Reference Manual

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Chapter 6
Control Valve Noise
Noise has always been present in control valves.
It is a natural side effect of the turbulence and
energy absorption inherent in control valves. This
chapter will address how noise is created, why it
can be a problem, and methods to attenuate noise
created in control valves.
The major problem with industrial noise is its affect
on humans. Companies usually build town border
stations on sites remote from residential
developments. Isolation, however, is not always
possible, and noise prevention is a must.
The U.S. Occupational Safety and Health Act
(OSHA) establishes maximum permissible noise
levels for all industries whose business affects
interstate commerce. These standards relate
allowable noise levels to the permissible exposure
time. Notice in table 6-1 that the maximum
permissible levels depend upon the duration of
exposure. For example, the maximum sound level
a person should be exposed to for an eight hour
day is 90 dBA. These maximum sound levels have
become the accepted noise exposure standard for
most regulatory agencies. Thus, they have
become the standard by which much noise
generating equipment has been specified and
measured.
Table 6-1. Maximum Permissible Noise Levels
Duration of Exposure
(Hours)
Maximum Sound Pressure
(dBA)
16 85
8 90
4 95
2 100
1 105
1/2 110
1/4 115
Decibels (dB) are a measure to give an indication
of loudness. The “A” added to the term indicates
the correction accounting for the response of the
human ear. The sensitivity of our ears to sound
varies at different frequencies. Applying this “A”
correction is called weighting, and the corrected
noise level is given in dBA.
The A-weighting factor at any frequency is
determined by how loud noise sounds to the
human ear at that particular frequency compared
to the apparent loudness of sound at 1000 hertz.
At 1000 hertz the A-weighting factor is zero, so if
the sound pressure level is 105 dB, we say it
sounds like 105 dB.
On the other hand, if we listen to a sound at 200
hertz with a sound pressure level of 115 dB, it
sounds more like 105 dB. Therefore, we say that
the A-weighted loudness of the noise with a sound
pressure level of 115 dB is 105 dBA.
Essentially, if two or more sounds with different
sound pressure levels and frequencies sound like
the same loudness, they have the same dBA,
regardless of what their individual, unweighted
sound pressure levels may be.
The effect of A-weighting on control valve noise
depends upon the flowing medium since each
develops its own characteristic spectrum. Noise
levels for hydrodynamic noise, or liquid flow noise,
have appreciable energy at frequencies below 600
hertz. When the levels are A-weighted, it makes
the low frequency terms more meaningful and the
government standards somewhat more difficult to
meet.
On the other hand, aerodynamic noise levels
produced by steam or gas flow are the same in
either dB or dBA. This is because aerodynamic
noise occurs primarily in the 1000 to 8000 hertz
frequency range. The human ear has a fairly flat
response in the frequency range of 600 to 10,000