Effect of Two-Phase Natural Circulation Distortion on Tube Failure in Steam Boilers

Two different cases of evaporator tube ruptures in power station boilers due to natural circulation distortion are presented. The first case discussed concerns a 110-MW, unit boiler with bottom evaporation tubing inclined at 15° to the horizontal. At the high heat fluxes present in the furnace, subcooled boiling occurs in inclined tubes. For these inclinations an insufficient flow rate causes local heat transfer deficiencies due to vapor-water separation. The introduction of internally finned tubes eliminates local heat transfer deficiencies and prevents further tube failures. The second case is that of circulation interruption due to blowdown during start-up. The water level in the drum of this second 110-MW, unit boiler was controlled by inlet header blowdown during start-up. Thus, natural circulation was interrupted, causing local overheating of evaporator tubing. The event was identified by an increase of the tube rupture frequency. After changing the blowdown procedure, the interruptions of natural circulation were avoided and the tube failure frequency decreased substantially.