Patent Number: 051788225
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 depicts a conventional once through steam generator 1, commonly referred to as an "OTSG." The OTSG of FIG. 1 is similar to an OTSG manufactured by Babcock and Wilcox, Model 177 FA as well as the OTSG described in U.S. Pat. No. 4,158,387 and others known in the art. The OTSG 1 is a large tube bundle heat exchanger. Heated primary coolant water from the nuclear reactor core enters OTSG 1 through primary inlet 2 into an inlet chamber 3, where it is distributed by upper tubesheet 4 through a plurality of generator tubes 5 and collected at lower tubesheet 6. From lower tubesheet 6 the primary water flows into outlet chamber 7 and then through primary outlet 8, from which it returns to the reactor 1 core to be reheated. The generator tubes 5 are collectively referred to as a tube bundle 9. Tube bundle 9 typically contains thousands of generator tubes 5. Heat from generator tubes is transferred to secondary feedwater which is injected into the OTSG through main feedwater nozzles 10. Tube bundle shroud 11 surrounds tube bundle 9 and outer casing 12 surrounds tube bundle shroud 11. An annulus 13 is thus formed between shroud 11 and casing 12. A partition ring 14 separates the annulus 13 into a downcomer annulus 15 and a steam annulus 16. Secondary feedwater is injected into downcomer annulus 15 through feedwater nozzles 10 and moves upwardly through tube bundle 9 within shroud 11. Steam is formed as secondary feedwater rises within shroud 11 and is heated by generator tubes 5. Steam then exits the OTSG through steam annulus 16 and steam outlets 17. As shown in FIGS. 1-3, generator tubes 5 are held in place by a plurality of generator tube support plates 18. Generator tube support plates 18 may take various forms, and are rigid support structures which allow secondary feedwater to flow around generator tubes 5 while providing stability to the tube bundle 9. Secondary feedwater flows through openings 19 (such as the broached openings shown in the Figures) in tube support plates 18. It should be understood that the term "tube support plate" refers to any tube support structure which performs the same function as those shown in the Figures, including without limitation drilled plates, support crates, support strips or similar structures. Chemical cleaning operations are aimed at removing sludge which has built up in openings 19 or sludge which has deposited on tubes and reduced the heat transfer capability of the steam generator. As stated previously, generator tubes 5 are commonly constructed of metal such as Inconel 600, while tube support plates 18 are constructed of a base metal such as carbon steel. As chemical cleaning takes place, it is critical that corrosion of tube support plates 18 be minimized, such that openings 19 are not overly enlarged, particularly at lands 84, so as to permit excessive vibration of generator tubes 5. Electronic measurements of corrosion during cleaning are monitored by a mean 31 for electronically determining corrosion within OTSG 1. Means 31 must be adapted such that it may be inserted and sealed within OTSG 1 during chemical cleaning, and must be electronically connected to a data acquisition means 32, such as a corrosion monitor 36 (such as a Petrolite.RTM. Instruments Model M-1010 or equivalent corrosion monitor for linear polarization measurements, and a Remote Galvanic Corrosion Monitor for zero resistance ammetry measurements). A dedicated computer 33 (such as an IBM.RTM. PC-XT or equivalent personal computer) having a software package for data storage and analysis may be used to recover and analyze data from each corrosion monitor. It is preferred that data acquisition means 32 include a primary unit and a backup unit powered by an uninterruptible power supply. It is also preferable that means 31 be capable of monitoring both free and galvanic corrosion. In the preferred embodiment, means 31 includes separate probes to monitor each form of corrosion. Free corrosion is monitored by a linear polarization ("LP") probe 26 and galvanic corrosion is monitored by a zero resistance ammetry ("ZRA") probe 34. LP probe 26 is a self-contained unit, and ZRA probe 34 is contained within mockup probe 25. Both probes 26,34 are electronically connected to data acquisition means 32 by cables 35. Actual corrosion during cleaning is measured utilizing mockup probe 25. Access to the interior of OTSG 1 is severely limited. When chemical cleaning takes place, chemical solvent is pumped into OTSG 1 typically through an access port such as manway 20. Solvent exits OTSG 1 at a convenient point such as handhole 21. Typically, limited access to the interior of OTSG1 is provided through other handholes around the periphery of OTSG 1, such as those shown at 22-24 in FIG. 1, with similar handholes located on the opposite side of OTSG 1. Preferably, three mockup probes 25 and one probe 26 are utilized to monitor corrosion during cleaning operations, in order to provide redundancy and comparative readings on galvanic corrosion. LP probe 26 can be located at handhole 23. One mockup probe 25 should be located as near as possible to the solvent inlet point, such as at handhole 22. A second mockup probe 25 should be placed at a separate point, such as at handhole 24. A third mockup probe 25 (not shown) should be placed at a central location, such as the handhole (not shown) on the opposite side of OTSG 1 from handhole 23. FIG. 4 shows a perspective view of mockup probe 25 attached to a flange 27 (for bolting onto handhole flange 38), via curved conduit 28. When installed, curved conduit 28 allows mockup probe 25 to be positioned in the limited space of downcomer annulus 15. FIG. 5 shows a perspective view of LP probe 26 attached to flange 29 (for bolting onto handhole flange 38), via conduit 30. When installed, LP probe protrudes through handhole 23 and into downcomer annulus 15. LP probe 26 provides real-time, in-generator monitoring of free corrosion rates during chemical cleanings of OTSG 1, independent of ZRA probe 34. The LP probe 26 shown in FIG. 5 provides instrumentation for three data channels in OTSG 1 during the iron dissolution steps and/or copper dissolution steps of chemical cleaning. All electrodes 37 are mounted in one 7-electrode chuck 87, and are electronically connected to acquisition means 32 via shielded cables 35, with connections which pass through conduit 30. Electrodes 37 are protected by a protective screen 85. In actual operation at Arkansas Nuclear One --Unit 1(ANO-1) at Russelville, Ark., corrosion of A515 Gr 70 support plate base metal was measured by two of the LP test circuits (two A515 Gr 70 electrodes 41 and one auxiliary electrode 42 per circuit), and E7018 weld-metal corrosion was measured by a third test circuit (two E7018 electrodes 43 and one auxiliary electrode 42 (i.e. one A515 Gr 70 base metal electrode as a meter prover) per circuit). The auxiliary electrode 42 is shared by all three test circuits. ZRA probe 34 provides real-time, in-generator monitoring of galvanic corrosion rates during chemical cleanings of OTSG 1, independent of LP probe 26. The ZRA probe 34 shown in FIGS. 6 and 8 provides instrumentation for six data channels in OTSG 1 during the iron dissolution steps and/or copper dissolution steps of chemical cleaning. A ZRA probe 34 for two data channels is located in each of the three mockup probes 25. Each data channel includes a cylindrical base metal (such as carbon steel [515 Gr 70]) anode 39 centered inside a length of Inconel (Alloy 600) tubing, which serves as a cathode 40. Slots 44 are provided in each cathode 40 to assure circulation of cleaning solvent within ZRA probe 34. Anodes 39 and cathodes 40 are sized to maintain the Inconel-to-carbon-steel surface area ratios that exist in the steam generator (between tubes 5 and support plates 18 and shroud 11) in order to achieve the desired galvanic efect. Anodes 39 and cathodes 40 are supported by electrode mount 45, preferably molded from a plastic, such as polysulfone. Upper plate 46 and lower plate 47 are also constructed of polysulfone and serve to position ZRA probe 34 within mockup probe 25. Holes 48 are formed in plates 46,47 to allow for circulation of cleaning solvent through ZRA probe 34 and mockup probe 25. Anodes 39 and cathodes 40 are electronically connected to data acquisition means 32 via cables 35, with connections which pass through conduit 28. It should be noted that, for both ZRA and LP measurements, data from the copper dissolution steps should not b included in the calculations of total cumulative corrosion. Galvanic corrosion currents measured by ZRA probe 26 and LP probe 34 during the copper steps are due to the dissolution of copper, and to oxidation and reduction reactions occurring in the solvent at the surface of electrodes, and do not involve the dissolution (i.e. corrosion) of electrodes. As shown in FIG. 6, mockup probe 25 is designed to approximate as closely as possible the physical relationship between generator tubes 5 and generator tube support plates 18. The basic components of mockup probe 25 are probe tube support plate 49 and at least one probe tube 50. Probe tube support plate 49 has and upper side 51 and a lower side 52, and should have substantially the same thickness and should be constructed of substantially the same material as generator tube support plates 18. Probe tubes 50 should have substantially the same diameter and should be constructed of substantially the same material as generator tubes 5. Each probe tube 50 passes through an opening 53 in probe tube support plate 49, which should have substantially the same size and shape as openings 19 in generator tube support plates 18 (e.g., if openings 19 are broached openings, opening 53 should be a breached opening of the same size and configuration). It is preferred that a plurality of openings 53 and probe tubes 50 are provided in order to provide multiple sites for actual corrosion measurement. While engineering judgment may vary as to the preferred number of probe tubes 50, seven probe tubes 50 were used in each mockup probe 25 used in corrosion measurements at ANO-1. The spacing between openings 53 should be geometrically similar to the corresponding spacing on generator tube support plates 18. It is also preferred that probe tubes 50 be of such length that the galvanic corrosion of probe tube support plate 49 during exposure to chemical cleaning solvent approximates that of generator tube support plates 18. In order to accommodate installation of mockup probe 25 in the limited space of OTSG 1, the length of probe tubes 50 was reduced by providing openings 54 in probe tube sidewalls 55. Openings 54 expose the surface area inside probe tubes 50 to the solvent providing the same galvanic effect as longer probe tubes 50. It is preferred that each probe tube have openings 54 above and below probe tube support plate 49. While engineering judgment may vary as to the preferred length of probe tubes 50, a length of seven and one-half inches was used for probe tubes 50 (with openings 54) in corrosion measurements at ANO-1, where the average spacing between generator tube support plates 18 is approximately three feet. Mockup probe 25 is encased by an upper protective sleeve 56 and a lower protective sleeve 57, which are connected to probe tube support plate 49 with screws 58. Flange 60 on probe tube support plate 49 assures a flush exterior surface for mockup probe 25 when protective sleeves 56 and 57 are installed. A base plate 59 (see FIGS. 4 and 7) is fixedly attached across the bottom of lower protective sleeve 57 by conventional means such as welding. Protective sleeves 56,57 and base plate 59 are preferably constructed of stainless steel. Probe tubes 50 are grounded to probe tube support plate 49 by tack-welding tubes 50 to base plate 59. Openings 61 are provided in base plate 59 to allow free chemical solvent circulation around and inside of probe tubes 50. Openings 62 are also provided in sidewalls 63 of protective sleeves 56,57 for the same purpose. It is preferable that upper insulating sleeve 64 (abutting upper side 51 of probe tube support plate 49) and lower insulating sleeve 65 (abutting lower side 52 of probe tube support plate 49) are provided, each having a sidewall 66 surrounding probe tubes 50 and separating them from protective sleeves 56,57. Insulating sleeves 64,65 are constructed of an insulating material, such as polysulfone, and serve the purpose of insulating the galvanic fields within mockup probe from exterior reactions and from protective sleeves 56,57. Openings 67 in sidewalls 66 are aligned with openings 62 in protective sleeves 56,57. Lower insulating sleeve 65 is held in position by positioning screw 80, which passes through positioning screw hole 81 in lower protective sleeve 57 and into threaded positioning screw hole 82 in lower insulating sleeve 65. It is preferred that both insulating sleeves 64,65 and both protective sleeves 56,57 extend upward at least to the upper ends 68 and downward at least to lower ends 69 of probe tubes 50. Of course, alternate designs are possible, wherein a single insulating sleeve and/or a single protective sleeve surrounds both probe tube support plate 49 and probe tubes 50. Lower plate 47 of ZRA probe 34 fits matingly within upper insulating sleeve 64, and upper plate 46 rests on top of upper insulating sleeve 64, positioning ZRA probe 34 just above upper ends 68 of probe tubes 50. ZRA probe 34 is thus contained by upper insulating sleeve 64 and upper protective sleeve 56. ZRA probe 34 and upper insulating sleeve 64 are held in position by positioning screw 75, which passes through positioning screw hole 77 in upper protective sleeve 56, positioning screw hole 78 in upper insulating sleeve 64 and into threaded positioning screw hole 79 in ZRA electrode mount 45. As can be seen in FIG. 8, it is preferred that positioning screw 75 together with all other exterior screws 58,71,80 are provided with a retention hole 86. During use of mockup probe 25 within OTSG 1 all screws 58,71,75,80 may be connected together by a single wire (not shown) passing through retention holes 86, preventing a loose screw from detaching and dropping into OTSG 1. Adapter 70, preferably constructed of stainless steel, rests on top of upper plate 46 and is matingly contained by upper protective sleeve 56. Adapter 70 is held in place by adapter screws 71, which pass through adapter screw holes 74 in upper protective sleeve 56 and into threaded adapter screw holes 76 in adapter 70. Electronic connection cables 35 from ZRA probe 34 pass through threaded cylindrical neck 72 of adapter 70, which attaches to conduit 28. Flow holes 73 allow chemical solvent to flow through adapter 70. In operation, mockup probes 25, each containing a ZRA probe 34, are installed in OTSG 1, as is LP probe 26. All cable connections are made to data acquisition means 32, preferably including separate computers 33 to monitor LP and ZRA readings. Chemical cleaning operations are initiated, with personnel monitoring the real-time readings on computers 33. Of course, should excessive corrosion be detected, cleaning operations can be suspended and mockup probes 25 removed to examine actual corrosion of probe tube support plates 49. Upon completion of chemical cleaning operations, mockup probes 25 are removed and actual corrosion of probe tube support plate lands 83 is carefully measured. Thus, real-time electronic readings are verified and the post-cleaning condition of OTSG tube support plates 18 is established. Other alternate embodiments of the invention will occur to those skilled in the art, and are intended to be included within the scope and spirit of the following claims.