Page 66 - Energize April 2022

TECHNICAL

Sound suppression for gas or

steam pipes

BY BILL FLOWERS, EMERSON

igh aerodynamic noise levels
are a natural result of turbulent
Hflow. As gas or steam flows
through a control valve, velocity
increases in the flow passages and then
slows, creating pressure fluctuations
and sound waves. These waves are
emitted from the valve and piping,
radiating in all directions.
At low velocities the sound is minor
and barely noticeable, but at higher
levels the noise becomes irritating to
personnel in the immediate area. Very Figure 1: Source control often uses valve trims with small holes (left), multiple flow paths (middle), or
loud noise levels can permanently downstream diffusers (right) to reduce aerodynamic noise. The methods spread the pressure drop and
damage hearing and destroy valve flow across multiple points to reduce vapour velocity and minimise noise.
internals due to excessive vibration.
Lighthill’s law states that drop through the valve itself (Figure 1).
aerodynamic noise varies as the eighth An alternative source control method is flow division. Flow division breaks up the
power of the gas velocity , so high flow single flow path into multiple flow paths, thus reducing the overall velocity of each
1
and high pressure drop applications path. The reduced velocity results in lower levels of sound. In either case, source control
get very loud, very quickly. Most plants often requires a much more complex valve internal design, which is costly and prone
address high control valve noise in one to plugging. External diffusers also add cost and usually incorporate very small flow
of two methods, either source control passages, which can plug as well.
or path control. Each method has
varying levels of cost and effectiveness. Sound suppression
The second method of aerodynamic noise reduction is path control. This method
Addressing the source attempts to muffle the sound and keep it from radiating to the surrounding area. The
Source control attempts to diminish techniques can be as simple as thick-walled pipe, pipe insulation, or encasing the pipe
noise levels by eliminating or at least with acoustic blankets or materials designed to absorb the sound.
reducing the generated sound. This is Alternatively, one can employ specially designed silencers which either absorb the
often accomplished using pressure drop sound or use resonant chambers to cancel the noise through destructive interference.
staging or flow division. Pressure drop A car muffler is an example of a resonant chamber silencer that is common in everyday
staging reduces the overall sound by use. Silencers work but often require a high pressure drop and tortuous path, and
dividing the total pressure drop over they may include sound absorbing materials which degrade or become saturated with
a number of steps, rather than taking process fluids. Acoustic blankets wear with exposure to the elements, and are often
the full drop in at a single point. The damaged or improperly installed after maintenance, so their performance declines over
smaller pressure drops create smaller time.
vena contractas with correspondingly
2
smaller pressure variations, and thus The price of silence
less noise. Both source and path controls do work, but they come with significant costs and
This can be accomplished with a limitations. Low noise valve trims usually require a more expensive globe valve, and its
multistage low noise valve trim design, intricate internals raise the valve price considerably. Silencers and diffusers add pressure
or it can include pressure-reducing drop by design and their peak performance can be limited to certain ranges of flow.
inline diffusers on the inlet and/or None of these options are capable of passing a pipeline pig, so a pig capture and bypass
outlet to reduce the overall pressure chamber are required.

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