Reference Manual

6−7
Figure 6-4. WhisperFlo Technology
W7065
W7056
Fisher WhisperFlot trim (figure 6-4) is well-suited
for applications that have high noise levels and
require large Cvs. It is effective in applications that
have a pressure drop ratio up to 0.99. When a
pressure drop ratio of .94 or higher is expected,
and WhisperFlo is desired, the noise calculations
will be performed by the engineering experts at
Emerson Process Management. This design is a
multi-path, two-stage design that has the
capability of reducing noise up to 50 dBA. The key
factor behind this attenuation is allowing the
pressure to recover between stages. This allows
for the pressure drop ratio of the second stage to
be less than the pressure drop ratio of the first
stage. In achieving this, along with special
passage shapes, the frequency is shifted to a
higher spectrum, velocities are managed, and the
jets maintain independence.
All of the Whisper Trim cages and WhisperFlo
trims are designed for sliding stem valves. In
applications requiring rotary valves that have high
noise, an attenuator, diffuser, or combination there
of may be applied. Applications with ball valves
can apply an attenuator to obtain up to 10 dBA
reduction in noise. These attenuators are
designed to reduce both aerodynamic and
hydrodynamic noise. With butterfly valves you can
only attenuate aerodynamic noise utilizing an
inline diffuser. As mentioned above, these
diffusers can provide up to a 25 dBA reduction in
noise.
Figure 6-5. Vee-Ball Noise Attenuator
W6116
For control valve applications operating at high
pressure ratios (ΔP/P
1
is greater than 0.8), a
series approach can be very effective in
minimizing the noise. This approach splits the total
pressure drop between the control valve and a
fixed restriction (such as a diffuser) downstream of
the valve. In order to optimize the effectiveness of
the diffuser, it must be designed for each unique
installation so that the noise levels generated by
the valve and diffuser are equal.
Control systems venting to atmosphere are
generally very noisy, as well. This is because of
the high pressure ratios and high exit velocities
involved. In these applications, a vent silencer
may be used to divide the total pressure drop
between the actual vent and an upstream control
valve (figure 6-6). This approach quiets both the
valve and the vent. A properly sized vent silencer
and valve combination can reduce the overall
system noise level by as much as 60 dBA.
Path Treatment
Path treatment can be applied where source
treatment is more expensive, or in combination
with source treatment where source treatment
alone is inadequate. Path treatment consists of
increasing the resistance of the transmission path
to reduce the acoustic energy that is transmitted
to the environment. Common path treatments
include the use of:
D Heavy walled pipe
The noise attenuation possible with heavy-walled
pipe varies with the size and schedule used. As an
example, increasing a pipeline from schedule 40
to schedule 80 may reduce sound levels by
approximately 4 dB.