Determining deaerator integrity
The original Burns & Roe plant design provided two dedicated deaerator systems: one for the HP steam system (providing deaeration for both HRSG HP economizers) and the second for both LP steam systems. Each deaerator system provides boiler feed pump suction for three 50% boiler feed pumps.
The steam turbine is single-pressure; LP steam is used today only for the deaerator steam supply. Vessels and tanks were provided by L.A. Water, a fabricator owned at the time by Chromalloy Corp.
Originally, the plant was designed with a “soft water” system, which was not unusual for the relatively low HP steam system operating pressure (about 650 psig). In the mid-1990s, this system was replaced with a demineralized water system to address recurrent problems with high silica levels in the steam drums.
The upgraded water treatment system reduced the water’s silica content but yielded water conditions that at times can be highly aggressive, particularly in high-turbulence and high-flow components such as the feedwater pumps and deaerators. Periodic water chemistry transients have also increased the frequency of aggressive water conditions that can further accelerate corrosion damage to these components.
Periodic inspections of the deaerators were performed by the plant maintenance department, a local nondestructive examination firm, and the plant’s ASME Code mechanical contractor. Both deaerator systems had a history of recurring damage that required numerous repairs (Tables 1 and 2). As the steam plant approached its end of design life (25 years), the frequency and severity of damage increased, and related damage was occurring in both boiler feed pump systems.
Table 1. Excerpts of the ASME Code repairs made to the HP deaerator vessel. Source: Chugach Electric Association Inc.
Table 2. Excerpts of the ASME Code repairs to the LP deaerator vessel. Source: Chugach Electric Association Inc.
Tube leaks in the air-cooled condenser have also occurred periodically, but routine cleaning and temporary repairs have maintained the reliability of this critical component.
Oxygen levels as high as 100 ppb were occasionally recorded in the mid-1990s. Management responded by instituting annual helium leak testing and routine vacuum decay testing on the steam turbine as well as periodic isolation of pumps to identify sources of air in-leakage. As the plant ages, these activities become critical to maintaining sufficiently low air in-leakage to allow the deaerator systems to perform as designed.
The LP deaerator was abandoned in 2004 after extensive pitting and stress cracking had damaged the vessel shell to the point where efforts to make repairs were unsuccessful. At that time, a modification was made to the plant feedwater piping to modify the remaining HP deaerator to supply suction feedwater to both the LP and HP feedwater pumps. This modification was reasonably simple: Two tees were installed to provide a connecting branch between the HP and LP pump suction lines. Calculations by plant engineering staff confirmed that adequate head existed to prevent pump cavitation and that the original (overcapacity) storage tank was large enough to provide a sufficient source of suction flow.
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