Reference Manual

6−3
Table 6-2. Combined Noise Corrections
dBA
1
- dBA
2
dBA Adder to Loudest Noise Source
0 3.01
1 2.54
2 2.12
3 1.76
4 1.46
5 1.2
6 <1
To use table 6-2:
1. Determine the noise level of each source at the point where you want to
determine the combined noise level.
2. Determine the arithmetic dB difference between the two sources at the
location of interest.
3. Find the difference between the two sources in the table.
4. Read across the table to find the dB factor to be used. Add this factor to the
louder of the two sources. This value is the combined dB of the two sources.
Let’s put this table to work to illustrate how noise
sources combine. Two interesting examples help
illustrate how sound levels combine:
1. When two noise sources with equal sound
pressure levels of 90 dB are combined, the
correction factor is 3.01. Therefore, the resultant
combined noise level is 93 dB.
2. If two sources have considerably different
noise levels, say 95 dB and 65 dB, the correction
factor is nearly zero. Therefore, the combined
noise level is essentially the same as the louder of
the two sources, that is, 95 dB. This leads us to
the first rule of noise control: Preventing or
controlling the loudest noise sources first.
While this appears obvious, in practice it is not the
easiest path.
Sources of Valve Noise
Control valves have long been recognized as a
contributor to excessive noise levels in many fluid
process and transmission systems. The major
sources of control valve noise are mechanical
vibration noise, aerodynamic noise, and
hydrodynamic noise.
Mechanical noise generally results from valve plug
vibration. Vibration of valve components is a result
of random pressure fluctuations within the valve
body and/or fluid impingement upon the movable
or flexible parts. The most prevalent source of
noise resulting from mechanical vibration is the
lateral movement of the valve plug relative to the
guiding surfaces. The sound produced by this type
of vibration normally has a frequency less than
1500 hertz and is often described as a metallic
rattling. In these situations, the physical damage
incurred by the valve plug and associated guiding
surfaces is generally of more concern than the
noise emitted.
Another source of mechanical vibration noise is
resonant vibration, which occurs when a valve
component resonates at its natural frequency.
Resonant vibration produces a single-pitched tone
normally having a frequency between 3000 and
7000 hertz. This type of vibration produces high
levels of mechanical stress that may produce
fatigue failure of the vibrating part. Valve
components susceptible to natural frequency
vibration include contoured valve plugs with hollow
skirts and flexible seals.
The noise caused by the vibration of valve
components is usually of secondary concern, and,
ironically, may even be beneficial because it gives
warning when conditions exist that could produce
valve failure. Noise resulting from mechanical
vibration has for the most part been eliminated by
improved valve design. Most modern control
valves employ cage guiding and more precise
bearings to eliminate vibration problems. Testing
helps isolate and eliminate resonant frequency
problems before installation.
The second type of noise is hydrodynamic noise.
Hydrodynamic noise results from liquid flow and is
caused by the implosion of vapor bubbles formed
in the cavitation process. Vapor bubble formation
occurs in valves controlling liquids when the
service conditions are such that the local static
pressure, at some point within the valve, is less
than or equal to the liquid vapor pressure.
Localized areas of low static pressures within the
valve are a result of the pressure-to-velocity-head
interchange that occurs at the valve vena
contracta. When the vapor bubbles move
downstream, they encounter pressures higher
than the vapor pressure and collapse. The rapid
implosion can result in severe damage to adjacent
valve or pipeline surfaces, and generate high
noise levels.
Hydrodynamic noise sounds similar to that of
gravel flowing through a pipe. Intense cavitation
can cause noise levels as high as 115 dBA, but
such cavitation would not be tolerated because
cavitation damage would drastically shorten the
operating life of the installation. Therefore, control
valve damage is normally of more concern than
the noise produced in cavitating services.
Aerodynamic noise is generated by the turbulence
associated with control of gas, steam, or vapors.
While generally thought of as accompanying high
capacity, high pressure systems, damaging noise