Combustion turbines provide the most power from the smallest footprint. Their modular design also makes them an economical choice across all land-based industries, as well as marine applications.
Wherever they are located, there must be a balance between power output and noise control. Overall, the exhaust system for combustion turbines must provide a high level of acoustic performance, low restriction to flow, potentially a maintainable catalyst system and have an anticipated life of at least 15-20 years.
Silencer systems are typically comprised of parallel (rectilinear) baffles. The baffles are suitable for a wide range of conditions, including aero-acoustical performance, system pressure and high flow rates, thermal stresses, environmental conditions (ocean, seismic, and wind), flow-induced vibration, corrosion, and fatigue. The baffles are housed in a structure located just prior to or directly in the stack, which also has special requirements. For example, with exhaust gas temperatures as high as 1250° F (675° C), it is necessary to limit shell temperatures and exhaust leakage to prevent harm to plant personnel, as well as prevent fire or explosion of nearby combustible materials. These, along with local structural requirements, are all key factors in developing a silencer system that achieves optimal performance metrics.
This article focuses on natural gas turbine applications with exhaust systems utilizing silencers with parallel (rectilinear) baffles. Frequency energy is a key factor of the aerodynamic, acoustical, and structural design, in order to silence the engine effectively.
Turbine exhaust produces a frequency energy that is broadband in nature with a larger energy spread, from 280 Hz to 4 kHz. In contrast, reciprocating engine exhaust systems produce a tonal exhaust signature dominated by lower frequency energy at multiples and submultiples of the cylinder firing frequencies, the majority of which fall below 250 Hz to 280 Hz.
The basics of parallel baffle design for silencers are relatively straightforward, with three basic parameters to control attenuation:
- Thickness of the baffle
- Width of the air gap between baffles
- Length of the baffle.
Low frequencies require thicker baffles with large air gaps, while higher frequencies use thin baffles with small air gaps. As such, it is possible to tune baffles to meet the specific acoustical spectra. The baffle length then determines the total attenuation achieved. Besides the three basic parameters noted above, attenuation is further affected by the choice of perforation geometry, packing material (temperature, chemical tolerance, friability), and pack retainer (cloth, wire screen, needle-mat, etc.). Special attention to pressure drop analysis is also required to optimize exhaust gas flow and turbine performance.
Computational fluid dynamic (CFD) modeling is a useful tool in airflow analysis and determining pressure drop. Knowledge based on flow calculations of similar systems and careful calibrations for local conditions can then be compared with the CFD modeling, which is often used for directional purposes.
Silencers fall into two categories, inlet and exhaust applications. Each has different operating conditions and require different materials.
Inlet systems have an allowable pressure-loss limit for the entire system as a way to prevent the turbine from stalling. Most inlet systems typically include a filtration system and sometimes evaporative cooling, depending upon the environment (cooling, heating, humidity, and water removal), which can add to system pressure loss.
Regarding the exhaust system, it is more flexible because combustion turbines typically have no problem pushing the exhaust gas out. However, low-pressure loss is desirable for system efficiency.
Aerodynamic design is a key factor in engineering inlet and exhaust systems, particularly for high-performance engines with substantial gas flows. However, in many legacy industrial applications, system ducting sized at the outset is often configured to be the smallest possible so as to minimize materials and construction costs. Unfortunately, original designs often give little to no attention to aerodynamic and acoustical requirements of future upgrades.
An improperly designed silencer system will have a shorter lifespan, reduce turbine performance, and potentially result in a catastrophic failure of the power plant. Careful analysis and efficient design of lined silencer baffles is required when retrofitting gas turbine exhaust systems utilizing silencers with parallel (rectilinear) baffles. While internal flow velocity and aerodynamics, vibration, and seismic loads are all critical factors, each driven by significant variables, newly designed baffles should deliver improved structural (strength and stiffness), acoustic, and aerodynamic performance.
Turbine Silencer Exhaust System—Project Example
The following case example from Dürr Universal describes the analysis processes for a complex silencer exhaust system replacement/retrofit project with specific review of the rectilinear acoustical baffles.
After less than five years in service, measurements and field inspections by Dürr Universal revealed the original silencer baffles exhibited a combination of perforated baffle sheet buckling, vibration failure, and thermal fatigue. In addition, there were signs of external shell overheating due to the movement of insulating pack material, as well as thermal fatigue cracks on several exhaust duct components.
Examination of original baffles suggested premature baffle failures were caused by excessive local buckling of the pack module perforated sheet. The buckling occurred due to the fatigue load combination, including self-weigh, thermal, and disproportionate lateral vibration of the pack module perforated sheet.
Management of the power plant was concerned about the findings and the impact on availability of the unit. Safety was also a key concern because old and failing exhaust baffles posed a potential safety risk if torn steal pieces are ejected from the stack near personnel or adjacent equipment.
An upgraded exhaust system, including high-performance exhaust baffles with absorptive insulation packing was commissioned to replace the existing exhaust silencer system.
The new retrofit exhaust system featured a similar design as the original system, but the baffle size and geometry were changed to meet stringent requirements for noise attenuation and the need for a more robust structure to avoid potential vibration failures. The new baffle design eliminates raised surfaces on the perforated sheets.
Often-used tack welds of flat sheets can expose leading edges to the high-temperature turbulent exhaust gases, which can cause premature baffle failure and safety concerns with torn steel being ejected from the stack. The new silencer system is supported on reinforced steel concrete pedestals via ordinary concentrically braced steel frames.
Modeling of the baffle boundary conditions/support points accounts for thermal expansion. In additional, the lateral restraint of the baffle at the support seat and guide allow for the free movement within a determined gap. Mechanical means were then used to maintain enough support tension, but still allow for thermal growth.
Along with the seismic load analysis, mode shapes in this baffle example are important because they were used to check for potential vortex shedding around the baffle nose and perforated sheet caused by the internal flow velocity between the baffles. Internal flow velocity can produce low frequencies in the baffle areas, which in turn may overlap with the low structural frequencies of the baffle perforated sheet.
When the flow-induced resonances in the silencer duct are coupled with the significant structural resonances of the baffle perforated plate at the lower frequencies, it may result in violent vibration levels. This ultimately can lead the perforated pack sheet to fail first at weld connections, cause extensive local buckling and failure of pack braces, as well as create the potential for severe deformation and failure of the baffle. Perforated sheet buckling typically occurs when the baffle pack migrates (loss of pack) and interconnected braces fail.
It is important to note, every turbine exhaust system retrofit project has a unique set of performance structural conditions. Each unit requires careful analyses and a customized plan. Effective noise control for gas turbine applications call for a systematic engineering approach to ensure local noise requirements are satisfied in a safe and cost-effective manner.