SVI DYNAMICS has extensive experience in the design, engineering, and manufacturing of gas turbine exhaust stacks and ducts. These items can vary in design based on the type of industrial application they are used in, but there are standard compositions usually based on the temperature of the gas. A good example is in a combined cycle gas turbine application where the duct construction may change throughout the system:
• Inlet duct to gas turbine pulling ambient air – unlined steel ductwork
• HRSG inlet duct where temperatures can well exceed 1000 F – internally lined double wall ducts
• HRSG stack where temperatures have been reduced from the heat recovery device typically under 400F – unlined steel stack
- Internal liner of solid sheet stainless steel to handle design temperature
- Layer of insulation with thickness determined by thermal performance requirement
- Outer casing plate typically of a lower grade steel (carbon steel) and designed to meet 140F average temperature for personnel protection
The double wall duct or stack has an inner and outer steel wall that contains insulation between the two using a stud system to join the system together.
While most of the time the lined ductwork design is based on meeting project goals for thermal performance, when acoustic performance is needed to meet noise criteria, SVI DYNAMICS can utilize the lined ductwork to meet project noise targets. The two sources of noise from ductwork and stacks are from noise radiated through the materials (casing/ductwork radiated) or out through the system discharge (stack exit) to atmosphere.
- Ductwork radiated noise – can typically be a problem in the near-field environment related to meeting the standard 85 dBA off the equipment. SVI DYNAMICS will base material thickness selections for the inner liner, insulation and outer casing to get the needed transmission loss (TL) from the composition to meet the noise specifications. The materials are usually thicker, which may increase cost, but the acoustic ducting can help avoid the need to add additional treatments like a separate acoustical panel system.
- Stack exit noise – is commonly handled with a parallel baffle silencer which is a series silencer panels with perforated facings across the gas path designed to meet need noise reduction (see absorptive silencer). With respect to internally lined turbine exhaust duct, the internal liner of the system can utilize a perforated duct liner instead of a solid, which would provide some acoustic performance. This perforated duct approach by itself in larger gas turbine system usually does not provide enough acoustic performance but can be used in conjunction with a gas path silencer.