Flexible joint

A flexible joint is disclosed which comprises a first rigid wall portion, a second rigid wall portion, a flexible wall portion extending between the first and second rigid wall portions, the three wall portions defining a longitudinal passage through the joint and being arranged along the length of the passage, a sleeve secured at one end to the first rigid wall portion and extending through the passage so as to overlap the second rigid wall portion, the sleeve being spaced from the flexible wall portion and free to move relative to the second rigid wall portion. In a preferred embodiment, a layer of insulating material is provided on a surface of the sleeve while leaving the space between the sleeve and the flexible wall portion to be occupied by process fluid when in use. In another embodiment, the length of the portion of the sleeve overlapping the second rigid wall portion is at least five times the distance of the sleeve from the second rigid wall portion, as measured perpendicular to the sleeve.

FIELD OF THE INVENTION 
The present invention relates to a flexible joint for use in conduits and 
vessels through which a fluid flows. In particular, the present invention 
relates to a flexible joint for use under aggressive conditions, such as 
high temperatures and the handling of fluids of a corrosive or abrasive 
nature. 
The need for flexible joints in conduits and vessels arises frequently in 
the design and construction of chemical process plants. In many 
circumstances, conventional vessel and pipe constructions are too rigid 
for the prevailing process operating conditions and operation of such a 
rigid construction would result in unacceptably rapid deterioration and 
failure of the process equipment. Accordingly, there is a need for a 
flexible joint which may be incorporated in the wall of a vessel or pipe 
which combines the necessary degree of flexibility with the required 
integrity to withstand the process operating conditions. Designs of 
flexible joints have been available for many years. Reference is made to 
Chemical Engineers' Handbook, Fifth Edition, Edited by R. H. Perry and C. 
H. Chilton at page 6-48 for a general discussion of flexible joints for 
use in piping constructions. A typical flexible joint for use in a vessel, 
pipe or the like comprises a first and second rigid wall portion connected 
by a flexible wall portion. The three wall portions define a longitudinal 
passage through which a process fluid can flow and are arranged along the 
length of the passage. 
The flexible wall portion is typically in the form of an undulant wall, 
often referred to in the art as "bellows". 
More recently, as chemical processes advance, it has been found necessary 
to provide flexible joints which are able to withstand ever harsher 
environments, such as exposure to fluids at high temperatures, corrosive 
fluids and abrasive fluids. One example of a particularly harsh process 
environment may be found in the fluid catalytic cracking of hydrocarbons, 
in which it is necessary to contain and transport fluids containing 
hydrocarbon vapors and entrained catalyst particles at high temperatures. 
Typical process operating temperatures are of the order of 550.degree. C. 
in the reaction stage and 750.degree. C. in the regeneration stage. It 
will be readily appreciated that such high operating temperatures and the 
presence of an abrasive component, such as the cracking catalyst give rise 
to a very aggressive environment for the flexible portion of the flexible 
joint. 
DESCRIPTION OF THE INVENTION 
In order to prevent premature failure of the flexible portion of the joint, 
it has been found necessary to construct the joint from suitably resistant 
materials, for example alloys of steel, and protect the flexible portion 
from direct exposure to the process fluid. Accordingly, a variety of 
designs of flexible joints have been proposed for use in such harsh 
environments. A first design incorporates a protective sleeve disposed 
between the flexible portion of the joint and the process fluid, thereby 
preventing the process fluid from directly contacting the flexible 
portion. The protective sleeve is fixed at one end to the first rigid wall 
portion. The second end portion of the sleeve extends into the region 
defined by the second rigid wall portion and overlaps the second rigid 
wall portion, but is left free to allow for relative movement between the 
rigid wall portions. The sleeve is spaced from the flexible portion so as 
to form a cavity between the sleeve and the flexible portion. The cavity 
is open to the interior of the joint at the free end of the sleeve and, in 
operation, is filled with process fluid. This provides, in use, a degree 
of thermal insulation of the flexible portion from the high temperature of 
the flowing process. This design has the advantage of being simple to 
construct, install and maintain. However, the design does not always 
provide adequate insulation of the flexible portion from the high 
temperatures prevailing in many processes, such as the fluid catalytic 
cracking process mentioned above. 
In a modification of this first design, one or more layers of insulating 
material are provided on the surface of the sleeve in contact with the 
process fluid and in the cavity between the sleeve and the flexible 
portion of the joint. A further modification is to provide a seal, 
typically a braided hose, between the free end of the sleeve and the 
second rigid wall portion in order to retain such insulating material in 
place. The seal also serves to limit the ingress of process fluid and 
debris into the cavity. These modifications provide improved thermal 
insulation for the flexible portion of the joint. However, they are more 
complicated to construct and are more difficult to maintain. In addition, 
it has been found that the seals deteriorate due to corrosion, 
embrittlement and/or relative movement between the sleeve and the second 
rigid wall portion, leading to an eventual failure of the seal. Debris 
from the seal may then be carried by the process fluid into other parts of 
the plant, causing blockages and damage to plant equipment. Designs have 
been proposed which comprise external insulation and electrical heating 
elements as a means of regulating the temperature of the flexible portion 
of the joint. These designs also suffer the disadvantage of being 
complicated to construct and maintain. 
Accordingly, there is a need for a flexible joint which combines both 
simplicity of construction and ease of maintenance, while at the same time 
providing the necessary protection for the flexible portion of the joint 
against the high process fluid temperature being handled. In particular, 
there is a need for a flexible joint which allows the precise degree of 
insulation of the flexible wall portion to be controlled. This is 
important in such applications as the fluid catalytic cracking process 
mentioned above, where the flexible wall portion must be maintained at a 
temperature significantly below the temperature of the process fluid, 
while being at a temperature above the dew point of the hydrocarbon vapors 
being handled. Too high a level of insulation will result in the 
temperature of the flexible wall portion falling below the dew point of 
the process fluid, causing condensation to occur on the inside of the 
flexible joint. This may in turn lead to corrosion and premature failure 
of the flexible wall portion. 
According to a first aspect of the present invention there is provided a 
flexible joint comprising a first rigid wall portion, a second rigid wall 
portion, a flexible wall portion extending between the first an second 
rigid wall portions, the three wall portions defining a longitudinal 
passage through the joint and being arranged along the length of the 
passage, a sleeve secured at one end to the first rigid wall portion and 
extending through the passage so as to overlap the second rigid wall 
portion, the sleeve being spaced from the flexible wall portion and free 
to move relative to the second rigid wall portion, wherein a layer of 
insulating material is provided on a surface of the sleeve while leaving 
the space between the sleeve and the flexible wall portion substantially 
free to be occupied by process fluid when in use. 
Surprisingly, it has been found that careful selection and arrangement of 
the insulating material, together with the provision of a space between 
the sleeve and the flexible wall portion, allows the desired level of 
temperature reduction to be achieved. The flexible joint of the present 
invention also obviates the need for seals to be provided between the 
sleeve and the second rigid wall portion, thus simplifying the 
construction and maintenance of the flexible joint. Inspection of the 
joint during a plant shutdown is also facilitated by the space between the 
sleeve and the flexible wall portion being left substantially empty. 
The flexible joint may be incorporated in the wall of a vessel or, more 
typically, in a conduit or pipeline. In cross-section, the wall portions 
of the flexible joint may have any suitable shape, adapted to match the 
cross section of the vessel or conduit into which the joint is to be 
installed. Most typically, the rigid wall portions will be cylindrical in 
cross-section. The rigid wall portions are typically constructed from the 
same material as the adjacent apparatus. Typical materials include mild 
steel, stainless steel and other steel alloys, with the selection 
depending upon the prevailing process conditions and the fluid being 
handled. 
The flexible wall portion is disposed between the first and second rigid 
wall portions and is intended to absorb relative movement between the two 
rigid wall portions. Such movement typically arises, for example, due to 
vibration and pressure and temperature differentials which occur in the 
process equipment. The flexible wall portion may be of any suitable form. 
Most typically, the flexible wall portion is in the form of an undulant in 
longitudinal section, more commonly referred to as "concertina" or 
"bellows". The size and form of the undulations are selected according to 
the degree of flexibility and displacement required. Techniques for the 
selection and design of the flexible wall portions are well known in the 
art. The flexible wall portion may be constructed from any suitable 
material, including those mentioned hereinbefore in connection with the 
construction of the rigid wall portions. The flexible wall portion, being 
generally thin, may be susceptible to higher levels of stress corrosion 
and embrittlement cracking than the rigid wall portions. Accordingly, more 
resistant, and hence expensive, materials may need to be selected for the 
flexible wall portion. It is, however, an advantage of the present 
invention that more commonly available materials can be employed, for 
example Inconel and Incoloy alloys. The flexible wall portion may be 
attached to the two rigid wall portions by any suitable means known in the 
art. 
The sleeve extends along the passage within the flexible joint. At one end, 
the sleeve is secured to the first rigid wall portion. Any suitable 
securing means may be employed. Welding is a most suitable means for 
securing the sleeve to the first rigid wall portion. The sleeve extends 
within the passage so as to overlap the second rigid wall portion. The 
sleeve is spaced from the flexible wall portion. The remaining end of the 
sleeve is not secured, thereby allowing the sleeve to move relative to the 
second rigid wall portion. Typically, the end portion of the sleeve 
overlapping the second rigid wall portion will be spaced from the second 
rigid wall portion. 
In a preferred arrangement, the sleeve extends from the first rigid wall 
portion in a manner providing a continuous wall surface, thereby 
presenting little or no obstacle to the flow of fluid through the flexible 
joint, that is, the sleeve and a portion of each of the first and second 
rigid wall portions combine to form a substantially smooth sided passage 
for the flow of-fluid through the joint. The required spacing of the 
sleeve from the flexible wall portion is accommodated by having the width 
of the passage defined by the flexible wall portion greater than that of 
the nominal width of the flexible joint, that is the width of the smooth 
sided passage formed by the sleeve and a portion of each of the first and 
second rigid wall portions. An alternative, less preferred arrangement 
comprises a sleeve which extends into the passage, thereby creating a 
constriction in the fluid flowpath along the joint. 
The sleeve may be formed from any suitable material which offers the 
required resistance to the process operating conditions. Typically, the 
sleeve will be of the same material as the first and second rigid wall 
portions. 
It is the intention that, in operation, process fluid is allowed to occupy 
the space between the sleeve and the flexible wall portion. Process fluid 
may enter this space between the free end of the sleeve and the second 
rigid wall portion. The process fluid in said space is relatively still 
and acts as an insulating medium for the flexible wall portion. In order 
to allow the process fluid in said space to remain still, it is preferred 
that the first rigid wall portion is at the upstream end of the flexible 
joint, when installed and in use. 
The insulating material is provided on a surface of the sleeve. The 
material may be attached to the surface of the sleeve in direct contact 
with the process fluid to be handled. Preferably, the insulating material 
is attached to the surface of the sleeve facing the flexible wall portion. 
Insulating material may be applied to both surfaces, if desired. The 
insulating material may consist of one layer of material or may comprise a 
plurality of layers. If a plurality of layers is employed, the layers may 
be of the same or different material. The selection of the number and 
position of the layers and the type of insulating material will depend 
upon the process operating conditions to be experienced by the flexible 
joint, the material from which the flexible wall portion is constructed 
and the nature of the process fluid being handled. The insulating material 
should have a low thermal conductivity at high operating temperatures, 
thereby allowing a relatively thin layer of the material to provide the 
required level of insulation. The material should preferably have as low 
an ash, sulphur and chloride content as possible. The material should be 
suitable for use under the prevailing process conditions, for example, 
conditions of high temperature. The insulating material is preferably a 
fibrous ceramic material. Suitable ceramic materials are fibers of silica, 
alumina, zirconia, magnesia, calcia and mixtures thereof. Suitable 
materials are available commercially, for example under the trade names 
SAFFILL and ZIRCAR. Alternative, less preferred materials for use as the 
insulating material include graphite and ash paper. Again, such materials 
are available commercially. 
The layer of insulating material preferably covers substantially all of the 
surface of the sleeve onto which it is mounted, thereby avoiding the 
formation of hot zones which in turn reduce the effectiveness of the 
insulation. The layer of insulating material may be secured to the sleeve 
by any suitable means known in the art. 
In a preferred embodiment of the present invention, the layer of insulating 
material is covered by a protective sleeve. The sleeve preferably covers 
substantially all of the insulating material, such that a sandwich of 
sleeves and insulating material is formed. In a further embodiment, the 
protective sleeve is not secured to the layer of insulating material, 
thereby allowing the protective sleeve to move relative to the insulating 
material. This allows for relative movement between the components 
arising, for example, from the effects of differential thermal expansion. 
Where appropriate, the flexible joint is provided with a layer of 
erosion-resistant refractory oxide covering the surface of the sleeve 
adjacent the process fluid when the joint is in use. Suitable refractory 
materials are available commercially and are well known in the art. 
Suitable materials include silica, alumina, titania, zirconia, calcia, 
magnesia and mixtures thereof. 
One suitable material comprises a mixture of alumina and silica and is 
available under the trade name CURAS 90 PF. The refractory oxide is 
typically applied in the form of a cement or in tiles or blocks. The layer 
of refractory oxide is relatively thick, compared with that of the layer 
of insulation material. The refractory oxide is porous, allowing some 
access of the process fluid to the underlying surface. A primary purpose 
of the refractory oxide is to provide protection against the abrasion and 
erosion of the underlying surface. If desired, the layer of refractory 
oxide may be applied over a layer of insulating material covering the 
surface of the sleeve. If no refractory oxide layer is provided, it is 
most preferred that the surface of the sleeve to be contacted directly by 
the process fluid is left bare and that the layer of insulating material 
is applied to the surface of the sleeve facing the flexible wall portion. 
In a preferred embodiment of the present invention a layer of insulating 
material is provided on the surface of the second rigid wall portion. The 
layer of insulating material, if present, should cover at least that 
portion of the surface of the second rigid wall portion overlapped by the 
sleeve. More preferably, the layer of insulating material extends over 
substantially more of the surface of the second rigid wall portion. 
Any of the materials discussed above for use as the layer of insulating 
material on the sleeve may be employed. Again, one or more layers may be 
used, the layers being comprised of the same or different material. In 
addition, it may be preferred to cover the layer of insulating material 
with a layer of refractory oxide, as discussed hereinbefore with respect 
to the refractory oxide applied to the surface of the sleeve. 
As mentioned hereinbefore, the sleeve is spaced from the flexible wall 
portion, forming a space into which process fluid is allowed to flow 
during operation. The process fluid in the space forms a reservoir of 
relatively still fluid which acts to further insulate the flexible wall 
portion. For the proper operation of this reservoir of fluid as an 
insulator, it is important that it remains relatively still. It has now 
been found that the design of the free end portion of the sleeve and its 
relationship with the second rigid wall portion plays an important role in 
the insulating properties of the process fluid in said space, in 
particular the extent by which the sleeve overlaps the second rigid wall 
portion. 
According to a second aspect of the present invention, there is provided a 
flexible joint comprising a first rigid wall portion, a second rigid wall 
portion, a flexible wall portion extending between the first and second 
rigid wall portions, the three wall portions defining a longitudinal 
passage through the joint and being arranged along the length of the 
passage, a sleeve secured at one end to the first rigid wall portion and 
extending through the passage so as to overlap the second rigid wall 
portion, the sleeve being spaced from the flexible wall portion and free 
to move relative to the second rigid wall portion leaving the space 
between the sleeve and the flexible wall portion substantially free to be 
occupied by process fluid when in use, wherein the length of the portion 
of the sleeve overlapping the second rigid wall portion is at least five 
times the distance of the sleeve from the second rigid wall portion, as 
measured perpendicular to the sleeve. 
Preferably, the length of the portion of the sleeve overlapping the second 
rigid wall portion is at least 100 mm, more preferably at least 200 mm, 
under any condition. 
It will be readily understood that the clearance between the free end of 
the sleeve and the second rigid wall portion must be sufficient to 
accommodate any relative movement between the two components when the 
joint is in use. In a preferred embodiment, the sleeve and a portion of 
both the first and second rigid wall portions combine to form a 
substantially smooth sided passage for the flow of fluid through the 
joint. In such cases, it is preferred that the distance of the free end of 
the sleeve from the second rigid wall portion, as measured along a line 
parallel to the longitudinal axis of the sleeve, is at least 10 mm, more 
preferably 25 mm under any condition. 
A most preferred flexible joint comprises the first and second aspects of 
the present invention. 
The flexible joint of the present invention may be applied in any 
application requiring a flexible joint. As mentioned hereinbefore, the 
joint is particularly suited to use in the handling of fluids under 
aggressive conditions, that is in handling fluids of a corrosive nature 
and/or fluids under conditions of high temperature. The use of refractory 
oxide materials also provides the joint with a high level of resistance to 
abrasive and erosive fluids. The joint may be applied in all manner of 
chemical and refining plants. As mentioned hereinabove, the joint is of 
particular use in processes employing fluidized solid media, such as the 
fluid catalytic cracking of hydrocarbons and the like. Other specific 
applications include the handling of gases at high temperatures, for 
example the handling of hot flue gases.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, a generally cylindrical flexible joint for use in a 
process pipeline, generally identified as 2, comprises a first rigid wall 
portion 4 having a narrow cylindrical outer end portion 6 and a wide 
cylindrical inner end portion 10 connected by a central frustoconical wall 
portion 8. A generally cylindrical flexible wall portion 12 extends from 
the inner end of the first rigid wall portion 4 to a second rigid wall 
portion 14, having inner, central and outer wall portions (16, 18, 20 
respectively) to: the first rigid wall portion 4, but in the opposite 
orientation. The first, flexible and second wall portions 4, 12, 14 
respectively are symmetrically arranged about a single longitudinal axis. 
The flexible wall portion 12 has an undulant form in cross-section commonly 
applied in the art and often referred to as "bellows". 
A cylindrical sleeve 22 is disposed co-axially within the wall portions 4, 
12, 14 and is secured at one end to the first rigid wall portion 4 in the 
region of the junction of the outer end portion 6 and the central portion 
8 by welds (not shown). The sleeve 22 extends past the flexible wall 
portion 12 and overlaps the inner end portion of the second rigid wall 
portion. A layer of insulating material 24, for example a ceramic fibre 
felt, is disposed on the outer surface of the sleeve 22, so as to lie 
between the sleeve 22 and the flexible wall portion 12. A layer of 
refractory oxide 26 is disposed on the inner surface of the sleeve 22. A 
layer of insulating material 28, for example a ceramic fibre felt, is 
disposed on the inner surface of the second rigid wall portion 14. A layer 
of refractory oxide 30 is disposed over the layer of insulating material 
28. 
It is to be understood that conventional fittings to flexible joints, such 
as the expansion limiting rods, leak detection system, weather shrouding 
and the gas purge connection have been omitted for the sake of clarity. 
In use, the flexible joint 2 of FIG. 1 is arranged so that process fluid 
flows in a direction from the first rigid wall portion 4 to the second 
rigid wall portion 14, 30 as indicated by the arrow. Process fluid enters 
the space remaining between the sleeve 22 and the flexible wall portion 12 
through the annulus between the sleeve 22 and the inner end portion 16 of 
the second rigid wall portion 14. 
Referring to FIG. 2, there is shown an enlarged view of the sleeve 22 and 
the insulating material 32 disposed thereon in the region adjacent the 
first rigid wall portion 4. In the embodiment of FIG. 2, the sleeve 22 has 
a layer of insulating material 32 disposed on its inner surface. A layer 
of refractory oxide 34 extends over the layer of insulating material 32. 
Referring to FIG. 3, showing a similar view to FIG. 2 of an alternative, 
particularly preferred embodiment, the sleeve 22 has a layer of insulating 
material 36 disposed on its surface facing the flexible wall portion 12. A 
cylindrical protective sleeve 38 extends over the layer of insulating 
material 36. A layer of refractory oxide 40 is disposed on the inner 
surface of the sleeve 22. The layer of insulating material 36 and the 
protective sleeve 38 should preferably extend as close as possible to the 
first rigid wall portion 4, while still allowing sufficient space to 
accommodate any relative movement between the various components. Space 
will be required, for example, in the case in which the protective sleeve 
38 is not rigidly secured to the layer of insulating material 36. 
Typically, for the embodiments shown in FIGS. 1, 2 and 3 to be adapted for 
application in a pipeline in the fluid catalytic cracking process referred 
to above, the sleeve 22 has a thickness of the order of 10 mm. The layer 
of insulating material or multiple layers thereof, if employed, has a 
total thickness of the order of 3 to 10 mm. The layer of refractory oxide 
has a thickness of the order of 25 mm. The protective cover, if present, 
has a thickness of the order of 3 to 6 mm. It will be readily understood 
that the thickness of the various components will be determined by the 
application in question. 
FIG. 4 shows a detail (not to scale) of a flexible joint according to the 
second aspect of the present invention. Identical components to those 
described above with reference to FIG. 1 have the same reference numeral 
in FIG. 4. Thus, in FIG. 4, the sleeve 22 extends so as to overlap the 
inner end portion 16 of the second rigid wall portion 14. A layer of 
insulating material 24 extends over the outer surface of the sleeve 22. 
The distance "d" in FIG. 4 is the distance between the surface of the 
sleeve assembly (in this case the surface of the insulating layer 24) and 
the inside surface of the inner end portion 16, measured perpendicular to 
the longitudinal axis of the sleeve 22. The overlap of the sleeve 22 
within the second rigid wall portion, represented by the distance l in 
FIG. 4, is at least five times the distance "d". Preferably, 1 is at least 
200 mm. The distance "a" measured from the end of the sleeve 22 to the 
second rigid wall portion 14 along a line parallel to the longitudinal 
axis of the sleeve 22, is at least 25 mm. 
EXAMPLE 
The present invention will be further described by way of the following 
illustrative example. 
The operating temperature of the flexible wall portion of a flexible joint 
according to the present invention can be calculated. A flexible joint was 
considered having the general structure shown in FIG. 1, but employing the 
arrangement of layers of insulating material, 36 protective sleeve 38 and 
layer of refractory oxide 40 shown in FIG. 3. The joint was considered as 
being installed in the catalyst stand-pipe of the regenerator of a 
conventional fluid catalytic cracking plant. The process fluid being 
handled was assumed to comprise hydrocarbon gases and entrained catalyst. 
As a worst case scenario, the space between the sleeve and the flexible 
wall portion was assumed to be filled with gas, with no catalyst being 
present. The flexible joint was assumed to have a geometry at the free end 
of the sleeve in accordance with the requirements of the second aspect of 
the present invention, as shown in FIG. 4. Details of the construction of 
the joint, the operating temperature and the resulting temperature of the 
flexible wall portion of the joint are shown in the Table below. 
For comparison purposes, a similar calculation was conducted for a flexible 
joint of identical structure, but with the omission of the insulating 
materials. The details and results of this experiment are also set out in 
the Table. 
TABLE 
______________________________________ 
Example 
Comparative Example 
______________________________________ 
Sleeve Thickness (mm) 
3 10 
Ceramic Fibre Insulating Layer 
3 None 
Thickness (mm) 
Protective Sleeve Thickness (mm) 
3 None 
Weather Enclosure Yes Yes 
Maximum Process Temperature 
750 750 
(.degree. C.) 
Maximum Temperature of Flexible 
324 497 
Wall Portion (.degree. C.) 
______________________________________ 
From the data in the Table, it can clearly be seen that the flexible joint 
constructed according to the present invention would result in a 
significantly lower temperature being experienced by the flexible wall 
portion of the joint. This in turn would lead to prolonged life of the 
joint and the opportunity to employ more commonly available, and hence 
more economical, materials of construction.