By Robert Swanekamp, P.E., Contributing Editor
The “boom” in gas-fired power plant construction of the late 1990s and early 2000s led to capacity margins exceeding 20 percent throughout much of North America. This capacity glut, combined with soaring natural-gas prices, drove many existing combined-cycle plants off-line for months at a time-essentially mothballing them until brighter economic conditions might return. Those fortunate to run at all typically ran at unusually low capacity factors, with daily start/stops becoming the norm for formerly baseload facilities.
By 2006, moderated natural gas prices and diminished capacity margins-helped by the “bust” in new construction and a sweltering summer-enabled many combined-cycle plants to return to service. In accounting offices, that turnaround means a potentially profitable year. In maintenance shops, it means a potentially busy fall outage season.
The extended shutdowns elevated the risk of equipment damage, especially in heat-recovery steam generators (HRSGs) where intricate steam/water circuits are susceptible to lay-up problems. The frequent start/stops at cycling plants also introduced risks to HRSGs-such as high stress levels and repeated thermal shocks to the superheaters, reheaters and economizers.
 An HRSG exhaust stack. Photo courtesy HRSG User’s Group. |
With the summer peak behind them, HRSG users now need to closely inspect their units for signs of trouble. Visual inspection by experienced eyes is the first and foremost method of HRSG checkup. Users also should understand and apply the many non-destructive evaluation (NDE) techniques that have proved effective for HRSGs.
Lay-Up Concerns
Shutdown periods-whether they are months long for a mothballed plant or overnight for a cycling unit-pose unique risks to HRSGs. Proper lay-up of both the gas- and the water-side is essential to minimizing these risks.
The most important gas-side lay-up procedure, according to Robert Anderson of Competitive Power Resources Corp., a former utility-plant manager and current chairman of the HRSG User’s Group, is to securely close off the exhaust stack to keep out rainwater and ambient humidity throughout the shutdown period.
“A tightly sealed and well-maintained stack damper is the best way to accomplish that,” Anderson says.
Plants that lack a stack damper have successfully used such low-tech solutions as boat covers and weather balloons. At least one vendor, GR Werth & Associates Inc., manufacturers a balloon specifically for this task. Marketed as “duct balloons,” they have been installed at numerous plants in pairs-one at the front end and one at the back end of an HRSG-or more commonly as a single unit in the exhaust stack (see accompanying illustration).
Water-side lay-up is more of a concern, because conditions promoting corrosion are more likely and because the effects of corrosion are more costly to repair. The most common water-side corrosion processes caused by lay-up are general (or “uniform”) corrosion, crevice corrosion and oxygen pitting.
To prevent water-side corrosion during HRSG lay-up, users must separate the water, the oxygen and the metal, says David Daniels, senior consulting scientist at Mechanical & Materials Engineering.
“If you have these three things in the same place at the same time, corrosion is inevitable,” he says.
Of course, the metal in an HRSG is intended to stay there, so users must prevent either the water or the oxygen from contacting the metal during lay-up. Daniels says this can be done either by removing all moisture from the water/steam circuits and maintaining a dry environment (a “dry lay-up”) or preventing oxygen from entering the steam/water circuits during shutdown and subsequent lay-up (a “wet lay-up”). Nitrogen blanketing is the preferred method of preventing oxygen ingress. In wet lay-up, corrosion is further minimized by establishing water in the HRSG with a high pH-between 9.4 and 9.6-and a low conductivity prior to shutdown.
Looking for Signs
Because some of these tasks are easier said than done, no plant correctly implements 100 percent of its prescribed lay-up conditions during 100 percent of its shutdowns, Anderson said. That’s why HRSG inspections are so important.
Anderson recommends proactively scheduling inspections to find and correct minor problems before they become major. Take a three-pronged approach, he advises, relying on the Authorized Inspector for statutory inspections; plant personnel for frequent, brief inspections whenever plant schedule allows; and HRSG specialists for thorough inspections during scheduled maintenance outages.
Lester Stanley, systems engineer for HRST Inc. is one such specialist. Stanley said that thorough inspections with well-organized reports are a valuable maintenance planning and justification tool. Spotting problems early and then explaining the consequences of “no-action” helps plant managers decide where to spend their outage resources wisely.
 Illustration of a duct balloon with a flexible tube to vent dehumidification equipment installed at a New Jersey plant. Courtesy of GR Werth & Associates Inc. |
At a minimum, he says, visual inspections should include a “crawl-through” of the complete gas path, accessible components above and below the gas path (the interconnecting piping such as jumpers, risers, drains and vents), the gas baffles, all piping hangers and the drum internals. Stanley organizes his inspections into specific areas: inlet duct areas; superheaters, reheaters, evaporators and economizers; header crawl space enclosure; firing ducts; selective catalytic reduction and carbon catalyst (if installed); stack; high- intermediate- and low-pressure steam drums; tube internals; casings; casing pipe penetration seals; module supports; and drain systems.
Improving Your Vision
In addition to visual inspections by experienced eyes, HRSGs should be inspected by appropriate non-destructive evaluation (NDE) techniques-some highly sophisticated, others wonderfully simple. According to Joe Frey, staff consultant for Stress Engineering Services Inc., NDE technologies successfully used on HRSGs include those discussed below. Frey emphasizes that no single NDE technique serves all purposes. Also realize that just because a test is more esoteric and more complex does not necessarily make it better.
- Liquid-Penetrant Testing (PT). While the unaided eye can evaluate surface finish and see moderate-sized pits and cracks, examining microscopic-level flaws requires the help of special devices-such as PT. Liquid penetrants can locate cracks from grinding, welding or casting processes; fatigue cracks caused while a unit is in service; local porosity; and lack of bonding between two materials. The technique often is used in HRSGs to find cracking in tube-to-header weld joints and in downcomer-to-evaporator distribution pipes.
- Magnetic-Particle Testing (MT). While PT is effective on fine surface discontinuities, MT is needed to detect larger surface flaws or those present just below the surface. This NDE technique depends on the fact that when a test piece is magnetized discontinuities oriented in a direction transverse to the magnetic field will distort that field, producing a flux “leakage” pattern directly above it. The leakage field is detected by applying finely divided magnetic particles over the surface. While MT is limited to use only on ferromagnetic material, it is sensitive to flaws almost without limitation to the size, shape, composition or treatment history of the part under test.
- Ultrasonic Testing (UT) has been used at power plants for many years to detect corrosion by measuring wall thicknesses. The technique uses a transducer to introduce ultrasonic energy at an angle perpendicular to the inspection surface. A range of relatively inexpensive UT devices can be used for simple thickness checks, said NDE specialist John D. McMillan of Mechanical Integrity Inc. But with these devices, any discontinuities in the metal will introduce errors. McMillan says that a more sophisticated UT instrument with an “active trace display” will enable the inspector to discriminate between discontinuities and actual corrosion, thus providing more useful results. In HRSGs, UT is helpful in measuring wall thicknesses of headers, riser pipes and drum internals-such as belly pans, baffles and cyclone separator cans. In fact, UT is a key tool in monitoring flow-accelerated corrosion in vulnerable locations such as economizer headers and low-pressure steam drums. Unfortunately, the presence of finned tubing in HRSGs severely limits this NDE technique’s use on the boiler tubes themselves.
- Radiography. Most NDE methods search for defects at or close to a component’s surface. More deep-seated flaws require methods capable of deeper penetration. Radiographic testing is the most familiar and widely applied technique for these purposes. In fact, radiography’s wide range of capabilities has brought it to the forefront of NDE techniques. Flaws as small as 2 percent of the work-piece thickness are readily detected. Sensitivities as high as 0.5 percent to 1 percent are achievable. Radiographic testing is used extensively in the power industry to verify the quality of castings, forgings and piping welds.
- Metallurgical Replication examines the microstructure of a suspect metal surface to confirm the correct metallurgical structures or to detect very early stages of creep damage, cracking, and other failures. The technique has existed for decades, but only recently has it been put to use in the combined-cycle industry.
Producing a replica begins with abrasive preparation of the metallographic specimen using successively finer grits of small (typically one-inch diameter) grinding disks, followed by a final polish using a diamond-impregnated cloth. Specimen preparation is completed by etching the polished surface with an acid. A sampling procedure then records and preserves the specimen’s topography as a negative relief on a plastic film (the replica).
Replication has been used successfully to determine the mechanism of cracking in HRSG tubes, headers and piping. The state of the weld and base material also can be examined to determine whether it has been damaged by improper heat treatment, exposure to fire or operation at excessive temperatures.