Abstrict The vortex frequency flow meter has a tube body which comprises
a flow passage for the medium of which the flow is to be measured.
In the flow passage a bluff body is arranged which generates Karman
vortices of which the recurrence frequency is proportional to the
flow velocity of the medium. A sensor which responds to the pressure
fluctuations generated by the vortices is arranged in a cavity which
communicates via passages with the flow passage so that the pressure
fluctuations are conveyed to the cavity. The passages can be closed
by a shutoff member which is adjustable from outside by an actuating
member. It is thus possible to remove the sensor from the cavity
after the closure of the passages without the flow medium being
able to emerge.
Claims I claim:
1. Vortex frequency flow meter having a tube body which comprises
a flow passage for the medium of which the flow is to be measured,
a bluff body arranged in the flow passage for generating vortices
having a recurrence frequency which is proportional to the flow
velocity of the medium, and at least one sensor which responds to
the pressure fluctuations generated by the vortices and is arranged
in a cavity which communicates via closable passages with the flow
passage, characterized in that the cavity is formed in the interior
of the bluff body, that the closable passages are formed in the
bluff body, that a rotatable shutoff member is provided for closing
the closable passages, that an actuating member is provided for
rotating the shutoff member from the outside of the tube body, and
that the shutoff member is a hollow cylindrical blocking bush which
is arranged rotatably in the cavity and which has openings formed
therein which by rotation of the blocking bush can be brought into
or out of coincidence with the closable passages.
2. Vortex frequency flow meter having a tube body which comprises
a flow passage for the medium of which the flow is to be measured,
a bluff body arranged in the flow passage for generating vortices
having a recurrence frequency which is proportional to the flow
velocity of the medium, and at least one sensor which responds to
the pressure fluctuations generated by the vortices and is arranged
in a cavity which communicates via closable passages with the flow
passage, characterized in that the cavity and the closable passages
are arranged in the tube body, that a rotatable shutoff member is
provided for closing the closable passages, that an actuating member
is provided for rotating the shutoff member from the outside of
the tube body, and that the shutoff member is a circular member
which closes an end of the cavity and which has openings formed
therein which by rotation of the circular member can be brought
into or out of coincidence with the closable passages.
3. Vortex frequency flow meter according to claim 2 characterized
in that the shutoff member is a disc which is rotatably mounted
at the end of the cavity in a wall of the tube body.
4. Vortex frequency flow meter according to claim 3 characterized
in that the actuating member comprises a rack which engages into
a toothing disposed on the disc.
5. Vortex frequency flow meter according to claim 2 characterized
in that the bluff body is rotatably mounted in the tube body and
that the shutoff member is a collar which is disposed at an end
of the bluff body adjacent the end of the cavity and which has openings
formed therein which by rotation of the bluff body can be brought
into or out of coincidence with the closable passages.
6. Vortex frequency flow meter according to claim 5 characterized
in that at the end of the bluff body opposite the collar a cylindrical
extension is disposed which is rotatably mounted in a bore in the
wall of the tube body and that the actuating member is attached
to the end of the cylindrical extension outside the tube body.
Description The invention relates to a vortex frequency flow meter having a
tube body which comprises a flow passage for the medium of which
the flow is to be measured, a bluff body arranged in the flow passage
for generating vortices having a recurrence frequency which is proportional
to the flow velocity of the medium, and at least one sensor which
responds to the pressure fluctuations generated by the vortices
and is arranged in a cavity which communicates via closable passages
with the flow passage.
The invention originates from the field of flow measurement utilizing
the principle of the Karman vortices. According to this principle,
when a suitable bluff body opposes a turbulent flow as an obstacle
periodically and locally alternating vortices detach from said body.
The frequency of said vortex detachments and the resulting pressure
fluctuations is proportional to the flow velocity.
In the practical operation of such flow meters it may occur that
the sensor responding to the pressure fluctuations must be removed
from the cavity in which it is arranged for the purpose of cleaning,
maintenance or repair. To prevent the cavity from continuing to
be subjected to the pressure of the flowing medium through the passages
which connect said cavity to the flow passage and thus prevent the
possible exit of flowing medium via the cavity it is known from
US-PS 4434668 to make the passages closable. For this purpose
in the bluff body a cavity is formed which accommodates the sensor
in its upper portion communicating via the passages with the flow
passage whilst in the lower portion a piston-like closure member
is mounted which is pressed by a spring against the end of the sensor.
When the sensor is removed from the cavity the spring relaxes and
the piston-like closure member moves upwardly through the cavity.
In doing so the piston-like closure member moves over the passages
and if the sensor has been completely removed from the cavity the
piston-like closure member closes from the outside the portion of
the cavity connected via the passages to the flow passage. When
the sensor is again introduced into the cavity it presses the piston-like
closure member downwardly against the force of the spring. Since
in this known vortex frequency flow meter the movement of the piston-like
closure member in the closure direction is effected only by the
spring force during the removal of the sensor, in the case of flow
media which cause deposits there is a danger of the closure member
getting stuck so that the communication between the flow passage
and the outer space is not interrupted. The spring force must be
relatively small since otherwise it opposes the introduction of
the sensor with too great a resistance and this obstructs precise
introduction of the sensor and with sensitive sensors involves a
risk of mechanical damage. Furthermore, it is not possible to block
the connection between the flow passage and the cavity accommodating
the sensor without removing the sensor. Also to be considered a
disadvantage is that the passages are not closed until the sensor
has already been partially removed from the cavity and that during
the introduction of the sensor they already open again. Finally,
the known solution is suitable only for vortex frequency flow meters
of which the bluff body is so large that a cavity can be formed
therein having a cross-section and length sufficient to accommodate
both the sensor as well as the piston-like closure member and the
spring. Such large bluff bodies are found only in vortex frequency
flow meters with large nominal widths, i.e. large diameters of the
flow passage.
The problem underlying the invention is to provide a vortex frequency
flow meter of the type mentioned at the beginning in which a reliable
closure of the passages is possible independently of removal of
the sensor and which is suitable also for small nominal widths or
diameters.
This problem is solved according to the invention in that for closing
the passages a shutoff member adjustable by an actuating member
from the outside is provided.
In the vortex frequency flow meter according to the invention the
passages which connect the flow passage to the cavity receiving
the sensor can be closed and opened by means of the actuating member
irrespective of whether the sensor is in the cavity or not. The
closure force is governed by the force exerted on the actuating
member, which is not restricted to the force of a spring and in
the case of a resistance caused for example by deposits can be made
as large as desired. The passages can be completely closed before
the removal of the sensor has started and they are not opened again
until the sensor has been completely introduced into the cavity.
In this manner even sensitive sensors can be introduced easily and
precisely into the still closed cavity without any mechanical stress.
The invention is suitable both for vortex frequency flow meters
of large nominal widths in which the bluff body has such a large
cross-section that a cavity adequate for accommodating the sensor
and the shutoff member can be formed therein, as well as for vortex
frequency flow meters of small nominal widths in which the cavity
receiving the sensor is formed in the wall of the tube body.
Advantageous further developments and embodiments of the invention
are characterized in the subsidiary claims.
Further features and advantages of the invention will be apparent
from the following description of examples of embodiment which are
illustrated in the drawings, wherein:
FIG. 1 shows a longitudinal section through a vortex frequency
flow meter according to a first embodiment of the invention,
FIG. 2 is a sectional view along the line II--II of FIG. 1 the
shutoff member being in the blocking position,
FIG. 3 is the same sectional view as FIG. 2 the shutoff member
being in the open position,
FIG. 4 is a longitudinal section through a vortex frequency flow
meter according to a second embodiment of the invention, the shutoff
member being in the open position,
FIG. 5 is a sectional view along the line V--V of FIG. 4
FIG. 6 is a partial sectional view along the line VI--VI of FIG.
4
FIG. 7 is a longitudinal section through the vortex frequency flow
meter of FIG. 4 the shutoff member being in the blocking position,
FIG. 8 is a sectional view along the line VIII--VIII of FIG. 7
FIG. 9 is a sectional view along the line IX--IX of FIG. 7
FIG. 10 is a longitudinal section through a vortex frequency flow
meter according to a third embodiment of the invention, the shutoff
member being in the open position,
FIG. 11 is a sectional view along the line IX--IX of FIG. 10
FIG. 12 is a longitudinal section through the vortex frequency
flow meter of FIG. 10 the shutoff member being in the blocking
position and
FIG. 13 is a sectional view along the line XIII--XIII of FIG. 12.
The vortex frequency flow meter 10 illustrated in longitudinal
section in FIG. 1 comprises a tube body 12 which is inserted into
a conduit or pipeline, not illustrated, so that the medium of which
the flow is to be measured flows in the direction of the arrow F
through the cylindrical flow passage 14 of the tube body 12. In
the tube body 12 a bluff body 16 is arranged in such a manner that
it extends transversely to the flow direction diametrically through
the flow passage 14. The bluff body 16 is mounted at both ends on
the inner wall of the tube body 12 for example by welding. The
bluff body 16 is a prismatic body of which the cross-section is
substantially trapezoidal, as apparent in the sectional views of
FIGS. 2 and 3 and the face of the prism corresponding to the major
base line of the trapezium points upstream and lies perpendicularly
to the flow direction. As is known, a bluff body of this type produces
in the medium flowing past Karman vortices which downstream of the
bluff body form two parallel vortex trails, the vortices of the
one vortex trail being offset with respect to the vortices of the
other vortex trail. The measurement of the flow velocity of the
medium is based on the fact that the distance between consecutive
vortices in a vortex trail is approximately constant over a large
range of flow velocities. The recurrence frequency of the Karman
vortices is thus proportional to the flow velocity. The vortex frequency
flow meter 10 is therefore so designed that it furnishes an electrical
signal which is characteristic of the recurrence frequency of the
Karman vortices. To generate said electrical signal use is made
of the fact that the vortices cause pressure fluctuations with the
same frequency. The vortex frequency flow meter 10 includes a sensor
assembly 18 which responds to the pressure fluctuations generated
by the vortices and generates an electrical signal dependent on
the frequency of said pressure fluctuations.
In the upper portion of the bluff body 16 a cylindrical cavity
20 is formed which extends from the end of the bluff body axially
over a portion of the length thereof. Running through the wall of
the tube body 12 coaxially with the cavity 20 is a bore 22 which
has the same diameter as the cavity 20. In the bluff body 16 passages
or holes are formed which from the side faces of the prism open
transversely to the flow direction into the cavity 20. A first pair
of passages 24 is formed near the lower end of the cavity 20 and
a second pair of passages 26 is disposed spaced thereabove near
the upper end of the cavity 20. The passages 24 and 26 may have
a circular cross-section as indicated in dashed line in FIG. 1.
The sensor assembly 18 comprises a sensor support 28 which is mounted
outside the tube body 12 and on which the sensor 30 responsive to
the pressure fluctuations is arranged in such a manner that it projects
through the bore 22 into the cavity 20 of the bluff body 16 and
its free end lies substantially at the level of the lower passages
24. The sensor 30 may be formed in any manner known per se in vortex
frequency flow meters. For example, the part of the sensor 30 visible
in FIG. 1 may be a sensor sleeve which is closed at the lower end
and which is set in oscillation by the pressure fluctuations which
are caused by the vortices and are transmitted through the passages
24 and 26 into the cavity 12 and in the interior of which an oscillation
sensor not shown in the drawings is arranged which converts the
oscillations of the sensor sleeve into an electrical signal which
has the same frequency as the pressure fluctuations and is transmitted
via a cable 32 to an evaluating device, not shown.
The oscillation sensor may be constructed in a manner known per
se as piezoelectric or capacitive sensor.
As further essential component of the vortex frequency flow meter
10 a shutoff member 34 is provided which consists of a blocking
bush 36 which is connected at one end to an actuating member 38.
The blocking bush 36 is a hollow cylinder which is closed at the
lower end and is inserted through the bore 22 into the cavity 20.
The outer diameter of the blocking bush 36 corresponds to the diameter
of the cavity 20 so that it fits substantially free of play but
rotatably in the cavity 20 and the length of the blocking bush
36 is so dimensioned that its sealed lower end extends up to the
bottom of the cavity 20 when the actuating member 38 bears on the
outer surface of the tube body 12. In the blocking bush 36 two pairs
of radial passages 40 and 42 are disposed in such a manner that
they lie at the level of the passages 24 and 26 respectively of
the bluff body 16 when the blocking bush 36 is inserted into the
cavity 20. A seal 44 which is inserted into an annular groove at
the outer surface of the wall of the tube body 12 surrounds the
end of the blocking bush 36 connected to the actuating member 38.
The sensor 30 is inserted in turn into the hollow interior of the
blocking bush 36. Its outer diameter is smaller than the inner diameter
of the blocking bush 36 so that between the sensor 30 and the blocking
bush 36 an annular intermediate space 46 is present. The lower end
of the sensor 30 lies spaced from the bottom of the blocking bush
36 when the sensor 30 is inserted completely into the blocking bush
36. Between the actuating member 38 and the portion of the sensor
support 28 adjoining said member a seal 48 is provided which is
inserted into an annular groove at the upper side of the actuating
member 38.
When the parts assume the position illustrated in FIGS. 1 and 2
the passages 40 and 42 of the blocking bush 36 are turned through
90.degree. with respect to the passages 24 26 of the bluff body
16. In this position the blocking bush 36 interrupts the connection
between the flow passage 14 and the cavity 20 so that no flow medium
can pass from the flow passage 14 into the cavity 20. Accordingly,
the pressure fluctuations caused by the vortices and indicated in
FIG. 2 by the arrows P are also unable to act on the sensor 30 projecting
into the cavity 20. The vortex frequency flow meter 10 is thus out
of operation in this position of the blocking bush 36. It is however
possible to remove the sensor assembly 18 from the flow meter and
take the sensor 30 out of the cavity 20 without flow medium being
able to emerge. This enables maintenance or repair work to be carried
out on the sensor without having to remove the bluff body 16 and
without any necessity of interrupting the flow of the medium through
the flow passage 14.
When the sensor assembly 18 has been correctly installed in the
flow meter 10 again so that the sensor 30 assumes the position shown
in FIG. 1 the flow measurement can be started simply by turning
the blocking bush 36 through 90.degree. into the position shown
in FIG. 3 in which the passages 40 coincide with the passages 24
and in corresponding manner the passages 42 coincide with the passages
26. In this position of the blocking bush 36 flow medium can flow
out of the flow passage 14 through the passages into the intermediate
space 46 and the pressure fluctuations caused by the vortices can
act on the sensor 30 and set the latter in oscillation as indicated
by a double arrow in FIG. 3.
The rotation of the blocking bush 36 may for example be by hand
by means of an actuating member 38. Conveniently, at a suitable
point on the tube body 12 or the bluff body 16 on the one hand and
the shutoff member on the other stops are arranged which limit the
90.degree. rotational movement of the shutoff member.
Since the sensor 30 has a certain minimum diameter the embodiment
according to FIGS. 1 to 3 is suitable mainly for flow meters of
relatively large nominal widths (diameters of the flow passage 14)
in which the bluff body 16 is large enough to have room in its interior
for a cavity 20 which can accommodate the sensor 30 in addition
to the blocking bush 36. In flow meters with small nominal widths
the bluff body is so small that the sensor can no longer be accommodated
in the interior thereof. Below embodiments are described which are
suitable for flow meters with small nominal widths.
The vortex frequency flow meter 50 illustrated in FIGS. 4 to 9
has a tube body 52 in the flow passage 54 of which a bluff body
56 is disposed. The bluff body 56 is solid and provided at the lower
end with a cylindrical extension 58 which is mounted in a bore in
the wall of the tube body 52 for example by welding. For simplification,
the sensor assembly is not shown in FIGS. 4 to 9. The cavity 60
intended for accommodation of the sensor is not formed in the interior
of the bluff body 56 in this case but in alignment with the axis
of the bluff body in the wall of the tube body 52 which has a thickening
53 at this point to obtain adequate axial length of the cavity 60.
The cavity 60 comprises in its lower region adjacent the bluff body
56 a somewhat greater diameter than in the upper region and into
said region of larger diameter a bush 62 is inserted of which the
inner diameter is equal to the small diameter of the upper region
of the cavity 60. In the bush 62 two diametrically opposite slots
64 are formed which extend from the lower end of the bush over the
greater part of the length thereof upwardly. Said slots 64 lie in
a diameter plane of the flow passage 52 lying perpendicularly to
the flow direction.
At the upper end of the bluff body 56 a disc-shaped collar 66 is
integrally formed and has the same outer diameter as the bush 62
and is located beneath the bush 62 in the widened region of the
cavity 60. At the edge of the collar 66 two diametrically opposite
cutouts 68 are disposed in such a manner that they lie in axial
alignment with the slots 64.
Between the facing end faces of the bush 62 and of the collar 66
as shutoff member a rotatable blocking disc 70 is inserted which
has the same diameter as the bush 62 and the collar 66. At the edge
of the blocking disc 70 two diametrically opposite cutouts 72 are
disposed which have the same form as the cutouts 68 of the collar
66. Between the cutouts 72 the blocking disc 70 is provided over
a portion of its periphery with a toothing 74 (FIGS. 6 and 9) into
which engages the toothing of a rack 76 which is mounted longitudinally
displaceably in a bore 78 in the wall of the tube body 52. By displacing
the rack 76 the blocking disc 70 can thus be turned through a limited
angle.
In FIGS. 4 5 and 6 the blocking disc 70 is set by means of the
rack 76 in such a manner that the cutouts 72 thereof align with
the cutouts 68 of the collar 66 and with the slots 64 of the bush
62. In this position of the components the flow medium can flow
from the flow passage 54 through the cutouts 68 and 72 and through
the slots 64 into the cavity 60 and the pressure fluctuations caused
by the vortices can act on the sensor (not shown in the drawings)
located in the cavity 60.
In contrast, in FIGS. 7 8 and 9 the blocking disc 70 is turned
by means of the rack 76 in such a manner that its cutouts no longer
align with the cutouts 68 of the collar 66 and the slots 64 of the
bush 62. The blocking disc 70 thus interrupts the connection between
the flow passage 54 and the cavity 60 so that no flow medium from
the flow passage 54 can enter the cavity 60 and the pressure fluctuations
caused by the vortices can no longer be transmitted to the cavity
60. In this position of the blocking disc 70 the sensor assembly
can be removed without any danger of flow medium emerging.
FIGS. 10 to 13 show a modification of the embodiment of FIGS. 4
to 9 corresponding parts being provided with the same reference
numerals as in FIGS. 4 to 9 and not being described again. The embodiment
of FIGS. 10 to 13 differs from that of FIGS. 4 to 9 only in the
following respect:
the blocking disc 70 and the rack 78 are omitted and the collar
66 engages directly at the lower end of the bush 62;
the bluff body 56 is rotatably mounted by means of the cylindrical
extension 58 in the tube body 52 and the rotation can for example
be made manually by means of an actuating member 80 attached to
the cylindrical extension 58 a seal 82 effecting the necessary
sealing.
In this embodiment the collar 66 forms the shutoff member. When
the bluff body 56 assumes the position illustrated in FIGS. 10 and
11 in which it has the correct position for generating the Karman
vortices, the cutouts 68 in the collar 66 align with the slots 64
of the bush 62 so that flow medium from the flow passage 54 can
enter the cavity 60 and the pressure fluctuations caused by the
vortices can be transmitted to the cavity 60. If however the bluff
body 56 is rotated from this position through 90.degree. so that
it assumes the position illustrated in FIGS. 12 and 13 the cutouts
68 are offset with respect to the slots 64 so that the connection
between the flow passage 54 and the cavity 60 is interrupted. In
this position of the bluff body 56 the sensor assembly can be removed
without the flow medium being able to emerge.
As can be seen in the embodiments of FIGS. 4 to 13 the cross-section
of the cavity 60 intended for accommodation of the sensor can be
of the same order of magnitude as or even larger than the cross-section
of the bluff body. Consequently, this embodiment is also particularly
suitable for flow meters with small nominal widths. |