Flow accelerated corrosion, corrosion fatigue, and stress corrosion are the three most common failure mechanisms for HRSG boiler tubes and components. The causes of these failure mechanisms in HRSGs are discussed and examples of each mechanism provided. Challenges associated with predicting high risk locations for inspection are discussed. Guidance is available to diagnose these failure mechanisms in the field. Areas of research needed to assist designers, operators, and maintenance staff in predicting and preventing failures are also identified.
Heat recovery steam generators (HRSG) remove heat from exhaust gases, typically from a gas turbine, and convert the energy to steam. The steam may be used for industrial processes or to drive a turbine generator to produce electricity. Failures of boiler tubes and other piping in HRSGs are a growing problem.
Many HRSGs were originally designed for base load operation. Recent economic factors have led to drastically increased cycling of many units. This has exacerbated the rate of failures. Other factors contributing to the high rate of tube failures are issues related to design, quality control during construction, operation, and water chemistry.
This paper examines the most common corrosion related failure mechanisms in HRSGs. The intent is to provide the owners and operators of HRSGs with some insight into prediction of potential tube failures, diagnosis of failures when they occur, and corrective actions to return to service and mitigate further failures.
TUBE FAILURE MECHANISMS COMMON TO THE HRSG
There are a number of failure mechanisms that are common to the boiler tube materials and construction practices employed for HRSGs. The two most common corrosion related failure mechanisms are flow accelerated corrosion and corrosion fatigue. Less common, but still a concern, is stress corrosion. While none of these mechanisms occur exclusively in HRSGs, there is a unique combination of design and construction practices, materials, and operating parameters which influences the failure mechanisms.
Flow Accelerated Corrosion
Flow accelerated corrosion (FAC) typically occurs in the low pressure (LP) section of the HRSG. The reason for this is primarily two-fold. First, the LP section is typically composed of carbon steel, which has a well-known high susceptibility to FAC. Second, all volatile (AVT) water treatments, like ammonia and hydrazine, are typically employed for pH and oxygen control in the feedwater and LP evaporator section of the HRSG and may not adequately protect the steel in contact with the liquid phase.
Trace Element Influence on FAC. The susceptibility of carbon steel to flow accelerated corrosion is strongly influenced by the trace elements present in the material. The trace concentration of chromium has the largest effect with higher concentrations increasing the resistance to FAC. Copper and molybdenum concentrations have similar effects, but reduced in magnitude. The carbon steel tubes used in most HRSGs are either SA 178 welded tubes or SA 192 seamless tubes as specified by the ASME code1. The specifications for these materials do not control chromium concentrations. Concentrations vary from heat to heat of material and range anywhere from a high of about 0.25% to undetectable levels, with an average of approximately 0.15%. Consequently, the FAC performance of a particular tube in service can vary greatly. In addition, since the ASME specification does not require identification of the chromium level, the concentration of chromium in a given tube and hence the tube?s relative resistance to FAC is generally unknown. Figure 1 shows the relative wear rates for ch
