The present invention relates to a lining in which a material with
a certain coefficient of linear expansion constitutes lining for another material
with a different coefficient of linear expansion. In particular, the invention
aims to solve the problem with the relative linear displacements in connection
with cyclic changes in temperature of the respective materials.
In industrial applications there is often a need to line certain
elements with a material which exhibits properties quite different from the material
from which the element is made. As an example may be mentioned tubes, tanks, nozzles
or similar devices which are to withstand, for example, high temperature, variations
in temperature, erosion from particles flow, or some other type of degrading influence
on the material.
The present invention is applicable within each technical field in
which linings of the above-mentioned kind are desirable. To describe the problems
which arise in connection with lining and the solutions that are currently applied,
known technique involving ceramic lining of tubes exposed to high temperatures
and/or erosive particle flows is chosen.
In boiler plants, flue gas tubes, cleaning plants for flue gases
etc., usually some steel quality is used in walls and casings. If these casings
were to be exposed to the influence of very hot flue gases containing erosive particles
for a substantial period of time, and under the effect of temperature fluctuations,
these casings would be worn out relatively quickly. For this reason, cyclones,
for example, which constitute dust cleaning devices in, inter alia, fossil fuelled
power plants, are usually provided with a lining, usually of a ceramic material,
as described for example in EP-A-0 094 098.
In a cyclone, a vortex of, for example, flue gases moves from a higher
to a lower level inside a cyclone wall with a downwardly tapering circular cross
section. During the downward flow, the speed of the gas flow increases, causing
heavier particles present in the gas vortex to be thrown out against the cyclone
wall and then to fall down into cyclone legs which form dust outlets from the cyclone.
The cleaned gas is discharged at the upper part.
For operation of gas turbines connected to cyclones of the above-mentioned
type, the highest possible gas temperature is desired at the inlet of the turbine.
This means that gas cleaners which separate dust from combustion gases operate
at the temperature of the combustion gases when these leave a bed in a combustion
A combustion plant of, for example, PFBC type operates with a gas
temperature which may amount to 950°C. This high temperature entails heavy stresses
in the cyclones for cleaning of the combustion gases before these are supplied
to a turbine. The problems are particularly great in the lowermost part of the
cyclone and in the cyclone legs. The high speed of the greatly abrasive, erosive
particles in the gas mass and the high temperature reduce the strength of the
cyclone material and deteriorate its resistance to wear.
Despite different forms of cooling of cyclones and different designs
of cyclones and cyclone legs, the problem with the heavy wear on the cyclone material
from dust in the gas remains. This has made it necessary to provide cyclones with
an erosion-resistant material, usually in the form of a lining. This lining may
be formed from ceramic material, which has long been known in the art. In already
existing PFBC energy plants, the cyclones have been internally lined with a high-resistant
One way of providing cyclones or other corresponding devices with
a ceramically resistant material according to known technique is to apply a steel
net with hexagonal meshes to the surface which is to be coated. The net is spot
welded to this surface. The net has a certain thickness, since the net is formed
from steel bands. Inside each mesh there are central holes in the steel band. After
application, the meshes in the steel net are filled with a ceramic material, usually
aluminium oxide, which is fixed in position by the ceramic material penetrating
also the holes in the steel band. The ceramic gives the surface good resistance
to erosion and provides good protection against fires which may arise under certain
conditions. In addition, the ceramic withstands temporary increases in temperature.
One problem, however, are the different coefficients of linear expansion of the
ceramic and the lined material.
Upon start-up, cyclones are heated from room temperature to operating
temperature for a relatively long time. When the ceramic gradually reaches the
operating temperature, the temperature of the cyclone wall has risen to about 850°C
or around 350°C depending on whether insulation has been applied outside the cyclone
wall or between the ceramic and the steel wall.
Prior to start-up of the plant, there are small gaps, at a temperature
of about 20°C, between the hexagonal ceramic plates inside the steel meshes and
the steel bands of these meshes. During a heating period and because of the greater
liner expansion of the steel material, the width of these gaps is increased, dust
from the flue gases being packed into these gaps. During the subsequent shrinkage
of the materials during an interruption of the operation, or a reduction of the
temperatures of the two materials for some other reason, stresses in the ceramic
material will arise because of the above-mentioned packing of dust into the gaps,
which results in the ceramic being easily broken.
This problem, of course, is aggravated by repeated increases and
falls in temperature.
Another problem arises with the existing temperature gradient across
the inside and outside of the ceramic. Under certain conditions, the temperature
difference beween inside and the outside of the ceramic is very great, which causes
cracking of the ceramic material. One reason for these temperature differences
arising is that flue gas is not allowed to sweep around the back of the ceramic
Nor are the currently used ceramics for the above-mentioned technique
sufficiently erosion-resistant. More erosion-resistant ceramics are available but
require a different application.
Another variant of the solution to the problem of two materials in
a lining expanding to differing degrees during heating is taken from an example
with a ceramic lining of a steel casing. Ceramic plates are provided with cast-in
steel holders. These holders are welded to the steel casing such that a certain
gap arises between the steel casing and the ceramic. The space formed by this gap
is filled with insulation. In this way, the ceramic and the steel casing may be
arranged to be maintained at different temperatures. The two sides of the ceramic
assume, for example, the temperature 850°C whereas the steel casing is allowed
a maximum temperature of, for example, 350°C. By controlling the temperature which
is adopted by the respective material, it will be possible to impart to each material
the same linear expansion. This causes the two materials to be expanded to the
same degree, so there will be no mutual displacements between the two materials.
However, also this solution has its disadvantages, since what is stated above only
applies to the steady state. Under heating or cooling conditions, stresses may
arise or a dust-accumulating growth of gaps may arise in the ceramic.
A specification of requirements may be drawn up for a lining, the
aim of which is to reach a solution to the problems described above. It is, for
example, desirable to have large, smooth, continuous surfaces while avoiding joints
in the ceramic lining. Another desire is to have as few contact points as possible
between the lining and the casing. In addition, it is advantageous to provide a
gap between the lining and the casing. In this way, in the example using cyclones,
a small amount of gas may sweep over the back of the lining, causing the back to
adopt the same temperature as the front to avoid temperature gradients over the
lining material. By means of such a gap, dust penetrating in between the lining
and the casing may also be allowed exit out of the gap.
Aimed at a solution to the above-mentioned problems, a new design
relating to a ceramic lining has been developed. However, the principle of the
solution is of such a general nature that it may be applied to a plurality of technical
SUMMARY OF THE INVENTION
Briefly the invention may be described as a lining of one element
which constitutes an outer casing by means of another element which constitues
an inner lining for the casing, the materials in the two elements having different
coefficients of linear expansion. The lining is characterized in that the linear
expansion of both materials is adapted to emanate from one single common fixed
point and that all contact points between the two elements are positioned on imaginary
cones or polygonal pyramids, the tips of the cones or the pyramids coinciding with
the fixed point, such that the contact points between the two elements are located
on the generatrices of the cone, and under relative linear displacements, caused
by temperature changes in any of the elements, the contact points have been given
a possibility of free movement along the generatrices through the respective contact
During heating of a body, if the body is homogeneous and is uniformly
heated, each point of the body moves under linear displacements, caused by temperature
fluctuations of the body, along a ray which runs from an optional fixed point
inside the body. That is to say, if each point in the body is viewed from this
fixed point, it seems as if, for example during thermal expansion of the body,
each point belonging to the body moves away rectilinearly outwards from the fixed
point. The corresponding situation, of course, applies to a contraction of the
body, when the points move inwardly towards the fixed point. The displacement of
the points of the body in the longitudinal direction is in direct proportion to
the distance of the respective point from the fixed point. From what has been stated
follows that a homogeneous body during an expansion or a contraction retains the
same shape and proportions. What is of interest in this connection is what happens
when a body is fixed at a point on the surface of or inside the body. When the
body is fixed in such a way, each section through the body with a changed volume
will be a uniformity reproduction of the corresponding section through the body
before the change in volume with the fixed point in a corresponding uniform position.
The known physical principles described above may be utilized to
solve problems with linings. As mentioned, it is desirable, for example, to have
larger sections of ceramics as lining according to the example above. According
to this invention, it is possible to form a ceramic lining in one single unit
or in large sections, where the lining is made such that all external surfaces,
which are to function as supporting points, which in some way are to contact the
casing to be lined, are located on the envelope surface of an imaginary cone.
This cone then constitutes a limiting surface for these supporting points. In the
same way the interior of the casing is formed, such that this exhibits supporting
points for the lining, all of these supporting points also being located on the
envelope surface of an imaginary cone. These imaginary cones for the supporting
points for the lining as well as for the supporting points for the casing are
allowed to coincide, that is, they form one and the same imaginary cone with a
common tip. In addition, both the lining and the casing are fixed to the tip of
this imaginary cone. In case of relative thermal changes in volume between the
lining and the casing, these supporting points will then slide in relation to each
other along the envelope surface on the imaginary cone. To be more precise, each
individual supporting point slides along a generatrix to the cone. The only point
which, in principle, is not subjected to any move because of thermal movements
of the materials is the fixed point which is common to the respective element.
The described principle for designing a lining may be applied to
all technical fields where thermal movements of materials, which are connected
to each other in some way, occur. It may, for example, apply to the lining of tubes
with erosion-resistant materials at exposed bends, end stops and the like.
What has been stated above may, of course, be reversed such that
a lining is placed outside an element enclosed by the lining according to the same
principle. In addition, the invention is not limited to two materials. The same
principle may be used even if more than two materials are to coat one another.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a section through a ceramic lining of a steel shell
intended to be used in a cyclone for cleaning flue gases. The lining is divided
into several sections.
Figure 2 illustrates a section through a part in the supporting device
for the ceramic lining according to Figure 1.
Figure 3 illustrates a cross section through a supporting yoke according
to section A-A in Figure 2.
Figure 4 shows the embodiment of the connection between the upper
part of the lining and the shell.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the accompanying figures, a number of preferred
embodiments of the present invention will be described below. As already mentioned,
the principle of a lining according to the invention is applicable to different
technical fields. In the following only examples of ceramic linings will be described,
but the technical principle employed can be easily transferred to closely related
technique for solving the lining problems.
A lining of a cyclone leg in a flue gas plant is shown in Figure
1. A ceramic lining 1 is enclosed in a steel shell 2. The steel shell constitutes
a cyclone leg and/or the lower conical part of a cyclone leg. Since it is subjected
to high temperature, the steel shell is normally lined around 850°C because of
the hot, flowing gas vortex. In addition, the gas in the vortex contains greatly
erosive particles. Without a lining, the material in the steel shell would be
rapidly worn and deformed.
The ceramic in the lining may be made in one piece or, as in the
preferred embodiment, be made in several sections 1a, 1b, 1c, which are stacked
on top of each other. Each section 1a, 1b, 1c is made in the form of a tube piece
with an internally cylindrical or conical envelope surface. Externally, the respective
tube section may be given an optional shape within the scope of the available space
The end surfaces of the tube sections are given a good planeness,
allowing the tube sections to be stacked one above the other and providing good
sealing between the joints. The tube sections rest freely on one another and each
individual tube section is only influenced by forces of gravity from tube sections
positioned above, which is the intention of this construction since ceramics has
a greater capacity to withstand compressive forces than, for example, tensile
forces. The length of each section may suitably be in the interval 700 mm to 900
mm but has no other significance than to contribute to a practically manageable
solution of the problems of installation and adjustment.
The ceramic tube sections 1a, 1b, 1c need not be manufactured on
the spot and can therefore be made in any optional material. It is fully possible
to choose the most erosion-resistant and high-strength materials. As examples
of such materials may be mentioned silicon carbide, SiC, or silicon nitride, SiN.
Each ceramic tube section is externally provided with at least two
annular conical sections 3a - 3e, so that the limiting surface of each such conical
section forms a frustum of a cone. These conical sections are chosen such that
all generatrices 4a-4e along the envelope surface of each such frustum of a cone,
which forms the conical sections of the tube sections, intersect at one and the
same point, called fixed point 5. The conical sections are suitably located at
the ends of the respective tube sections, with the exception of the tube end where
the ceramic lining is supported, where the embodiment of the end of the corresponding
tube section will be described separately below. If the ceramic lining is made
as one coherent section, an external conical section 3e may, in principle, be sufficient,
but the number may be freely chosen according to the circumstances.
On a level with each of the above-mentioned conical section 3a-3e,
the surrounding steel shell 2 exhibits a number of radial supports 6a-6e. For each
separate level these radial supports are arranged in a ring around the steel shell,
such a ring of radial supports serving as side supports for the ceramic lining
in that each conical section on the lining in principle makes contact with an associated
ring of radial supports. The radial supports 6a may be angled such that the longitudinal
axis 7a through the respective radial support is perpendicular to the nearest generatrix
4a of the associated conical section of the lining, as shown in the figures. Since
such an embodiment requires greater effort during manufacturing, it is more suitable
that only the contact surfaces on the supports 6a-6e are arranged to be parallel
to a tangential plane through the generatrix of the associated conical section
of the lining. In this way, the necessity of an accurate alignment of the longitudinal
axes of the radial supports is dispensed with.
The lowermost one 1a of the tube sections of the ceramic lining is
formed with a collar 8, at a certain distance above the lowermost tube end, with
a larger outside diameter than the outside diameter of this tube section below
the collar. This collar has a substantially horizontal lower surface.
In the lower part of the steel shell, a certain distance below the
level of the collar 8, the steel shell changes into a conical/cylindrical section
with a smaller outside diameter. At this transition a substantially horizontal
shelf 9 is formed in the steel shell (see Figure 2). On this shelf there rests
a circular support ring 10 with an appearance resembling that of a ring-half for
a radial bearing. The support ring 10 has a surrounding circular and cupped surface
11 directed upwardly and inwardly and towards the centre of the ring.
To support the ceramic lining 1, a number of supporting elements
(hereinafter called flexible supports) 12 are arranged in a ring between the ceramic
lining 1 and the steel shell 2. Each flexible support 12 rests with its lower
end in the cupped surface 11 of the support ring 10 and with its upper end against
a rounded part of the ceramic lining which is formed in the angle between the lowermost
outer neck of the ceramic lining and the collar 8. By this mounting, the flexible
supports 12 will carry the weight of the ceramic lining 1 and transfer this weight
to the support ring 10 and further to the shelf 9 on the steel shell 2.
The purpose of the loose flexible supports 12 is to bring about an
accurate centering of the ceramic lining 1 in the steel shell 2, so that the symmetry
axes for the lining 1 and the steel shell 2 coincide and extend through the above-mentioned
fixed point 5. If, for example, the steel shell 2 is expanded to a major extent
in relation to the lining 1, the lower supporting legs on the respective flexible
supports will be displaced horizontally outwards from the centre, the flexible
supports 12 then adopting a somewhat more inwardly sloping position than earlier.
In this way, the lining is lowered to some small extent in relation to the steel
shell, but the lining is still maintained centered by the uniform influence from
all the flexible supports.
For centering the lining and a uniform adoption of pressure from
the collar 8 of the lining, the vertical and horizontal positions of the support
ring 10 are adjusted by means of both horizontal adjusting screws 13 through the
wall of the steel shell and vertical adjusting screws 14 through the shelf 9.
The flexible supports 12 are formed as yokes with the yoke legs downwardly
rounded and resting in the cupped surface 11. The flexible supports are rounded
also at the top to conform well to the cupped shape below the collar of the lining.
The flexible supports 12 are placed close to each other so that each yoke leg on
a flexible support in the lateral direction supports against an adjacent flexible
support in the ring of such supports.
The main principle according to the invention is that the two different
materials in the ceramic and in the steel casing, respectively, should have one
single common point, from where all linear expansion or contraction caused by
changes in temperature emanates for the different materials. In the present invention,
the lining and the steel casing are adapted to include such a point, which is the
only point which is common and coinciding under relative temperature displacements
between the two materials. This point is the same as the above-mentioned fixed
point 5. In this case the fixed point 5 has been located at the point of intersection
between a plane 15 through the lower side of the ceramic collar 8 and the common
axis of rotation for the steel shell 2 and the ceramic lining 1. Under temperature
movements the two materials are allowed to be displaced from this common fixed
point, which means that the plane 15 through the collar of the lining cannot be
allowed to become displaced in the vertical direction. If the steel shell expands
to a larger extent than the lining, the flexible supports 12 will adopt a more
inwardly inclined position, causing the lining to be somewhat lowered. However,
this vertical lowering of the lining is only of marginal importance and is negligible.
In the case of a ceramic lining, the flexible supports 12, which are made of steel,
also have a greater linear expansion than the ceramic, vertical relative movements
thus cancelling each other out.
In the case of relative changes of volume because of temperature
fluctuations, the two materials, the steel shell and the lining, will be able to
move in relation to each other since the conical sections 3a-3e are able to slid
along the radial supports 6a-6e along the above-mentioned generatrices 4a-4e,
which converge at the fixed point 5. This is independent of which of the two materials
expands or contracts. The lining material will not be subjected to any stresses.
This is true provided that the two materials are homogeneous and not influenced
by any residual stresses.
Between the lining 1 and the shell 2 a natural gap 16 arises, which
allows a small amount of gas to flow along this space via the gap 18. In this way,
the outer side of the lining will be subjected to the same temperature as its
inner side. Also this fact contributes to reduce the risk of temperature gradients
arising in the lining material and the risk of stresses therein.
Since the gas also contains dust, there is a risk that this dust
clogs the space in the gap 16. At the lower end of the lining a gap 17 is arranged,
through which gas and dust are able to flow out. At the upper part of the gap 16,
the gas pressure is higher than at the lower part, causing dust to be blown out
of the space between the lining and the shell through the lower gap 17. The width
of this gap 17 may be chosen freely by making the downwardly projecting throat
below the collar 8 on the lining longer or shorter.
The flexible supports 12 have been deliberately designed as yokes,
thus forming openings, in this case between the legs of the yokes and between the
flexible supports. This allows gas and dust in the gap 16 to traverse the ring
of flexible supports and flow out through the lower gap 17.
In the upper part of the lining, a gap 18 is arranged. As indicated
in Figure 4, the steel shell 2 can be made with an inner collar which hangs down
over the end of the lining as a lip 2a, between which lip and the uppermost lining
end the upper gap 18 is formed. Since both the inside of the end of the lining
tube and the outside of the overhanging lip 2a towards the lining end are angled
in the direction of the fixed point 5, the gap 18 maintains the same width also
in case of temperature movements of the two materials. The width of this gap 18
may be freely chosen.
In an alternative embodiment of the ceramic according to the above
embodiment, it is possible to utilize two separate ceramics in the lining. For
example, a more inexpensive material with lower erosion resistance but with higher
strength in case of a large volume of the body may be used as an external frame
in a lining according to the above. The inside of the frame is then coated with
smaller plates of a different ceramic with the desired properties, such as erosion
resistance. Plates with, for example, silicon carbide may then internally coat
a frame of aluminium oxide.