Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NOISE REDUCTION DEVICE FOR FLUID FLOW SYSTEMS
This is a division of co-pending Canadian Patent Application
No. 2,478,097 filed on March 19, 2003.
BACKGROUND OF THE INVENTION
Technical Field of the Invention
This invention relates to noise reduction devices in fluid flow systems
and more particularly to a differential velocity device for use downstream of
a
valve in a fluid flow system.
Description of Related Art
Control valves are used in process industries to control flow of fluids,
both liquids and compressible fluids. Aerodynamically generated noise is
inherent in the throttling process of gases and vapors. Throttling occurs by
opening or closing a selected valve in a fluid flow system.
It is generally accepted that exposure to high levels of noise can
damage the hearing of individuals working near fluid flow systems. In the
United States, the Occupational Safety and Health Administration (OSHA)
limits noise levels of worker exposure for the purpose of hearing
conservation.
For example, presently noise levels are limited to 90 decibels on the A
weighted scale (dBA) for eight hour exposure. Some other countries limit
exposure to 85 dBA.
Since noise generation is inherent in the throttling process, many
control valves require some method of noise reduction. Often globe type
valves are supplied with low noise trim using cages with a multiplicity of
small
drilled holes.
A more cost effective solution is desirable for moderate service
conditions. Specifically some form of noise reduction that can be obtained at
moderate cost is desirable for rotary control valves.
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SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention there is
provided a device to reduce noise transmitted from a piping system, said
device comprising: an inner central section comprising a plurality of
apertures
on an upstream side of the inner section and a plurality of apertures on a
downstream side of the inner section; a surrounding outer section comprising
a plurality of apertures on an upstream side of the outer section and a
plurality of apertures on a downstream side of the outer section, the total
cross-sectional area of the upstream apertures of the outer section being less
than the total cross-sectional area of the upstream apertures of the inner
section, the outer section reducing the velocity of a fluid flow relative to
said
inner section; and a fin downstream of the downstream apertures of the inner
section and of the downstream apertures of the outer section and between the
inner section and the outer section, the fin facilitating separation between
fluid
flow from the inner section and fluid flow from the outer section.
In accordance with another aspect of the present invention there is
provided a noise reduction device for fluid flow systems, the device
comprising: an inner section with a plurality of apertures on an upstream side
and a plurality of apertures on a downstream side; an outer annular section
with a plurality of apertures on an upstream side and a plurality of apertures
on a downstream side, the total cross-sectional area of the downstream
apertures of the outer section being less than the total cross-sectional area
of
the downstream apertures of the inner section; and a fin downstream of the
downstream apertures of the inner section and of the downstream apertures
of the outer section and between the inner section and the outer section, the
fin facilitating separation between fluid flow from the inner section and
fluid
flow from the outer section.
In accordance with yet another aspect of the present invention there is
provided a method for reducing noise transmitted from a fluid flow system
having at least one valve, the method comprising: separating a fluid flow
downstream of the at least one valve into an inner core fluid flow and a
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surrounding outer annular fluid flow by passing a portion of the fluid flow
through a device having a plurality of first apertures through a central
portion
of the device; reducing the velocity of said outer annular fluid flow relative
to
said inner core flow by passing a portion of the fluid flow through a device
having a plurality of second apertures disposed annularly, the inner core flow
having a larger total cross-sectional area than the outer annular fluid flow;
and
facilitating separation between the inner core flow and the outer annular flow
by passing the inner core flow on a first side and the outer annular flow on a
second side of a fin downstream of the apertures.
In accordance with still yet another aspect of the present invention
there is provided a fluid flow system comprising: a valve with an upstream
inlet and a downstream outlet; a fluid flow from said upstream inlet through
said downstream outlet with a certain velocity; a noise reduction device
disposed in the downstream outlet, said noise reduction device having an
inner section and an annular outer section, a plurality of apertures on a
downstream side of the inner section and a plurality of apertures on a
downstream side of the outer section, the outer section reducing the velocity
of the fluid flow to form a slower annular fluid flow and having a total flow
cross-sectional area less than the total flow cross-sectional area of the
inner
section; and a fin downstream of the downstream apertures of the inner
section and of the downstream apertures of the outer section and between the
inner section and the outer section, the fin facilitating separation between
fluid
flow from the inner section and fluid flow from the outer section.
A noise reduction device comprising a central section and an outer
annular section is provided. The outer section is designed to reduce the
velocity of fluid flow through the device and create an annular fluid flow
that
has a reduced velocity when compared to the core fluid flow of the system.
The preferred method for reducing the velocity of the annular flow is a staged
pressure reduction wherein the fluid flow passes through an upstream
aperture into a pressure reduction chamber and then through an offset
downstream aperture of larger cross sectional area than the upstream
aperture. The core flow of the system passes through a plurality of apertures
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in a central section of the noise reduction device to increase the frequency
of
the noise in the core flow. The device creates a flow regime with an annular
flow surrounding a core flow, the annular flow having a reduced velocity
compared to the core flow.
The present invention is intended to provide noise reduction of 15-20
decibels over a wide range of operating conditions. The one piece device is
readily machined from wrought material such as austenitic stainless steel. In
spite of the drilled holes the thick sections provide, an extremely high
natural
frequency to prevent failure due to flow induced vibration.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a cross sectional side view of the preferred embodiment of
current invention.
Fig. 2 is a front view of the device in Fig. 1 from the upstream side.
Fig. 3 is a rear view of the device of Fig. 1 from the downstream side.
Fig. 4 is a side cross sectional view of a second embodiment of this
invention.
Fig. 5 is a front view of the embodiment at Fig. 4 from the upstream
side.
Fig. 6 is a rear view of the embodiment in Fig. 4 from the downstream
side.
Fig. 7 is a cross sectional top view of the embodiment of a valve
system in accordance with the invention.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
OF THE INVENTION
Reference is now made to the Drawings wherein like reference
numerals denote like or similar parts throughout the Figures.
Referring now to Figs. 1, 2, and 3, the noise reduction device 10
comprises a circular disc having a central section 12 and an outer annular
section 14. The central section 12 contains a plurality of central apertures
16
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extending through the disk. The outer annular section 14 has upstream
apertures 18 communicating with a pressure reduction chamber 20 which
communicates with downstream apertures 22. The noise reduction device 10
has an outer circumferential surface 24 into which a groove 26 is cut around
5 its entire circumference. Groove 26 forms pressure reduction chamber 20
when the device is placed within a fluid flow system, as is illustrated in
Fig. 7.
Central section 12 in the embodiment shown in Fig. 1 further includes an
upstream recess 28 and a downstream recess 30. Outer annular section 14
may have a downstream fin 32 and an annular recess 34. Annular recess 34
communicates with downstream apertures 22 and is separated from
downstream recess 30 by downstream fin 32. Recesses 28, 30 and 32, in
conjunction with downstream fin 32 enhance the separation between a core
fluid flow through the central section 12 and an annular fluid flow through
the
annular section 14.
Each upstream aperture 18 has an axis 36 which extends generally
parallel to the flow direction. Each downstream aperture 22 has an axis 38
which extends generally parallel to the flow direction. In the preferred
embodiment shown in Figs. 1, 2, and 3, upstream axes 36 are offset from
downstream axes 38 by 5 degrees. The offset between upstream axes and
downstream axes enhances the pressure reduction in chamber 20 and is
shown in each of the figures. In Fig. 3 aperture 22 is shown while aperture 18
is in shadow, offset from aperture 22 by 5 degrees of rotation. In Fig. 2
aperture 18 is shown while aperture 22 is in shadow, offset from aperture 18
by 5 degrees of rotation. Fig. 1 shows aperture 22 as a part of the main
cutaway, but aperture 18 is shown as a part of a partial cutaway, indicating
that it is not in the same plane as aperture 22. The partial cutaway is used
to
show the passage of fluid in the annular section 14 first through aperture 18
into chamber 20 and then out of chamber 20 through aperture 22. Figures 4,
5 and 6 are drawn in similar fashion to show the same offset.
Referring now to Figs. 4, 5, and 6, a second embodiment of the
invention is shown. Noise reduction device 40 is similar to noise reduction
device 10. Noise reduction device 40 has a central section 42 and an outer
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annular section 44. Central section 42 has central apertures 46. Outer
annular section 44 has upstream apertures 48 which communicate with
pressure reduction chamber 50 which communicates with downstream
apertures 52. Noise reduction device 40 has an outer circumferential
surface 54 into which a groove 56 has been cut to form noise reduction
chamber 50.
Noise reduction device 40 does not have an upstream recess,
downstream recess, downstream fin, or annular recess as shown in noise
reduction device 10. Noise reduction device 40 relies on the pressure
differential created between the outer section 44 and central section 42 for
separation and velocity reduction. Upstream apertures 48 have upstream
axes 66 and downstream apertures 52 have downstream axes 68. Upstream
axes 66 are offset from downstream axes 68 by 5 degrees as shown in
Figs. 5 and 6.
Noise reduction devices 10 and 40 illustrate two embodiments of the
invention. Other embodiments may include selected features of each. For
example, a third embodiment may be similar to device 10, but without annular
recess 34 and downstream fin 32. Such a third embodiment may be
described as similar to device 40, but adding upstream recess 28 and
downstream recess 30 from device 10. As will be appreciated by one skilled
in the art, many other embodiments are within the scope of this invention.
Referring now to Fig. 7, a valve system 70 is shown with an upstream
inlet 72 and a cylindrical downstream outlet 74. The noise reduction
device 10 of the present invention is shown as inserted into downstream
outlet 74. Downstream outlet 74 may be threaded with outlet threads 76 and
noise reduction device 10 may have mating threads 78 on outer surface 24.
Threads 78 engage outlet threads 76 to restrain noise reduction device 10 in
downstream outlet 74. It will be appreciated by those skilled in the art that
other methods of securing noise reduction device 10 in the outlet may be
used.
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Noise reduction device 10 and noise reduction device 40 are both
designed to separate the flow in a fluid flow system into an inner core fluid
flow and an outer annular fluid flow. Devices 10 and 40 and other
embodiments thereof reduce the pressure in the outer annular fluid flow in a
staged manner and thereby reduce the velocity of outer annular flow relative
to inner core flow.
Noise reduction device 10 or 40 achieves a reduction in the noise
transmitted to the air surrounding the exterior of a piping system by three
identifiable mechanisms. The first mechanism is reduced noise generation in
the fluid. The difference in velocity between the annular flow and the core
flow reduces aerodynamically generated noise as compared to a device that
produces a singular flow field. In subsonic flow, the noise reduction is due
to
the reduced strength of turbulent eddies that create noise. In sonic flow
conditions, the noise reduction is due to the reduced interaction of turbulent
flow with shock cells. Through these fluid mechanisms the fluid generated
noise is reduced.
The second mechanism is through the generation of high frequency
noise. Flow through small apertures, such as apertures 16, produces high
frequency noise. Pipe wall transmission loss is dependent upon the driving
frequency. The minimum transmission loss for a particular pipe size and wall
thickness, and with a given fluid, is at the lowest coincidence frequency. The
lowest region of transmission loss falls between the lowest coincidence
frequency and the ring frequency. Flow through small apertures, such as
central apertures 16 or 46, produces high frequency noise that is intended to
be well above the coincidence and ring frequencies of the downstream piping.
The resulting increased transmission loss is very beneficial toward
reduction of the perceived noise in the air surrounding the exterior of a
piping
system.
The third mechanism is the effect of downstream velocity adjacent to
the pipe wall. Normally the perceived noise outside the piping increases with
increased downstream velocity even with the same internal noise level. The
annular flow velocity along the downstream pipe wall is lowered by pressure
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staging in the noise reduction device 10 or 40. Lower velocity along the
downstream pipe wall reduces the downstream velocity effect thereby
reducing the perceived noise level.
The downstream apertures of the inner section are downstream of the
downstream apertures of the outer section.
While the invention has been described and illustrated in at least two
embodiments there are modifications that can be made to these embodiments
while still remaining within the scope of the invention. It will be
appreciated by
one skilled in the art that all of the apertures in the preferred embodiments
are
circular, this reflects the easiest known method of manufacturing the
preferred
embodiments, which is a drilling procedure. The device would work equally
well with other shapes of apertures such as rectangular apertures, square
apertures, oval apertures, hexagonal apertures, etc. It is also noted that the
apertures in the central section of the device are arranged hexagonally to
create even spacing between apertures. This hexagonal spacing is the
preferred method for achieving even spacing between the apertures although
other spacings would also work. A further modification within the scope of
this
invention would be to further adjust the offset between the upstream apertures
and the downstream apertures. While the methods, apparatus and system
shown and described have been characterized as being preferred
embodiments, it will be readily apparent that various changes and
modifications can be made therein without departing from the scope of the
invention.