Abstrict The present invention relates to a magnetic-inductive flow meter
with a measuring tube, which can be fitted into a pipeline system
by using connecting means, with at least two measuring electrodes
that are fitted into the wall of the measuring tube opposite each
other in an electrically isolated manner and are intended for sensing
a measuring voltage, a magnet unit, which is likewise arranged on
the outside of the measuring tube, generating a magnetic field that
is aligned substantially perpendicularly in relation to the direction
of flow of the conductive flow medium to be measured. In order to
provide here a magnetic-inductive flow meter for low-pressure applications
which can be easily produced and can also be easily fitted into
a pipeline system, it is proposed here according to the invention
that the the measuring tube (1) is produced from a semifinished
tube made of high-density polyethylene (HDPE), which can be fitted
into the pipeline system (2) without any flanges.
Claims 9. A magnetic-inductive flow meter for flangeless fitting into
a pipeline system comprising: a measuring tube produced from a semi-finished
tube made of high-density polyethylene (HDPE), said measuring tube
comprising with at least two measuring electrodes that are fitted
into a wall of said measuring tube opposite each other in an electrically
isolated manner, said at least two measuring electrodes for sensing
a measuring voltage; and a magnet unit arranged on the outside of
the measuring tube, said magnet unit generating a magnetic field
that is aligned substantially perpendicularly in relation to the
direction of flow through said measuring tube of a conductive flow
medium to be measured.
10. The magnetic-inductive flow meter of claim 9 further comprising
means for flangelessly fitting said flow meter into said pipeline
system.
11. The magnetic-inductive flow meter of claim 10 wherein said
means for flangelessly fitting said flow meter into said pipeline
system is an unreleasable connecting means.
12. The magnetic-inductive flow meter of claim 10 wherein said
means for flangelessly fitting said flow meter into said pipeline
system is a releasable connecting means.
13. The magnetic-inductive flow meter of claim 12 wherein said
measuring tube and said pipeline system have the same outer diameter
and said releasable connecting means is a sleeve connection.
14. The magnetic-inductive flow meter of claim 12 wherein said
releasable connecting means is a clamp connection when said measuring
tube is fitted into said pipeline system.
15. The magnetic-inductive flow meter of claim 9 wherein said measuring
tube further comprises either an integral metal shielding or a metal
shielding arranged on the outer surface of said measuring tube.
16. The magnetic-inductive flow meter of claim 15 wherein said
metal shielding arranged on the outer surface of said measuring
tube is either a metal plate or a metal foil.
17. The magnetic-inductive flow meter of claim 16 wherein said
metal plate or said metal foil consists of aluminium of alloys thereof.
18. The magnetic-inductive flow meter of claim 15 wherein said
metal shielding arranged on the outer surface of said measuring
tube is electrically connected to grounds electrodes.
19. The magnetic-inductive flow meter of claim 16 wherein said
metal shielding arranged on the outer surface of said measuring
tube is electrically connected to grounds electrodes.
20. The magnetic-inductive flow meter of claim 17 wherein said
metal shielding arranged on the outer surface of said measuring
tube is electrically connected to grounds electrodes.
21. A method for making and using a magnetic-inductive flow meter
for flangeless fitting into a pipeline system comprising: producing
a measuring tube from a semi-finished tube made of high-density
polyethylene (HDPE), said measuring tube comprising with at least
two measuring electrodes that are fitted into a wall of said measuring
tube opposite each other in an electrically isolated manner, said
at least two measuring electrodes for sensing a measuring voltage.
22. The method of claim 21 further comprising: arranging a magnet
unit on the outside of the measuring tube, said magnet unit generating
a magnetic field that is aligned substantially perpendicularly in
relation to the direction of flow through said measuring tube of
a conductive flow medium to be measured.
23. The method of claim 21 further comprising: flangelessly fitting
said flow meter to said pipeline system.
24. The method of claim 23 further comprising unreleasably flangelessly
fitting said flow meter to said pipeline system.
25. The method of claim 23 further comprising releasably flangelessly
fitting said flow meter to said pipeline system when said measuring
tube and said pipeline system have the same outside diameter.
26. The method of claim 23 further comprising releasably flangelessly
fitting said flow meter to said pipeline system by fitting said
measuring tube into said pipeline system.
Description [0001] The present invention relates to a magnetic-inductive flow
meter with a measuring tube, which can be fitted into a pipeline
system by using connecting means, with at least two measuring electrodes
that are fitted into the wall of the measuring tube opposite each
other in an electrically isolated manner and are intended for sensing
a measuring voltage, a magnet unit, which is likewise arranged on
the outside of the measuring tube, generating a magnetic field that
is aligned substantially perpendicularly in relation to the direction
of flow of the conductive flow medium to be measured.
[0002] A magnetic-inductive flow meter is preferably used as a
flow meter for liquids, slurries and pastes which have a specific
minimum electrical conductivity. This type of flow meter is distinguished
by quite accurate measuring results, without any pressure loss being
caused in the pipeline system by the measurement. Furthermore, magnetic-inductive
flow meters do not have any movable components or components protruding
into the measuring tube, which are particularly liable to wear.
The area of use of the flow meter of interest here extends primarily
to applications in the chemical industry, pharmaceuticals and the
cosmetics industry as well as communal water and waste-water management
and the food industry.
[0003] Faraday's law of induction forms the physical basis for
the measuring method of a magnetic-inductive flow meter. This natural
law states that a voltage is induced in a conductor moving in a
magnetic field. When this natural law is exploited in measuring
technology, the electrically conductive medium flows through a measuring
tube in which a magnetic field is generated perpendicularly in relation
to the direction of flow. The voltage induced in the medium is picked
up by an arrangement of electrodes. Since the measuring voltage
obtained in this way is proportional to the average flow rate of
the flowing medium, the volumetric flow of the medium can be determined
from this. Taking the density of the flowing medium into account,
its mass flow can be ascertained.
[0004] EP 0 869 336 A2 discloses a magnetic-inductive flow meter
of the generic type. Its arrangement of electrodes interacts with
two opposite solenoids, which generate the required magnetic field
perpendicularly in relation to the direction of flow in the measuring
tube. Within this magnetic field, each volume element of the flowing
medium moving through the magnetic field, with the field strength
that is present in this volume element, makes a contribution to
the measuring voltage picked up by means of the measuring electrodes.
The measuring voltage is fed to the input side of downstream evaluation
electronics. Within the evaluation electronics, firstly a signal
amplification takes place by means of an electronic differential
amplifier, the differential amplifier operating here with respect
to the reference potential, which usually corresponds to ground
potential. On the basis of the measuring voltage, the evaluation
electronics produce a value for the volumetric flow of the medium
flowing through the measuring tube. The measuring tube is fastened
by means of flange regions on both sides to corresponding flange
ends of the pipeline system with the assistance of sealing rings
and a number of screws distributed around the periphery of the flange
connections.
[0005] The measuring tube of such a magnetic-inductive flow meter
usually consists of a metal, in order to ensure adequate pressure
stability. This is so since the pipeline system in which a flow
meter is fitted is usually under pressure to make the medium flow.
Customary metals are, in particular, steel, titanium, tantalum,
platinum-iridium or alloys thereof and also lightweight metal, such
as aluminum or alloys thereof. All these metals are sufficiently
corrosion-resistant and, in particular, pressure-stable for the
common applications of a magnetic-inductive flow meter. For the
electric isolation of the measuring electrodes extending through
the wall of the measuring tube with respect to the metallic, and
to this extent conductive, measuring tube, the latter is lined.
The lining in this case consists of a non-conductive material; so-called
liners, that is thin-walled plastic tubes that are drawn into the
metallic measuring tube, are used for this purpose.
[0006] A disadvantage here is that a measuring tube constructed
in such a way requires quite high expenditure on material, if only
because of the quite expensive types of metal. In addition, the
drawing of the liner into the measuring tube is quite a complex
technical production operation. Close dimensional tolerances have
to be maintained.
[0007] On the other hand there is the recognition that, for many
applications--in particular in the water and waste-water area--a
measuring tube with comparatively low pressure stability would be
adequate.
[0008] It is therefore the object of the present invention to provide
a magnetic-inductive flow meter for low-pressure applications which
can be easily produced and can also be easily fitted into a pipeline
system.
[0009] The object is achieved on the basis of a magnetic-inductive
flow meter according to the preamble of claim 1 together with its
defining features. The dependent claims which follow present advantageous
developments of the invention.
[0010] The invention includes the technical teaching that the measuring
tube is produced from a semifinished tube made of high-density polyethylene
(HDPE), which can be fitted into the pipeline system without any
flanges.
[0011] The advantage of the solution according to the invention
is, in particular, that a commercially available semifinished tube
made of a special material is used for the specific application
of interest here. Tests have shown that the material HDPE meets
all the requirements in the low-pressure area--in particular in
the water and waste-water area. The special material is distinguished
by a low density with at the same time good toughness. It is consequently
quite lightweight and stable. In addition, the material HDPE has
very good chemical resistance, which is a necessary condition in
the waste-water area in particular. The comparatively quite low
operating temperature of the material HDPE of about 80.degree. C.
is also acceptable in the low-pressure area. In addition, a semifinished
tube made of HDPE has a smooth inner tube surface and consequently
offers virtually no contact points for abrasion. The semifinished
tube is also suitable for use as a measuring tube in a magnetic-inductive
flow meter because it has adequate flexibility to adapt to offset
tube connections. It is unlikely to be affected by damage or brittle
fracture caused by the length of its service life. The measuring
tube according to the invention can be obtained in a simple way
by cutting the semifinished tube to length. This is the precondition
required for fitting the measuring tube obtained in this way into
the pipeline system entirely without flanges.
[0012] The flange-free connection of the measuring tube to the
pipeline system can preferably take place according to the following
three suggestions:
[0013] Firstly, the measuring tube may be fitted into the pipeline
system without any flanges by using unreleasable connecting means,
in that the connection between the measuring tube and the adjoining
pipeline system is configured as an integral welded connection.
A precondition for a welded connection is that the material of the
measuring tube and the material of the adjoining pipeline system
are identical. This means that the pipeline system must also consist
of HDPE. The choice of a welded connection as a connecting means
ensures particularly high strength of the connecting location.
[0014] As an alternative to this, it is also possible that the
measuring tube can be fitted into the pipeline system without any
flanges by using releasable connecting means. One way of doing this
is by using a sleeve connection. A sleeve connection presupposes
that the measuring tube and the adjoining pipeline system have the
same outside diameter, which can be enclosed by the connecting sleeve
in a sealing manner. The connecting sleeve is usually closed by
using releasable fastening means--such as screws. Furthermore, it
is also conceivable to use releasable connecting means for a clamp
connection for fastening the measuring tube in the pipeline system.
A precondition for this is that the measuring tube can be inserted
into the end of the adjoining pipeline system on both sides. Subsequently,
the connecting location is placed over the insertion region and
closed by using releasable connecting means--such as screws.
[0015] According to a further measure, improving the invention,
the measuring tube is equipped with an integrated metal shielding,
or a metal shielding arranged on the outer surface, in particular
for purposes of diffusion protection against contaminants. The metal
shielding may take the form here of a metal plate or a metal foil.
Aluminum or alloys thereof have proven to be particularly suitable
as the material for the metal plate or the metal foil. A metal shielding
that is integrated directly in the metal tube can be provided in
a simple way by encapsulating the hollow-cylindrical, prefabricated
metal shielding with the HDPE material. The use of a metal foil
leads to considerable material savings here in comparison with the
use of a metal plate, while at the same time the desired function
is reliably performed. In the case of an arrangement of a metal
shielding on the outer surface of the measuring tube, a metal plate
is to be given preference for reasons of stability. This is so since
a metal plate is more resistant than a metal foil to external mechanical
loads.
[0016] Apart from the diffusion protection created in this way,
the metal shielding of the measuring tube of a magnetic-inductive
flow meter may also be used for the electrical shielding of the
electrodes. All that is required for this is that the metal shielding
is electrically connected to the ground electrodes, in order to
shield the useful voltage in the measuring tube from the excitation
voltage generated by the magnet unit.
[0017] The most significant advantage resulting from this is that,
with the measuring tube according to the invention, electrically
the same functions as in the case of a metallic measuring tube can
be performed, while on the other hand the benefits of simple production
can also be exploited. In addition, a magnetic-inductive flow meter
with the measuring tube formed according to the invention is also
distinguished by itself having a comparatively low weight.
[0018] Further measures improving the invention are described in
more detail below together with the description of a preferred exemplary
embodiment of the invention on the basis of the single figure. The
figure shows a schematic longitudinal section through a magnetic-inductive
flow meter with a measuring tube made of plastic.
[0019] According to the figure, the magnetic-inductive flow meter
has a measuring tube 1 which is fitted into a pipeline system 2.
The measuring tube 1 is flowed through by a flowable flow medium
3 and, to conform to the magnetic-inductive flow measuring principle,
the flow medium 3 has at least slight electrical conductivity. Also
provided, on the outside of the measuring tube 1 is a magnet unit
4a, 4b, which comprises magnets lying opposite each other and serves
for generating a magnetic field extending perpendicularly in relation
to the axis of the measuring tube. The magnet unit 4a, 4b corresponds
with two measuring electrodes 5 arranged lying opposite each other
on the measuring tube 1 (of which only one measuring electrode can
be seen in this sectional representation). The measuring electrodes
5 are aligned perpendicularly in relation to the axis of the magnetic
field and serve for measuring measuring voltage induced as a consequence
of the flow of the flow medium 3. The measuring signal is fed to
a downstream electronics unit 6 which serves as an electrical interface
with further signal-processing devices.
[0020] The measuring tube 1 is produced according to the invention
from a semifinished tube made of high-density polyethylene (HDPE)
and has a substantially hollow-cylindrical basic shape. For electrical
shielding and as diffusion protection against contaminants, the
measuring tube 1 has an integrated metal shielding 7. The metal
shielding 7 in each case comprises a metal foil and is integrated
in the measuring tube 1 by encapsulating it with HDPE. The metal
foil consists here of aluminum. The metal shielding is electrically
connected to ground electrodes--not represented any further--in
order that the useful voltage in the measuring tube 1 is shielded
from the excitation voltage generated by the magnet unit 4a, 4b.
[0021] The measuring tube 1 is fitted into the surrounding pipeline
system 2 without any flanges. For the connection between the measuring
tube 1 and the adjoining pipeline system 2 of the same diameter,
a sleeve connection 8 is used here, by which the measuring tube
1 is releasably installed in the pipeline system 2. |