Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1064141
The present invention relates to apparatus for the detection of
- temperature variations above or below a predetermined value.
A known phenomenon associated with ferromagnetic materials is
the very large change in magnetic permeability at a temperature identified
as the Curie Point, from being ferromagnetic to being paramagnetic or
vlce versa.
When a ferromagnetic material is employed having a Curie Point
at some preselected value, if the said material is at a temperature below
its Curie Point it will have a relatively high magnetic permeability and
a relatively large current will be induced in the second electrical con-
ductor. When the said material is at a temperature above its Curie Point
it will have a very low magnetic permeability and as a result a negligible
current will be induced. Monitoring the current induced in the said second
electrical conductor will therefore give an indication of whether the
ferromagnetic material is at a temperature above or below its Curie Point,
and temperature changes through that temperature can be immediately detected
and recorded.
1064~
Thus, according to the present invention, there is provided
apparatus for detecting temperature viarations above or below a predetermined
value at a plurality of areas, comprising, at the areas, a quantity of
ferromagnetic material having a Curie Point approximately equivalent to the
predetermined temperature; a network of electrical conductors consisting
of two groups arranged in pairs of conductors, one conductor of each pair
being from each group, such that the two conductors of each pair are in
combination with the ferromagnetic material at a respective area in mutual
inductive relationship whereby when the ferromagnetic material at the
respective area is at a temperature below its Curie Point and a first vary-
ing electrical current is passed along a first one of the two conductors a
second varying electrical current is induced in a second of the two con-
ductors; means for passing a first varying electrical current along one
groups of conductors and means for monitoring the second varying electrical
current induced in the other group of conductors.
The ferromagnetic materials we have so far discovered to be most
suitable for use according to the present invention are those ferromagnetic
oxides generally identified as soft ferrites. In particular the magnetical-
ly soft ferrite materials having a cubic crystal structure are especially
useful. These materials have the structure of the mineral spinel and are
sometimes referred to as spinel ferrites. They have the general formula
MeFe204 where Me usually represents one or, in mixed ferrites, more than
one of the transition metals, Mn, Fe, Co, Ni, Cu and Zn, or Mg and Cd.
Other combinations, of equivalentvalency, are possible and it is also possible
to replace some or all of the trivalent iron ions with other trivalent metal
~",
.
1064~41
ions. For the present application maganese zinc ferrites (MnZn ferrite)
or nickel zinc ferrites (NiZn ferrite) are most suitable. The magnetic
properties of these spinel ferrites arise from interactions between metallic
ions occupying particular positions relative to the oxygen ions in the
crystal structure of the ferrite. Full details of ferrite materials of this
type and of various others, together with a discussion of their magnetic
properties can be found in the publication "Soft Ferrites" Properties and
Applications by E. C. Snelling, BSc(eng), C.Eng., F.I.E.E., published by
Iliffe Books Ltd., London. Soft ferrites are known having Curie Points
between -20C and +350C (theoretically between -100C and +350C) and
; material having a particular predetermined temperature for the present use
may be selected from the tables of recorded data.
For example the ferromagnetic material may be a toroidal shaped
piece of ferrite and the first and second electrical conductors may be
single coils of wire around opposite sides of the toroid or may even be
single strands of wire threaded through the centre hole of the toroid. Any -~
suitable source of alternating current may be connected to the ends of the
first electrical conductor and a lamp bulb may be connected to the ends of
the second electrical conductor. When the ferrite toroid is at a temper-
ature below its Curie Point, the lamp bulb will light and when the temper-
ature rises above the Curie Point of the ferrite the lamp bulb will extin-
guish. With the development of computers and microelectronics very many
forms of the combination of ferromagnetic material with first and second
electrical conductors suitable for use with the apparatus of the present
invention can now be envisaged, but any of these when used in combination
with a means producing a varying electrical current and a monitoring means,
as discussed above, fall within the generaldescription of the present
invention.
: "
1064141
The individual detectors so far described will monitor only a
relatively small area depending upon the quantity of the magnetically soft
ferromagnetic material used and the thermal conductivity of the object with
which they are in contact. In order to monitorlarger areas, such as the
walls of tanks, detectors can be joined together in the form of a network.
For example, a first electrical conductor can be coiled in sequence around
a number of toroidal shaped pieces of ferrite. Several strings of toroids
like this can be made and placed side-by-side and a second electrical con-
ductor can be coiled around the first toroid in each string, a further one
around the second toroid in each string and so on to form a two-dimensional
network. When a first varying electrical current is passed along any of
the first electrical conductors and the second electrical conductors monitor-
ed, if the temperature in the vicinity of any of the toroids is such that
the magnetically soft ferromagnetic material is below its Curie Point, a
current will be detected in the second electrical conductor coiled around
the toroid in question and it can be immediately identified.
The present invention can be further illustrated and explained
by reference to examples of use to which the apparatus can be put, which
are hereafter described and illustrated with reference to the accompanying
drawings.
A first use for the present apparatus is to detect failures in
insulation systems surrounding cold materials which can lead to the form-
ation of cold spots. This is a particularly important use, especially when
the insulation in question is surrounding a cryogenic material and leakage
of cold through the insulation could lead to the outside container wall
becoming subjected to cryogenic
-5
.:
. ~
.
1064141
.
temperatures and cracking. More particularly a system of cryogenic
insulation exists where the material of the insulation (e.g. poly-
urethane foam) acts as the actual container for a cryogenic
liquid and failure in the insulation could allow cracks to form
through which cryogenic liquid could flow, perhaps even into contact
with the outside container wall with catastrophic results. The
'~ C~ S
- present invention provides 2r~K~P~ by which such failures in the
insulation can be detected at a very early stage and moreover can
be measured and monitored.
Apparatus according to the present invention can be installed
either within the insulation itself or on the inside or outside of
the outer container wall. Whilst the temperature in the vicinity
of the ferromagnetic material is greater than the predetermined
temperature (the Curie Point of the ferromagnetic material selected),
a first varying electrical current passed along the first electrical
conductor will not induce a current in the second one. If a failure
occurs in the insulation in the vicinity of ferromagnetic material
cold will flow through it cooling the said material. The ferromagnetic
material will be cooled to below ltS Curie Point and its magnetic
permeability will be greatly increased. A first varying electrical
current passed along the first electrical conductor w-ill now cause
a second varying electrical current to be induced in the second
electrical conductor.
In the case of a very large tank such as those installed in the
liquid petroleum gas (LPG) and liquid natural gas (LNG) tankers at
present being built and projected for the future, individual detectors
according to the present invention can be included into the insulation
or attached to the tank wall in the form of a network as discussed
1064141
.. :
above in such a manner that the whole of each tank wall can be
monitored from one point by pas6ing a current through a first
series of electrical conductors each in association with a
- -~ quantity of ferromagnetic material and monitoring the induced
currents in a second series of electrical conductors.
The ability of the apparatus of the present invention to respond
to a decrease in temperature can be used in order to detect a failure
in a vessel or pipe containing a gas or liquid under pressure.
In this situation a small hole will allow the contained material
to jet from the container and as a result of the Joule Thomson effect,
the area surrounding the hole becomes cooled. A well-known phenomenon.
- The lowering in temperature at this point is detectable by the detectors
of the present apparatus in the same manner as a cold spot in an insulation
system. In the event of a catastrophic failure, having explosive force
which might damage the detector, this is also detected since a
discontinuity is created at the scene of the damage and the first
electrical current does not flow around its conductor.
Alternatively the present mcthc~ ~nl apparatus may be used to
detect a failure in a thermal insulation system, where failure can
lead to the formation of a hot spot. Hot spots can be especially
dangerous because they can become ignition sources. Examples of thermal
- insulation systems which can be protected by the apparatus of the present
- invention include those surrounding chemical distillation plant,
furnaces, boilers, reaction vessels and heated storage vessels. The
method of detecting a failure is similar to that for detecting a failure
in a cryogenic insulation system except that the detector detects the
presence of a magnetic discontinuity caused by the ferromagnetic material
in the area of the ~ailure passing through its Curie Point and becoming
paramagnetic. -
_ 7 _
- ~
~ 1064141
- , As well as changes in temperature due to insulation failure the
method and apparatus of the present invention can also detect a change
brought about by an undesirable change in state, for example caused by
faulty heating, running, cooling or electrical equipment, as the result
of a chemical reaction going out of control or as the result of an out-
break of fire, or as a result of bearing failure, or as a result of short
circuiting, etc.
An added advantage of the present detection apparatus is that
individual detectors, comprising a quantity of ferromagnetic material
together with one or more associated electrical conductors can be made
extremely small and can be interconnected by very thin electrical
conductors. They are thus able to be 'built-into' a wide range of
~ 5 ~ ss~ e_ r~
insulation systems, such as fible-g~a~ m~t8 and foams and can
effortlessly be made to follow very complex contours.
~; Referring now to the accompanying drawings, Figure 1 shows
apparatus according to the invention and Figure 2 shows a typical
network configuration of detectors according to the invention arranged
on the surface of an insulation system having a fault therein.
Figure 1 shows a toroidal shaped element 1 of ferromagnetic
material, for example a ferrite, around which a first electrical
conductor 2 has been wound to form a loop 3. The two ends 4 and 5,
of the first electrical conductor 2, are connected to a means 6,
for producing a varying electrical current. A second electrical
conductor 7 has been wound around the toroid shaped element 1, so as
to form a loop 8, and the two ends 9 and 10, of the second electrical
conductor 7, are connected to a monitoring means 11.
~ hen the apparatus of Figure 1 is in use a varying electrical current,
produced by the means 6, passes through the first electrical conductor 2,
and the loop 3, around the toroidal element l. Provided that the
-- 8 --
~064141
... .
~- toroidal shaped element 1 is at a temperature above the Curie Point
of the selected ferromagnetic material, a second varying electrical
current will be induced in the loop 8, and the second electrical
conductor 7, and will be monitored by the monitoring means 11.
~: 5 Figure 2 shows how the toroidal elements ehown in Figure 1
can be arranged on a surface in the form of a network, reducing the
connections necessary to employ them and enabling a single source
of varying electrical current and a single monitor to be used with
a plurality of elements. In Figure 2, four toroidal elements 20, 30,
40 and 50, are shown arranged in a regular pattern on the surface 15,
of an insulation system. A first electrical conductor 21, having
two ends 22 and 23, is passed first through a first toroidal element
20, and then through a second element 30. A second electrical conductor
41, having two ends 42 and 43, is similarly passed first through a
third element 40, and then through a fourth element 50. A third
electrical conductor 25, having two ends 26 and 27, is passed first
-~ through the first toroidal element 20, and then through the third 40.
A fourth electrical conductor 45, having two ends 46 and 47, is passed
first through the second toroidal element 30, and then through the
: 20 ~ourth 50 Suitable connections, not shown, enable a means, not shown,for producing a varying electrical current to be connected across
any of the pairs of electrical conductors 22, 23; 42, 43; 26, 27; or
46, 47, of any of the electrical conductors 21, 41, 25 or 45, or
- alternately a monitoring means, not shown, may be similarly connected.
Also shown in Figure 2 is a shaded area 55, indicating a cold spot
on the surface 15, of the insulation system. It will be understood
from the description that, although only four toroidal elements
have been shown in Figure 2, any number of elements may be similarly
wired together and monitored in a network such as this and the ~-
1~64141
element or elements affected identified easily. It will also be
understood that the attachment positions of the means for producing
a varying electrical current and the monitoring means may be fully
interchangeable.
In use, the means, not shown, for producing a varying electrical
current is connected across the two ends 22 and 23, of the first
electrical conductor 21, or the two ends 42 and 43, of the second 41.
If the first toroidal element 20 is below its Curie Point then a
second varying electrical current will be induced in the third electrical
conductor 25, and will be detected when the monitoring means, not
shown, is connected to its two ends 26 and 27. ~o current will be
induced in the fourth electrical conductor 45. If the second toroidal
element 30 is below its Curie Point a second varying electrical current
will be induced in the fourth electrical conductor 45, and this will
be detected when the monitoring means is connected across its two ends
46 and 47.
Similarly if the means for producing a varying electrical
current is connected across the two ends 42 and 43, of the second
electrical conductor 41, and either the third or the fourth toroidal
elements 40 or 50, is below its Curie Point a second varying electrical
current will be induced in either the third or the fourth electrical
conductor 25 or 45, depending on which toroidal element is cold, and
this current will be detected only when the monitoring means is
connected across the two ends of that conductor.
In the situation shown in Figure 2, where a cold spot 55 exists
on the surface 15, of the insulation system in the vicinity of the
fourth toroidal element 50, the fourth toroidal element 50 will be
-- 10 --
1064141
cooled to below its Curie Point whereas the other three elements
20, 30 and 40 will remain above theirs. ~hus when a means for
producing a varying electrical current is connected across the
two ends 42 and 43, of the second electrical conductor 41, and
the monitoring means is connected across the two ends 46 and 47, of
the fourth electrical conductor 45, an induced current will be
detected. No induced current will be detected in the third electrical
conductor 25, or, on connecting the means for producing a varying
electrical current across the two ends 22 and 23, of the first electrical
10 conductor 21, in either the third or the fourth conductors 25 or 45.
~ .
- 11 _
