Background
In 1971, a rupture occurred in a deaerator heater installed in a power plant. In 1979, the head of a deaerator storage tank separated suddenly around the shell circumference alongside the girth weld. Two additional deaerator heaters ruptured in 1983 in the United States, with catastrophic consequences to personnel working at the respective plants. In 1998, another catastrophic rupture occurred in a deaerator located in Belgium. The rupture caused severe damage to the plant facility. This was followed in 1999 by another catastrophic rupture of a deaerator located in the Dominican Republic.
Because of the ruptures of the deaerator heaters and deaerator storage tanks that have occurred, and the discovery of cracking in a significant number of other deaerator heaters and deaerator storage tanks, the National Board of Boiler and Pressure Vessel Inspectors have expressed serious concerns about the integrity of this type of equipment. As a result of the catastrophic failures, a number of insurance underwriters, trade associations, and publications have been recommending the inspection of deaerator heaters and deaerator storage tanks.
Failure Causes and Considerations
Cracking in deaerator heaters and/or deaerator storage tanks is typically the result of mechanical fatigue or mechanical shock conditions. The mechanical fatigue is the result of cyclic stresses created by such operating conditions as full load rejection, temperature/pressure fluctuations, and water hammer. Full load rejection occurs when the turbine trips and turbine extraction steam is lost. This may cause flooding of the downcomers, resulting in water being blown upward against the heater trays. Trays may become dislodged and in some cases, bent or broken beyond repair.
Moreover, when a unit trips, the pressure is reduced inside the deaerator heater and storage tank, resulting in steam flashing. This produces a water hammer effect, which will result in mechanical shock loading of the deaerator heater and storage tank, the pipe connections and the structural supports. These shock conditions may introduce stresses of as much as 30,000 psi. Flow induced vibrations or water hammer may also occur in the piping connected to these vessels. Severe stresses in the deaerator can occur from temperature/pressure fluctuations resulting from either an influx of cold water or a unit trip. Large influxes of cold water entering the deaerator can cause a sudden drop in deaerator pressure and a subsequent flashing of water in the storage tank. Again, the result is movement and distortion of internal components in the deaerator heater as well as mechanical shock loading of the deaerator storage tank. Water hammer may also occur when high-temperature condensate comes in contact with cooler make-up water. The result is damage to deaerator internals, seams and supports. Necessary precautions include cooling the returning condensate either through the direct injection of cool make-up water or through a make-up heat exchanger.
In some instances, where the cracks have been widened by slight corrosion, failures have been attributed to corrosion fatigue, even though mechanical fatigue or shock represents the actual primary cause. Nevertheless, magnetite (Fe3O4) generally is formed in the fatigue cracks. Thus, magnetite formation is considered a contributor to the cracking of deaerators and deaerator storage tanks.
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