Abstrict An electromagnetic flow meter comprises a ceramic measuring pipe
formed into a straight cylinder through which fluid to be measured
flows, electromagnetic field generating device disposed adjacent
to a central portion of an outer surface of the central portion
of the ceramic measuring pipe for generating an electromagnetic
field into a radial direction of the ceramic measuring pipe, electrodes
for detecting an electric potential generated in the fluid, a case
which accommodates the ceramic measuring pipe, the electromagnetic
field generating device and the electrodes, and sealing members
which are disposed between inner surfaces of the flanges of the
case and ends of outer circumference of the ceramic measuring pipe.
The case has a length longer than that of the ceramic measuring
pipe and flanges at both ends thereof. Inner diameters of the flanges
are slightly larger than an outer diameter of the ceramic measuring
pipe. Portions of the sealing members outwardly project over the
end surfaces of the flanges of the case.
Claims What is claimed is:
1. An electromagnetic flow meter comprising:
a ceramic measuring pipe having a pipe body portion and a flange
portion, said pipe body portion defining a hollow cavity through
which fluid to be measured flows;
electromagnetic field generating means disposed adjacent to a central
portion of an outer surface of said pipe body portion and for generating
an electromagnetic field into a radial direction of said ceramic
measuring pipe;
electrodes for detecting an electric potential generated in said
fluid;
a case which accommodates said ceramic measuring pipe, said electromagnetic
field generating means and said electrodes;
earth rings for making an initial electric potential of said fluid
to be an earth potential; and
sealing members which are interposed between said earth rings and
respective end surfaces of said ceramic measuring pipe, wherein
each sealing member has an outer diameter which is substantially
the same as an outer diameter of said pipe body portion of said
ceramic measuring pipe and which has a predetermined thickness such
that when said sealing members are interposed between said earth
rings and said respective end surfaces of said ceramic measuring
pipe, an axially compressive force is applied to said ceramic measuring
pipe substantially only to said pipe body portion, and said flange
portion has substantially no axially compressive force applied thereto,
and wherein
at least one earth ring of said earth rings and said ceramic measuring
pipe has a stepped portion in which a portion of an outer periphery
of said sealing member is fit.
2. An electromagnetic flow meter according to claim 1 wherein
said ceramic measuring pipe is held at at least one end portion
thereof displaceably relative to said case in the axial direction
thereof.
3. An electromagnetic flow meter according to claim 1 wherein
said earth ring has a stepped portion in which a portion of a gasket
as said sealing member is fit, said stepped portion having an outer
diameter which is substantially the same as the outer diameter of
said pipe body portion of said ceramic measuring pipe.
4. An electromagnetic flowmeter as claimed in claim 1 wherein
said predetermined thickness of said each sealing member is sufficiently
thick such that a gap is left between said earth rings and a majority
of each said flange portion.
5. An electromagnetic flowmeter as claimed in claim 1 wherein
said ceramic measuring pipe is substantially cylindrical.
6. An electromagnetic flow meter according to claim 1 wherein
said earth ring comprises a first noble metal disc portion which
comes into contact with fluid flowing in said ceramic measuring
pipe and a second metal disc portion in which said gasket is fit,
said first and second disc portions being joined together.
7. An electromagnetic flow meter comprising:
a ceramic measuring pipe having a pipe body portion, said pipe
body portion defining a hollow cavity through which fluid to be
measured flows;
electromagnetic field generating means disposed adjacent to a central
portion of an outer surface of said pipe body portion for generating
an electromagnetic field into a radial direction of said ceramic
measuring pipe;
electrodes for detecting an electric potential generated in the
fluid;
a case which accommodates said ceramic measuring pipe, said electromagnetic
field generating means and said electrodes, said case having a length
longer than that of said ceramic measuring pipe for receiving an
axially compressive force applied thereto during a mounting of said
electromagnetic flow meter in a working position and substantially
preventing said axially compressive force from being applied to
said ceramic measuring pipe, said case further having flanges at
both ends thereof of which inner diameters are larger than an outer
diameter of said ceramic measuring pipe, and
sealing members which are disposed between inner surfaces of said
flanges of said case and ends of an outer circumference of said
ceramic measuring pipe, wherein said ceramic measuring pipe has
substantially no axially compressive force applied thereto and at
least a portion of said ceramic measuring pipe is normally axially
displaceable with respect to said case.
8. An electromagnetic flow meter according to claim 7 further
comprising second sealing members disposed between the outer circumference
of said ceramic measuring pipe and the inner surfaces of said flanges
of said case.
9. An electromagnetic flow meter according to claim 7 wherein
said sealing members comprise "O" rings.
10. An electromagnetic flowmeter as claimed in claim 7 wherein
each sealing member of said sealing members is only partially disposed
between said case and said ceramic measuring tube and is partially
disposed beyond an axial length of said ceramic measuring pipe.
11. An electromagnetic flowmeter as claimed in claim 10 wherein
said ceramic measuring pipe is substantially straight and has substantially
uniform wall thicknesses.
12. An electromagnetic flowmeter as claimed in claim 10 wherein
said ceramic measuring tube is substantially cylindrical.
13. An electromagnetic flowmeter as claimed in claim 7 wherein
an entire said ceramic measuring pipe is normally axially displaceable
with respect to said case.
14. An electromagnetic flowmeter as claimed in claim 7 wherein
said ceramic measuring pipe is substantially straight and has substantially
uniform wall thicknesses.
15. An electromagnetic flowmeter as claimed in claim 7 wherein
said ceramic measuring tube is substantially cylindrical.
16. An electromagnetic flow meter as claimed in claim 7 further
comprising earth rings disposed adjacent axial ends of said case,
wherein said earth ring comprises a first noble metal disc portion
which comes into contact with fluid flowing in said ceramic measuring
pipe and a second metal disc portion in which said gasket is fit,
said first and second disc portions being joined together.
17. An electromagnetic flow meter comprising:
a ceramic measuring pipe through which fluid to be measured flows;
electromagnetic field generating means disposed adjacent to a central
portion of an outer surface of said ceramic measuring pipe and for
generating an electromagnetic field into a radial direction of said
ceramic measuring pipe;
electrodes for detecting an electric potential generated in said
fluid;
a case which accommodates said ceramic measuring pipe, said electromagnetic
field generating means and said electrodes;
earth rings for making an initial electric potential of said fluid
to be an earth potential; and
sealing members which are interposed between said earth rings and
both end surfaces of said ceramic measuring pipe, each of which
has an outer diameter which is substantially the same as an outer
diameter of a barrel portion of said ceramic measuring pipe and
which has a thickness which causes gaps to be left between said
earth rings and said ceramic measuring pipe when said sealing members
are interposed between said earth rings and the both end surfaces
of said ceramic measuring pipe;
wherein at least one of said earth ring and said ceramic measuring
pipe has a stepped portion in which a portion of an outer periphery
of said sealing member is fit; and
wherein said earth ring comprises a first noble metal disc portion
which comes into contact with fluid flowing in said ceramic measuring
pipe and a second metal disc portion in which said gasket is fit,
said first and second disc portions being joined together.
18. An electromagnetic flow meter comprising:
a ceramic measuring pipe having a pipe body portion, said pipe
body portion defining a cavity through which fluid to be measured
flows;
electromagnetic field generating means, disposed on an outer surface
of said ceramic measuring pipe, for applying a magnetic field to
said fluid;
electrodes for detecting an electric potential generated in said
fluid;
a case receiving said ceramic measuring pipe therein and having
a flange at each of axial ends thereof and a length longer than
that of said ceramic measuring pipe, said flange having an inner
diameter larger than that of said ceramic measuring pipe, said case
being axially compressively interconnected to process pipes; and
seal means for sealing between an inner surface of said flange
and an outer surface of said ceramic measuring pipe;
wherein an axial compressive force applied by said process pipes
to said electromagnetic flowmeter is substantially not applied to
said ceramic measuring pipe.
Description BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flow meter,
and, more particularly, to an electromagnetic flow meter of a type
having a ceramic measuring pipe capable of satisfactorily measuring
the flow rate of a hot fluid, a corrosive fluid or an abrasive fluid
or the like.
The electromagnetic flow meter is based on the principle that the
flow rate can be measured by detecting the electric potential, which
is generated when a conductive fluid to be measured flows perpendicular
to the magnetic field, and which is in proportion to the product
of the intensity of the applied magnetic field and the flow rate
of the fluid, the detection being performed by a pair of electrodes.
The measuring pipe of the flow meter, which comes into contact with
the fluid to be measured, has a rubber or an ethylene fluoride resin
lining on the inner surface thereof in order to have corrosion resistance
and to achieve an electrical insulation. In particular, electromagnetic
flow meters of a type having a ceramic measuring pipe have been
produced recently in order to improve the heat resistance and the
wear resistance.
FIG. 1 illustrates the structure of a conventional ceramic electromagnetic
flow meter. Referring to FIG. 1 reference numeral 1 represents
a ceramic measuring pipe through which a fluid to be measured flows.
The measuring pipe 1 has a pair of electrodes 4 facing each other
in the radial direction thereof in such a manner that their leading
portions project into the inside portion of the measuring pipe 1.
The measuring pipe 1 has thick flange portions at the two end portions
thereof so as to be secured to a case member 7 by an organic or
inorganic adhesive at the outer circumferential surface portions
of the flange portions. Electromagnetic coils 2a, 2b and cores 3
are disposed adjacent to the outer surface of the central portion
of the measuring pipe 1 the electromagnetic coils 2a, 2b and the
cores 3 acting to generate a magnetic field which penetrates the
measuring pipe 1. When a fluid flowing in the measuring pipe 1 traverses
the aforesaid magnetic field, an electric potential is generated
in the fluid. The electric potential thus generated is detected
by the pair of the electrodes 4. A wire ejection portion 9 is formed
in the central portion of the case member 7 so that wires for exciting
the electromagnetic coils 2a and 2b and wires, through which detection
signals supplied from the electrodes 4 are connected to outer terminal
portions.
The electromagnetic flow meter is clamped and fastened by bolts
12 and nuts 13 while interposing earth rings 5a and 5b and gaskets
10a and 10b disposed on the two sides of the earth rings 5a and
5b in a state where the electromagnetic flow meter is held between
a pair of piping flanges 11a and 11b. The earth rings 5a and 5b
are in contact with the fluid and are connected to an earth line
of the electromagnetic flow meter so that an initial electric potential
of the fluid and the earth potential of the electromagnetic flow
meter are made to be the same. The gaskets 10a and 10b, in terms
of fluid, seals the portions between the process piping flanges
11a, 11b and earth rings 5a, 5b and the portions between the earth
rings 5a, 5b and the ceramic measuring pipe 1 of the electromagnetic
flow meter.
However, the conventional electromagnetic flow meter encounters
the following problems.
When the electromagnetic flow meter is mounted to the process piping,
each of the bolts is fastened at a tightening torque of thousands
N.cm so as to apply compressive force to each gasket in the axial
direction (in the direction of the thickness) in order to obtain
fluidtight performance. Since the aforesaid fastening or clamping
force is received by the surface of each flange of the ceramic measuring
pipe, the surface of the flange is applied with a large plane pressure
of tens Mpa to hundreds Mpa.
The plane pressure thus applied generates compressive stress (FIG.
2) in the cylindrical portion of the ceramic measuring pipe in the
axial direction and bending moments acting on the flanges. The bending
moments result an axial tensile stress and a circumferential tensile
stress.
In general, ceramics have the tensile strength about one-tenth
of the compressive strength. Therefore, the aforesaid tensile stresses
are the largest cause for the breakage of the ceramic measuring
pipe. In actual practice, the ceramic measuring pipe is usually
broken at positions shown in FIG. 3.
Therefore, an upper limit of the tightening torque for the bolt
at the time of fastening the electromagnetic flow meter structured
as shown in FIG. 1 to the process piping is usually specified. If
the bolt is fastened with a tightening torque larger than the specified
torque, the ceramic measuring pipe can be broken. Even if the bolts
are fastened with a tightening torque smaller than the specified
torque, a so-called "unevenly fastened state", in which
only either of the bolts is tightly clamped, will generate excessive
tensile stress, causing the ceramic measuring pipe to be broken.
Further, the gasket 10b held between the flange of the ceramic
measuring pipe and the earth ring deforms outwardly due to the clamping
force applied thereto, that is, a so-called "cold flow"
takes place. As a result, another problem arises in that the pressure
applied to the gasket is undesirably reduced and therefore the degree
of the fluidtight sealing deteriorates.
Another problem arises in that excessively large thermal stress
is generated in the flange portion of the ceramic measuring pipe
if the temperature of the fluid, which flows in the ceramic measuring
pipe, has been rapidly changed because the flange portion has a
large thickness. It also leads to a fact that the ceramic measuring
pipe is broken.
Accordingly, an object of the present invention is to provide an
electromagnetic flow meter, which can be fastened to process piping
while preventing generation of excessively large tensile stress
in a ceramic measuring pipe thereof, and in which deterioration
of fluidtight sealing performance due to cold flow of gaskets interposed
between flanges of the ceramic measuring pipe and earth rings can
be prevented.
Another object of the present invention is to provide an electromagnetic
flow meter, which can be fastened to process piping while preventing
generation of excessively large tensile stress in a ceramic measuring
pipe thereof, in which deterioration of fluidtight sealing performance
due to cold flow of gaskets interposed between flanges of the ceramic
measuring pipe and earth rings can be prevented, and in which generation
of excessively large thermal stress can be prevented even if temperature
of the fluid has been changed rapidly.
SUMMARY OF THE INVENTION
An earth ring for an electromagnetic flow meter according to the
present invention has a stepped portion in which a portion of a
gasket is fit, the gasket having an outer diameter which is substantially
the same as an outer diameter of a barrel of a ceramic measuring
pipe of the electromagnetic flow meter.
When an electromagnetic flow meter is fastened to a process piping
by using the earth rings according to the present invention, the
clamping load acts, as an axial directional compressive force, on
only the body of the ceramic measuring pipe and no load acts on
outer portions of the flange portions of the ceramic measuring pipe.
Hence, no bending moments act on the flanges, so that the breakage
of the ceramic measuring pipe experienced with the conventional
technology can be prevented. Furthermore, the arrangement in such
that the outer periphery of each of the gaskets is restricted by
the stepped portion of the earth ring prevents the gaskets from
being outwardly deformed. Therefore, satisfactory fluidtight sealing
performance can be maintained. If the gasket is deformed outwardly,
a bending moment generated due to the clamping force acts on the
outer portions of the flanges of the ceramic measuring pipe to cause
the ceramic measuring pipe to be broken. However, the arrangement
according to the present invention is made in such a manner that
the outward deformation of the gasket is prevented as described
above and prevents the aforesaid problem of the breakage. Furthermore,
a conventional electromagnetic flow meter can be adapted to the
present invention while eliminating a necessity of changing the
design.
The earth ring may comprise a first noble metal disc portion which
comes into contact with fluid which flows in the ceramic measuring
pipe and a second metal disc portion into which the gasket is fit.
As a result of the structure thus made, another effect can be obtained
in that the manufacturing cost of the earth ring can be reduced.
An embodiment of an electromagnetic flow meter according to the
present invention comprises: a ceramic measuring pipe through which
fluid to be measured flows; electromagnetic field generating means
disposed adjacent to a central portion of an outer surface of the
ceramic measuring pipe for generating an electromagnetic field into
a radial direction of the ceramic measuring pipe; electrodes for
detecting an electric potential generated in the fluid; a case which
accommodates the ceramic measuring pipe, the electromagnetic field
generating means and the electrodes; earth rings for making an initial
electric potential of the fluid to be an earth potential; and sealing
members which are interposed between the earth rings and the extremity
surfaces of the ceramic measuring pipe, each of which has an outer
diameter substantially the same as an outer diameter of the ceramic
measuring pipe and a thickness which causes gaps to be left between
the earth rings and the ceramic measuring pipe when the sealing
members are interposed between the earth rings and the end surfaces
of the ceramic measuring pipe, wherein at least one of the earth
ring and the ceramic measuring pipe has a stepped portion into which
a portion of outer periphery of the sealing member is fit.
When the electromagnetic flow meter according to the present invention
is fastened to the process piping, the load due to the clamping
acts, as an axial compressive force, on only the body of the ceramic
measuring pipe and no load acts on outer portions of the flange
portions of the ceramic measuring pipe. Hence, the breakage of the
ceramic measuring pipe experienced with the conventional technology
can be prevented. Furthermore, as the outer periphey of each sealing
member is restricted by the stepped portion of the earth ring and/or
the ceramic measuring pipe, the sealing members are prevented from
being outwardly deformed. Therefore, satisfactory fluidtight sealing
performance can be maintained. Furthermore, as the outward deformation
of the sealing members is prevented similarly to the aforesaid embodiment,
no bending moment is applied to the flange of the ceramic measuring
pipe.
Another embodiment of an electromagnetic flow meter according to
the present invention comprises: a ceramic measuring pipe formed
into a straight cylinder through which fluid to be measured flows;
electromagnetic field generating means disposed adjacent to a central
portion of an outer surface of the ceramic measuring pipe for generating
an electromagnetic field into a radial direction of the ceramic
measuring pipe; electrodes for detecting an electric potential generated
in the fluid; a case which accommodates the ceramic measuring pipe,
the electromagnetic field generating means and the electrodes, the
case having a length longer than that of the ceramic measuring pipe
and flanges at both ends thereof of which inner diameters are slightly
larger than an outer diameter of the ceramic measuring pipe, and
sealing members which are disposed between inner surfaces of the
flanges of the case and ends of an outer circumference of the ceramic
measuring pipe, and portions of which outwardly project over the
end surfaces of the flanges of the case.
In the embodiment, the straight cylindrical ceramic measuring pipe
can be freed from breakage because the clamping force for fastening
the electromagnetic flow meter is applied to only the case and no
load is applied to the straight cylindrical ceramic measuring pipe.
Furthermore, the ceramic measuring pipe can be freed from the generation
of the thermal stress due to the change of the thermal change of
the fluid because the straight cylindrical ceramic measuring pipe
is supported by sealing members in such a manner that it can be
displaced in the radial and axial directions, that is, it is floatingly
supported. In addition, the generation of the thermal stress can
be prevented from another reason that the ceramic measuring pipe
is formed into a straight cylinder having no thick portion.
Although the case is longer than the ceramic measuring pipe, it
cannot be corroded because it is sealed from the fluid by means
of the sealing members.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an axial cross sectional view of a conventional electromagnetic
flow meter;
FIG. 2 is a perspective view of an axially sectioned ceramic measuring
pipe of the conventional electromagnetic flow meter in which shown
is the distribution of stresses generated when the ceramic measuring
pipe is fastened to a process piping;
FIG. 3 a perspective view similar to FIG. 2 and showing a state
of breakage of the ceramic measuring pipe;
FIG. 4 is an axial cross-sectional view of an electromagnetic flow
meter according to the present invention having earth rings;
FIG. 5 is a cross-sectional view which illustrates an essential
portion of another example of the earth ring of the electromagnetic
flow meter according to the present invention;
FIG. 6 is an axial cross-sectional view of an embodiment of an
electromagnetic flow meter according to the present invention; and
FIG. 7 is an axial cross sectional view of another embodiment of
the electromagnetic flow meter according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 4 is a cross sectional view which illustrates an electromagnetic
flow meter according to the present invention. A body of the electromagnetic
flow meter is the same as that of the conventional electromagnetic
flow meter shown in FIG. 1 and the common elements are given the
same reference numerals. Reference numeral 1 represents a ceramic
measuring pipe molded from ceramic raw material and arranged to
allow fluid to be measured to flow. A pair of electromagnetic coils
2a and 2b are mounted adjacent to a central portion of an outer
surface of the ceramic measuring pipe 1. Furthermore, cores 3 are
disposed outside of the electromagnetic coils 2a and 2b so as to
surround the electromagnetic coils 2a and 2b. The electromagnetic
coils 2a, 2b and the cores 3 generate an electromagnetic field which
vertically traverses the ceramic measuring pipe 1. In general, the
electromagnetic coils 2a and 2b are excited by a direct current
which is formed into rectangular waves and which is repeatedly turned
on/off or an alternating current. Furthermore, a pair of electrodes
4 are disposed in the central portion of the ceramic measuring pipe
1. The pair of electrodes 4 are mounted in a direction perpendicular
to the direction of the electromagnetic field and a direction of
the flow of the fluid. An electric potential generated in the fluid
can be detected by means of the electrodes 4. Thick flange portions
15a and 15b are formed at both ends of the ceramic measuring pipe
1.
One of the flange portions 15a and 15b is firmly secured to an
inner surface of a cylindrical portion of a casing member 7 by bonding,
or shrinkage fitting or the like, while the residual flange portion
is held by, for example, sticking with a silicon adhesive in such
a manner that it can be displaced in the axial direction. If necessary,
both flange portions may be displaceably held in the axial direction.
The casing member 7 has an opening 9 formed at a central portion
of the cylindrical portion thereof. The opening 9 is used to connect
wires through which electric currents are supplied to the electromagnetic
coils 2a and 2b and signals transmitted from the electrodes 4 are
supplied to an external amplifier portion. On the other hand, the
flange portions 15a and 15b are coaxially positioned on and in contact
with gaskets 16a and 16b each having an outer diameter sufficiently
smaller than an outer diameter of each of the flanges portions 15a
and 15b. It is ideal that the aforesaid outer diameter be the same
as an outer diameter of the body of the ceramic pipe. The gaskets
16a and 16b are inserted into stepped portions (groove portions)
18a and 18b of earth rings 17a and 17b which come into contact with
the fluid in order to make an initial electric potential of the
fluid to be measured to be the same as an earth electric potential.
The depth of the stepped portion is made to be smaller than the
thickness of each of the gaskets 16a and 16b so that the earth rings
17a and 17b do not directly come into contact with the ceramic measuring
pipe 1. Furthermore, the earth rings 17a and 17b are fastened to
the casing member 7 by means of threads in such a manner that the
separation of the earth rings 17a and 17b can be prevented.
The electromagnetic flow meter is held between flanges 11a and
11b of the process piping while interposing gasket members 10a and
10b between the electromagnetic flow meter and the earth rings 17a
and 17b. The electromagnetic flow meter is clamped by bolts 12 and
nuts 13 so as to be, in terms of fluid, hermetically secured to
the process piping.
Then, the description will be made about the action of force generated
in the electromagnetic flow meter structured as described above
when the electromagnetic flow meter is fastened to the process piping
by bolts 12 and nuts 13. The compressive force applied to the earth
rings 17a, 17b via the flanges 11a, 11b and the gasket members 10a,
10b when the bolts 12 and nuts 13 are tightened is then applied
to both extremity surfaces of the ceramic measuring pipe 1 via the
gaskets 16a and 16b, causing axial directional compressive force
to act on the ceramic measuring pipe. At this time, no clamping
force is transmitted to the outer portions of the side surfaces
of the flange portions 15a and 15b of the ceramic measuring pipe
1 because the gaskets 16a and 16b do not come into contact with
the outer portions of the side surfaces. Therefore, no moment to
bend the flange portions 15a and 15b, is generated. Hence, in the
ceramic measuring pipe 1 there is no portion where excessively
large tensile stress results and only compressive stress acts thereto,
so that the breakage can be prevented.
Furthermore, the gaskets 16a and 16b and the ceramic measuring
pipe 1 can be accurately disposed on the same axis, and the outward
deformation of the gaskets 16a and 16b which takes place due to
the application of the clamping force can be prevented because the
gaskets 16a and 16b are received in the stepped portions 18a and
18b of the earth rings 17a and 17b. As a result, deterioration of
the fluidtight sealing performance which takes place due to cold
flow of the gasket can be prevented. Also generation of bending
moment in the flanges of the ceramic measuring pipe due to the enlargement
deformation of the outer diameter of the gasket can be prevented.
When the fluid has corrosion influences, the earth rings must be
made of noble metal. In such a case, if the whole of the earth ring
is made of noble metal as is in the aforesaid embodiment, it becomes
very expensive. This is not economical. An earth ring shown in FIG.
5 is one to cope with the aforesaid case.
An earth ring 19 has a first disc portion 20 which comes into contact
with the fluid to be measured and a second disc portion 21 in which
the gasket 23 is fit. The first disc portion 20 is made of a foil
of the noble metal. The second disc portion 21 is made of metal
such as stainless steel. The first and the second disc portions
20 and 21 are integrally fastened by a rivet 22 or the like at their
outer peripheries.
The structure thus arranged will reduce the cost of the earth ring
in addition to the advantages obtainable from the aforesaid embodiment.
A modification of the aforesaid embodiment will now be described
with reference to FIG. 6.
The difference from the aforesaid embodiment lies in that electrodes
4' for detecting the electric potential are disposed on an outer
surface of a ceramic measuring pipe 26 and that stepped portions,
to which portions of the gaskets are fit, are formed at the extremity
surfaces of flanges 26a and 26b of the ceramic measuring pipe 26.
The electrodes 4' are applied to the outer surface of the ceramic
measuring pipe 26 and detect the electric potential generated in
the fluid to be measured via an electrostatic capacity created between
the fluid to be measured and the electrodes 4'. The flanges 26a
and 26b of the ceramic measuring pipe 26 have, at the extremity
surfaces thereof, stepped portions 25a and 25b to which portions
of the corresponding gaskets 16a and 16b are fit. Each of the gaskets
16a and 16b has a thickness which allows gaps to be formed between
the flange 11a of the process piping and the flange 26a of the ceramic
measuring pipe 26 and between the flange 11b and the flange 26b
when the electromagnetic flow meter is fastened between the flanges
11a and 11b of the process piping.
Also in this embodiment advantages the same as the embodiment shown
in FIG. 4 are obtained.
Another embodiment of the electromagnetic flow meter according
to the present invention will now be described with reference to
FIG. 7.
A ceramic measuring pipe 29 is formed into a straight cylindrical
shape and has a thickness of 5 to 10% of an inner diameter thereof.
A pair of electromagnetic coils 2a and 2b mounted adjacent to a
central portion of an outer surface of the ceramic measuring pipe
29. Furthermore, cores 3 are disposed on outside the electromagnetic
coils 2a and 2b so as to surround the electromagnetic coils 2a and
2b. The electromagnetic coils 2a, 2b and the cores 3 generate an
electromagnetic field which vertically traverses the ceramic measuring
pipe 29. Similarly to the aforesaid embodiment, electrodes 4' for
detecting the electric potential of the fluid to be measured are
applied to substantially the central portion of the outer surface
of the measuring pipe 29.
A case 28 is formed into a substantially straight cylindrical shape
having, at both ends thereof, flanges 28a and 28b each having an
inner diameter larger than an outer diameter of the ceramic measuring
pipe 29. The case 28 has an overall length slightly longer than
the ceramic measuring pipe 29. Furthermore, an opening through which
wires connected to the electromagnetic coils 2a, 2b and the electrodes
4' are taken out of is formed in the central portion of the case
28.
The ceramic measuring pipe 29 is inserted into the case 28 in such
a manner that the ceramic measuring pipe 29 is able to be displaced
in the radial direction and the axial direction by a pair of sealing
members 27a and 27d disposed at both ends of the outer surface of
the ceramic measuring pipe 29. That is, the ceramic measuring pipe
29 is held by the pair of sealing members 27a and 27d in such a
manner that the ceramic measuring pipe 29 is floated.
As described above, the pair of sealing members 27a and 27d are
interposed between the both ends of the outer surface of the ceramic
measuring pipe 28 and the flanges 28a and 28b and further, portions
of the pair of sealing members 27a and 27d project outwardly over
the end surfaces of the flanges 28a and 28b of the case 28. It is
preferable that the sealing members 27a and 27d be "O"
rings.
In the embodiment shown in FIG. 7 second sealing members 27b and
27c are disposed between the outer surface of the ceramic measuring
pipe 29 and the inner surfaces of the flanges 28a and 28b of the
case 28. They may be omitted from the structure because they are
ones used in response to needs.
When the electromagnetic flow meter thus structured is fastened
to flanges 11a and 11b of the process piping by a plurality of bolts
12 and nuts 13 while interposing earth rings 5a, 5b, gaskets 10a
and 10b, the compressive force applied to the earth rings 5a, 5b
via the flanges 11a, 11b and the gaskets 10a, 10b compresses the
whole of the case 28 in the axial direction. However, the force
is not transmitted to the ceramic measuring pipe 29 because gaps
are present between the ceramic measuring pipe 29 and the earth
rings 5a, 5b. That is, any force applied at the time of fastening
the electromagnetic flow meter to the process piping is not applied
to the ceramic measuring pipe 29.
If the temperature of the fluid to be measured and flowing in the
ceramic measuring pipe 29 is rapidly changed, a difference in the
temperature takes place between the outer surface of the ceramic
measuring pipe 29 and the inner surface of the same, causing thermal
stress to be generated in the ceramic measuring pipe 29. The level
of the thermal stress is in proportion to the thickness of the measuring
pipe 29 and the degree of the uneven thickness. Since ceramics generally
have poor durability against a tensile force in comparison with
a compressive force, ceramics can be most easily broken when the
inner surface of the ceramic measuring pipe 29 is rapidly cooled.
Since in this embodiment, the ceramic measuring pipe 29 is formed
into a simple cylinder having a thin wall, generation of the internal
thermal stress can be reduced.
Although the difference in the temperature and the linear expansion
coefficient between the ceramic measuring pipe 29 and the case 28
ought to cause the dimensions of them to be changed in the axial
and radial directions and generate thermal stress between them,
only the gap between them is changed but no force is applied to
the ceramic measuring pipe 1 because the ceramic measuring pipe
29 is floated in the case 28 via the "O" ring which are
soft.
In the case where the earth rings 5a and 5b are not required, another
arrangement may be employed in which also the gaskets 10a and 10b
are removed and the end surfaces of the flange 28a and 28b are directly
brought into contact with the piping flanges 11a and 11b of the
process piping.
Even if the fluid to be measured has corrosiveness, a necessity
for the material of the case to have specific corrosion resistance
can be eliminated, because the case is sealed from the fluid to
be measured by means of the "O" rings. Therefore, the
case can be made of a general material such as stainless steel.
Furthermore, the arrangement according to this embodiment enables
simple flat rings to be used as the earth rings and therefore a
corrosion resisting electromagnetic flow meter can easily be obtained
by making the earth rings of a corrosion resisting noble metal film
such as platinum.
Since the sealing members 27a and 27b can be easily removed when
they comprise the "O" rings, it is possible to use them
by selecting their material in response to the degree of the corrosiveness
of the fluid to be measured.
The conventional electromagnetic flow meter shown in FIG. 1 and
the electromagnetic flow meters according to the present invention
and shown in FIGS. 4 and 7 were subjected to a compressive load
test by using a loading test machine. As a result, the ceramic measuring
pipe of the conventional electromagnetic flow meter was broken when
a compressive load of 9.4 tons was applied, that of the electromagnetic
flow meter according to the present invention and shown in FIG.
4 was broken when a compressive load of 32 tons was applied, and
that of the electromagnetic flow meter according to the present
invention and shown in FIG. 7 was broken when a compressive load
of 137 tons was applied. That is, the electromagnetic flow meters
according to the present invention respectively have strength of
4.2 times and 137 times the strength of the conventional electromagnetic
flow meter. |