in high temperature sections of reheaters and superheaters. The scale thickness within these sections may vary because of the range of temperatures. The thickest scale may be expected on tubes that are in the hottest sections of the boiler. Thermocouple measurements taken in the boiler may be used to plan oxide scale inspections.
A history of prior long term overheat failures should also be used to select tubes for oxide scale inspection. Locations that have experienced a large number of high temperature failures should be selected for inspection.
Selecting the circumferential location
for scale thickness measurement is very important. Because of the
temperature variations, the scale thickness may vary around the
tube circumference. Oxide scale measurements should be performed
at circumferential locations which are expected to have
experienced the highest temperature.
UT Technique
The ultrasonic
method for measuring scale thickness is based on transmitting a
wave through the tube thickness. The thickness is calculated by
measuring the time difference between the signals reflected from
the steel/scale interface and the tube ID surface.
Remaining Life Assessment (RLA)
General methodology of remaining life calculation involves the following steps. Empirical formulas that correlate scale thickness to temperature are first used to compute the tube metal temperature. Larson Miller Parameter (LMP) is then determined for the hoop stress in the tube. Knowing the LMP and the tube metal temperature, the remaining life is calculated (1).
Since the ID scale grows over a period of time, the life estimation is not done for a scale thickness value at a specific time. The life is calculated incrementally over the expended time. This is done by dividing the expended life into smaller time interval and scale thickness estimated in each interval. Both the scale thickness and stress values are extrapolated to initial conditions using linear extrapolation. The tube metal temperature and the hoop stress is calculated for each interval. Knowing the tube metal temperature and the LMP, the life fraction expended for each interval is computed. These life fractions are summed to determine the expended life fraction upto present.
NDE Associates, Inc. provides inspection services for measurement of ID oxide scale and remaining life assessment.
- R. Viswanathan, Damage Mechanisms and Life Assessment of High-Temperature Components, ASM International, Metals Park, OH, 1989
NDE Associates, Inc.
515 Tristar Drive
Webster, TX 77598
Phone: 281-488-8944 Fax: 281-488-8485
