U.S. patent application number 15/088840 was filed with the patent office on 2016-07-28 for corrosion-resistant bimetallic tube and its use in tube bundle equipment.
This patent application is currently assigned to SNAMPROGETTI S.p.A.. The applicant listed for this patent is SNAMPROGETTI S.p.A.. Invention is credited to Alessandro GIANAZZA, Luca MAIRANO, Giuseppe MERELLI, Domenico SANFILIPPO.
Application Number | 20160216050 15/088840 |
Document ID | / |
Family ID | 37585561 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160216050 |
Kind Code |
A1 |
GIANAZZA; Alessandro ; et
al. |
July 28, 2016 |
CORROSION-RESISTANT BIMETALLIC TUBE AND ITS USE IN TUBE BUNDLE
EQUIPMENT
Abstract
A bimetallic tube consisting of at least one tubular element in
a first metal resistant to the corrosive and/or erosive action of a
process fluid with which it is put in contact, having at least one
end, or an area close to an end, externally coated with a layer of
a second metal, different from the first and more suitable, with
respect to this, for being seal-welded to a support. Tube bundle
equipment to be used for thermal exchange operations at high
temperatures and pressures, under conditions of high aggressiveness
of the process fluids, wherein the tube bundle comprises at least
one tube having the above characteristics. Said equipment is
particularly used as a heat exchanger and decomposer, for example
as a stripper, in the cycle of urea synthesis processes where there
are conditions of high pressure, high temperatures, high
aggressiveness of the process fluids, and in which the tube bundle
consists of at least one tube having the above characteristics.
Inventors: |
GIANAZZA; Alessandro;
(Legnano (MI), IT) ; MAIRANO; Luca; (Milan,
IT) ; MERELLI; Giuseppe; (Vertova (BI), IT) ;
SANFILIPPO; Domenico; (Paullo (MI), IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SNAMPROGETTI S.p.A. |
San Donato Milanese (MI) |
|
IT |
|
|
Assignee: |
SNAMPROGETTI S.p.A.
San Donato Milanese (MI)
IT
|
Family ID: |
37585561 |
Appl. No.: |
15/088840 |
Filed: |
April 1, 2016 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11761735 |
Jun 12, 2007 |
9341419 |
|
|
15088840 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28F 2275/062 20130101;
F28F 9/18 20130101; Y02P 80/156 20151101; F28F 21/081 20130101;
F28F 2275/068 20130101; F16L 13/0236 20130101; F28F 19/06 20130101;
Y10T 29/49718 20150115; B23P 15/26 20130101; F28F 21/084 20130101;
F28F 21/086 20130101 |
International
Class: |
F28F 19/06 20060101
F28F019/06; F16L 13/02 20060101 F16L013/02; B23P 15/26 20060101
B23P015/26; F28F 21/08 20060101 F28F021/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 2006 |
IT |
MI2006A 001223 |
Claims
1: A bimetallic tube resistant to the corrosive action of a process
fluid with which it is put in contact on its internal surface,
comprising a first tubular element E.sub.1 (1) facing said internal
surface, consisting of a metal M.sub.1 selected from Zr, Ta, Nb and
Al or an alloy of said metals, homogeneously extending for the
whole length of the tube, and at least a second tubular element
E.sub.2 (2), consisting of a second metal or alloy M.sub.2
different from M.sub.1, arranged circularly outside said first
tubular element, in a position close to one of its ends, for a
section of less than a third of the length of the tube itself, and
seal attached to said element E.sub.1.
2: The bimetallic tube according to claim 1, comprising two of said
tubular elements E.sub.2, of the same material or different
materials, each situated close to one of the ends of the tube.
3: The bimetallic tube according to claim 1, wherein said element
E.sub.1 consists of zirconium or an alloy with at least 60% by
weight of zirconium.
4: The bimetallic tube according to claim 1, wherein said metal
M.sub.2 of the element E.sub.2 is selected from titanium, a
titanium alloy and urea grade stainless steel.
5: The bimetallic tube according to the previous claim 4, wherein
said metal M.sub.2 is selected from titanium or a titanium
alloy.
6: The bimetallic tube according to claim 1, comprising, in
addition to said elements E.sub.1 and E.sub.2, at least a further
metallic tubular element positioned around E.sub.1 and in contact
with its outer surface.
7: The bimetallic tube according to claim 6, wherein said further
tubular element, consisting of a metal selected from titanium, a
titanium alloy or a stainless steel, is contiguous to E.sub.2 and
extends in the central section of the tube itself.
8: The bimetallic tube according to claim 7, wherein said further
tubular element consists of a metal compatible with the welding
with the metal M.sub.2 of E.sub.2.
9: The bimetallic tube according to claim 1, wherein the central
section of said element E.sub.1 has a thickness ranging from 1 to
15 mm.
10: The bimetallic tube according to claim 1, wherein each element
E.sub.2 has a length ranging from 0.2 to 20% of the total length of
the tube.
11: The bimetallic tube according to claim 1, wherein said element
E.sub.2 has a uniform thickness ranging from 1 to 15 mm.
12: A method for the production of the bimetallic tube according to
claim 1, starting from a tube E.sub.0 comprising for the whole of
its length at least one tubular element E.sub.0, consisting of said
metal M.sub.1, comprising the following steps: a) arrangement of
the outer surface of at least one of the ends of E.sub.o, for a
length sufficient for receiving a second tubular element E.sub.2,
so as to produce a suitable insertion seat, preferably having an
outer diameter of the tube smaller than the initial diameter; b)
positioning of a tubular element E.sub.2, having a length equal to
or less than a third of the length of E.sub.o, consisting of a
metal M.sub.2 different from M.sub.1, arranged as a ring around at
least a segment of said insertion seat; c) joining of the surfaces
of the metals M.sub.1 and M.sub.2, for at least a part of the
contact surface between them, so as to form a seal joint on the
whole perimeter of the bimetallic tube thus obtained.
13: The method according to claim 12, wherein, in step a), a
metallic layer having a thickness ranging from 0.1 to 2 mm is
removed from the insertion seat.
14: The method according to claim 12, wherein, in said step c), the
two metals M.sub.1 and M.sub.2 form a metallurgical bond with each
other extending over all the contact area of the surfaces of the
elements E.sub.1 and E.sub.2.
15: Use of the tube according to claim 1, in equipment for
processing corrosive fluids in an industrial plant.
16: Use according to claim 15, for the manufacturing of a tube
bundle for heat exchange inserted in said equipment.
17: Use according to claim 15, wherein said equipment forms an
ammonium carbamate stripper in the high pressure cycle of plants
for the production of urea.
18: Industrial equipment for processing a highly corrosive fluid at
high pressure, characterized in that it comprises at least one
bimetallic tube, preferably a series of bimetallic tubes, according
to claim 1, whose internal surface is put in contact with said
fluid.
19: The equipment according to claim 18, consisting of a
tube-bundle heat exchanger.
20: The equipment according to claim 19, wherein said tube bundle
comprises from 100 to 6,000 bimetallic tubes having a diameter
ranging from 10 to 100 mm.
21: The equipment according to claim 18, comprising a tube plate on
which said bimetallic tubes are welded, which is coated with at
least one metallic layer resistant to the corrosion of said process
fluid.
22: The equipment according to claim 21, wherein said metallic
coating layer consists of a metal M.sub.3 compatible with the metal
of the element E.sub.2 of said bimetallic tube.
23: The equipment according to claim 22, wherein said metallic
coating layer is seal welded with said element E.sub.2.
24: The equipment according to claim 22, wherein said coating and
said element E.sub.2 consist of a metal M.sub.2 selected from
titanium or a titanium alloy.
25: A method for repairing or improving tube-bundle chemical
equipment suitable for the treatment of a corrosive fluid, wherein
said fluid is in contact with the internal wall of the tubes
forming the tube bundle, comprising the substitution of at least
one of said tubes with a bimetallic tube according to claim 1.
26: The method according to claim 25, wherein said equipment
comprises a tube plate coated with a metal selected from titanium,
a titanium alloy or stainless steel.
27: The method according to claim 25, wherein said equipment forms
a stripper in the high pressure cycle in a urea synthesis
process.
28: The method according to claim 25, comprising the removal of at
least one of the pre-existing tubes, cleaning of the cavity thus
formed in the tube plate, insertion of a bimetallic tube according
to the present invention, having a suitable length, in each cavity,
positioning the mouth of each tube that it protrudes for a short
distance, usually from 3 to 50 mm, and welding the coating of the
tube plate with the outer surface of the element E.sub.2 of each
tube.
Description
[0001] The present invention relates to a corrosion-resistant
bimetallic tube and its use for the production of tube bundle
equipment.
[0002] More specifically, the present invention relates to a
bimetallic tube consisting of a metal resistant to the corrosive
and/or erosive action of a process fluid with which it is put in
contact, structured so as to be conveniently inserted in tube
bundle equipment for thermal exchange operations under conditions
of high pressure and high erosive and/or corrosive aggressiveness
conditions.
[0003] The construction technique of high pressure equipment,
whether it consists of decomposers, reactors, separators, boilers,
and other apparatuses in which a thermal exchange is effected,
normally comprises the assembly of a compact reinforcement body
capable of tolerating the operating pressures, guaranteeing the
maximum safety and duration with time of the mechanical
specifications, equipped with the necessary passages for the inlet
and outlet of the process fluids.
[0004] The most widely used material for the construction of the
reinforcement body is carbon steel, due to its excellent
combination of optimum mechanical properties, its relatively low
cost and commercial availability. In order to maximize the exchange
surface, a tube bundle is normally inserted inside the
reinforcement body, terminating, at each end, with a plate or
perforated drum facing a collection or fluid distribution chamber.
The thermal exchange by means of a tube bundle is effected with a
second fluid at a different temperature, generally with reduced
aggressiveness characteristics, circulating in the other side of
the exchanger, directly in contact with the outer surface of the
tubes.
[0005] In chemical processes which treat highly aggressive fluids,
at least one of the two surfaces of each tube and tube plate and at
least a part of the internal surface of the reinforcement body are
exposed to the direct contact with said fluids and their surfaces
must therefore be adequately coated by a protective layer
consisting of a suitable metal and/or metal alloy.
[0006] Some of the methods and equipment generally used for
effecting thermal exchange in these cases are described, among
other things, in the technical publication "Perry's Chemical
Engineering Handbook", McGraw-Hill Book Co., 6.sup.th Ed. (1984),
pages 11-18. A typical example of this equipment is represented by
the stripper inserted in the high pressure cycle of urea synthesis
processes.
[0007] The problem of corrosion and/or erosion has been faced with
various solutions in existing industrial plants, and others have
been proposed in literature. There are in fact numerous metals and
alloys capable of resisting for sufficiently long periods the
extremely aggressive conditions produced inside equipment in
processes involving highly corrosive fluids, such as for example in
the synthesis of nitric acid and urea. Among these metals, lead,
titanium, zirconium, tantalum, niobium and their alloys in various
grades, can be mentioned, together with numerous stainless steels,
such as for example, austenitic stainless steel (AISI 316L urea
grade), stainless steel of the type 25/22/2 Cr/Ni/Mo,
austenitic-ferritic stainless steels. [0008] In spite of their
higher cost with respect to stainless steels, metals such as
titanium and zirconium, due to their high corrosion resistance and
satisfactory mechanical properties, are frequently preferred for
the production of tubes in high pressure thermal exchange equipment
used in the synthesis of urea and nitric acid. Zirconium, in
particular, is known for its excellent resistance to both chemical
corrosion and the erosive action of process fluids with which it
comes into contact, whereas titanium has a corrosion resistance
substantially similar to that of zirconium, but a lower resistance
to erosive action.
[0009] One of the problems which most frequently arise in the known
art, when special materials are used in the designing and
manufacturing of heat exchangers of the type described above,
consists in the design and production of long-lasting sealing
joints between the various surfaces exposed to corrosive action. It
is well known in fact that weldings always represent preferential
attack points for corrosive fluids, as the crystalline structure of
the metal in the joining areas has a higher number of
imperfections. The joining between different metals is also
extremely problematical, as diffusion areas of one metal in another
can be easily formed, together with instabilities due to the
different chemical power, incompatibility in forming an alloy (such
as, for example, between titanium or zirconium, on the one hand,
and stainless steel or carbon on the other).
[0010] In the particular case of a tube bundle exchanger, such as
for example, the stripper included in the high pressure (loop)
cycle of the synthesis of urea, the solution to problems of
corrosion is extremely complex due to the particular geometry of
the equipment aimed at allowing distribution with maximum control
and reproducibility of the temperatures and compositions of the
fluids, especially when the thermal exchange is associated with
chemical reactions. Also in this case, relatively successful
attempts have been made to prevent corrosion with suitable coatings
of the surface of the tube plate and other surfaces of the
reinforcement body in contact with the corrosive fluids, but
without succeeding in producing at reasonable costs equipment which
is capable of enabling a further extension of the operating times
without repair interventions.
[0011] U.S. Pat. No. 4,899,813 (assigned to the Applicant)
describes the construction and use of vertical tube bundle
equipment especially suitable for the high pressure stripping
operation of the urea solution coming from the synthesis reactor.
In order to prevent corrosion in the internal area of the tubes,
where the thermal exchange and decomposition of the carbamate take
place and where the chemical and erosive aggressiveness of the
fluid is therefore at its maximum, a tube bundle is used,
consisting of bimetallic tubes, i.e. consisting of an external part
made of stainless steel, and an internal part, having a finer
thickness (0.7-0.9 mm) made of zirconium, which mechanically
adheres to the former but is not welded to it. The remaining part
of the exchanger/stripper in contact with the urea solution, on the
other hand, is constructed with the reinforcement body by means of
the normal carbon steel technique, internally coated with a
suitable stainless steel. Problems linked to corrosion and erosion
inside the tubes are therefore solved, due to the excellent
resistance of zirconium, without however running into difficulties
associated with the formation of special steel/zirconium joints
which cannot be efficiently welded directly to each other, and at
the same time maintaining the production of the equipment
economically reasonable.
[0012] In spite of the excellent results obtained by applying this
latter technology, it has been found however that in certain areas
of the exchanger, especially concentrated around the lower tube
plate of the stripper and in the corresponding chamber,
unpredictable corrosion phenomena still occur under extremely
aggressive conditions of the fluids. The same problem can also
arise with time in other tube bundle equipment operating under
comparable aggressive conditions.
[0013] In tube bundle exchangers operating under extreme
conditions, it has also been proposed to use tubes integrally made
of a high performance metal, such as zirconium, niobium or
tantalum, whereas the other surfaces of the exchanger, i.e. the
surface of the tube plate and walls of the fluid collection and
distribution areas, exposed to less aggressive conditions, can be
produced with a coating consisting of a different more convenient
and/or available material, such as titanium or stainless steel, but
with lower performances in terms of resistance to corrosion and/or
erosion. An analogous situation can be found in the case of
maintenance or repair of pre-existing exchangers, in which the
corroded or degraded tubes are substituted with new tubes made of
more resistant materials than that used originally, preserving, on
the other hand, the remaining surfaces of the equipment which are
less degraded, in the same original material. In the latter case,
the necessity of forming a long-lasting connection between
different metals is even more pressing as it is practically
impossible to intervene on the structure of the pre-existing plate
with a new coating, due to the processing difficulties deriving
from the large number of tubes per surface unit.
[0014] In both of the above cases, however, there is still the
problem of the sealing connection of the tubes with the protective
coating of the tube plate. The small maneuvering space available in
the assembly of the tubes onto the plate, where they are positioned
at a short distance from each other, complicates the use of special
joining techniques, such as cold welding, solid welding, or by
explosion or co-extrusion, often necessary for seal joining metals
which are not compatible with traditional welding.
[0015] Patent application EP 1577632 describes a tube bundle
exchanger suitable for the treatment of ammonium carbamate in
plants for the synthesis of urea, in which the bundle consists of
titanium tubes coated with a thin layer of zirconium on the side in
contact with the corrosive fluid, and seal fixed on the tube plate
by means of titanium-titanium welding. The zirconium layer is not
necessarily extended for the whole length of the tubes, but can be
arranged in the area of the tube subject to the most intense
aggressive attack. Methods for obtaining said tubes can comprise
hot welding or forging, to favour the formation of a metallurgic
bond between the zirconium layer and the surface of the titanium.
The solution proposed in this patent application however is not
entirely satisfactory as far as the mechanical characteristics of
the titanium tubes are concerned, which require greater
thicknesses, thus reducing the efficiency of the thermal exchange.
The problem becomes even more serious due to the fact that titanium
has a lower thermal conductivity with respect to that of
zirconium.
[0016] Patent application US 2006027628 proposes a different
solution to this problem by the production of a tube bundle with
tubes comprising an intermediate tubular metallic element,
essentially consisting of an anti-corrosion high performance metal
which is welded in the solid state to one or both ends, with a
second double-layered coaxial tubular element in which one layer is
of the same kind of metal as the intermediate element, and the
other layer is suitable for welding with the metal of the plate
coating.
[0017] No completely satisfactory answer is provided, however, for
the request for pressure equipment comprising tubes in contact with
extremely corrosive fluids, especially tube bundle equipment used
in the urea synthesis cycle, having an excellent combination of
high durability, design and construction simplicity, with a
consequent cost reduction and respect for the most pressing safety
criteria. Furthermore, some of the construction problems connected
with maintenance, restoration and improvement interventions of
existing tube equipment designed for high performances still remain
partially unsolved.
[0018] During its continuous activity for improving its own
technology, the Applicant has now found that the above demands and
problems associated therewith are adequately satisfied, especially
in relation to tube bundle equipment with tubes comprising an
anticorrosive material different form stainless steel, adopting a
particular type of tube configuration.
[0019] A first object of the present invention therefore relates to
a bimetallic tube comprising a first tubular element E.sub.1,
consisting of a metal M.sub.1 selected from Zr, Ta, Nb and Al or an
alloy of said metals, suitable for resisting the aggressive action
of a process fluid in contact with its internal surface,
homogeneously extending for the whole of its length, and at least a
second tubular element E.sub.2, consisting of a second metal or
alloy M.sub.2 different from M.sub.1, circularly arranged outside
said first tubular element, in a position close to one of its ends,
for a section less than a third of the length of the tube itself,
and seal fixed with said element E.sub.1.
[0020] A second object of the present invention relates to a method
for the manufacturing of the above bimetallic metal starting from a
tube comprising for the whole of its length at least one tubular
element E.sub.0 consisting of said metal M.sub.1, comprising the
following steps: [0021] a) arrangement of the outer surface of at
least one of the ends of E.sub.0, for a length sufficient for
receiving a second tubular element E.sub.2, so as to produce a
suitable insertion seat, preferably having an outer diameter of the
tube smaller than the initial diameter; [0022] b) positioning of a
tubular element E.sub.2, having a length equal to or less than a
third of the length of E.sub.0, consisting of a metal M.sub.2
different from M.sub.1, arranged as a ring around at least a
segment of said insertion seat; [0023] c) joining of the surfaces
of the metals M.sub.1 and M.sub.2, for at least a part of the
contact surface between said tubular elements E.sub.0 and E.sub.2,
so as to form a seal joint, preferably forced, on the whole
perimeter of the bimetallic tube thus obtained.
[0024] A further aspect of the present invention relates to tube
bundle equipment suitable for efficiently effecting thermal
exchange, under high pressure and temperature conditions, between
at least two fluids, one of which, having characteristics of high
aggressiveness under the process conditions, is in contact with the
internal walls of the tubes of the bundle, comprising a hollow body
or reinforcement body, suitable for tolerating the operating
pressures and consisting of a material subject to corrosion by
contact with said highly aggressive fluid, in whose central area a
tube bundle is fixed, supported at the ends by two tube plates
hinged onto the reinforcement body and coated with a metal M.sub.3
resistant to corrosion on the surface in contact with said
corrosive fluid, characterized in that said tube bundle comprises
at least one bimetallic tube according to the present invention
inserted in the tube plate so that at least one of the ends
comprises a seal welding between the metal M.sub.2 of said tubular
element E.sub.2 and said metal M.sub.3 of the coating of the tube
plate.
[0025] Yet another object of the present invention relates to a
method for the manufacturing of said equipment and the application
of the method itself in the variant for effecting the restructuring
or repair of pre-existing equipment with the introduction of said
bimetallic tube.
[0026] Other objects of the present invention will appear evident
for experts in the field in the following present description.
[0027] The term "alloy" as used herein with reference to a certain
metal, refers to a metallic composition comprising said metal in a
quantity of at least 40% by weight.
[0028] The term "corrosion" and "corrosiveness" as used in the
present description and claims with reference to the action of a
process fluid in contact with a surface of a certain metal or
alloy, in intended in the general meaning of removal or
modification of the properties of the material forming the surface
and comprises both the corrosion action deriving from a chemical
attack of the surface and also the erosive action deriving from a
physical removal process due to impact forces, friction and
cutting.
[0029] In accordance with the present description, the term
"corrosion resistant" referring to a material with respect to a
fluid under certain process conditions, defines a material which
has a corrosion index lower than 0.1 mm/year measured according to
the regulation ASTM A 262 dossier C (HUEY TEST). Corrosion indexes
for materials of normal industrial use are cited in various
handbooks known to experts in the field, such as, for example, in
tables 23-22 to 23-24, of the above-mentioned "Perry's Chemical
Engineering Handbook", under the item Ammonium Carbamate.
[0030] The term "force welding" and "seal welding", as used in the
present description and claims, refer to the following definitions
taken from the regulations ASME VIII Div. 1 UW20: [0031] a force
welding is a welding with characteristics which are such as to
satisfy the project prescriptions, on the basis of the mechanical
characteristics and stress deriving from expansion of the welded
parts; [0032] a seal welding is effected with the aim if avoiding
losses and its dimensions are not determined on the basis of the
loads previously expressed for force weldings.
[0033] The term "homogeneous" and "homogeneously", as used herein
with reference to a tubular element E.sub.1, indicate the lack of
any discontinuity deriving from welding or another seal or force
joining method between different parts of the metal M.sub.1. This
definition does not exclude that the section or thickness of said
tubular element can be different in various regions of the
tube.
[0034] The term "metallurgically attached" as used herein with
reference to the interaction between two metallic bodies joined to
each other (such as, for example, any two bodies selected from a
tube, a tubular element, a metallic coating, a plate or a metallic
layer), indicates the presence of a contact surface or section
between said metallic bodies, wherein the respective constituents
(which can be the same metal or different metals) are joined
directly or indirectly to each other so as to form a joint with
characteristics of mechanical and detachment resistance in the same
order of magnitude as at least one of said metals. Examples of
metallurgically attached bodies according to this definition are
those in which the respective metals are joined by molten welding,
with or without a melting rod, brazing, cold welding (friction
welding, explosion welding), co-extrusion, hot-drawing and
analogous techniques.
[0035] The tube according to the invention is not limited to a
particular form of its transversal section, which can therefore be
circular, ovoidal, rectangular or having other shapes, possibly
also irregular, according to application demands. For reasons of
processing and installation, and also for reaching the best
mechanical characteristics, a circular section is preferred for the
whole length of the tube. Furthermore, the tube according to the
present invention is not limited to a linear form in a longitudinal
sense, but can also have arched, elbow or flexed forms, even if the
linear form is most commonly used for the sake of manufacturing and
installation simplicity.
[0036] For its numerous applications, the dimensions of the tube in
question can vary within wide limits. For an optimum performance in
the presence of a high pressure differential, usually ranging from
2 to 30 MPa, between the outer surface (mantle side, in contact
with a thermal fluid, normally with low, medium or high pressure
vapour) and internal surface (in contact with the corrosive fluid),
the internal diameter (or maximum width of the section) of the tube
ranges from 5 to 150 mm, preferably from 10 to 100 mm, and the
thickness of the tubular element E.sub.1 in the central area of the
tube, where the element E.sub.2 is absent, preferably varies within
a range of 1 to 15 mm, preferably from 1.5 to 10 mm, except for
when further tubular elements concentric to E.sub.1 are
present.
[0037] Preferred metals for the element E.sub.1 are zirconium and
niobium, especially zirconium and its alloys comprising at least
60% of Zr, such as Zircalloy.COPYRGT. and Zircadyne.COPYRGT., due
to the excellent resistance to both corrosion and erosion, and its
satisfactory commercial availability.
[0038] The tube according to the present invention can also
comprise, in addition to said elements E.sub.1 and E.sub.2, other
tubular elements forming corresponding layers arranged
concentrically and externally to the element E.sub.1, and extending
for the whole length of the tube or sections having a shorter
length. The tubular element E.sub.1, in this case, can conveniently
have thicknesses even less than 1 mm, for example ranging from 0.3
to 5 mm. Around the element E.sub.1 formed by the metal M.sub.1, in
this case, there is one or more layers having a tubular form
integral with and adjacent to M.sub.1, consisting for example of a
third metal or alloy (for example stainless steel) suitable for
tolerating the pressure differential, but conveniently less costly,
which can be metallurgically attached to the metal M.sub.1 or
simply in contact with it forming a structure in which E.sub.1 is
pressure supported. The metal of said one or more additional layers
is preferably selected from metals or alloys included in those
defined above in relation to the metal M.sub.2, but is not
necessarily the same metal which forms the tubular element E.sub.2,
even if it is preferable for said third metal to form
metallurgically attached weldings or joints with the metal
M.sub.2.
[0039] A non-limiting example of a multilayer tube with several
superimposed tubular elements according to the present invention,
is schematically represented in FIG. 3. In this case, the further
tubular element is adjacent to the element E.sub.2, and extends in
the central section of the length of the tube, but also included in
the present invention is the solution in which the further tubular
element extends for the whole length of E.sub.1, and the element or
elements E.sub.2 are inserted at the ends of the tube, arranged
circularly on the surface of said further tubular element.
[0040] The length of the tube according to the present invention
can vary within wide limits, in relation to the dimensions of the
equipment where it is used. In general, the length is at least 5
times greater than the diameter and preferably varies from 1 to 20
meters, more preferably from 2 to 10 meters. Whereas the first
tubular element E.sub.1 substantially extends for the whole length
of the tube, the second element E.sub.2 extends to one or both
ends, or close to these, for a length preferably ranging from 0.2
to 20%, more preferably from 1 to 10% of the total.
[0041] The thickness of E.sub.2 can be conveniently selected in
relation to the mechanical characteristics and operating conditions
projected for its use. Normal thicknesses can vary from 1 to 15 mm,
preferably from 2 to 10 mm.
[0042] Said element E.sub.2 consists of a metal or alloy M.sub.2
different from M.sub.1 and suitably selected in relation to the
constituents of the equipment in which the bimetallic tube is
included. In general M.sub.2 is advantageously selected from metal
or alloys compatible with the welding with the protective coating
of the equipment in the areas in contact with the corrosive fluid
close to the connection of the tube. In the case of an exchange of
the stripper type for urea, for example, said metal M.sub.2 is
preferably selected from titanium or one of its alloys, or
stainless steel urea grade, in relation to the metal which forms
the coating of the distribution and collection chamber of the
stripper. Typical, non-limiting examples of these metallic
materials are, in addition to titanium and its relative alloys,
AISI 316L steels (urea grade), INOX 25/22/2 Cr/Ni/Mo steel, special
austenitic-ferritic steels.
[0043] Particularly preferred M.sub.2 metals are titanium and its
alloys resistant to corrosion on the part of ammonium
carbamate.
[0044] According to the present invention, as described in more
detail hereunder, said element E.sub.2 can be conveniently seal
welded with the coating of the tube plate in a heat exchanger. As
commonly used in the art, said welding preferably also forms the
force attachment area of the tube on the plate, resistant to the
mechanical stress deriving from the pressure differential. In
relation to the use of the exchanger and its structure, the element
E.sub.2 can be conveniently positioned so that one of its ends
coincides with the end of the tube, or it can be circularly
inserted around the element E.sub.1 in a position close to the
mouth of the tube so that the latter only consists of the element
E.sub.1 (as represented by the element 3 in FIG. 1B).
[0045] According to a preferred embodiment, said element E.sub.2
homogeneously extends for the whole length of the terminal section
of the bimetallic tube, forming a continuous layer up to its
ends.
[0046] According to another embodiment of the present invention,
said element E.sub.2 can, on the other hand, protrude for a short
distance, preferably from 0.1 to 15 cm, beyond the length of the
element E.sub.1.
[0047] The element E.sub.2, moreover, can also have an outer
diameter greater than that of the bimetallic tube in the central
area, in order to form a wider and supporting joining surface for
the possible seal welding on the tube plate. Said element E.sub.2
preferably has a thickness ranging from 0.5 to 8 mm, more
preferably from 1 to 4 mm, adequate for forming the base for the
welding of the bimetallic tube on the respective support, for
example on the tube plate of a heat exchanger or decomposer.
[0048] In a particularly preferred embodiment, the tube of the
present invention comprises a tubular element E.sub.1 made of pure
zirconium or an alloy comprising at least 60% of zirconium, and at
least one tubular element E.sub.2 made of titanium or one of its
alloys, arranged circularly around E.sub.1 close to an end of the
tube, and metallurgically seal and preferably force attached
thereto, on the contact surface and welded at least in the area
closest to the mouth of the tube.
[0049] According to a particularly preferred embodiment, said
bimetallic tube comprises two tubular elements E.sub.2, of the same
material or different materials, each positioned close to one of
the ends of the tube itself. This configuration is convenient in
the production of tube bundle exchangers, in which both of the tube
plates comprise M.sub.3 metallic coatings different from the metals
or alloys included in the definition of M.sub.1 which are adopted
for forming the internal wall of the bimetallic tubes.
[0050] The tube according to the present invention can be produced
according to the usual metallurgic techniques, suitably adapted to
each case by experts in the field. The Applicant has now found
however a particular original and efficient method for the
production of said tube, which represents a second object of the
present invention, as already mentioned above.
[0051] In step (a) of said method, a segment of the external
surface of the tube E.sub.o, positioned at one or both ends, is
subjected to treatment for allowing it to house a second tubular
element E.sub.2 consisting of the metal M.sub.2. The treatment can
consist of a surface cleaning of the segment of interest in order
to obtain an efficient adhesion with the surface of the element
E.sub.2, or it can include a treatment for the removal of a thin
layer of metal from the surface, for example by abrasion or
turning, in order to obtain an insertion seat having a diameter (or
equivalent dimension, when the tube is not circular) smaller than
the original one, preferably from 0.1 to 2 mm smaller (or even
greater according to the geometrical details), which can better
house the element E.sub.2. Suitable cleaning and abrasion
techniques are those normally known in the art for metals of the
M.sub.1 type.
[0052] The surface preparation technique is effected on a segment
of the tube having a length suitable for the dimension of the
element E.sub.2 and relative assembly techniques. In general, it is
preferable to prepare an insertion seat having a length from 1 to
20 mm greater than the extension of the superimposition between
E.sub.1 and E.sub.2 in the bimetallic tube.
[0053] In step (b) of the present manufacturing method, the tubular
element E.sub.2 is positioned on the insertion seat prepared
according to step (a). The element E.sub.2, for this purpose, if
preformed, has an inner diameter corresponding to that of the
insertion seat, with the exception of possible small deformations
following the inserting phase, when effected under stress or
compression.
[0054] In the following step (c), the surfaces of the elements
E.sub.1 and E.sub.2, in contact with each other, are processed in
order to obtain a sealing joint, capable of supporting the
projected axial stress on the whole surface perimeter, with the
formation of a metallurgic bond. This connection can be effected by
means of a welding, according to the known techniques for welding
metals of the M.sub.1 type to those of the M.sub.2 type, for
example Ti with Zr, or Al with Zr, etc . . . or it can be obtained
by explosion (so-called "explosive bonding", according to the usual
English term), by means of vacuum and/or hot drawing, or by means
of another adhesion and connection technique of different metals,
so as to produce a metallurgic bond between the surfaces of the two
elements E.sub.1 and E.sub.2, consequently guaranteeing a stable
sealing under the operating conditions of the metallic tube. Even
if not necessary, it is preferable, according to the present
invention, for the connection zone (i.e. the area where the
surfaces of M.sub.1 and M.sub.2 are seal adhered) to be extended
onto the largest surface possible, more preferably coinciding with
the whole contact and superimposing area between M.sub.1 and
M.sub.2.
[0055] According to a particular embodiment of said method, the
bimetallic tube of the present invention can be produced by
arranging a welding deposit of the metal M.sub.2 in the insertion
area prepared as in (a), subsequently effecting the necessary
finishing operations. This variation allows steps (b) and (c) of
said method of the present invention, to be effected
contemporaneously.
[0056] Other variations in the above method and other manufacturing
methods of said bimetallic tube can be effected by experts in the
field, by adapting knowledge in the area to the desired embodiment.
This includes the possibility of producing a bimetallic tube having
a longer length with respect to the operating length and
subsequently removing the exceeding parts.
[0057] According to a further aspect of the present invention, said
bimetallic tube is produced so that the element T.sub.1 and the
element T.sub.2 are attached to each other, preferably forming a
metallurgic bond, along a contact surface having a
truncated-conical profile, rather than cylindrical. In this case
step (a) of the method claimed for the manufacturing of said
bimetallic tube comprises the production, for example by means of
turning, of an insertion seat on E.sub.0 having a truncated-conical
shape, preferably with length of 20 to 50 mm and a progressive and
continuous reduction in the diameter of E.sub.o, along the
truncated-conical profile, for a total of 0.5 to 6 mm, preferably
from 1 to 3 mm. An element E.sub.2, whose inner surface is
correspondingly truncated-conically shaped in order to fit and be
attached to the surface of E.sub.o, is then superimposed and fixed
on said conical seat, in accordance with steps (b) and (c) of the
present method, respectively.
[0058] The tube according to the present invention can be used in
several industrial chemical processes, due to its original and
advantageous characteristics which allow a seal connection to be
obtained between its ends and the anti-corrosion layer of at least
one part of the equipment wherein said tube emerges, maintaining at
the same time a high resistance to corrosion/erosion of the process
fluids, along its whole length, due to the presence of an integral
tubular element E.sub.1, consisting of a very high performance
material, with no joining areas, or, in any case, non-homogeneous
on the surface prepared for contact with the corrosive fluid, in
connection with an element E.sub.2, metallurgically bound to
E.sub.1, prepared for seal insertion on the outlet support, for
example a tube plate, forming an assembly which is resistant to
corrosion, as a whole, under standard process conditions.
[0059] It can therefore be used, for example, as a connection line
between equipment in which corrosive fluids flow under pressure,
or, preferably, for the manufacturing of the tube bundle of a heat
exchanger suitable for processing corrosive fluids under medium to
high pressures. A particularly preferred use is for the production
of heat exchangers in which chemical reactions or phase transitions
also take place, comprising the formation of several phases in
contact with each other, in which both the corrosive action due to
oxidative chemical attack and the erosive action due to the
turbulence and friction against the walls are considerable.
Equipment of this type include carbamate strippers in plants for
the synthesis of urea.
[0060] This latter equipment operates under pressures normally
ranging from 1 to 40 MPa and temperatures between 70 and
300.degree. C., in the presence of mixtures containing water,
ammonia, carbon dioxide and ammonium carbamate, which is the
condensation product of said compounds, according to the
reaction:
[2NH.sub.3+CO.sub.2+nH.sub.2O.fwdarw.NH.sub.4OCONH.sub.2.nH.sub.2O]
[0061] The operating conditions are preferably a pressure of 12-25
MPa and a temperature of 120 to 240.degree. C.
[0062] In normal industrial plants for the production of urea, to
which the present invention particularly refers, the
above-mentioned equipment included in medium or high pressure
sections, normally contains volumes ranging from 2,000 to 100,000
liters.
[0063] A further object of the present invention therefore relates
to equipment comprising a series tubes for heat exchange between
two fluids (tube-bundle heat exchanger), wherein the inner wall of
said tubes is suitable for contact with a fluid having
characteristics of high corrosion with respect to normal stainless
steels (corrosion index>0.2 mm/year), characterized in that at
least one, preferably a portion of said tubes, consists of the
bimetallic tube in accordance with claim 1 of the present
invention. More preferably, all of said tubes are bimetallic tubes,
in accordance with the present invention.
[0064] The pressure equipment according to the present invention
can have various geometrical forms, both internally and externally,
depending on the functions for which it is used. It is preferably
constructed according to the typical criteria of tube-bundle heat
exchangers for high or medium pressures. It therefore normally has
a cylindrical form, with two semispherical caps (heads) at the ends
of the cylinder, for a better distribution of the pressure.
Openings are suitably produced in the semispherical caps and along
the cylindrical body, for the inlet and outlet of fluids, the
introduction of possible sensors and an opening for inspections
(manhole). According to use, it can be horizontally or vertically
oriented, the latter as in case of the urea process strippers
mentioned above.
[0065] The outer wall of the equipment, which almost entirely
supports the pressure thrust, consists of a thick wrapping made of
a high mechanical performance metal or alloy, normally carbon
steel, also called reinforcement body, having a thickness
calculated in relation to the pressure to be tolerated, normally
ranging from 20 to 350 mm. In high pressure exchangers, the outer
wall can suitably have different thickness according to the
pressure to be effectively tolerated. Normally, the central
cylindrical area, in contact with the saturated vapour at pressures
ranging from 0.2 to 5 MPa, preferably has thicknesses ranging from
20 to 100 mm, whereas the wall of the caps and of the cylinder
close to this, which has to support higher pressure from the
process fluids, proportionally has higher thicknesses, preferably
from 80 to 300 mm. The outer wall can consist of a single layer or
several layers of carbon steel, assembled according to any of the
known techniques.
[0066] The area comprising the series of tubes, or tube-bundle, can
be distinguished inside the equipment, as they are normally grouped
parallel to each other, inserted on two septa or plates suitably
positioned transversally to the main axis of the equipment, also
including a flat element suitable for tolerating the pressure
difference, normally made of carbon steel, with a thickness of 40
to 500 mm. In the most common case, each of the two plates are
situated close to one of the two caps and define a central volume
having an essentially cylindrical geometry. Each plate is seal
fixed on the circular wall by welding, so that there can be no
exchanges of material between contiguous cavities. Alternatively,
the tube-bundle can be U-curved and connected to the same plate,
defining on the same an inlet and an outlet area of the fluid,
separated by a septum, but substantially at the same pressure.
[0067] In the tube-bundle equipment object of the present
invention, a series of tubes are fixed between two tube-plates or
sections of the same plate, which are suitably perforated so as to
allow the passage of a fluid between the two cavities at the ends
of the tubes. A second fluid, normally a water/vapour mix, is
circulated in the intermediate cavity, usually on the mantle side,
to effect thermal exchange through the tube wall.
[0068] The number of said tubes varies according to the project
specifications, but normally ranges from a minimum of 2 to about
10000 for larger equipment. There are preferably from 100 to 6000
tubes, and their diameter varies from 10 to 100 mm. The length of
the tubes normally coincides with the length of the central body of
the equipment and preferably ranges from 1 to 20 m, their form is
generally linear, but tubes comprising curved or toroidal parts are
not excluded and the thickness can vary, depending on the load to
be supported and diameter, from 2 to 25 mm. Intermediate septa
(also called "baffles") can be positioned in the intermediate
cavity to support the tubes. These are normally made of carbon
steel and have a thickness of a few millimeters, as they do not
have to support any pressure thrust.
[0069] According to a preferred aspect of the present invention,
all the tubes of said thermal exchange equipment are bimetallic
tubes according to the present invention.
[0070] The process fluid with characteristics of high
corrosiveness, for example an aqueous solution of carbamate and
urea, or a solution of concentrated nitric acid, is situated inside
the caps positioned at the end of the equipment, and flows inside
said tubes, forming a higher pressure fluid. Saturated water vapour
is normally fed into the intermediate cavity at pressures varying
from 0.2 to 5 MPa, which, on condensing, releases the necessary
quantity of heat, for example for decomposing the carbamate.
[0071] In the equipment in question, the bimetallic tubes are
conveniently force welded onto the tube plate in order to guarantee
the necessary mechanical and sealing stability. The tube plate
normally consists of a thick layer or several layers of carbon
steel, perforated for the passage of the tubes, and one or more
anti-corrosion coating layers on the side in contact with the
process fluid. At least one of said anti-corrosion layers
preferably consists of a metal or alloy compatible with the metal
or alloy forming the element E.sub.2 of the tube of the present
invention, i.e. with said metal or alloy, it can form a welding or
seal connection with satisfactory mechanical properties and
corrosion resistance.
[0072] The tube plate, for example, is coated on one or both sides
of the tube bundle with a layer of titanium or titanium alloy,
possibly fixed by explosion bonding with an intermediated layer of
stainless steel. Said layer is force and seal welded with the
element E.sub.2 of each bimetallic tube close to the outlet on the
surface, optionally allowing a short section of E.sub.2, for
example from 1 to 5 cm, to protrude from the surface of the plate.
The thickness of the anti-corrosion layer is suitably selected so
as to resist corrosion for an adequate period of time, it
preferably varies from 2 to 20 mm, preferably from 3 to 15 mm.
[0073] Suitable techniques for effecting the welding between the
anti-corrosion layer of the plate and the end of the tube are
generally known to experts in the field. These are special but
well-known techniques for the joining of parts made of titanium or
titanium alloys.
[0074] The bimetallic tube according to the present invention can
be advantageously used for totally or partly substituting the tubes
of a tube bundle in a pre-existing heat exchanger.
[0075] According to a further embodiment of the present invention,
the tube in question can be conveniently used in the substitution
of one or more tubes of a tube bundle of a pre-existing heat
exchanger, according to normal practice in maintenance or
modernization interventions (or revamping), generally applied in
industrial plants. Said revamping operation can achieve the double
objective of restoring the functionality of the exchanger by
replacing pre-existing tubes which for some reason no longer
function and/or are no longer sufficiently integral (for example,
due to thinning or perforation deriving from corrosion, which have
caused their closure), and also improving the performances and
safety of the equipment by substituting pre-existing tubes produced
with less resistant materials.
[0076] A further object of the present invention therefore relates
to the repair or improvement of the performances of tube-bundle
chemical equipment suitable for the treatment of a corrosive fluid,
wherein said fluid is in contact with the internal part of the
tubes forming the tube bundle, comprising the substitution of at
least one of said tubes with a bimetallic tube according to the
present invention.
[0077] The equipment on which said maintenance or modernization is
effected is preferably a heat exchanger, more preferably a stripper
of the urea synthesis cycle, whose tube plate is coated with
titanium or one of its alloys. According to a preferred embodiment,
the method according to the present invention comprises the removal
of at least one of the pre-existing tubes, cleaning and boring of
the cavities thus formed, the insertion of a bimetallic tube
according to the present invention, having a suitable length, in
each cavity, positioning the mouth of each tube so as to protrude
for a short section, normally from 0.3 to 5 cm, and finally welding
the coating of the tube plate with the outer surface of the element
E.sub.2 of each tube.
[0078] The enclosed figures provide some illustrative and
non-limiting examples in scale of embodiments of the present
invention. Parts having the same function in the figures are
indicated with the same number.
[0079] FIG. 1 schematically represents a view of two longitudinal
sections of tubes according to the present invention, respectively
having: [0080] (A) an element E.sub.2 positioned at only one end
[0081] (B) an element E.sub.2 positioned at each end, above, up to
the terminal mouth of the tube, below, in a slightly withdrawn
position, allowing a section homogeneously consisting of the metal
M.sub.1 of the element E.sub.1 to protrude.
[0082] FIG. 2 schematically represents two examples of sectional
views of an insertion detail on a tube plate of the tube according
to the present invention, in which, respectively: [0083] (A) the
element E.sub.2 is positioned in the terminal area of the tube,
close to the welding with the coating of the plate; [0084] (B) the
element E.sub.2 extends for an external section along the axis of
the tube, beyond the thickness of the tube plate.
[0085] FIG. 3 schematically represents an analogous detail to that
of FIG. 2, but relating to a tube according to the present
invention, consisting, in the intermediate section between the
ends, of two coaxial layers of different metals, of which the
innermost is the tubular element E.sub.2.
[0086] For the sake of greater simplicity and figurative clarity of
the details, the proportions between the different elements
appearing in the figures do not correspond to the actual
values.
[0087] In the following description, relating to some illustrative
and non-limiting examples of tubes and installations according to
the present invention, the orientation of the figures and relative
position of the different parts to which reference is made is
neither representative nor limiting of the configurations of the
objects described in the practical embodiment of the invention.
[0088] With reference to FIG. 1(A), the bimetallic tube according
to the present invention comprises a homogeneous and continuous
tubular element 1, having a cylindrical form, which extends for the
whole length of the tube and consisting of the metal M.sub.1 as
defined above, preferably zirconium or one of its alloys. Said
element, obtained by means of one of the normal tube-manufacturing
techniques suitable for tolerating high pressures, in addition to
providing the desired resistance to corrosion of the fluid in
contact with the internal wall, exerts the function of fluid
containment, entirely sustaining the pressure force for most of the
length of the tube, and is therefore produced with an adequate
thickness for the process pressure. In the upper part of said
bimetallic tube, the thickness of a section of the wall of the
element 1 is hollowed, for a length ranging from 2 to 10% of the
whole tube, to concentrically adapt the second tubular element 2,
consisting of the metal M.sub.2, preferably titanium or one of its
alloys, on the outer surface. The metals M.sub.1 and M.sub.2,
specifically zirconium and titanium, are metallurgically seal bound
on the contact surface between the elements 1 and 2. The two
elements 1 and 2 are concentrically arranged as far as the upper
mouth of the tube (A). The thickness of the element 2, which is
this case partly contributes to contrasting the internal pressure
of the tube, preferably ranges from 20 to 50% of the thickness of
1.
[0089] With reference to Figure (B), the bimetallic tube comprises
a homogeneous and continuous tubular element 1, consisting of the
metal M.sub.1, having a cylindrical form, which extends for the
whole length of the tube, on whose upper part there is a second
tubular element 2, consisting of the metal M.sub.2, analogously to
what is indicated in FIG. 1(A). In the lower segment of said tube,
the element 1 is shaped so that the outer surface comprises a
cavity for a length preferably ranging from 2 to 10% of the whole
tube, produced so that a small final part of the element 1, for
about 0.5-3% up to the mouth of the tube, remains unaltered with
respect to the central section thereof. The tubular element 3 made
of titanium or one of its alloys, preferably metallurgically
attached to the element 1 by means of one of the techniques listed
above, is arranged in said cavity concentrically with respect to
the element 1.
[0090] With reference to FIG. 2(A), the bimetallic tube according
to the present invention is represented by the combination of the
tubular element 1, made of the metal M.sub.1, of which only a
portion is represented, the element 2, made of the metal M.sub.2,
positioned in the terminal part of the tube, and the duct 4 for the
passage of the fluid, delimited by the tube itself. Said tube is
fixed onto the support consisting of the tube plate of a typical
heat exchanger which treats a highly corrosive fluid, under
pressure, such as, for example, a stripper of ammonium carbamate in
the industrial synthesis process of urea. In this case, the tube
plate comprises the reinforcement body 5, normally a perforated
sheet made of carbon steel, having a high thickness, suitable for
contrasting the pressure thrust, and the anticorrosive coating 6,
consisting of a corrosion-resistant metal, preferably such as to
form long-lasting seal connections with M.sub.2, by means of
welding or another method. In the example represented in FIG. 2(A),
the coating 6, which, if necessary, can also comprise several
metallic layers, according to what is already known in the art, for
example, WO03/095060, is seal and force attached, preferably by
means of a welded joint 7, onto the tubular element 2 present
externally on the outlet of the bimetallic tube.
[0091] Different variants of the example represented in FIG. 2(A)
are possible, all equally included within the scope of the present
invention and not shown in the figure as they can derive from
experts in the field applying the known art. It is possible, for
example, to insert one or more peep-holes into the plate and other
elements suitable for improving the safety of the equipment.
[0092] FIG. 2(B) shows a variant of FIG. 2(A), in which the tubular
element E.sub.2 of the tube according to the present invention
(again indicted with 2 in the figure), extends beyond the thickness
of the tube plate 5, so that the latter is in only contact with the
metal M of the outer layer.
[0093] In a tube-bundle exchanger comprising the technical solution
represented in FIGS. 2(A) and 2(B), the seal connections of the
tube with the tube plate can be effected between similar metals and
compatible with the welding, as the element E.sub.2, in the metal
M.sub.2, is arranged on each tube by connecting techniques with the
element E.sub.1 which can be effected directly on the tube before
insertion onto the tube plate, fully satisfying the strict safety
requirements envisaged for pressure equipment of this type.
[0094] In this way, it is not necessary to effect any connection
between the metal M.sub.1 and the metal forming the coating of the
plate, making the production of the exchanger much easier and more
economical when said metals cannot be easily welded to each other,
or form a welding with a lower corrosion resistance to that of each
metal. According to the present invention, the connection between
E.sub.1 and E.sub.2 can in fact be easily effected with the
techniques described above, in environments and with suitable
equipment, without drawbacks due to encumbrance and limited
operating spaces typical of a tube plate, where the high space
density of the tubes (on an average at a distance of 3 to 5 cm from
each other) and the overall dimensions of the unit make it
impossible to use techniques different from traditional
welding.
[0095] FIG. 3 shows a further variant of an assembly of the same
type represented in FIG. 1. In this case, however, a tube can be
observed, consisting of a continuous and homogeneous tubular
element 1, corresponding to the tubular element E.sub.1 of the
present invention, which has a lesser thickness than that of the
previous case and it is therefore preferably inserted, for most of
the section between the ends, for a portion of 80 to 95% of the
total length, inside a tube 8 consisting of a more economical and
easily available metal or alloy than M.sub.1, having good
mechanical properties but a lower resistance to corrosion. In the
area close to the end, said tube 8 is substituted by the tubular
element 2, consisting of M.sub.2, according to the procedures
described above with reference to FIG. 2. The metals of the element
2 and element 8 preferably form a connection in the contact area
between each other, which in this case does not require particular
corrosion resistance, as it is normally in contact with pressurized
vapour.
[0096] For the sake of graphical simplicity, FIGS. 2(A), 2(B) and 3
schematically represent only one of the insertion areas of the end
of the tube into the tube plate, as indicated by the sketch of the
drawing of the tubes on the opposite side of the tube plate.
[0097] Embodiments of the present invention, different from those
described above, can be effected by experts in the field with
adaptations to various applicative demands, forming obvious
variants, in any case included in the scope of the subsequent
claims.
* * * * *