Patent Number: 053612841
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring to FIGS. 1 a, b and c, displaying three embodiments of the corrosion accelerator adapted for use in the heat exchangers of PWR's, a tube 12 is provided of similar dimensions and composition as in a system whose heat exchange tubes are to be monitored. The U-tubes in PWR steam generators heat exchangers are typically 5/8"-7/8" in diameter. The exterior of the tube has a plurality of corrosion sites 30 spaced apart along its length. Mounted on the exterior at the tube at the axial positions of the corrosion sites are structures for retaining a corrosive medium in contact with the exterior surface of the tube. One such structure is a member 28 that can be mounted either longitudinally or as an annular ring, as depicted in FIG. 1a. A corrosive sludge accumulates in the crevice 30 between the member and the exterior surface and also in the corners between their surfaces. The member may be made of the same material as support structures for heat exchange tubes in the heat exchanger to be monitored. In PWR's, the support plates for heat exchanger tubes are usually made of carbon steel or stainless steel. In a preferred embodiment, the member is an annular ring having a width of about 3/4 inches and an outer diameter of about inches. In another embodiment, depicted in FIGS. 1b and 1c, the structure for retaining a corrosive medium in contact with the exterior surface of the tube is a consolidated, porous medium 38. This medium is preferably the sludge that accumulates in the heat exchanger to be monitored or a material simulating the sludge. In PWR's the sludge comprises oxides of aluminum, zirconium, and iron, plus metallic copper. Therefore, a consolidated, metal oxide powder, such as a sinter of one or more of those materials, is preferred. This material serves as a medium for retaining a concentrated solution of dissolved chemicals that, in combination with the solution, simulates the sludge accumulations found in the operating heat exchangers of PWR's. Also depicted in FIGS. 1b and 1c, mounted in close proximity to the structure for retaining a corrosive medium, are measuring probes 40 for monitoring corrosion conditions of the tube. Several different probes can be used to monitor the progression of corrosion of the sealed tube. The probes can be used to monitor at least some of the following characteristics of the tube indicative of corrosion conditions: potential noise, current noise, coupling current, zero resistance current, AC impedance and electrochemical potential. The probes 40 are shown to be electrically insulated from each other by insulator 50 and from the tube in FIG. 2 by the consolidated, porous medium 38. The probes may be shaped as annular rings or annular sections, as depicted. These annular structures are also 3/4 inches wide and 2 inches in diameter. Probes can also be used in conjunction with other types of structures for retaining a corrosive medium, such as those depicted in FIG. 1a. In fact, properly shaped probes can themselves provide the structure for retaining a corrosive medium. It is important to note that the shape of the consolidated, porous medium 38, probes 40 or members 28 is not important, so long as the structure provides a means for retaining the corrosive medium in contact with the tube 12. The annular members 38 or the probes can be held in place at a fixed axial position with a variety of attachment means. Side brackets 52, designed so as to not block the crevice, such as the teflon brackets in FIG. 1, hold the annular structures in position with set screws. A strap 46, as depicted in FIG. 2, or some other means may be utilized to retain the probes and consolidated porous medium in together. The data reduction software and hardware can be supplied by Capcis March, Ltd. of Manchester, England. The probes should be capable of directly monitoring the form and rate of corrosion of the tubing. The data reduction analysis techniques required to interpret the data generated by the probes are known in the art and are not included in this specification. Positioned within the tube is a heater assembly. One type of heater assembly, depicted in FIGS. 1a and 1b, is constructed of a support tube 16 having an outer diameter slightly less than that of the inner diameter of the sealed tube. In a preferred embodiment, the annular gap between the tubes is about 0.025 inches. The support tube has positioned within it a heater at each of the axial positions of the corrosion sites. In this embodiment, each heater comprises a pair of electrical cartridge heaters 14 spaced about the corrosion site. A continuous hecter may also be used. The heaters are used to provide a superheat condition at each of the corrosion sites on the exterior of the tube which is comparable to or above that found on the hot leg side of the PWR steam generator. Typically, such temperatures are about 620.degree. F. This will assure that chemicals are concentrated to the same levels in the Corrosion Accelerator as in the crevices of the heat exchanger. A spacing of less than about 1/2" between elements of a heater pair provides an almost uniform heat distribution around each of the corrosion sites. The cartridge heaters, which may be hollow as depicted in the figure, are held at fixed positions within the support tube by swaging down on the support tube at the heater sites, thereby clamping the support tube down on the heater at that site. The heaters can be plated with a high thermal conductivity metal, such as copper, to provide a uniform heat distribution about each cartridge heater. A thermocouple (not shown) positioned in the center of a heater can provide a reference temperature. The heater assembly, comprising the pairs of cartridge heaters 14, the support tube 16 and wire leads 18, is reusable. The heater assembly is mounted toward a first end of the tube. The first end of the tube is sealed with a cap or end-plug 48, against which the heater assembly may abut. The heater wires 18 extend along to the opposite second end of the tube and exit through or terminate at a sealed fitting 34. In this embodiment, the fitting is a CONAX type fitting, which allows a simple feedthrough for the heater wires. The wires may then be connected to a heater control unit to provide power to the heaters. Provided near the second end of the tube 12, opposite the heaters, is a gas inlet port 20. This allows connection to a source of pressurized gas. The interior of the tube is a sealed chamber that will be pressurized with gas such that the internal pressure of the tube will be at least as high as the pressure corresponding to the fluid pressure on the primary side of the heat exchanger being monitored. For PWR's, this pressure is typically 2250 psi or above. Helium is the preferred pressurizing gas because of its high thermal conductivity and inert nature. In another embodiment of the heater assembly, depicted in FIG. 1c, tube heating and pressurization is provided by a thermosiphon. In place of the cartridge heaters and helium gas source, the sealed tube has contained within it a smaller diameter inner tube 150 carrying steam. The inner tube has an opening 152 proximate to its termination point near the first end of the sealed tube. Its opposite end connects to an inlet port 154 near the opposite, second end of the sealed tube. Also near the second end, the sealed tube has an outlet port 156. High pressure steam enters the inner tube through the inlet port, flows through the inner tube and out through the opening near the termination point. The steam flows back through the annular space between the inner tube and the sealed tube, thereby heating the sealed tube. At least some of the steam condenses on the inner walls of the sealed tube, also providing heat to the tube. Condensates and steam exit the sealed tube through the outlet port. Gravity helps to drain condensates when the sealed tube is oriented vertically with its first end higher than its second end. Since intergranular corrosion processes are a strong function of the tubing stress, the desired degree of corrosion acceleration can be established as a function of the initial tubing pressure. Laboratory testing can be performed to establish both the recommended heater power level and the helium backpressure to achieve the appropriate local chemical concentrations and desired degree of corrosion acceleration. Each grouping of members 28 or sinter 38 with probes 40 about a crevice corrosion site comprises a crevice assembly. Depending upon the needs of the operator, the device may have a single type of crevice assembly or a variety. The simplest configuration has no probe means, and depicted in FIG. 1a. In this version, the members 28 provide crevices 30 for the corrosion sites. To monitor corrosion prior to tube failure, the tube must be removed from the heat exchanger for nondestructive or destructive testing. If probe means are used, as depicted in FIG. 1b, then corrosion can be monitored within an operating heat exchanger. In either case, an operator can vary the conditions for creating a corrosive condition by varying the temperature at the corrosion sites with the heaters and by varying the pressure stress within the tube with the gas pressure. The tube 12 is mounted to a flange 22. The first end of the tube, having the heaters and the crevice corrosion sites, extends on a first side of the flange into the tubelane region of a heat exchanger. It may also be located elsewherein the steam generator, or in a vessel connected to the blowdown line. The second end of the tube, having the gas inlet port 20 and the sealed fitting for the electrical leads 18, extends on the opposite second side of the flange 22. A leak-tight seal to the flange is provided by a swage fitting 36. The first side of the flange mates with a port in the secondary side of the heat exchanger. The wire leads from the probes 42 extend through a probe lead port 44 provided on the flange. The probe lead port may also be provided with a CONAX type fitting. The flange mounts to a mating flange 24 providing a port 26 to the interior of a heat exchanger. Depending upon the length of the tube, an anchor 32 may be provided on the first end of the tube, anchoring the tube to the blowdown line or tubesheet in the secondary side of the PWR steam generator. The anchor can reduce the risk of damage to the apparatus by buffeting from the strong hydraulic flow in the secondary side. Although not shown, the anchor may be required to rotate into and out of position so that it can pass through the shell of the steam generator. Tubelane blocking devices may also be attached to the tube. Referring now to FIG. 3, the corrosion accelerator 10 is emplaced within an operating heat exchanger 100 in the form of a steam generator of a PWR. Two hydraulic flow paths, the primary and the secondary sides, are separated by a tube sheet 104, near the bottom of the pressure vessel 102, and U-tubes 106. Water from the reactor core enters the primary side 108 through inlet 108a, then circulates through the U-tubes and out of the vessel 102, returning to the core through outlet 108b. The inlets and outlets for the U-tubes 106 are isolated from each other by divider plate 110. The secondary side 112 defines a second flowpath for nonradioactive water that can contain corrosive components. Feedwater for the secondary side enters the containment vessel 102 through inlet 112a, flows down around the inner wall of the containment vessel 102, then up around and past the U-tubes lee. steam emerges from outlet 112b to drive the turbine-generator (not shown). Corrosive sludges accumulate in the crevices between the support plates 114 and the U-tubes 106. The fluid conditions (entailing fluid chemistry, flow fields and the nature of the debris carried by the fluid) experienced by the corrosion accelerator depend upon its placement. As illustrated in FIG. 3, the anchor 32 is hooked over the blowdown line 130 for support. Placement in close proximity to the heat exchange tubes 106 will more closely produce conditions experienced by the heat exchange tubes. However, as noted earlier, the accelerator can also be placed in a vessel connected to the blowdown line 130. A pressurized gas supply 120, preferably helium, is connected to the gas inlet port. Helium is non-corrosive and provides good thermal contact between the heaters and the tube. A gas valve 122 permits simple connection and disconnection of the gas supply. Also schematically depicted in FIG. 3 is a heater control and display means 124 which connects to the heater wires 18. A person skilled in the art will be able to calibrate the heaters 14 and control the output power of the heaters to provide a desired temperature at the surface of the tube 12. Automatic controls can also be used. Also schematically depicted in FIG. 3 is a probe signal processor 126 for receiving data from the various probe means arrayed about the tube. The processing electronics are available from Capcis March, Ltd. The corrosion accelerator can be mounted to any type of reservoir containing a corrosive medium. Although this will normally be part of a heat exchanger, such as the secondary side of a PWR steam generator, the device can be used in a laboratory setup. The crevices may be prepacked with a sludge or left clear, however, prepacking is preferred. If left clear, and monitoring probes are used, the probe surface must be electrically insulated from the tube. The end of the tube having the corrosion sites extends into the reservoir. While the tube is in the reservoir it is pressurized with helium gas to accelerate corrosion. The pressure within the U-tubes of operating PWR steam generators is about 2200 psi, therefore, the pressure would be adjusted to at least about this level for this application. Similarly, the heaters are energized to heat the corrosion sites to at least about the temperatures experienced by the heat exchanger tubes being characterized by the device. A variety of different pressure/stress conditions can be created simultaneously if several different tubes are utilized. Higher temperatures will generate greater concentrations of corrosive chemicals at the corrosion sites because the accumulated sludges diminish the heat removal capability of the reservoir. Higher pressures cause greater stress, also accelerating corrosion. These operating temperatures and pressures will be determined by the operator. Corrosion monitoring is accomplished either in situ with the probes or by removal of the device from the reservoir for nondestructive and/or destructive testing. If the device is mounted in a side line of the secondary side of a heat exchanger capable of being valved off from the secondary side, then the device can be removed without shutting down the heat exchanger by valving off the sideline. As mentioned above, bench-top testing of materials can also be accomplished with this invention, eliminating the heat exchanger system altogether. Virtually the only expendable part in this device is the tube. The probes 40, the member 28, the heater assembly and the flange are all reusable. Thus, this invention provides a simple, inexpensive device for creating a corrosive condition. The methods described herein allow an operator to monitor corrosion in heat exchangers, or, if desired, to accelerate the corrosion process to be able to predict corrosive failure. The corrosion accelerator of this invention, while having special applicability to the operation and maintenance of the heat exchangers in PWR steam generators, also has broad application to other types of fluid heat exchangers. For example, nuclear reactors used for power generation, in addition to the heat exchangers used in steam generation, have residual heat removal heat exchangers for removing heat from a reactor when the reactor is shut down. The corrosive conditions experienced by the tubes in the residual heat removal systems differs from that in the steam generator system. This apparatus and method can be utilized equally as well for the maintenance of those tubes. Heat exchangers are not unique to the nuclear power industry, and this invention can be used for characterizing corrosion in many types of heat exchanger tubing. The apparatus and methods described herein can also be useful to researchers and engineers for characterizing corrosion in a broad range of materials that are subject to corrosion. It is understood that various modifications of this device and method may occur to those skilled in the art and it is intended that this invention be limited only by the scope of the claims.