Increasing flow resistance rippling formation of the oxide layer is sometimes observed in high pressure supercritical boilers, resulting in a lower boiler efficiency. In 1968 Grosskraftwerk Mannheim (GKM) started a detailed research project on operational boiler tubes (Schuster, 1971; Schoch et al., 1972). A few years later, the University of Stuttgart joined GKM in a study in greater depth the results of which have been published too (Pfau et al., 1978).
In the German study flow resistance had increased to approx. 50% due to rippling in
the oxide layer on the tube wall across the direction of flow. Ripple height was 1-5 ltm
and ripple intervals were 30-100 pm. Ripples in the samples appeared as undulations
in the oxide layer. The steel oxide front was straight and there was no corrosion
below the thinner oxide areas.
KEMA has started a program to complement the German research and focuses on
the effects of steel quality and heat flux. One experiment was done under
supercritical and one under subcritical conditions. Because of experience gained
from the German studies, a variety of steel and tube types were used in the KEMA project.
See paper nr 28.