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A silencer is a component installed in a piping system to
reduce noise. The amount of noise reduction achieved depends
upon the silencer design, the characteristics of the noise
source, and the interaction of other piping components.
While it is not possible to state the exact performance for
all situations, some general guides are presented to help
you make a proper silencer selection. Typical attenuation
values (in dBA) are given on the selection menus for the
silencers. If you are unsure which model to select or need
a specific recommendation, we suggest you contact MAXIM at
the address shown at the end of this program or our
representative in your area. The pressure drop program is
provided to determine the pressure drop of simple piping
systems such as those used in conjunction with silencers and
heat recovery units. It is limited in capability and should
only be used within the following limits:
- Piping components are in series.
- Pipe and fittings are all the same size.
- Single phase flow.
- Pressure drop for air does not exceed 1/10 of
upstream pressure.
- Elevation change between inlet and outlet is
negligible.
- Temperature change between inlet and outlet is
negligible.
- Pressure should not exceed ratings for silencer or
heat recovery unit.
- Atmospheric pressure is near sea level (14.7 psia)
When silencers are to operate at high altitudes (over
about 1800 ft.), a correction factor must be applied to the
calculated pressure drops to compensate for the reduced
density of the gas. One method of doing this, for silencers
open to the atmosphere, is to determine the pressure drop
based on a flowrate in acfm at standard atmospheric pressure
and multiply the results by the ratio of (14.7/Patm), where
Patm is the absolute atmospheric pressure at the operating
altitude in psia. A similar correction would apply to blower
silencers that operate under pressure. The correction factor
for these silencers is [(P+14.7)/(P+Patm)], where P is the
operating pressure in psig. These same correction factors
apply to piping pressure drop calculations that involve
gases.
The VT series of silencers are designed to reduce the
noise produced by high velocity gases discharging to the
atmosphere. They operate at essentially atmospheric
pressure, i.e.,the outlet of the silencer is always open to
the atmosphere. It is not a pressure vessel and should not
be used in a pressurized line. In special applications, a
high pressure diffuser can be incorporated into the inlet of
the silencer to reduce inlet pressure to atmospheric and to
restrict flowrate. A restrictive diffuser should not,
however, be used when the silencer is installed on the
outlet of a safety valve.
The unsilenced noise calculations are based on a
correlation developed by Maxim from noise measurements made
of gases discharging to the atmosphere. Data for steam, air
and natural gas were used in its development. Upstream
pressures ranged from 100 psig to 3000 psig. The noise level
and frequency spectrum are presumed to be best for upstream
pressures in this range.
Except for steam, gas density inside the silencer is
calculated by the perfect gas law equation. This is
generally applicable for gases well above their boiling
points. It is assumed that the temperature of the gas inside
the silencer is the same as its upstream value. Steam
properties are calculated from thermodynamic relations
published by the ASME and assume a constant enthalpy
expansion from upstream conditions to atmospheric pressure.
Conditions which result in wet steam are not permitted.
In order to minimize packing erosion, the internal
velocity in the silencer is limited to a maximum of 250 fps.
When the program does the sizing, this criteria is observed.
For sizes specified by the user, a caution statement is
displayed beside the model number when this criteria is
exceeded.
Occasionally low noise level requirements make it
necessary to select a silencer for a lower velocity.
Turbulence inside the silencer produces noise which is
additive with the silenced primary noise. When this "self
noise" causes the combined noise level to exceed the
permissible level, the next larger size silencer should be
considered. If self noise is at least 10 dB less than the
silenced primary noise, its effect is negligible.
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