Steam generation plays a critical role at many industrial facilities. Unfortunately, the high-temperature and high-pressure environment of large steam generators makes them susceptible to corrosion. Even seemingly minor impurity ingress can cause problems . Fortunately, plants can take advantage of a lot of lessons learned about preventing corrosion in boilers. However, an often overlooked issue is the risk of severe corrosion occurring during those times when a steam generator is down due to lack of steam demand or for maintenance. This article outlines several techniques for protecting steam generators at these times. Our examples come from a power plant, Lincoln Electric System’s Terry Bundy Generating Station in Lincoln, Nebraska, but the technologies suit process plants as well.
The Terry Bundy plant has three GE LM6000 combustion turbines, two of which are equipped with heat recovery steam generators (HRSGs) that drive a supplemental steam turbine. Total plant capacity is 170 MW. The units cycle on and off, often on a daily basis during the summer, according to the requirements of the South West Power Pool. At other times, a unit may be down for several days or perhaps even weeks during periods of mild weather or for maintenance outages.
For normal operating chemistry, the HRSG feedwater is on an all-volatile treatment oxidizing [AVT(O)] program, with ammonium hydroxide injection to maintain feedwater pH within a 9.6–10 range. High-pressure evaporator chemistry complies with the Electric Power Research Institute’s phosphate guidelines, with trisodium phosphate as the only phosphate species and control within a 1–3 ppm range. The high-pressure evaporator pH control range is 9.5–10. Free caustic concentrations are maintained at or below 1 ppm to minimize the risk of caustic gouging.
For short-term outages, the HRSGs must remain full of condensate to enable quick startup. On the other hand, a maintenance outage requires draining of the unit, preferably while it is hot so it flash-dries. However, even such drying still leaves some areas exposed to moist conditions and, thus, vulnerable unless additional protective methods are used.
The equipment and processes outlined below all are designed to protect the unit from oxygen corrosion during any outage. Oxygen attack is extremely serious.
The corrosion mechanism can induce severe metal loss in those areas of high oxygen concentration. The attack often takes the form of pitting (Figure 1), where the concentrated corrosion can cause through-wall penetration and equipment failure in a short period of time.
In addition, oxygen attack will generate corrosion products that then carry over to the steam generator during startups. Deposition of iron oxides in the waterwall tubes leads to loss of thermal efficiency and, most importantly, establishes sites for under-deposit corrosion, such as very insidious hydrogen damage, acid phosphate corrosion (in units with poorly maintained or monitored chemistry — see: “Don’t Get Steamed"), and caustic gouging .
Oxygen also can infiltrate steam generators at startup when collected condensate or fresh demineralized water is needed for filling or boiler top-off. These high-purity waters typically are stored in atmospherically vented tanks. The water absorbs oxygen and carbon dioxide, often to the saturation point, which may be up to 8 ppm in the case of oxygen. When the makeup is injected into a cold steam generator, significant oxygen attack is possible.
To prevent oxygen ingress and corrosion, the Terry Bundy combined-cycle plant relies on four of the best techniques: nitrogen blanketing; periodic water circulation; dissolved oxygen removal from makeup condensate and demineralized water; and warm air recirculation to protect the low-pressure turbine.