Gas Turbine Handbook : Principles and Practices

162 Gas Turbine Handbook: Principles and Practices
also accelerates material breakdown and basalts generally require
thermal shock protection usually in the form of a needle-mat layer
between the screen mesh and the basalt. Poor aerodynamics can also
cause the silencer panels to lose fill by the exhaust flow beating on
the panels and duct vibration has the same detrimental effects. Some
OEMs require higher operating temperatures in order to have the
materials withstand hot starts; thus, fibrous glass batting products
may not be suitable, and exhaust gas chemistry must also be considered
in the material selection.
The ability of the sound wave to penetrate into the acoustical
fill is critical in obtaining the desired performance. The material exhibits
an acoustical resistance that attenuates the sound energy that
is categorized as airflow resistance and is measured in rayls/meter.
The property “ rayls/meter” is a measurement of the airflow resistance
a material exhibits; it is actually a hydrodynamic measurement 15
that is used as an indicator of acoustical performance and the lower
the value the better the performance, generally. Most acoustical materials
on the market today have flow resistance values less than
40,000 rayls/meter; but, for certain designs, it may be desirable to
use materials having higher flow resistances. One must be careful
in using airflow resistance values, as they are dependent upon the
environmental air or gas temperature and chemistry. Hot exhaust
gases typically have viscosity values that are four to six times higher
than that of ambient air, (thus, resulting in a higher flow resistance
values) and as can be well imagined, material testing is conducted
at ambient (room) conditions.
Silencer panels need periodic maintenance and replacement,
particularly in exhaust systems. In turbines that operate continuously
(thus, free of frequent thermal cycling) the panels may last 10
or more years. However, in turbines that start and stop frequently
the thermal cycling causes stress on the silencer construction (welds,
etc.) that lead to failures and loss of fill, and may need repair or replacement
in just a few years. These expenses need to be considered
in the operating system costs and the duct design should allow access
to the silencer section.
Duct Wall Design
The duct system is also a key factor in acoustical design as it
principally houses the silencer section, controls the flow and direc-