Tube to tubesheet connection system

A tube to composite tubesheet connection system for preventing fluid leakage from the shell to the tube side and from the tube side to the shell side of a shell and tube heat exchanger. Tubes are inserted through and fastened to an inner tubesheet structure. An outer tubesheet structure constituting a plurality of outer tubesheet covers cooperates with the tubes which protrude therethrough and with the inner tubesheet to form a plurality of cavities. The inter-tubesheet cavities about each row or column of tubes are interconnected in a variety of ways. Grooving at least one of the tubesheet structures about its tube holes provides groove-cavities which may be selectively interconnected by networks of holes formed in at least one of the tubesheet structures. Additional cavity interconnection systems include outer tubesheet covers grouped together in sets of cover strips which enclose furrows formed between selected cavities in the outer surface of the inner tubesheet and metallurgically partially refilled portions of a plurality of channels formed between selected cavities in the outer surface of the inner tubesheet. Cavities may also be provided by assembling concave or channel-shaped outer tubesheet covers with the tubes and the inner tubesheet structure. Such cavities must be interconnected by a plurality of hole networks primarily concealed within the inner tubesheet but which connect selected cavities. A final technique for selectively connecting the inter-tubesheet cavities is grooving at least one of the tubesheet structures about the tube holes so as to cause the grooves to intersect and thus connect, or more correctly, form the cavities. The aforementioned grooving connection, however, requires nonstandard tube pitches to limit the interconnections to predetermined, localized areas which facilitate isolation of tube leaks to such predetermined areas as single rows or columns of tubes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to shell and tube heat exchangers, and more 
particularly, to means for producing and joining composite tubesheets to 
intersecting tubes. 
2. Description of the Prior Art 
Shell and tube heat exchangers are widely used in industry and, in 
practice, handle a variety of fluids on the shell and tube sides. Single 
tubesheets have often been joined to intersecting tubes by welding or 
rolling the tubes to the tubesheet. Operating experience has shown some of 
those joints to be susceptible to leaks which permit intrusion of the 
higher pressure fluid (shell or tube side) into the lower pressure fluid 
(shell or tube side). Depending upon the heat exchanger application, the 
nature of the shell and tube side fluids, and the extent of the leak, it 
may be necessary to suspend operation of the heat exchanger and repair the 
leak or leaks. In the particular case of a heat exchanger in a power plant 
cycle, shutdown of such heat exchanger and interruption of the associated 
power cycle can add further strain to the already serious energy shortage 
in addition to resulting in losses of $100,000 per day and more at typical 
cost figures. Leak monitoring for heat exchangers with single tubesheets 
is difficult at best and often provides leak detection for only 
substantially sized leaks. 
Double tubesheets were developed to provide greater reliability and better 
leak detection than is available for single tubesheet structures. Double 
tubesheets usually constitute two tubesheets which have tubes protruding 
through both and a gap arranged therebetween. The tubes are secured in 
sealing contact with each of the tubesheets and thus cause the intervening 
gap to be fluidtight. Two types of double tubesheet structures exist: (1) 
an integral double tubesheet fabricated from a single plate with 
interconnecting annular grooves formed interiorly about the tube holes 
which extend entirely through both tubesheets and (2) two substantially 
planar tubesheets which are fabricated so as to be joined together at 
their outer peripheries and have a fluidtight gap therebetween with the 
tubes extending across the gap and protruding through both tubesheets. The 
former prior art double tubesheet structure is characterized by German 
Pat. No. 624,385 which issued in 1935 and the second structure is 
characteristic of many heat exchangers now in service. The aforementioned 
double tubesheet structures provided greater reliability than single 
tubesheet structures due to the multiple joint formations between each 
tube and the tubesheets. Such double tubesheets also provided better leak 
detection than the single tubesheet structures by virtue of the fact that 
detection devices could monitor the presence of any fluid within the gap, 
a change in the pressure of buffer fluid placed in the gap, or any other 
parameter which reflected the condition of the intervening gap or resident 
fluid therein. Leak detection in single tubesheet structures is often 
difficults since the leaking fluid tends to beome diluted in the 
intrudedupon fluid due to the leaking fluid's usually comparatively small 
leaking flow rate. 
While prior art double tubesheet systems provide relatively greater 
reliability and superior leak detection over single tubesheet systems, 
those prior art double tubesheet systems suffer from inaccessibility to 
the inner tube joints when maintenance is necessary and provide little, if 
any, leak detection localization. The joints between the tubes and the 
inner tubesheet of integral double tubesheets cannot be welded and thus 
must be rolled or otherwise mechanically connected. Such mechanically 
connected joints are more susceptible to leaks than welded joints and thus 
substantially reduce the normally greater reliability of double tubesheet 
systems. Furthermore, accessibility to the inner joint for the parallel, 
separate tubesheet system requires removal of the entire outer tubesheet 
structure and destruction of all the tubes' joints therewith. Since both 
prior art double tubesheet structures contain single large cavities 
between the tubesheets and about all the tubes, finding leaks during 
inspection can be difficult in heat exchangers having an appreciable 
number of tubes. 
The problems associated with prior double tube-sheet systems stem directly 
from their relatively inaccessible inner tube-to-tubesheet joints and the 
accompanying difficulty in welding those joints. Additional disadvantages 
of the prior art include poor localization of the tube joints which 
require leak testing after a leak has been detected and low tube joint 
flexibility. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved heat exchanger 
apparatus and method for forming its tube-to-tubesheet connection system 
are provided for securing tubes from a shell and tube heat exchanger in a 
composite tubesheet structure. Cavities formed about the tubes between the 
composite tubesheets' inner and outer joints are selectively 
interconnected for improved leak detection and localization of leaks while 
permitting high accessibility to the inner joint for subsequent repair. 
Detection of leaks within channels that interconnect selected cavities 
reduce the number of tube-to-tubesheet joints which are candidates for 
earlier detected leaks and thus increase the precision of locating and 
decrease the time to find leaking joints during inspection and 
maintenance. The invention generally comprises a shell structure with 
tubes disposed therein, a tubesheet apparatus which cooperates with the 
shell to isolate the shell's interior from the tube's interior with that 
tubesheet apparatus including an inner and an outer tubesheet structure 
each of which have a plurality of holes therein for receiving said tubes, 
and means for fluidly interconnecting selected cavities which are formed 
between the tubesheet structures about the tubes. The cavities result from 
grooving at least one of the tubesheet structures about the tubes, 
utilizing outer tubesheet covers whose inner surface is generally concave, 
or both, or partially refilling furrows formed between tube holes on at 
least one of the tubesheet structures. Selected cavities are 
interconnected by channels disposed therebetween with such channels 
including a plurality of furrows formed at the interfacing, cooperating 
surfaces of said outer tubesheet covers and said inner tubesheet and a 
plurality of concealed openings in at least one of said tubesheet 
structures which result from drilling holes therein or forming furrows 
whose outer portions are metallurgically refilled so as to leave voids 
along those furrows' inner portions. 
Single wall and duplex or double wall tubes having inner and outer walls 
can be used with the present invention's composite tubesheet structure. 
Single wall tubes are joined to the inner and outer tubesheet structures 
and the inner and outer tubesheet structures are likewise joined. The 
inner wall of a duplex tube is joined to the outer tubesheet structure, 
the outer wall of the duplex tube is joined to the inner tubesheet 
structure, and the inner and outer tubesheet structures are joined such 
that the cavities surrounding the tubes are fluidly connected with gaps 
separating the inner and outer walls of the duplex tubes. The composite 
tubesheet structure of the present invention provides improved access to 
the inner tube to tubesheet joint, permits the use of a welded inner joint 
for increased reliability, provides localization of tube joint candidates 
for leak testing after leak detection, and increases tube-tubesheet joint 
flexibility.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is concerned primarily with joining tubes to 
composite tubesheets which constitute parts of shell and tube heat 
exchangers. Accordingly, in the description which follows the invention is 
shown embodied in a shell and tube heat exchanger. It should be 
understood, however, that the invention may be utilized for any 
application which requires tubes to be joined in a fluidtight and 
optimally flexible manner to a tubesheet or tubesheets. 
FIG. 1 illustrates a partial sectional view of a shell and tube heat 
exchanger 20 which has a tubesheet 22 (not sectioned), a plurality of 
U-tubes 24, 26, 28, a shell structure 30, and a plurality of tube supports 
32, 34, and 36. Although U-tubes are illustrated in FIG. 1, it is to be 
understood that straight tubes could be utilized equally well in the 
present invention. Tubes 24-28 are joined in fluidtight relationship with 
tubesheet structure 22 so as to prevent fluid leakage from the tube side 
to the shell side and vice versa. Tube side fluid normally enters inlet 
nozzle 38 and flows through the inlet portion of chamber 40, through the 
U-tubes 24-28, departing heat exchanger 20 through the outlet portion of 
chamber 40, and exiting chamber 40 through outlet nozzle 42. The inlet and 
outlet portions of chamber 40 are normally separated by a partition plate 
or other fluidtight sealing member (not shown). Shell side fluid normally 
enters nozzle 44, passes over the exterior of tubes 24, 26 and 28, and 
exits the shell size through nozzle 46. The shell side fluid and tube side 
fluid exchange heat therebetween through the walls of the tubes. Typical 
examples of such heat exchangers commonly used in power plant cycles are 
condensers, feedwater heaters and nuclear steam generators. 
FIG. 2 is a sectional view of one embodiment of the present invention. A 
portion of tubesheet structure 22 is illustrated in FIG. 2 and includes 
inner tubesheet 22a which is adjacent shell side 30' and outer tubesheet 
structure 22b adjacent tube side 40'. Grooves 48 are formed about each 
tube 24 and thus provide cavities which are bounded by inner tubesheet 
22a, outer tubesheet covers 22b, and tube 24. Tubesheet 22a and 22b are 
sealed together by fluidtight joints 50 which commonly constitute fillet 
welds. Tube 24 is sealed to both tubesheet 22a and tubesheet 22b by fillet 
welds 52 and 54 respectively. For purposes of the present invention, 
however, it is to be understood that joints 52 and 54 may be welded with 
other weld geometries, rolled or formed in a manner other than welding. 
Cavities 48 situated about each tube between the tubesheet structures are 
fluidly connected to other selected cavities 48 (in this case the tube row 
into and out of the paper) by networks of connecting holes 56 which in 
this case include transverse headers 58 and secondary conduits 60. Hole 
networks 56 and connected cavities 48 may be monitored for leaks through 
the inner and outer joints 50-54 associated therewith. Normally, the 
cavities are maintained under vacuum or pressure and predetermined changes 
in that vacuum or pressure may be interpreted as leaks from the shell or 
tube side for the group of tubes whose cavities 48 are interconnected. 
Thus, the leaking tube-to-tubesheet or tubesheet-to-tubesheet joint can be 
localized to the particular set of tubes whose cavities 48 are fluidly 
connected to the hole network 56 where the leak is detected. Additional 
leak detection systems include monitoring the content of a third fluid 
which normally fills cavities 48 and is in the pure state. Any leak 
detection system, however, which separately monitors each of the hole 
networks 56 greatly simplifies any necessary leak repair and permits an 
orderly shutdown of the utilizing apparatus in preparation therefor. 
FIG. 3 illustrates an alternate embodiment whose outer tubeplate covers 22c 
are concave or channel-shaped. The purpose for such shape is two-fold; 
greater joint flexibility for reduced tube joint stresses during transient 
operation is provided and counterboring about each tube is obviated due to 
covers 22c having inherent cavities 62 on their side adjacent inner 
tubesheet 22a. 
FIGS. 4A to 4B illustrate alternate embodiments for transverse headers 58 
which permit elimination of secondary conduits 60. Transverse headers 58 
are seen to intersect with grooves formed about the tube holes as was 
illustrated in FIG. 2. Such transverse header formation is preferably 
accomplished prior to grooving and tube hole drilling. FIGS. 5A and 5B 
illustrate alternate embodiments of headers 58. FIGS. 5A and 5B require 
furrowing the outer surface of inner tubesheet 22a, insertion of welding 
backup strips 64, and subsequent filling of the furrows above the weld 
strips with weld metal. By suitably forming weld strips 64 the desired 
cross-section of headers 58 may be obtained. 
FIGS. 6A and 6B illustrate formation of headers 58 for inner tubesheets 22a 
when they are clad by welding and explosive or brazing techniques 
respectively. Weld cladding 66 deposited above weld strips 64 which were 
inserted in furrows previously formed in inner tubesheet 22a result in the 
formation of headers 58. FIG. 6B illustrates three alternate embodiments 
of headers 58 utilized when tubesheet 22a is clad by explosive techniques, 
roll bonding, or brazing. It can be seen in FIG. 6B that weld strips 64 
are not necessary when tubesheet 22a is clad with one of the above-used 
techniques under appropriate conditions. FIGS. 7A and 7B illustrate 
formation of headers 58 when tubesheets 22a are multiply clad by welding 
and explosive bonding respectively. Such double cladding may be useful in 
a variety of circumstances including when it is desired to provide 
corrosion resistant material about the headers 58 which are to be 
monitored for joint leaks. Headers 58 are most conveniently located at the 
interface or near the interface between outer weld cladding 70 and inner 
cladding 66 and also between outer explosively clad layer 72 and inner 
cladding 68. 
FIGS. 8A and 8B respectively illustrate a plan view and sectional view of 
duplex tubes 74 which are connected to inner and outer tubesheet 
structures 22a and 22b. Duplex tubes 74 constitute an outer wall 76 and an 
inner wall 78 separated by longitudinal space 80 which is in fluid 
communication with the grooved cavities 48. Furrows or indentations 82 
formed in the outward facing surface of inner tubesheet 22a provide fluid 
communication between selected cavities 48. Furrows 82, when covered with 
a series of outer tubesheet covers 22b which are grouped into strip sets 
84, are fluidly sealed by welds 50 disposed about the edges of strip sets 
84. Grouping of such covers 22b into strip sets 84 can reduce the total 
amount of welding required from that of the double tubesheet configuration 
illustrated in FIG. 2. 
FIGS. 9A and 9B are plan views and sectional views of duplex tubes 74 
connected with inner tubesheet 22a and outer tubesheet structure 22c. The 
channel-shaped, outer tubesheet structure 22c is analogous to that 
illustrated in FIG. 3 with two notable exceptions: (1) FIGS. 9A and 9B 
illustrate an embodiment of outer tubesheet structure 22c comprising 
channel-shaped cover strips 86 and (2) FIGS. 9A and 9B do not require 
headers 58 and secondary conduits 60 since fluid communication between 
cavities 48 about each tube are inherently present due to the shape of the 
longitudinally channel-shaped outer tubesheet cover strips 86. Although 
FIGS. 8A, 8B, 9A, and 9B illustrate a duplex tube structure 74, it is to 
be understood that a single wall tube could likewise be used with the 
present invention and still obtain all the advantages thereof. It is to be 
further noted that for an array of tubes arranged in a triangular pattern, 
the longitudinal shapes of the cover strips may have wavy rather than 
straight contours when seen from the plan view of the tubesheet structure. 
Such wavy contours are often useful in accommodating relatively small tube 
pitches. 
FIGS. 10A-10D sequentially illustrate typical steps in forming inner 
tubesheet 22a and fastening a mating tube thereto. FIG. 10A shows groove 
48 formation about the position of future tube holes and the cutting of 
furrows 82 which intersect such grooves. FIG. 10B illustrates a 
cross-section of inner tubesheet 22a subsequent to deposition of cladding 
layer 68. FIG. 10C shows cladded tubesheet 22a with tube holes drilled 
therethrough and FIG. 10D shows exemplary tubes 24 which have been 
expanded into inner tubesheet 22a and welded to outer tubesheet cladding 
layer 68. FIGS. 10E and 10F are sectional views taken along the 
appropriate section lines indicated in FIG. 10A. It is to be understood 
that furrows 82 are obviated when successive circular grooves 48 intersect 
in rows or columns. 
FIG. 11 illustrates schematically the preferred sequence of operations 
(from right to left) for forming an additional composite tubesheet 
configuration by cutting furrows 82 in tubesheet 22a and refilling a 
portion of those furrows. Furrows 82 are initially formed and are 
subsequently partialled refilled with weld metal. To facilitate weld metal 
refilling while simultaneously assuring the presence of headers 58 
thereunder, welding strips 64 are placed near the bottom of the groove and 
weld metal is deposited thereover. Such weld metal is then, preferably, 
ground flush with that face of tubesheet 22a. Tube holes 85 are then 
drilled to intersect with headers 58. Tubes 24 inserted in holes 84 are 
then secured to tubesheet 22a and the refilled weld metal by tube 
expansion and welding respectively. The joint connections made in FIG. 11 
rely on the presence of crevices which are situated about each tube and 
extend axially between the outer weld joint and the inner expanded joint 
to provide fluid communication to headers 58. 
It is to be understood that other weld joint geometries and outer tubesheet 
cover shapes such as butt joints and multidirectional cover pieces 
respectively, are considered to be within the scope of the present 
invention's composite tubesheet structure. Such additional joint 
geometries and tubesheet cover shapes provide a choice of joint 
flexibility appropriate to the application and a variety of physical 
configurations suitable to the inspection procedures desired. The shape of 
fluid streamlines into and out of the tubes constitute an additional basis 
for selecting the shape of the tubesheet cover pieces. 
It will now be apparent that an improved tube to composite tubesheet 
connection system has been provided in which relatively removable, outer 
tubesheets are utilized to obtain better access to the inner 
tube-to-tubesheet joints so as to make feasible the welding of those 
joints, to provide better tube-to-tubesheet leak localization for making 
repairs subsequent to leak detection, to secure greater flexibility at 
tube-to-tubesheet joints so as to alleviate residual and thermal stresses 
under a variety of transient service conditions, and to permit utilization 
of single or duplex tubes with or without tubeplate cladding.