Compact Model Steam Generator Having Multiple Primaries

1. Field of the Invention

This invention relates to an improved compact model steam generator which has the ability to accurately simulate a variety of separate thermo-hydraulic conditions within a full-scale steam generator in order to separately monitor the corrosion effects of these conditions on the heat exchange tubes within the full-scale steam generator.

2. Description of the Prior Art

Model steam generators for monitoring the amount of corrosion degradation occurring within the heat exchange tubes of nuclear steam generators are known in the prior art. Generally speaking, such model generators operate by subjecting an array of sample heat exchange tubes to the same set of heat, pressure and chemical conditions which surround the heat exchange tubes in such nuclear steam generators. If these conditions are accurately simulated, the amount and nature of corrosion which occurs in the sample tubes of the model steam generator will provide an accurate indication of the tube corrosion present in the nuclear steam generator being monitored. Such model steam generators are a particularly useful form of corrosion monitor because they obviate (or at least reduce) the need for shutting down a nuclear plant and sending technicians into the radioactive interiors of the generators.

However, such model steam generators are useful only insofar as they are capable of accurately simulating at least one set of the heat, pressure and chemical conditions which actually exist within a selected portion of the nuclear steam generator. Any material departures from these conditions will adversely affect the ability of the model steam generator to accurately monitor the amount of corrosion accumulating around the heat exchange tubes and support plates within the full scale steam generator. In order to understand the difficulties in building a practical model steam generator which provides an accurate monitor for such tube corrosion, one must first understand how nuclear steam generators are generally constructed, and what chemical, thermal and hydraulic conditions are responsible for the tube corrosion.

Nuclear steam generators are comprised of three principal parts, including a primary side, a secondary side, and a tubesheet in which the inlet and outlet ends of a plurality of U-shaped tubes are mounted. The tubesheet and U-shaped tubes define a pressure boundary between the primary and secondary sides. The primary side of the generator defines a first hydraulic flowpath through the inlets, outlets and interior surfaces of the U-shaped tubes, and includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. The secondary side includes a feedwater inlet, and defines a second hydraulic flowpath around the outside surfaces of these tubes. Hot radioactive water flowing out of the nuclear reactor core is admitted into the section of the primary side containing all of the inlet ends at the U-shaped tubes. This hot water flows into these inlets, up through the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the steam generator. The heated water transfers its heat through the walls of the U-shaped tubes to non-radioactive feedwater and recirculating water flowing through the secondary side of the generator, thereby converting it to non-radioactive steam. After the nuclear-heated water circulates completely around the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back into the core of the nuclear reactor. The inlet ends of the U-shaped tubes are known as the "hot legs" and the outlet ends of these tubes are known as the "cold legs". An illustration of this general arrangement is present in FIG. 1 of U.S. patent application Ser. No. 567,328, filed Dec. 30, 1983 and assigned to Westinghouse Electric Corporation, the entire specification of which is hereby expressly incorporated herein by reference.

Over a period of time, the heat exchange tubes of such nuclear steam generators can suffer a number of different types of corrosion degradation, including denting, stress corrosion cracking, intergranular attack, and pitting. In situ examination of the tubes within these generators has revealed that most of this corrosion degradation occurs in what are known as the crevice regions of the generator. Such crevice regions include the annular space between the heat exchange tubes and the tubesheet, as well as the annular clearance between these tubes and the various support plates in the secondary side which are used to uniformly space and align these tubes. It is believed that the corrosion which occurs in these crevice regions is caused from the corrosive chemicals in the sludge which accumulates in these regions. Deposits of sludge tend to collect in these crevices from the effects of gravity. Additionally, the relatively poor hydraulic circulation of the water in these regions tends both the create and to maintain the sludge in these crevices, as well as to create localized areas of elevated temperature (or "hot spots") in the tubes adjacent the sludge. These "hot spots" create local concentrations of corrosive impurities and act as a powerful catalyst in causing the exterior surface of the heat exchange tubes to react with the corrosive chemicals and the sludge. The resulting corrosion products tend to fill the crevices even more, thereby exacerbating the corrosion-producing conditions. While most nuclear steam generators can be sludge-lanced to periodically sweep the sludge out of the generator vessel, the sludges in the crevice regions are not easily swept away by the hydraulic currents produced by such systems. Despite the fact that the heat exchange tubes of such nuclear generators are typically formed from corrosion-resistant Inconel.RTM. 600 alloy, the combination of the localized regions of heat and corrosive sludges can ultimately cause the heat exchange tubes to crack, and leak radioactive water from the primary side into the non-radioactive water in the secondary side of the generator. However, such dangerous leakage need not occur if the heat exchange tubes are subjected to remedial action (such as plugging or sleeving) before corrosion causes cracks in the walls.

Model steam generators were developed in order to accurately monitor the amount of corrosion degradation occurring in the heat exchange tubes of a particular nuclear steam generator so that corrective actions may be taken before any of the tube walls crack. Such model steam generators have been found to be a particularly accurate way of ascertaining the amount of corrosion degradation occurring in the heat exchange tubes of a nuclear steam generator, because the particular amount of corrosion which the feedwater chemistry and thermohydraulics in a particular region of a given generator will induce in a particular set of tubes is virtually impossible to predict by purely theoretical models.

Unfortunately, prior art model steam generators are not without significant shortcomings. For example, none of these prior art model steam generators includes more than one primary system. Accordingly, if the plant operator wishes to simultaneously monitor the corrosion effects on heat exchange tubes in different regions of the full-scale generator, he must purchase and install two separate model steam generators if he is to obtain the information he desires. Another shortcoming associated with such prior art model steam generators is their relatively large size and weight, which makes them unwieldy not only with respect to installation, but to disassembly and reassembly after the completion of each monitoring test. One type of model steam generator which solves much of the size and weight problems by means of a simple and relatively inexpensive design is described and claimed in U.S. application Ser. Nos. 636,437, 636,438, 636,449 and 636,450, all of which were filed on July 31, 1984 and assigned to the Westinghouse Electric Corporation. However, this particular design of model steam generator still does not have the capacity to simultaneously simulate two or more sets of thermo-hydraulic conditions which may exist within the full-scale steam generator being monitored. Accordingly, a power plant operator wishing to simultaneously monitor the tube corrosion in both "hot leg" and "cold leg" portions of the full-scale generator would have to purchase, install and operate two of these types of model steam generators. Clearly, there is a need for a model steam generator which is capable of simultaneously simulating two or more sets of thermo-hydraulic conditions in order that the amount of tube corrosion occurring in different regions within the full-scale generator may be monitored. Ideally, such a model generator should also be relatively compact in size so that it could easily fit into areas of constricted space, and lightweight so that it could be easily installed on the feedwater system of the full-scale generator being monitored. Finally, it would be desirable if it were also easily dissembled and reassembled in order that the operators of the model generator might conduct their corrosion-monitoring tests with a minimum of awkward and time-consuming handling.

In its broadest sense, the invention is an improved model steam generator having multiple primary systems for simultaneously simulating one or more sets of thermo-hydraulic conditions within a full-scale steam generator in order to monitor the effects of these conditions on the heat exchange tubes at various locations within the full-scale generator. The invention generally comprises a boiler vessel having a primary side, a tubesheet, and a secondary side, a plurality of sample heat exchange tubes for conducting heat from the primary systems within the primary side to the secondary side of the boiler vessel, and a control means for separately controlling the heat flux by individually controlling the heat sources within each of the primary systems. Each of the primary systems is preferably individually pressure-sealed in order that the pressure differential between the inside of each of the sample tubes and the secondary side of the boiler may be separately controlled. This feature is particularly useful when one or more of the sample tubes develops a crack from either corrosion or denting in the secondary side of the boiler, because the loss of the pressure seal between one of the primary systems of the model steam generator does not necessitate a cessation of the test being carried out in the remaining sample tubes.

Each of the primary systems may include an elongated chamber for holding a reservoir of water, an electric heater for converting the water in the reservoir to steam, and a riser tube concentrically disposed within the sample tube for providing a thermosyphonic circulation between the steam generated by the electric heater and the resulting condensate which flows down the inside walls of the sample tube. In order to minimize the length of the primary side of the improved model steam generator, the electrical heater may include a high-density configuration of electrical resistance wire, such as a coil, in order to shorten the length of the longitudinal chamber used in each primary system. In order to render the primary side of the boiler vessel diametrically compact, the cross-sectional area of the longitudinal chambers of each of the primary systems is preferaby no more than about four times the cross-sectional area of its associated sample tube. In order to compact the longitudinal dimensions of the primary side even further, each of the primary systems may be housed within the tubesheet of the boiler vessel. More specifically, the longitudinal chambers of each of the primary systems may be formed from the tube-housing bores which normally exist in the tubesheet. Finally, in order to minimize the number of hydraulic components associated with each of the individual primary systems in the primary side of the boiler vessel, each of these primary systems may be charged by a single charging pump by way of a hydraulic manifold.

FIG. 1 is a cross-sectional side view of a first preferred embodiment of the model steam generator of the invention;

FIG. 2A is a cross-sectional side view of the primary side of the model steam generator illustrated in FIG. 1;

FIG. 2B is a cross-sectional view of the primary side illustrated in FIG. 2A along the line A--A;

FIG 2C is a detailed view of the area circled in the lower, left hand section of FIG. 2A;

FIG. 3A is a cross-sectional side view of the primary side of a second preferred embodiment of the invention, wherein the separate primary systems are integrated within the tubesheet;

FIG. 3B is an enlarged, detailed view of the portion of FIG. 3A surrounded by the lower dotted circle;

FIG. 3C is a cross-sectional view of the primary side illustrated in FIG. 3A taken along the line A--A;

FIG. 3D is an enlarged, detailed view of the portion of FIG. 3A surrounded by the upper dotted circle, and

FIG. 4 is a schematic representation of the control system used in each of the individual primary systems which form the primary side of each of both preferred embodiments of the invention.

With reference now to FIG. 1, wherein like components are designated with like numerals throughout all of the several figures, the first preferred embodiment of the improved, compact model steam generator 1 of the invention generally comprises a boiler vessel 3 having a primary side 5, a sample tubesheet 70, and a secondary side 80. The boiler vessel 3 houses four sample heat exchange tubes 4a, 4b, 4c and 4d for transferring heat from the primary side 5 of the vessel 3, through the sample tubesheet 70, and ultimately to the secondary side 80. The heat released from the sample tubes 4a, 4b, 4c and 4d is used to generate steam from feedwater which is introduced into the secondary side 80 through feedwater inlet port 86. As will be discussed in more detail hereinafter, the bottom end of each of the sample heat exchange tubes 4a, 4b, 4c and 4d is thermally coupled to separately controllable primary systems 9a, 9b, 9c and 9d in the form of independent thermosyphon boiler cells. Because the thermal output of each of the primary systems 9a, 9b, 9c and 9d is separately controllable, both the heat flux and the surface temperature of the upper portions of the sample tubes 4a, 4b, 4c and 4d which extend into the secondary side 80 may be separately controlled to simultaneously simulate the different thermo-hydraulic conditions present within different sections of the full-scale generator being monitored, such as "hot leg" or "cold leg" conditions.

The steam which is generated from the transfer of heat between the sample tubes 4a, 4b, 4c and 4d and the feedwater within the secondary side 80 of the boiler vessel 3 passes through a separator assembly 100 which removes water vapor from the steam. From there, the dried steam flows through a centrally disposed bore 90 in the flanged end cap 88 of the secondary side 80. The sludge which results from the constant evaporation of the feedwater within the secondary side 80 collects within a sludge cup (not shown) present on the top end of the tubesheet 70 where it accumulates around the sides of the sample tubes 4a, 4b, 4c and 4d, and within the annular space between the tubesheet bores 74a, 74b, 74c and 74d and the outside surfaces of the sample tubes 4a, 4b, 4c and 4d. The corrosive chemicals formed with the sludge will corrode the exterior walls of each of the sample tubes 4a, 4b, 4 c and 4d at a rate which is primarily dependent upon the individual heat flux and surface temperature of the tube, which in turn is individually selected for each of the tubes by means of the separate control system 120 which is illustrated in FIG. 4. At the outset, it should be noted that far more detailed descriptions of the tubesheet 70, the secondary side 80, and all of the various feedwater systems, condensing systems, blow-down systems, general control systems and mounting arrangements therefor are set forth in the text of U.S. patent application Ser. Nos. 636,437, 636,438, 636,449 and 636,450, all filed on July 31, 1984 and assigned to the Westinghouse Electric Corporation, the entire texts of which are hereby expressly incorporated herein by reference.

Because the principal differences between the model steam generator claimed and disclosed in these patent applications in the instant invention lies in the structure of the multiple primary systems 9a, 9b, 9c and 9d and their associated control systems 120a, 120b, 120v and 120d, the balance of this detailed description will focus on these two areas of the boiler vessel 3.

Turning now to FIGS. 2A, 2B and 2C, the primary side 5 of the boiler vessel 3 includes four separate primary systems 9a, 9b, 9c and 9d in the form of four individual thermosyphon cells. Each of these primary systems includes its own separately controllable electrical resistance heater 11a, 11b, 11c and 11d, respectively. Each of these electrical resistance heaters is formed from a rod heating element 13a, 13b, 13c and 13d coiled about a mandrel 15a, 15b, 15c and 15d. As shown in the preferred embodiment, each of the heating elements 13a, 13b, 13c and 13d is a Model BXX-0913-48-4T rod heater, manufactured by ARI Industries, Inc. located in Franklin Park, Illinois. Structurally, each of these heating elements 13a, 13b, 13c and 13d is formed from an inner electrical resistance element of aluminum oxide or manganese oxide sheathed in a heat- and corrosion-resistant material such as Incoloy.RTM.800. Each of these heating elements has a maximum energy output of 6 kilowatts, and is connected to a 220 volt power source 136 (which forms part of its associated control system to be discussed in detail hereinafter) by way of electrical connecting lead pins 14a, 14b, 14c and 14d. In the preferred embodiment, each of the rod heating elements 13a, 13b, 13c and 13d are preferably coiled (or otherwise densely configured) around a mandrel 15. Because the structure of each of the electrical resistance heaters 11a, 11b, 11c and 11d is identical, the following description of structural details will refer only to electrical resistance heater 11a, it being understood that this description applies equally to heaters 11b, 11c and 11d.

With specific reference now to FIG. 2C, the mandrel 15 of the electrical resistance heater 11a includes both a tube portion 17a having a hollow interior 18a and a hollow, cylindrical base 19. The heating element 13a is preferably coiled around the tube portion 17a at a pitch of approximately 5 and 3/8 loops per inch, which leaves approximately 3/32nds of an inch of space between adjacent loops. Although not expressly shown in the several figures, the outside surface of the tubular portion 17a preferably includes a helical groove for receiving and seating the loops of the rod-shaped heating element 13a. This helical groove advantageously maintains proper spacing between the loops formed by the heating element 13a by preventing these loops from "gathering" as a result of any incidental friction they experience when they are slid into the tubular pressure chamber 35a which houses the electrical resistance heater 11a. Turning now to the hollow, cylindrical base 19a of the mandrel 15a, the bottom portion of the base 19a terminates in a nipple 21a having a centrally disposed bore 23a through which the lower portion of the heating element 13a extends. The top portion of the base 19a includes a laterally disposed bore 32a through which the balance of the heating element 13a extends, as well as a pair of lateral fluid ports 29a and 29b which advantageously allow water to circulate upwardly through the hollow interior 18a of the tube portion 17a. Finally, in order to maintain the necessary pressure boundary between the interior of the cylindrical base 19a and the ambient atmosphere, a weld joint 27a formed from a circular bead of welding metal sealingly connects the end of the nipple 21a and the outer surface of the lower portion of the heating element 13a as shown. While the invention would be operable if the tube portion 17a of the electrical resistance heater 11a were a solid rod instead of a hollow tube, the provision of a hollow, tube portion 17a and the mandrel 15a facilitates the transfer of heat between the heating element 13a and the water in the primary system 9a by providing approximately 20% more surface contact between the heating element 13a and the water which normally surrounds this element.

With reference again to FIGS. 2A, 2B and 2C, each of the separate primary systems 9a, 9b, 9c and 9d is housed within an elongated, tubular chamber 35a, 35b, 35c and 35d. At their lower ends, each of these chambers is sealingly engaged around an annular recess 36a, 36b, 36c and 36d of a bottom plug 37a, 38b, 37c and 37d, by means of weld joints 39a, 39b, 39c and 39d, respectively. Like weld joint 27a, each of these weld joints 39a, 39b, 39c and 39d is formed from a circular bead of weld material. In order to accommodate the insertion of the electrical resistance heaters 11a, 11b, 11c and 11d within the separate primary systems, each of the bottom plugs 37a, 37b, 37c and 37d further includes a centrally disposed bore 41 a, 41b, 41c and 41d which flares out into a tapered, threaded mouth 43a, 43b, 43c and 43d for threadedly engaging the male portion 45a, 45b, 45c and 45d of a compression fitting 47a, 47b, 47c and 47d. In the preferred embodiment, each of the compression fittings 47a, 47b, 47c and 47d may be either a Hoke Gyrolok.RTM. fitting, manufactured by Hoke, Inc. of Cressel, N.J., or a Swagelok.RTM. fitting, manufactured by Crawford Tool Company of Solon, Ohio. As may best be seen with reference to FIG. 2C, each of the compression fittings 47a, 47b, 47c and 47d is sealingly engaged to the cylindrical base 19a, 19b, 19c and 19d of its respective electrical resistance heater by means of a weld joint 49a, 49b, 49c and 49d.

As may best be seen with reference to FIG. 2A, each of the tubular chambers 35a, 35b, 35c and 35d which houses the separate primary systems 9a, 9b, 9c and 9d includes both a lower fluid port 52a, 52b, 52c and 52d, as well as an uppr fluid port 54a, 54b, 54c and 54d, respectively. Each of the lower fluid ports 52a, 52b, 52c and 52d hydraulically connects the interior of its respective tubular chamber 35a, 35b, 35c and 35d to a separate differential pressure cell 126a, 126b, 126c and 126d, while each of the upper fluid ports 54a, 54b, 54c and 54d hydraulically connects its respective tubular chamber to an absolute pressure cell 122a, 122b, 122c and 122d. As will be discussed in more detail hereinafter, these hydraulic connections are part of the control system 120a, 120b, 120c and 120d of each of the primary systems 9a, 9b, 9c and 9d. In addition to the upper and lower fluid ports for the differential and absolute pressure cells, each of the tubular chambers 35a, 35b, 35c and 35d also includes a coupling (not shown) for hydraulically connecting the interiors of each of these chambers with the output conduit of the charging pump 146 of the control systems 120a, 120b, 120c and 120d in order that a proper amount of water may be maintained within each of these tubular chambers. The upper end of each of the tubular chambers 35a, 35b, 35c and 35d is received within a cylindrical bore 56a, 56b, 56c and 56d which is present in the lower end of the primary end cap 58. Each of these relatively large, cylindrical bores 56a, 56b, 56c and 56d connects with a relatively narrow, offset bore 59a, 59b, 59c and 59d through a funnel shaped recess, 61a, 61b, 61c and 61d, respectively. As will be presently explained, the purpose of this particular bore configuration is to accommodate the extensions of the riser tubes 60a, 60b, 60c and 60d which form a necessary part of the thermosyphon mechanism present in each of the separate primary systems 9a, 9b, 9c and 9d.

With reference again to FIGS. 2A, 2B and 2C, each of the tubular pressure chambers 35a, 35b, 25c and 35d includes a concentrically disposed riser tube 60a, 60b, 60c and 60d which completely circumscribes the electrical resistance heater 11a, 11b, 11c and 11d present in each of these cells. Each of the riser tubes 60a, 60b, 60c and 60d includes a relatively narrower riser tube extension 62a, 62b, 62c and 62d which is concentrically disposed within its respective sample tube 4a, 4b, 4c and 4d. In the preferred embodiment, each of the riser tube extensions 62a, 62b, 62c and 62d includes a completely open top end which extends to about 11/2 inches from the closed end of its respective sample tube. In order that all of the steam generated within the riser tube 60a, 60b, 60 c and 60d might be directed to the end of its respective sample tube, coupling plates 64a, 64b, 64c and 64d are sealingly engaged to the ends of each of the riser tubes 60a, 60b, 60c and 60d. Each of these coupling plates includes an offset bore 66a, 66b, 66c and 66d into which the end of a relatively short, connecting tube is engaged which fluidly connects each riser tube to its respective extension. As is indicated in FIG. 2A, each of these connecting tubes includes a flared end for frictionally receiving the end of the riser tube extension 62a, 62b, 62c and 62d. The provision of a non-permanent, friction fit between the bottom ends of the riser tube extension 62a, 62b, 62c and 62d in the flared upper ends of the coupling tubes renders it easy for the operator of the model steam generator to disconnect the primary side 5 of the boiler vessel 3 from the bottom end of the tubesheet 70 upon completion of one or more of the corrosion monitoring tests. As may best be seen with reference to FIG. 2C, the bottom section of each of the riser tubes 60a, 60b, 60c and 60d is received and joined within a complimentary, annular recess in bottom plug 37a, 37b, 37c and 37d. Just above the junction between the bottom edge of these riser tubes and their respective bottom plugs, each of these riser tubes includes a pair of fluid inlet ports 67a, 67b, 67c and 67d and 68a, 68b, 68c and 68d. As is indicated by the fluid flow arrows in FIG. 2C, the purpose of these fluid inlet ports is to allow recirculating condensate flowing down from between the annular space between the outer surface of the riser tube 60a, 60b, 60c and 60d and the inner surface of the tubular chambers 35a, 35b, 35c and 35d to flow either along the outside or inside surface of the tube portion 17a, 17b, 17c and 17d of the heater 11a, 11b, 11c and 11d. More specifically, these ports direct this recirculating flow both over the coil windings of the electrical heating element 13 a, 13b, 13c and 13d, and through the lateral fluid ports 27a, 27b, 27c and 27d located in the cylindrical bases of the heaters 11a, 11b, 11c and 11d in order that the recirculating condensate might be reconverted into steam by contacting the inner walls of the hollow tube portion 17a, 17b, 17c and 17d of the mandrels. In closing, it should be noted that the riser tube 60a, 60b, 60c and 60d in each of the primary systems 9a, 9b, 9c and 9d simultaneously functions as a "downcomer" tube by defining an annular flowpath between the outside surface of the tubes 60a, 60b, 60c and 60d and the inside surfaces of the surrounding chambers through which the returning condensate may circulate. This arrangement is a significantly more compact design that the provision of separate riser and downcomer tubes, and is one of several design features which considerably reduces both the size and weight of the primary side 5 of the boiler vessel 3.

Turning back to FIG. 2A, the tubesheet 70 of the boiler vessel 3 includes a cylindrical body 71 which is detachably coupled onto the top of the primary side 5 by means of a Grayloc.RTM. clamping assembly 72. The tubesheet 70 includes an array of bores 74a, 74b, 74c and 74d which are preferably mutually spaced from one another at the same (or greater) square pitch as the tube bores in the tubesheet of the full-scale generator being monitored, which is about 1.06 in. Additionally, these bores 74a, 74b, 74c and 74d are slightly larger in diameter than the outer diameter of the sample tubes 4a, 4b, 4c and 4d which they concentrically surround so that an annular space exists between the outer surface of the tube and the inner surface of its respective housing bore when the sample tubes 4a, 4b, 4c and 4d are mounted in the position illustrated in FIG. 2A. As is further indicated in FIG. 2A, these tube bores 74a, 74b, 74c and 74d are further registrable with the offset bores 59a, 59b, 59c and 59d so that the tube bores may be precisely aligned with the offset bores when the tubesheet 70 is clamped together with the primary side 5 by means of the Grayloc.RTM. clamping assembly 72. Finally, metal O-rings 75a, 75b, 75c and 75d are provided in the junctions between the tube bores 74a, 74b, 74c and 74d and the offset bores 59a, 59b, 59c and 59d and the offset bores 59a, 59b, 59c and 59d in order to pressure-isolate each of the primary systems 9a, 9b, 9c and 9d. In the preferred embodiment, each of these O-rings is fabricated from silver-plated or nickel-plated Inconel.

Turning back now to FIG. 1, the secondary side 80 of the boiler vessel 3 generally includes a cylindrical housing 82 having a lower flange which is connected to an upper flange of the tubesheet 70 by means of another Grayloc.RTM. coupling 84. The outside surface of to cylindrical housing 82 includes a feedwater inlet 86 which is connected to a source of feedwater (not shown) which is preferably substantially identical in chemical composition to the feedwater used in the full-scale steam generator. The secondary side 80 of the boiler vessel 3 further includes a flanged end-cap 88 which is detachably mounted onto the top edge of the cylindrical housing 82 by means of still another Grayloc.RTM. assembly 92. This flanged end-cap 88 includes a centrally disposed threaded bore for threadedly receiving a steam-outlet coupling (also not shown). Concentrically disposed within the interior of the cylindrical housing 82 of the secondary side 80 is a riser barrel 95 which terminates in a riser barrel cap 97. This barrel cap 97 includes a steam opening 99 which, as indicated in FIG. 1, is preferably in alignment with the centrally disposed threaded bore 90 in the flanged end cap 88. A separator assembly 100 is provided within the riser barrel 95 in order to separate the droplets of water from the steam generated within the secondary side 80. The separator assembly generally includes a plurality of large-droplet separator grids 102 at its bottom portion, as well as a plurality of small-droplet separator grids 104 at its upper portion. The large-droplet separator grids 102 are preferably inclined at a slight angle to the horizontal as shown in order to promote a flow of condensate down along the inner walls of the riser barrel 95 when the column of wet steam generated by the transfer of heat from the sample tubes 4a, 4b, 4c and 4d into the surrounding feedwater flowing through these grids. By contrast, the small-droplet separator grids are preferably substantially horizontally disposed as shown. These small droplet separator grids rely on the tendency of the droplets of water which impinge thereon to migrate laterally in order for the condensate formed therefrom to flow back along the inner walls of the riser barrel 95. For a more detailed description of both the structure and operation of the tubesheet 70, the secondary side 80 and the separator assembly 100, reference is again made to the model steam generator described and claimed in the aforementioned U.S. patent application Ser. Nos. 636,437, 636,438, 636,449 and 636,450.

Turning now to FIGS. 3A, 3B, 3C and 3D, the second preferred embodiment of the invention includes a combined primary side and tubesheet 110 which allows for an even greater amount of longitudinal compaction of the boiler vessel 3. Since the bores 74a, 74c, 74c and 74d not only house the sample tubes 4a, 4b, 4c and 4d but also serve the same function as the chambers 35a, 35b, 35c and 35d in the first embodiment, this configuration also has the added advantage of reducing the size of the primary side 3 along its diameter, since the chambers of the primary systems provided by the bores 74a, 74b, 74c and 74d are only slightly larger than the diameter of the sample tubes which they house. By contrast, the diameter of the chambers 35a, 35b, 35c and 35d of the first embodiment are almost twice the diameter of the sample tubes 4a, 4b, 4 c and 4d.

Turning now to a more detailed description of the tube housing bores 74a, 74b and 74c and 74d, the square pitch of these bores in the combined tubesheet and primary side 110 is preferably about the same as the square pitch of the tube housing bores of the tubesheet of the full-scale generator being monitored (which again is approximately 1.06 in.). Additionally, each of these bores 74a, 74b, 74c and 74d has a somewhat larger inner diameter than the outside diameter of the respective sample tube which it houses. An annular space (which may best be seen with respect to FIG. 3B) exists between the inside surface of each of these bores and the outside surfaces of each of the sample tubes 4a, 4b, 4c and 4d. The provision of such an annular space is important, since such annular spaces exits between the heat exchange tubes and the tubesheets of the full-scale generators being monitored, and are often the location of corrosive chemicals and sludge accumulation and consequent tube corrosion.

As may further be seen best with respect to FIGS. 3B, the lower ends of the sample tubes 4a, 4b, 4c and 4d are hydraulically expanded against the inner walls of their respective tube housing bores 74a, 74b, 74c and 74d for two reasons. First, such an expansion helps to mechanically secure the ends of the sample tubes within the combined tubesheet and primary side 110. Secondly, since such a tube expansion is present in the full-scale generator being monitored, the provision of such a expansion in the model steam generator enhances the ability of the model steam generator to accurately simulate the thermonhydraulic conditions with the full-scale generator. To this end, the length of this tube expandion should match the length of the expansion present in the full-scale steam generator. It should also be noted that such an expansion has the effect of providing a somewhat larger water reservoir around the cylindrical bases 19a, 19b, 19c and 19d of the electrical resistance heaters 11a, 11b, 11c and 11d. This in turn, facilitates the circulation of water through the thermosyphon mechanism contained within the sample tube bores 74a, 74b, 74c and 74d by lowering the amount of fluid resistance which the water experiences as it flows down through the annular space between each of the riser tubes 60a, 60b, 60c and 60d and the inside walls of its respective sample tube 4a, 4b and 4c 4d through the fluid ports 67a, 67b, 67c and 67d and 68a, 68b, 68c and 68d in these riser tubes.

Apart from the fact that the tubesheet in the second preferred embodiment doubles as the primary side, with the tube housing bores 74a, 74b, 74c and 74d displacing the function and structure of the previously described chambers 35, 35b, 35c and 35d, there are only two major differencs between the structure of the first preferred embodiment illustrated in FIGS. 1 and 2A, 2B and 2C and the second preferred embodiment illustrated in FIGS. 3A, 3B and 3C and 3D.

The first of these differences is that the riser tubes 60a, 60b, 60c and 60d maintain substantially the same diameter throughout their entire length due to the fact that the maximum outer diameter that these tubes can assume is, of course, limited by the extent of the inner diameter of the sample tubes 4a, 4b, 4c and 4d. As best shown with respect to FIG. 3D, each of the riser tubes 60a, 60b, 60c and 60d is in reality formed from two tubes having slightly different diameters which are joined at junctions 116a, 116b, 116c and 116d. The lower section of the riser tubes 60a, 60b, 60c and 60d houses the electrical resistance heaters 11a, 11b, 11c and 11d, and the somewhat larger outer diameter of the riser tube which surrounds the electrical resistance heaters 11a, 11b, 11c and 11d provides enough annular clearance between the outer diameter of these electrical resistance heaters and the inner diameter of these riser tubes so that the electrical resistance heaters may be easily inserted into their respective riser tubes without any mechanical binding, and a minimum of incidental friction. Such friction could damage the heating elements 13a, 13b, 13c and 13d, and might compress some of the coil windings of these heating elements 13a, 13b, 13c and 13d closer together at some points than at others, which in turn could result in undesirable "hot spots" or short circuiting along the longitudinal axes of the various heaters 11a, 11b, 11c and 11d. While the larger outer diameter could conceivably be used throughout the entire length of the riser tubes 60a, 60 b, 60c and 60d, the provision of a narrower section of tubing for the longitudinal section extending above the electrical resistance heaters 11a, 11b, 11c and 11d creates a larger annular gap between the outer surface of each of the riser tubes and the inner surfaces of its respective sample tubes. This larger annular gap in turn facilitates the circulation of condensate back down from the inner walls of the sample tubes to the reservoir created by the expanded portions 112a, 112b, 112c and 112d of the sample tubes 4a, 4b, 4c and 4d, respectively.

The second major structural difference between the second and first preferred embodiments of the invention is the provision of an emergency water level alarm in the form of a thermocouple 117a, 117b, 117c and 117d placed just above the upper end of the heating element 13a, 13b, 13c and 13d of each of the electrical resistance heaters 11a, 11b, 11c and 11d. In order to simplify FIG. 3A, the relative positioning of these thermocouples is shown only with respect to primary system 9a; however, an identical thermocouple 117b, 117c and 117d is present in the same location in each of the other primary systems 9b, 9c and 9d, respectively. Additionally, each of these thermocouples 117a, 117b, 117c and 117d are electrically connected to the microprocessor 121 of the control system 120a, as is schematically illustrated in FIG. 3A. Before a detailed description is given of the control system 120 of this second preferred embodiment of the invention, the control system used in the first embodiment will be considered.

FIG. 4 is a schematic representation of the control system 120a, 120b, 120c and 120d used in each of the primry system 9a, 9b, 9c and 9d to control the heat flux of the sample tubes 4a, 4b, 4c and 4d, the water levels within the pressure chambers 35a, 35b, 35c and 35d, and the presure differential between these chambers and the secondary side 80 of the boiler vessel 3. Since the structure of each of these control systems are identical reference will be made only to control system 120a in order to avoid prolixity.

Control system 120 includes a microcomputer 121 which is preferably a Model 550PM microcomputer manufactured by Texas Instruments, Inc., of Dallas, Texas. Preferably, the same microcomputer 121 is used in all four control systems 120a, 120b, 120c and 120d to avoid needless duplication of components. On its input and output sides, the microcomputer 121 may include one or more Model 7MT 100 and 7MT 200 Texas Instruments modules, respectively, connected together in a fashion well known to those skilled in the art of computer controls. This control system 120 further includes an over-pressure sensor and a water level monitor in the form of an absolute pressure cell 122, and a differential pressure cell 126. The absolute pressure cell is connected to the previously mentioned upper fluid port 54a by means of a pressure conduit 123. The electrical output of the absolute pressure cell 122 is electrically connected to the input of the microcomputer 121 by means of a pair of cables, as indicated. If this absolute pressure cell 122 should even transmit an electrical signal to the input of the microcomputer 121 indicative of an over-pressure condition within the chamber 35a, the microcomputer 121 is programmed to open the breaker circuit 138 so that all electrical power is disconnected from the electrical resistance heater 11a. However, as an additional safeguard against any such over-pressure condition, a rupture disc 124 is hydraulically connected to pressure conduit 123 as indicated. This rupture disc 124 is calibrated to burst if the pressure within the tubular chamber 35a should ever rise above a preselected point indicative of an imminent boiler-burst condition. Turning now to the lower differential pressure cell 126, one side of the cell is fluidly connected to the lower fluid port 52a of the tubular pressure chamber 35a by means of a pressure conduit 128a, while the other side is pneumatically connected to the aforementioned pressure conduit 123 via pressure conduit 128b. The electrical output of the lower differential pressure cell 126 is connected to the input of the microcomputer 121 by suitable electrical cables, as indicated. Because the two sides of the differential pressure cell 126a are connected across the lower and upper fluid ports 52a and 52b, the microcomputer 121 can compute the level of the water within the tubular pressure chamber 35a by monitoring any pressure transmitted to it by the cell 126. For the most part, this pressure reading should remain unchanged throughout any test which is conducted upon the sample heat exchange tube 4a. However, should sample tube 4a crack so that water from its primary system leaks into the secondary side 80 of the boiler vessel 3, the differential pressure between the lower and upper fluid ports 52a and 54b would change rapidly and substantially due to the small size of the liquid reservoir within each of the primary systems 9a, 9b, 9c and 9d. In the preferred embodiment, the microcomputer 121 of the control system 120 is programmed to deactuate the electrical resistance heater associated with any such cracked sample tube in order to minimize any contaminating flow between the water in the primary side 5 and the feedwater in the secondary side 80. When any one of the electrical resistance heaters 11a, 11b, 11c and 11d is deactuated by the microcomputer 121 in this manner, it should be noted that each of the other corrosion monitoring tests being carried out on the sample tubes 4b, 4c and 4d may be continued without any interruptions whatsoever since each of the primary systems 9a, 9b, 9c and 9d are pressure-isolated from one another.

Turning now to the manner in which each of the control systems 120a, 120b, 120c and 120d normally control the amount of heat flux flowing through its respective sample heat exchange tube, control system 120a further includes a thermocouple 130 disposed within the tubular chamber 35a above the electrical resistance heater 11a. The electrical output of this thermocouple is connected to the input of the microcomputer 121 by means of an electrical cable 132. Also connected to the input of the microcomputer 121 is the output of a watt meter 140. As is indicated in FIG. 4, this watt meter is serially connected to one of the power cables leading into the heating element 13a of the electrical resistance heater 11a. Either the thermocouple 130 or the watt meter 140 may be used to control the heat flux radiated out of the end of the sample tube 4a. Generally, the thermocouple method of control is preferred when the operator is concerned with maintaining a given sample tube temperature within the annular space between the tubesheet bore 74a and the outside surface of the sample tube 4a. However, the watt meter 140 may optionally be used in conjunction with a simple program which may be entered into the memory of the microprocessor 121 which converts a desired amount of heat flux to a specific electrical power input into the electrical resistance heater 11a, and which further regulates the gate of the SCR 134 so that this associated amount of electrical power is indeed admitted into the heating element 13a of the electrical resistance heater 11a. Regardless of which mode of control is used, it should be noted that the operator of the control system should compensate for the incidental heat migration which occurs in the tubesheet 70 when the various primary systems 9a, 9b, 9c and 9d are operated at different heat fluxes. For example, if the tubes 4a and 4b are being operated at fluxes equivalent to "hot leg" thermo-hydraulic conditions while tubes 4c and 4d are run at "cold leg" conditions, it may be necessary to operate the "hot leg" tubes at a slightly higher temperature, and the "cold leg" tubes at a slightly lower temperature than would normally be associated with a correctly-simulative heat flux due to the fact that some significant amount of heat will flow through the tubesheet 70 from the "hot legs" 4a and 4b to the "cold legs" 4c and 4d. Such a compensating operation should also be used with respect to the second preferred embodiment of the model steam generator, since this same undesired heat transfer between "hot leg" and "cold leg" sample tubes would occur in the combined primary system and tubesheet 110. To reduce (or at least minimize) this spurious heat flow, both the tubesheet 70 of the first embodiment and the combined primary system and tubesheet 110 of the second embodiment might be cut into quadrant-shaped sectors, with a cruciform ceramic insulator disposed between these sectors.

Before moving onto a detailed description of the charging system 144 which is part of the control system 120, it should be noted that the power cables leading to the electrical resistance heater 11a include both a circuit breaker 138, as well as a set of conventional fuses 142 in order to protect the electrical resistance heater 11a (and all of the power circuitry connected thereto) from a power surge in the event of a short-circuit or other malfunction.

The charging system 144 of the control system 120a generally includes a charging pump 146 having an inlet conduit 148, and an outlet conduit 150. The inlet conduit 148 is preferably hydraulically connected to a source of purified demineralized water in order to minimize the amount of scale or corrosion within the tubular chambers 35a, 35b, 35c and 35d. The outlet conduit 150 preferably includes a manifold structure which connects the outlet of the charging pump 146 to the charging ports (not shown) of each of the tubular chambers 35a, 35b, 35c and 35d. The provision of such a manifold structure obviates the need for multiple charging pumps. An electrically controlled valve 152 is serially connected between the output of the charging pump 146, and the charging port of each of the tubular chambers 35a, 35b, 35c and 35d. This electrically controlled valve 152 is connected to 110 volt power source 154 by means of electrical cables 156a and 156b. A normally open relay 158 is serially connected with an electrical cable 156a. The control leads of this normally open relay 158 are connected to microprocessor 121 of the control system 120a. Additionally, the charging pump 146 is connected to a 220 volt power supply by means of power cables 160a and 160b, and a normally-open relay 162 is likewise serially connected within power cable 160a. As was the case with the normally-open 110-volt relay 158, the output of the microcomputer 121 is electrically connected to the charging pump relay 162. The control system 120a controls the water level within the tubular chambers 35a, 35b, 35c and 35d by monitoring this water level in the manner previously described by means of differential pressure cell 126, and by periodically opening the electrically controlled valve 152 and actuating the charging pump 146 to administer additional primary water through the charging port within the particular chamber whenever the differential pressure cell 126 indicates that the water level within this chamber has fallen beneath a preselected level. While the charging system 144 may be used to continuously replenish the water within a tubular chamber connected to a cracked sample tube, this charging system 144 is normally used only in the initial portion of the test to put a proper initial charge of water within the tubular chambers 35a, 35b, 35c and 35d.

The control system used in the second embodiment of the model steam generator is the same as the previously described control system 120 used in the first preferred embodiment with one major exception. Specifically, in addition to using a differential pressure cell 126 to sense the water level in the chambers 35a, 35b, 35c and 35d formed by the tube housing bores in the combined primary side and tubesheet 110, this control system further includes the previously described thermocouples 117a, 117b, 117c and 117d which are spaced just above the heating elements 13a, 13b, 13c and 13d in each of the bores 74a, 74b, 74c and 74d and electrically connected to the microcomputer 121 of the control system 120. If the water level in one of the tube-housing bores should fall to such an extent that the tip of its respective heating element becomes de-submerged, the temperature signal of its respective thermocouple 117a, 117b, 117c and 117d will rise substantially in a very short period of time, due to the fact that the steam surrounding the tip of the de-submerged heating element is a considerably poorer heat conductor than the water which normally surrounds this tip. In response to such a signal, the microcomputer 121 will automatically de-actuate the electrical resistance heater associated with this thermocouple. In both the first and second embodiments, if the loss of water inventory in the chamber of the primary system is the result of a cracked sample tube, the operator will normally allow the pressure to equilibrate between the primary side and the secondary side of the particular primary system affected while continuing to carry out the monitoring test of the remaining primary systems. If, however, the loss of water inventory was the result of a primary-to-atmosphere leak, such as may occur around the pressure fittings 47a, 47b, 47c and 47d, the operator has the option of re-charging the primary system affected with the charging system 144, and continuing the test.

In actual tests conducted by the applicants, the improved, compact model steam generator of the invention was capable of generating heat fluxes in the secondary water above the tubesheet 70 in the range from about 1.times.10.sup.4 BTUs/ft..sup.2 per hour through about 1.times.10.sup.5 BTUs/ft..sup.2 per hour, with an associated secondary side pressure of approximately 1,100 psig. Additionally, in both embodiments, while the end of the sample tubes 4a, 4b, 4c and 4d extending into the secondary side 80 may be as long as 24 inches, and 8- to 12-inch tube extension into the secondary side 80 is preferred when electrical resistance heaters of the aforementioned specifications are used. Finally, it should be noted that both embodiments of the invention are completely retrofittable onto the secondary side of the model steam generator described and claimed in the aforementioned U.S. patent application Ser. Nos. 636,437, 636,438, 636,449 and 636,450.