Note: Descriptions are shown in the official language in which they were submitted.
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Vortex flow pickup
The invention relates to a vortex flow pickup for
measuring the volume flow, the mass flow or the flow
velocity of the fluid flowing in a direction of flow in
a measuring tube, with a bluff body which serves for
producing Karman vortices being arranged over a
diameter of the measuring tube.
The volume flow is defined as the volume of fluid
flowing through the cross section of the measuring tube
per unit of time and the mass flow is defined as the
mass of fluid flowing through the cross section of the
measuring tube per unit of time.
It is known that, during the operation of a vortex flow
pickup of this type, a Karman vortex street is produced
downstream of the bluff body and its pressure
fluctuations are converted by a vortex sensor into an
electrical signal, the frequency of which is
proportional to the volume flow or the flow velocity.
In US-A 60 03 384 there is a description of a currently
customary vortex flow pickup for measuring the volume
flow or the flow velocity of a fluid which is flowing
in a direction of flow in a measuring tube having a
tube wall, which vortex flow pickup comprises:
- a bluff body which is arranged along a diameter of
the measuring tube and
-- serves for producing Karman vortices and
-- is connected to the tube wall of the measuring tube
from the inside at a first and a second fixing
location, which lie opposite each other,
- a vortex sensor, which responds to pressure
fluctuations produced by the vortices, is fitted
downstream of the bluff body in a bore of the tube
wall of the measuring tube and seals off this bore,
- the center of the bore lying together with the center
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of the first fixing location of the bluff body on a
generatrix of the measuring tube and
- the vortex sensor comprising:
-- a diaphragm covering the bore, with a first surface
facing the fluid and a second surface facing away
from the fluid,
-- a wedge-shaped sensor vane, which is fastened on the
first surface of the diaphragm and
--- is shorter than the diameter of the measuring tube
and
--- has principal surfaces in line with the generatrix
of the measuring tube and also a front edge, and
-- a sensor element fastened on the second surface.
If the temperature of the fluid is also measured by
means of a temperature sensor, the mass flow can be
determined, for example calculated by means of a
microprocessor, from the volume flow, the type of fluid
and its properties as well as the temperature at any
given time.
This has already been described some time ago in the
case of vortex flow pickups with different types of
vortex sensors. For instance, US-A 40 48 854 and US-A
44 04 858 each show a temperature sensor which is
arranged on the tube wall of the measuring tube from
the inside in such a way that it is skimmed over by the
flowing fluid.
In JP-A 2000-2567 there is a description of a vortex
flow pickup for measuring the mass flow, the volume
flow or the flow velocity of a fluid which is flowing
in a direction of flow in a measuring tube having a
tube wall, which vortex flow pickup comprises
- a blade which is fixed on one side to the tube wall
from the inside by means of a base plate and
-- during operation produces Karman vortices,
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is shorter than a diameter of the measuring tube and
-- has parallel principal surfaces aligned
perpendicularly to the direction of flow and a
rounded front face,
--- on which a temperature sensor is arranged,
- first sensor elements, fastened in the vicinity of
the fixing location, for pressure fluctuations of the
flowing fluid produced by the Karman vortices and
- second sensor elements, fastened in the vicinity of
the fixing location, for deflections of the blade.
produced by the flowing fluid.
This temperature sensor is also skimmed over by the
flowing fluid and, as the inventors have found, is
consequently not resistant to all fluids encountered in
operation, i.e. some fluids corrode temperature sensors
arranged in such.a way.
These fluids which corrode the temperature sensor must
therefore be banned from use with the vortex flow
pickup by the manufacturer of the latter. However,
such a ban restricts the use of these vortex flow
pickups, that is the universality of their
applications, and consequently also their
attractiveness on the market.
One object on which some embodiments of the invention is based
is to specify vortex flow pickups with a bluff body and with
a vortex sensor fixed in the tube wall of the measuring
tube and with a temperature sensor which is arranged in
such a way that the respective vortex flow pickup may
also be used together with those fluids which corrode
the temperature sensor.
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According to an aspect of the invention, there is provided a vortex
flow pickup for measuring the mass flow, the volume flow or the flow velocity
of a
fluid which is flowing in a direction of flow in a measuring tube having a
tube wall,
which vortex flow pickup comprises: a bluff body which is arranged along a
diameter of the measuring tube and serves for producing Karman vortices and is
connected to the tube wall of the measuring tube from the inside at a first
and a
second fixing location, which lie opposite each other, a vortex sensor, which
responds to pressure fiuctuations produced by the vortices, is fitted
downstream of
the bluff body in a bore of the tube wall of the measuring tube and seals off
this
bore, the center of the bore lying together with the center of the first
fixing location
of the bluff body on a generatrix of the measuring tube, said vortex sensor
comprising: a diaphragm covering the bore, with a first surface facing the
fluid and
a second surface facing away from the fluid; a sensor vane, which is fastened
on
the first surface of the diaphragm and is shorter than the diameter of the
measuring tube, has principal surfaces in line with the generatrix of the
measuring
tube and also at least one front edge, and is provided with a blind hole, a
bottom
of which lies in the vicinity of the at least one front edge; a temperature
sensor,
which is fixed on the bottom of the blind hole, said temperature sensor being
disposed within said sensor vane such that it is separated from the fluid to
be
measured and said temperature sensor being capable to follow temperature
changes of the fluid flowing past the sensor vane; and a sensor element
fastened
on the second surface, said sensor element being operable to generate a signal
having a frequency being proportional to a time-related separation frequency
of
said vortices.
According to another aspect of the invention, there is provided a
vortex flow pickup for measuring the mass flow, the volume flow or the flow
velocity of a fluid which is flowing in a direction of flow in a measuring
tube having
a tube wall, which vortex flow pickup comprises: a first temperature sensor
and a
second temperature sensor; a bluff body which is arranged along a diameter of
the measuring tube and serves for producing Karman vortices and is connected
to
the tube wall of the measuring tube from the inside at a first and a second
fixing
location, which lie opposite each other; and a vortex sensor, which responds
to
pressure fluctuations produced by the vortices, is fitted downstream of the
bluff
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body in a first bore of the tube wall of the measuring tube and seals off this
bore,
the center of the first bore lying together with the center of the first
fixing location
of the bluff body on a generatrix of the measuring tube, said vortex sensor
comprising: a diaphragm covering the first bore, with a first surface facing
the fluid
and a second surface facing away from the fluid, a sensor vane, which is
fastened
on the first surface of the diaphragm and is shorter than the diameter of the
measuring tube and has principal surfaces in line with the generatrix of the
measuring tube and also at least one front edge, and a sensor element fastened
on the second surface, said sensor element being operable to generate a signal
having a frequency being proportional to a time-related separation frequency
of
said vortices; wherein the bluff body is provided with a blind hole, which is
in line
with a second bore in the tube wall and in which said first temperature sensor
is
fitted, said first temperature sensor being capable to follow temperature
changes
of the fluid flowing past the bluff body and said first temperature sensor
being
disposed within said bluff body such that it is separated from the fluid to be
measured; and wherein the second temperature sensor is disposed outside of
said measuring tube.
According to an embodiment of both variants of the invention, the
principal surfaces of the sensor vane form a wedge with a single front edge.
According to yet another aspect of the invention, there is provided an
apparatus for measuring a temperature of a flowing fluid and at least one of
volume flow, mass flow and flow velocity of said fluid, said apparatus
comprising a
measuring tube for conducting said fluid to be measured, a bluff body being
arranged within said measuring tube for producing Karman vortices within said
flowing fluid; a sensor vane being fitted within the measuring tube downstream
the
bluff body, said sensor vane being operable to respond to pressure
fluctuations
produced by said Karman vortices; a sensor element being coupled to the sensor
vane and being operable to generate a signal having a frequency being
proportional to a time-related separation frequency of said vortices; and a
temperature sensor being capable to follow temperature changes of the fluid
flowing past the sensor vane; wherein the temperature sensor is disposed
within
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said sensor vane such that said temperature sensor is separated from the fluid
to
be measured.
According to a further aspect of the invention, there is provided an
apparatus for measuring a temperature of a flowing fluid and at least one of
volume flow, mass flow and flow velocity of said fluid, said apparatus
comprising a
measuring tube for conducting said fluid to be measured, a bluff body being
arranged within said measuring tube for producing Karman vortices within said
flowing fluid; a sensor vane being fitted within the measuring tube downstream
the
bluff body, said sensor vane being operable to respond to pressure
fluctuations
produced by said Karman vortices; a sensor element being coupled to the sensor
vane and being operable to generate a signal having a frequency being
proportional to a time-related separation frequency of said vortices; and a
temperature sensor being capable to follow temperature changes of the fluid
flowing past the sensor vane; wherein the temperature sensor is disposed
within
said bluff body such that said temperature sensor being separated from the
fluid to
be measured.
According to still a further aspect of the invention, there is provided a
vortex flow sensor for measuring a fluid flowing in a pipe, particularly for
measuring a flow velocity, a volumetric flow rate, and/or a mass flow rate of
the
fluid, the vortex flow sensor comprising: a flow tube connected into the pipe
for
conducting the flowing fluid; a bluff body disposed in the lumen of the flow
tube
and serving to shed Karman vortices; a vortex sensor device responsive to
pressure fluctuations caused by the vortices and to convert pressure
fluctuations
into an electric vortex signal; and a first temperature sensor being capable
to
follow temperature changes of the fluid to be measured, and a second
temperature sensor; wherein the first temperature sensor is disposed within
said
bluff body such that said temperature sensor is separated from the fluid to be
measured, and wherein the second temperature sensor is disposed outside of
said measuring tube.
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One advantage of some embodiments of the invention is that the
temperature sensor has no chance of coming into contact with the
flowing fluid and consequently also cannot be corroded
by it. Nevertheless, the temperature sensor is
arranged so close to the fluid that it senses its
temperature with virtually no delay; it is in fact
separated from the fluid only by the thin wall of the
vortex sensor or of the bluff body and, like the
remaining parts of the vortex flow pickup, these parts
are produced from a metal, preferably stainless steel,
and are therefore good heat conductors.
A further advantage of some embodiments of the invention is that,
in a way corresponding to the book by F. P. Incropera and D. P.
DeWitt "Fundamentals of Heat and Mass Transfer", 4th
edition, 1996, ISBN 0-471-30460-3, pages 114 to 119 and
407, the temperature sensor arranged in the sensor vane
or in the bluff body can work together with a second
temperature sensor, which is fastened on the measuring
tube, preferably from the outside, that is to say
likewise does not come into contact with the fluid. As
known, if the second temperature sensor is provided, a
more exact temperature measurement is obtained than
with a single temperature sensor.
The invention and further advantages are now explained
in more detaii on the basis of exemplary embodiments,
which are represented in the figures of the drawing.
The same parts are designated in the differpnt figures
by the same reference numerals, which are omitted
however if necessary for the sake of clarity.
Figure 1 shows a vortex flow pickup corresponding to
the first variant of the invention in a
perspective view, as seen in the direction of
flow, and partly cut open,
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Figure 2 shows the vortex flow pickup from figure 1 in
a perspective view, as seen counter to the
direction of flow, and partly cut open,
Figure 3 shows the vortex sensor from figures 1 and 2
in a perspective view from below,
Figure 4 shows a perspective longitudinal section of
the vortex sensor of figure 3, and
Figure 5 shows in a way analogous to figure 2 a vortex
flow pickup corresponding to the second
variant of the invention in a perspective
view and partly cut open.
Figures 1 to 4 are described together below, since the
details cannot all be represented in each figure. The
perspective views shown first in figures 1 and 2, and
serving as an overview, of an exemplary embodiment of
the first variant show a partly cut open vortex flow
pickup 1, seen on the one hand in the direction of flow
(figure 1) and on the other hand seen counter to the
direction of flow (figure 2), with a vortex sensor 3
fixed to a tube wall 21 of a measuring tube 2 and
protruding through a bore 22. This is preferably a
dynamically compensated vortex sensor with a capacitive
sensor element, as is described in US-A 60 03 384, the
content of which belongs to the disclosure of this
application.
Arranged along a diameter of the measuring tube 2, in
the interior of the latter, is a bluff body 4, which is
firmly connected to the measuring tube 2, thereby
forming a first fixing location 41, which is
represented, and a second fixing location 41*, which is
concealed. The center of the bore 22 and the center of
the fixing location 41 lie on a generatrix of the
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measuring tube 2.
The bluff body 4 has an impact surface 42, against
which, in operation, a fluid to be measured, for
example a liquid, a gas or a vapor, flows. The bluff
body 4 also has two lateral surfaces, of which only one
(front) lateral surface 43 can be seen in figures 1 and
2. Two separation edges are formed by the impact
surface 42 and the lateral surfaces, only one (front)
separation edge 44 of these being completely visible
and one (rear) separation edge 45 being shown in an
indicative manner in figure 1.
The bluff body 4 of figures 1 and 2 has substantially
the shape of a straight triangular column, that is a
column with a triangular cross section. However, other
conventional shapes of the bluff body can also be used
in the invention.
The flow of the fluid against the impact surface 42
leads to the formation, downstream of the bluff body 4,
of a Karman vortex street in the fluid due to the fact
that vortices separate alternately at each separation
edge and are carried along by the flowing fluid. These
vortices generate local pressure fluctuations in the
fluid, the time-related separation frequency of which,
i.e. what is referred to as their vortex frequency, is
a measure of the flow velocity and/or the volume flow
of the fluid.
The pressure fluctuations are converted by means of the
vortex sensor 3 into an electrical signal, which is fed
to evaluation electronics, which calculate the flow
velocity and/or the volume flow of the fluid in the
customary way.
The vortex sensor 3 is fitted downstream of the bluff
body 4 in the bore 22 of the tube wall 21 of the
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measuring tube 2 and seals off the bore 22 from the
circumferential surface of the measuring tube 2, the
vortex sensor 3 being screwed to the tube wall 21.
Used for example for this purpose are four screws, of
which the screws 5, 6, 7 can be seen in figures 1 and
2, while associated bores 50, 60, 70, 80 are
represented in figure 3.
Of the vortex sensor 3, a wedge-shaped sensor vane 31,
protruding into the interior of the measuring tube 2
through the bore 22 of the tube wall 21, and a housing
cap 32 can be seen in figures 1 and 2. The housing cap
32 runs into an extension 322, by insertion of a
thinner-walled intermediate piece 323, cf. the cited
US-A 60 03 384.
The sensor vane 31 has principal surfaces, of which
only the principal surface 311 can be seen in figures 1
and 2. The principal surfaces are in line with the
mentioned generatrix of the measuring tube 2 and form a
front edge 313. The sensor vane 31 may also have other
suitable three-dimensional shapes; for example, it may
have two parallel principal surfaces, which form two
parallel front edges.
The sensor vane 31 is shorter than the diameter of the
measuring tube 2; furthermore, it is flexurally rigid
and has a blind hole 314 (can only be seen.in figure
4). In order that the blind hole 314 has an adequate
diameter, wall parts protrude from the principal
surfaces, of which the wall part 315 is indicated in
figure 2. The blind hole 314 reaches into the vicinity
of the front edge 313, where it has a bottom.
Also belonging to the vortex sensor 3 is a diaphragm
33, which covers the bore 22 and has a first surface
331, facing the fluid, and a second surface 332, facing
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away from the fluid, see figures 3 and 4. Fixed on the
surface 331 is the sensor vane 31 and fixed on the
surface 332 is a sensor element 35. Preferred are the
sensor vane 31, the diaphragm 33, the annular rim 333
of which and the part 351 of the sensor element 35 that
is fastened to the diaphragm 33 consisting of a single
piece of material, for example metal, in particular
stainless steel. The sensor element 35 generates the
abovementioned signal, the frequency of which is
proportional to the volume flow of the flowing fluid.
Fixed in the vicinity of the bottom of the blind hole
314 is a temperature sensor 34. Supply leads 341, 342
of the temperature sensor 34 lead centrally upward
through the vortex sensor 3.
One of the supply leads 341, 342 may be omitted if the
temperature sensor 34 is in electrical contact on one
side with the sensor vane 31, and is consequently at
the potential of circuit zero point. The temperature
sensor 34 is preferably a platinum resistor.
Since the sensor vane 31, and in particular its wall
part 315, can be made adequately thin and also
preferably consist of metal, the temperature sensor 34
is virtually at the temperature at any given instant of
the fluid flowing past the sensor vane 31 and, because
of the low thermal capacity of the arrangement, is also
very capable of following temperature changes of the
fluid adequately quickly and virtually without any
delay. Consequently, the mass flow can be calculated
in the customary way from the temperature of the fluid,
measured by the temperature sensor 34, and from the
signal of the vortex sensor 3.
In figure 5, a vortex flow pickup 1' corresponding to
the second variant of the invention is represented in a
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way analogous to figure 2 in a perspective view and
partly cut open. The parts of figure 5 which are of
the same type as the parts of figure 2 are not
explained again, but the reference numerals used for
them in figure 2 are provided with an apostrophe.
The differences between the exemplary embodiment of the
second variant of the invention and the exemplary
embodiment of its first variant are, on the one hand,
that the bluff body 4' is provided with a blind hole
46, which is in line with a second bore 24 in the tube
wall 2' and in which a temperature sensor 34' is
fitted, and, on the other hand, that the wedge-shaped
sensor vane 31' has two planar principal surfaces 311'.
The temperature sensor 34' has a supply lead 341'.
The blind hole 46 may be provided to any desired depth
in the bluff body 4'; its bottom 461 preferably lies in
such a way that the temperature sensor 34' is arranged
in the center of the bluff body 4'.
Since the bluff body 4' can be made adequately thin in
the region of the blind hole 46 and, like the sensor
vane 31 of figures 1 to 4, likewise preferably consists
of metal, in particular stainless steel, the
temperature sensor 34' is virtually at the temperature
at any given instant of the fluid flowing past the
bluff body 4' and, because of the low thermal capacity
of the arrangement, also very able to follow
temperature changes of the fluid adequately quickly and
virtually without any delay. Consequently, the mass
flow can again be calculated in the customary way from
the temperature of the fluid, measured by the
temperature sensor 34', and from the signal of the
vortex sensor 3'.
