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.