Combined-cycle natural gas generation is displacing King Coal. And although renewables promise a bright future, combined-cycle power plants (CCPP) are efficient, clean and inexpensive generating sources with the capacity to replace base-load generation in large scale.
The recent growth U.S. shale gas production along with pipeline network expansions have lowered and stabilized natural gas prices to the point where they are competitive with coal generation. Combined-cycle plants are relatively inexpensive to build and can achieve thermodynamic efficiencies exceeding 60%. Additionally, their fast-start ramping capabilities enable hundreds of megawatts to hit the grid faster than other sources.
The U.S. Energy Information Administration (EIA) projects that 2016’s power generation from natural gas, for the first time ever, will surpass coal’s share at 33% to 32%, respectively. Correspondingly, 2015 was the first year when domestic natural gas plant utilization exceeded that of coal at a capacity factor of 56% versus 55%.
The initial wave of CCPP construction that began in the 1990’s, anticipated low generating costs through baseload operation. Instead, natural gas price volatility and electricity demand variation forced most of these plants to catch emerging power sale opportunities by cycling (meaning they were off at night and on weekends). In recent years, with gas price stabilizing at around the $3 - $4/MMBtu range, many CCPPs are called on to follow load demand and even for baseload operation as they originally were designed.
At the same time, their associated heat recovery steam generators (HRSGs) have been used (and sometimes abused) to suit market needs. The HRSG is the boiler placed after the gas turbine to absorb remaining hot exhaust gasses and produce steam to drive an additional turbine/generator set. It enables the added generation and efficiency made possible by combined-cycles in the power plant.
Highly flexible operational practices--from periodic baseload operation to cycling the plant every day-- take their toll on HRSG pressure parts. Most in current operation were not designed with the flexibility to withstand the stress levels caused by faster startups, low-load operations and repeated thermal cycling. And the stressors are intensified by today’s larger, more efficient gas turbines.
Located directly downstream of these turbines, HRSGs sustain greater thermal and mechanical stress from increased exhaust gas temperatures and pressure changes. In addition to damages from failure mechanisms that have long-plagued conventional boilers, HRSGs are also prone to design, construction, operation and water chemistry deficiencies. Pressure part failures of tubes, headers and connecting piping represent some of the greatest reliability threats. Of critical importance are regular internal inspections to proactively identify failure mechanisms and root causes so that forced outages can be mitigated.
The following summarizes some of the most common types of HRSG system failure mechanisms and their causes.
HRSG tube failures (HTF) are the primary source of unavailability among combined cycles causing an average of six forced outage events per unit year as indicated by recent NERC GADS data. HTF repair events don’t necessarily require lengthy outages, but they can have large effects on in-market availability and prove costly, especially for merchant generators.
- Thermal Fatigue is a common damage mechanism among superheaters and reheaters caused by the thermal expansion and contraction of cyclic operation. Thermal fatigue occurs primarily at dissimilar metal welds and tube to header connections. Attemperator overspray and residual condensate among LP economizer sections cause steaming and quenching during startup and exacerbate thermal fatigue.
- Short-term Overheating results from exposures to highly elevated temperatures or when tubes are starved of flow as a result of some blockage. Improving temperature controls, preventing tube internal exfoliation or upgrading tube materials may mitigate overheating.
- Long-term Overheating (Creep) accumulates from temperature exposures in excess of design. Even minimal temperature exceedances accrue cumulatively and with time form microstructural creep voids that can shorten tube life. Improved temperature controls or upgrading tube materials are two strategies to mitigate creep.
- Creep Fatigue occurs due to the combined effect of overheating and cyclic stress. Often tough to diagnose, it typically initiates along the outside diameter of tubing in high-temperature locations. It can be mitigated by reducing cyclic loading and localized overheating.
- Bowing is a common failure mechanism caused by differential expansion, quenching and tube fabrication disparities. Reheater tubes in close proximity to attemperators or duct burners are especially susceptible to bowing. Some units experience tube bowing to the degree that crimping and local yielding results in premature tube failures.
- Acid Dewpoint Corrosion occurs from gas turbine exhaust gas moisture which condenses to water vapor and sulfuric acid. The corrosive mix rests upon the tubes and wastes away metal over time. This mechanism may be spared by upgrading tube materials or changing operation to increase tube temperatures.
Cycle Chemistry (CC) influences approximately 70% of the HRSG tube failures (HTF). Oxide growth and progressive deposition of water/steam impurities or oxide scale buildup contribute to a variety of damage mechanisms. Unfortunately, the system design or mode of operation increases susceptibility to tube internal deposition. The shutdowns and poor layup practices from cyclical operation introduce elevated temperatures, flow disruptions and contaminants. A suitable water treatment program helps to ensure feedwater quality maintains tube internal surfaces free of contamination and corrosion among all areas of the HRSG.
- Flow-Assisted Corrosion (FAC), a chemistry-related failure, causes 40% of all HRSG tube failures. It involves the single (water only) and two-phase (water/steam) variations. FAC originates from the loss of protective metal oxides within the tubes which enables wall loss. Proper boiler water chemistry is critical. In contrast to conventional boilers, FAC among HRSGs is found predominantly among tubes, headers and risers in low-pressure (LP) economizers and LP evaporators. External feedwater piping is also susceptible among HRSGs which take feed pump suction from the LP drum. The best approach to managing FAC metal loss is a combination of correct water chemistry control and regular assessment and trending of wall thicknesses among susceptible locations (most of which are internal to the HRSG box). Additionally, materials containing chromium are resistant to FAC. Increasing numbers of utilities are simply upgrading materials to chromium to enact a permanent fix.
- Under-Deposit Corrosion (UDC) occurs exclusively among HP evaporator tubing. It encompasses several water chemistry-related failure mechanisms which commonly cause significant problems when not adequately controlled. A combination of deposited material and corrosion products adhere to the internal tube surface and waste away tube material until eventual failure occurs. An understanding of these corrosion mechanisms is necessary to prohibit or reverse active corrosion. Appropriate cycle chemistry with negligible feedwater corrosion products and avoidance of localized elevated temperatures are the best defense against UDC.
- Acid Phosphate Corrosion is defined by a combination of internal deposits and phosphate salts leading to UDC and eventual tube failure. Chemistry controls using mono- or di-sodium phosphate is problematic.
- Caustic Gouging occurs when chemistry controls employ too highly concentrated caustic or caustic ingress occurs from the regeneration ion exchange process. The excess caustic dissolves the protective magnetite layer. The water in contact with iron attempts to restore this magnetite and traps the high caustic concentration. A continuous loss of metal ensues.
- Hydrogen Damage refers to a combination of internal deposits and contaminant ingress or an acidic concentration. Chloride frequently enters the cycle through condenser leakage.
- Pitting is characterized by localized corrosive metal loss illustrated by deep pits. The most common cause of pitting is poor drainage and layup between cycles. Oxygenated, stagnant water within numerous tube circuits is the usual culprit. It’s imperative during lengthy shutdowns that procedures are in place to drain and/or evacuate all water and protect the tube internals from any remaining moisture through dehumidification or nitrogen blanketing.
- Corrosion Fatigue is a leading cause of failures among LP evaporators and economizers. It’s usually identified where expansion is restricted such as among tube to header welds. Groups of cracks appear on the internal surface in a position perpendicular to the major strain. Corrosion fatigue is an “on again, off again” mechanism that reemerges when oxide laden cracks are exposed to concurrent strain along with poor water chemistry.
Grade 91 steel poses a particular problem. Containing 9% Chromium, it exhibits enhanced creep rupture, yield and ultimate tensile strengths in addition to toughness. Grade 91 enables elevated temperature operation, better lifetime performance and thinner materials in the design and manufacture of piping systems.
During the boom years of new HRSG construction, Grade 91 materials were viewed as a panacea for cycling-related thermal fatigue. Unfortunately, many complications have emerged since then. Grade 91 is more sensitive to variations in metallurgy and heat treatment than traditional materials. In particular, its material integrity was frequently compromised from microstructural damages sustained during manufacture, erection or as a result of operational issues experienced among HRSGs.
Premature cracking from creep degradation, especially among welds, is widespread. A majority of large HRSGs built from the 1990s have tubes, headers and high energy piping constructed from Grade 91. Failures among the larger components such as headers, major connecting piping, and steam piping frequently require lengthy outages with significant repair costs not to mention the loss of generation.
The recommended course of action, regarding P91 piping, is two-fold: First, for repairs, pay close attention to any and all heat treatment activities such that the correct microstructures are developed and maintained and, second, assess current risk(s) through timely and aggressive piping and weld inspections to determine creep degradation and confirm material properties.
Problematic designs and operating constraints introduce additional causes of HRSG failures. As previously discussed, the HRSG is located downstream of the combustion turbine. The consequences of sporadic temperature and pressure swings from exhaust gasses and the effect they have on tubes are an obvious place to recognize. But where else?
- Water Hammer occurs among many HRSGs. Improper steam attemperator spray controls, premature valve actuation or inadequately design condensate drain lines are often culprits. Water hammer can be a destructive force, often taking a toll on piping supports and sometimes exacerbating piping failure mechanisms.
- Thermal Quenching-Induced Fracture ensues when significant “off-design” events occur resulting in rapid thermal quenching and/or overloading failures, primarily at tube-to-header connections. Faulty control logic or damaging operational practices are often to blame. A root cause of failure analysis should be conducted following any such event to mitigate future occurrences.
- HRSG Economizers are vulnerable to a number of issues.
- Tube-to-Tube Stressors can occur from unbalanced economizer flow distribution. Upon initial startup, HP evaporators are yet to produce steam and do not require makeup water. Thus, since the feedwater control valve is closed, no water enters the economizer. Without this needed heat transfer, economizer metal temperatures can reach flue gas temperatures of 500–600F.
- Cold surges among the economizer pose significant sources of stress. They create large temperature differences between contiguous tube sections. Cooler tubes contract while the adjacent warm tubes do not. This phenomenon introduces stress risers, particularly among areas of tube geometrical changes, such as at a weld. These events, although short-lived, shorten the economizer’s remaining useful life at each instance.
HRSG reliability is maximized by understanding the conditions giving rise to potential damage mechanisms. HRSG design, along with its feedwater and attemperator control systems, water chemistry, and component materials, are variables critical to operational flexibility. By ensuring correct operational methods, such as controlled startups/shutdowns, the damaging thermal transients can be mitigated. Proactive, risk-based inspections of failure-vulnerable locations are critical to issue identification, necessary repair(s) and preventing unexpected outages.