Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 163
tion of the gas flows (air in and exhaust gas out), and must provide
a level of noise reduction to avoid breakout and flanking noises from
compromising the overall performance. Most have a common grasp
that the principal noise from the turbine is by way of its gas path
but importantly, as discussed in the introduction, turbines having
levels of 140 dB or more can have a significant amount of noise
breakout through the duct wall. So, in analysis of the total system
noise, the acoustical engineer or designer accounts for both the gas
path and the duct wall breakout noises. Also, the duct can act as a
noise short-circuit around silencers. This is referred to as flanking
noise where the silencers may be designed to achieve 60 dB noise
reduction but something less is measured because the turbine’s induct
(airborne) noise has coupled to the duct wall structure, travels
down the duct liner/walls and re-radiates into the duct downstream
of the silencers. This mainly becomes a concern when having to
achieve better than 60 dB noise reduction. In these cases a vibration
break or isolator must be installed in the duct wall to reduce or
eliminate noise from flanking around the silencer section. In cases
having extensive duct work exposed in a low noise environment an
additional barrier wall or enclosure may be require whereby the
main duct is structurally independent of the other acoustical barriers.
Unfortunately, duct walls are not simple to design, especially if
having to meet an external shell temperature limit. Most industrial
turbines use double wall construction and the internal duct noise is
mainly transmitted to the exterior wall by way of the structural connections
between the inner liner sheets and the outer duct wall as
illustrated in Figure 10-7. Increasing the insulation thickness only
plays a minor part in reducing the breakout noise; the transmission
loss (TL) of the insulation material alone is fairly low unless
using a fairly dense product. In certain cases it may be possible to
install an isolator between the structural connector and the liner
sheet to minimize noise transmission to the exterior but this is both
expensive and labor intensive as each isolator must be installed
carefully. Visco-elastic and polymer damping materials and lead
septums cannot be used on high temperature exhaust ducts, even
if on the exterior shell; the potential for fire, emissions of toxins,
and accelerated material degradation will occur.