Gas Turbine Handbook : Principles and Practices

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160 Gas Turbine Handbook: Principles and Practices • The insertion loss (dynamic if under flow conditions): IL = Lp2 – Lp4 • The noise reduction is: NR = Lp3 – Lp4 or Lp5 – Lp6 • The transmission loss (TL) is: Same as NR but assumes Lp4 is anechoic (free from echoes and reverberations) • Lp1 ≠ Lp3 because of the presence of the silencer And it is known that IL ≠ NR ≠ TL (but they are usually close). The NR is measured at convenient locations upstream and downstream of the silencers. TL is measured in the same manner as NR but the downstream end must have an anechoic termination; that is, no sound reflection at the end of the duct—the sound is completely absorbed. In measuring the NR and IL, the duct geometry and length play an important role as they affect performance but TL is less affected by the duct geometry since the duct termination must be anechoic. Most all field verification measurements are NR under no flow conditions; while measurements under flow conditions are possible there is variability and uncertainty involved in the process. So, when specifying silencer performance alone be sure to specify the conditions under which the performance is to be evaluated or verified. In certain cases smaller scaled model silencers or full scale can be tested in a laboratory to verify acoustical and aerodynamic performances. 14 A typical cross section of a silencer panel is shown in the following figure. Shown are the outer perforated sheet, a screen mesh or cloth used to retain the acoustical filler material and the center filled with the acoustical material. The choice of the perforated sheet open area ratio, the screen mesh or cloth, and the acoustical filler material all effect performance. Figure 10-6. Typical Silencer Partial Cross Section
Gas Turbine Acoustics and Noise Control 161 Table 10-4. General Uses and Properties of Acoustical Materials ———————————————————————————————————— Silencer Acoustical Retainer Temperature Thermal Durability Resistance to Application Material Material Range Cycling Friability [1] ———————————————————————————————————— Mineral Wool Cloth Mesh [2] N/A N/A Good Good Inlet Fibrous Glass Cloth Mesh [2] N/A N/A Good Good Blanket/Batt ———————————————————————————————————— Mineral Wool Wire Mesh ~ 800°F Good Moderate Moderate Ceramic Wire Mesh ~ 2,000°F Excellent Poor [3] Low [3] Exhaust Basalt Wire Mesh ~ 1,400°F Poor [4] Good [4] Moderate Fibrous Glass Wire Mesh 1,000°F Good Good Moderate Blanket/Batt typical limit ———————————————————————————————————— Notes: [1] Friability—the propensity to breakdown or disintegrate. [2] A very low flow resistance is required to maintain performance. [3] Packing of material and low vibration critical for longevity. [4] Protective thermal shock blanket required for longevity. The perforated sheet should have an open ratio greater than 35% for inlet applications and greater than 25% for exhaust applications. Table 10-4 provides a general guideline as to choice and applicability of materials. The user should always verify material properties. Exhaust system temperatures can be in excess of 540º C (1,000ºF), which calls for the careful selection of materials. Wire mesh is used in exhaust for material retention because at high temperatures the cloth materials begin to melt and fuse forming a glass sheet that degrades the silencer’s ability to attenuate noise. Too heavy or too tightly weaved retainer cloth can degrade performance dramatically and the challenge is to find a cloth suitable for retaining the filler material while still allowing the acoustical sound wave to penetrate the cloth. Mineral wools typically have binders that breakdown at about 425º C (800ºF), which causes the material to be blown out the exhaust. Thermal cycling (start-stop of the turbine)
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Gas Turbine Handbook: Principles an
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Dedication This book is dedicated t
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Contents PREFACE ..................
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Chapter 13—BOROSCOPE INSPECTION .
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Preface to the Third Edition The ne
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Acknowledgments I wish to thank the
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2 Gas Turbine Handbook: Principles
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Figure 2-3. Courtesy of United Tech
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Appendix A-2 304 Gas Turbine Handbo
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5. Weather Hood Material __________
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Appendix C-6 Air/Oil Cooler Specifi
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9. Drive Requirements A. Hydraulic
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22. Packaging For Shipment Per Spec
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Parameters Units NH 3 CO H 2 CH 4 C
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.055 Parameters Units 1/0 .88/.12 .
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Koppers Foster Blue Coal Gas Winkle
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