Abstrict A flow meter comprising a metallic conduit (1) through which, in
use, fluid flow takes place, and ultrasonic transducer (4) mounted
in a wall of the conduit (1) and operable to sense the presence
of fluid in the conduit, the transducer being mounted in the conduit
wall in metallic mounting (6) which are sealed in said wall by welding
(9).
Claims We claim:
1. A flow meter as defined herein comprising a metallic conduit
through which, in use, fluid flow takes place, and ultrasonic transducer
means mounted in a wall of the conduit and operable to sense the
presence of fluid in the conduit, the transducer means being mounted
in the conduit wall in metallic mounting means which are spaced
from said wall but sealed thereto by welding.
2. A flow meter according to claim 1 wherein the transducer means
comprise a pair of transducers mounted on opposed sides of the conduit,
with one transducer arranged as a transmitter and the other as a
receiver.
3. A flow meter according to claim 2 wherein each transducer is
mounted in an aperture through the wall of the conduit dimensioned
to receive the associated mounting means in the form of a cup having
its closed end facing into the conduit.
4. A flow meter according to claim 3 wherein the axis of the or
each aperture is inclined to the longitudinal axis of the conduit.
5. A flow meter according to claim 4 wherein said angle of inclination
is 7.degree..
6. A flow meter according to claim 3 wherein means are provided
within the or each cup to assist in the transmission/reception of
ultrasonic signals.
7. A flow meter according to claim 6 wherein said means is in
the form of a piece of synthetic plastics material having a silicon
grease applied to opposed sides thereof.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to flow meters and more particularly to
ultrasonic flow meters, which term is intended to cover fluid presence
sensors, i.e. sensors which indicate the presence of a fluid as
opposed to a continuous flow of a fluid.
2. Description of the Prior Art
Ultrasonic flow meters are well known and comprise ultrasonic transducer
means mounted in relation to a conduit, through which a fluid flows,
or in which a fluid is present from time to time, such that an ultrasonic
signal can be transmitted across the conduit. Usually, a pair of
ultrasonic transducers is employed, the two transducers being mounted
on opposed sides of the conduit with one acting as an ultrasonic
transmitter and the other as an ultrasonic receiver. Assuming a
cylindrical conduit, the two transducers are mounted diametrically
opposite and extreme care has been taken in the past to ensure that
the ultrasonic signal emitted by the transmitter is directed across
the conduit, and hence through any fluid therein, as opposed to
around the wall of the conduit, the latter being the path of least
resistance and thus the "preferred" path of the ultrasonic
signals. To this end, each transducer has been mounted in a cup
inserted into the wall of the conduit, the cup being sealded into
that wall with a silicon-based sealant, for example. If the cups
are metallic, then it is usual to provide a non-metallic mount therefor,
again to safeguard against the risk of an acoustic short circuit
around the wall of the conduit.
These precautions against acoustic short circuiting are tedious
but more importantly, it is impossible to achieve a leak-proof seal
between the cup and conduit for certain fluids and/or pressures.
One of the most difficult fluids in this respect is steam. Thus
whilst the short circuiting problem is solved by the use of an acoustic
insulator between each transducer and the conduit by way of the
non-metallic mount and/or sealant, the seal achieved is not viable
under certain conditions.
SUMMARY OF THE INVENTION
In addressing this problem, it was discovered that, contrary to
all expectations, the use of a metallic seal between a metallic
mount and a metallic conduit does not give rise to an acoustic short
circuit around the wall of the conduit. Thus, according to the present
invention there is provided a flow meter comprising a metallic conduit
through which, in use, fluid flow takes place, and ultrasonic transducer
means mounted in a wall of the conduit and operable to sense the
presence of fluid in the conduit, the transducer means being mounted
in the conduit wall in metallic mounting means which are spaced
from said wall but sealed thereto by welding.
The term "welding" as used herein is intended to cover
any process for joining together two metallic components, such as
brazing or soldering, for example.
The transducer means may comprise a pair of transducers, preferably
of the piezo-electric type, mounted on opposed sides of the conduit
which may be cylindrical, with one transducer arranged as a transmitter
and the other as a receiver. The or each transducer may be mounted
in an aperture through the wall of the conduit dimensioned to receive
the associated mounting means which may be in the form of a cup
having its closed end facing into the conduit.
BRIEF DESCRIPTION OF THE DRAWINGS
A flow meter according to the present invention will now be described
in greater detail, by way of example, with reference to the accompanying
drawings, in which:
FIG. 1 is a longitudinal cross section of the flow meter, and
FIG. 2 is a plan view of the underside of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The flow meter is of the Karman vortex shedding type and comprises
a cylindrical conduit 1 which may be of stainless steel, for example,
and which forms the body of the flow meter. Two diametrically opposed
apertures 2 are drilled in the wall of the conduit 1 the axes of
these apertures being inclined to the longitudinal axis of the conduit
1 by an angle of the order of 7.degree. for a purpose to be described.
However, this angle may be varied, or the axis of each aperture
2 may be at right angles to the longitudinal axis of the conduit
1. Each aperture 2 is counterbored 3. Transducer means in the form
of a pair of ultrasonic transducers 4 are mounted in the apertures
2 only one transducer being shown in FIG. 1 of the drawings. The
transducer 4 may be of any type but is preferably of the piezo-electric
type, with one transducer acting as a transmitter and the other
as a receiver. A pair of electrical leads 5 is provided for each
transducer 4 which is mounted in a metallic, for example stainless
steel, cup 6 having a body portion dimensioned to be received within
but spaced from the associated aperture 2 and a flanged rim 7 dimensioned
to be received within but again spaced for the associated counterbore
3. The closed end 8 of each cup 6 faces inwardly of the conduit
and is secured in position by a weld 9 around the periphery of the
flange 7. In order to accommodate tolerances, each transducer 4
is mounted within a sleeve 11 which is preferably of a synthetic
material such as PTFE, for example. Also, in order to assist in
the transmission of ultrasonic signals between each transducer 4
and the closed end of the associated cup 6 a slip of PTFE, for
example, may be provided in the base of the cup 6 having a silicon
grease, for example, applied to both sides thereof.
In as much as Karman vortex shedding type of flow sensor involves
the provision of a strut across the conduit, such is provided at
12 the strut being of triangular cross section with the apex of
the triangle facing the direction of the flow to be measured (indicated
by arrow F in FIG. 1), and with the transducers 4 being positioned
in the vortex street produced by the strut 12. The bores 2 in the
wall of the conduit 1 are inclined so that the faces of the cups
6 and hence the operative faces of the transducers 4 taper inwardly
so that these faces are directed towards the strut 12. The strut
12 extends diametrically across the conduit 1 and is mounted by
drilling through one side of the conduit wall and into, but not
through, the opposed wall so that the strut sits in a blind bore
on one side and in a through bore in the other side of the conduit
wall, the through bore being sealed by welding. A through bore may
be provided and the strut welded in position at each end.
As already discussed, the provision of a metallic conduit 1 and
metallic mounting cups 6 for the transducers 4 would normally lead
the skilled person to expect problems in use of the flow meter in
as much as the ultrasonic signals would be expected to take the
path of least resistance around the wall of the conduit 1 rather
than across the bore thereof, the latter path being necessary in
order for the flow meter to function. However, the welding of the
mounting cups 6 within the apertures 2 in the conduit has been found,
for some reason which cannot be fully explained, to prevent this
acoustic short circuiting and avoid the need to select appropriate
synthetic plastic mounting means and/or sealants and, more importantly,
provides a seal which remains viable under what would normally be
adverse conditions, such as relatively high pressure. More specifically,
the seal remains viable when handling steam which has proved to
be an extremely difficult medium to handle with known flow meters.
The reason why the weld appears to provide an acoustic insulator,
as opposed to the expected acoustic conductor, is believed to be
the weld presents an effective discontinuity in the path between
the mounting cups 6 and the body of the conduit 1.
In operation, one of the transducers 4 is driven to act as a transmitter
of ultrasonic pulses which are transmitted across the bore of the
conduit 1. The other transducer 4 acts as a receiver and the delay
in receipt of a transducer pulse provides an indication of the flow
of fluid in the conduit as is well know.
It will be seen that the present invention affords an extremely
simple, but totally unexpected, solution to the problem of being
able to handle all fluids at all pressures, which solution could
not have been forecast and indeed has been studiously avoided to
date. It is emphasised that the present invention is applicable
to fluid sensors as well as flow meters, as already explained and
it is also applicable to flow meters or fluid sensors which are
other than of the Karman vortex shedding type.
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