Retrofits & Upgrades - O&M

Critical maintenance: Combustion turbine outlet duct upgrades at a simple cycle plant

Maintenance contractor SVI BREMCO addressed critical issues in a plant’s duct and stack system, including silencer baffle repair, duct liner replacement and crack repairs, through a coordinated outage involving multiple specialists.
Critical maintenance: Combustion turbine outlet duct upgrades at a simple cycle plant
(Figure 1. New silencer baffles are being hoisted to the duct.)

Simple cycle power plants are an integral part of our current electrical generating network, as they can be quickly started and stopped to follow renewable energy load swings or to address peak load needs. The high temperatures generated by combustion turbines and the common on-and-off cycling of these units induce thermal and mechanical stresses within the turbine exhaust system in components such as duct liners, silencers, and support brackets. Regular maintenance is important, as this article explains.   

Some simple cycle design basics

Anyone who has flown in a modern commercial airliner has experienced the effects of combustion turbines. Jet engines, of course, generate thrust for flight, whereas in power units, the energy drives a stationary turbine to provide the mechanical energy for an electrical generator. An outlet duct and stack convey the turbine exhaust gas to atmosphere. Important components within the exhaust system include internally lined ductwork and stacks and silencers. Noise regulations are common for simple and combined cycle plants, as many facilities are located within or near residential areas.  Silencers reduce noise levels below local guidelines. Ducts also typically have an internal liner to inhibit transient heat transfer through the duct walls. 

The combustion products that power gas turbines reach very high temperatures, perhaps up to 2,700o F. Even though much of this energy is utilized by the turbine generator (and to drive inlet air compressors), the exhaust gas temperature may still exceed 1,100o F. The hot exhaust gas and the wide temperature ranges that occur from unit load cycling place much stress and wear on the outlet duct and internal components. Periodic inspections and maintenance are necessary to keep this equipment in reliable condition.   

The following list outlines critical outlet duct and stack maintenance issues that developed over time at the plant highlighted in this article:

  • Repair of silencer baffles and support brackets
  • Replacement of failed internally lined exhaust system sections
  • Installation of new test ports (depending on condition and new silencer design)
  • Repair of cracks in the casing

Solutions

Following a complete system survey, the maintenance contractor, SVI BREMCO, addressed each of these issues and others during a several-week maintenance outage. A primary task was to replace the silencer baffles and support brackets. SVI BREMCO replaced the traditional parallel baffle silencer design with a proprietary bar silencer design. The bar silencer design is very popular for simple cycle exhaust system retrofits because it provides better acoustics, reduces pressure drop (better turbine performance and project ROI), and optimizes aerodynamics (improving durability).

This work required coordination of multiple personnel, including equipment specialists, welders, crane operators and safety personnel. These specialists also installed new test ports and made duct liner repairs during the outage window.

Figure 2. New test ports.

Testports are critical for flue gas monitoring, including flow dynamics. Adjustments to test port location can often improve monitoring capabilities. Installation of new ports was a key part of this project, along with improved silencer design, as was the installation of liner material at these locations.

Figure 3. New test port liner installation. The SVI BREMCO website includes additional posts that provide information about more extensive liner replacements.

The maintenance crews also repaired duct/stack metal cracks in the casing. The peaking nature of simple cycle operation places much cyclic stress on liners, which in turn can cause cause liner failure and cracked casing. Cracks that breach welds or duct walls allow flue gas to escape. Once cracking begins, the damage typically continues to propagate.

Figure 5. Stack weld repair.
Figure 4. Transition duct cracking before repair.


The crews also repaired a crack in a generator lubricating oil line, whose discovery came from inspections apart from those of the duct and stack. As this author can attest from direct experience, maintenance issues may stay hidden without thorough inspections conducted per detailed procedures.

Safety – The overall critical issue

No matter how competently maintenance personnel and contractors perform the work, the foremost concern is that everyone arrives home safely at the end of the day. A sampling of the topics discussed during SVI BREMCO safety meetings (conducted at the start of each shift) includes fall protection, electrical safety, hot work issues, and lock out/tag out (LOTO) procedures.  
 
Conclusion

Even though a large amount of the energy generated in a simple cycle power plant is converted to mechanical energy in the combustion turbine, the exhaust duct, duct liners, silencers, support brackets, and other components are still subject to harsh conditions. As with all equipment, these components degrade over time and require maintenance. SVI BREMCO has a highly experienced staff to evaluate and offer solutions for these and many other simple and combined cycle maintenance needs. Contact information is available on the company website.
 
Industrial Contracting Services for HRSGs- SVI BREMCO (svi-bremco.com)

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Ill.) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kan., station. His work has also included eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and he has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, CTI, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and POWERGEN International.