Reference Manual

6−8
Figure 6-6. Valve and Vent Diffuser Combination
W2672
D Acoustical or thermal insulation
The noise level near the valve can be lowered by
applying insulation to absorb the noise. Insulation
absorbs much of the noise that would normally
reach the atmosphere, but does not absorb any of
the noise going up or down inside the pipe walls.
Thermal insulation can give 3 to 5 dBA noise
reduction per inch of insulation thickness to a
maximum attenuation of 12 to 15 dBA. Acoustical
insulation can give 8 to 10 dBA noise reduction
per inch of blanket type insulation. The maximum
attenuation that should be expected is 24 to 27
dBA.
Path treatments such as heavy-walled pipe or
external insulation can be a very economical and
effective technique for localized noise abatement.
However, they are effective for localized noise
reduction only. That is, they do not reduce the
noise in the process stream, but only shroud it
where the treatment is used. Noise propagates for
long distances via the fluid stream and the
effectiveness of the treatment ends where the
treatment ends.
D Silencers
The silencer differs from other path treatments in
that it does actually absorb some of the noise
energy. Therefore, it reduces sound intensity both
in the working environment and in the pipeline. In
gas transmission systems, in-line silencers
effectively dissipate the noise within the fluid
stream and attenuate the noise level transmitted
to the solid boundaries. Where high mass flow
rates and/or high pressure ratios across the valve
exist, an in-line silencer is often the most realistic
and economical approach to noise control. Use of
absorption-type in-line silencers can provide
almost any degree of attenuation desired.
However, economic considerations generally limit
noise attenuation to approximately 35 dBA.
Hydrodynamic Noise
The primary source of hydrodynamic noise is
cavitation. Recall that cavitation is the formation
and subsequent collapse of vapor bubbles in a
flowing liquid. This phenomenon sounds similar to
that of gravel flowing down the pipe.
Source treatment for noise problems associated
with control valves handling liquid is directed
primarily at eliminating or minimizing cavitation.
Cavitation and its associated noise and damage
can often be avoided at the design stage of a
project by giving proper consideration to service
conditions. However, where service conditions are
fixed, a valve may have to operate at pressure
conditions normally resulting in cavitation. In such
instances, noise control by source treatment can
be accomplished by using one of several methods;
multiple valves in series, a special control valve, or
the use of special valve trim that uses the series
restriction concept to eliminate cavitation.
Cavitrol Trim is a source treatment solution as it
eliminates cavitation across the control valve. This
is achieved by staging the pressure drop across
the valve so the pressure of the fluid never drops
below its vapor pressure (figure 6-7). Cavitrol Trim
is only effective in clean processes. If a process
contains particulate, it will require Dirty Service
Trim (DST). DST also operates on the concept of
staging the pressure drops (figure 6-8).
While path treatment of aerodynamic noise is
often an economical and efficient alternative, path
treatment of hydrodynamic noise is not generally
recommended. This is because the physical
damage to control valve parts and piping produced
by cavitation is generally a much more serious
issue than the noise generated.