Patent Number: 056053614
Section: summary

BACKGROUND OF THE INVENTION The invention disclosed herein relates to nozzles for vessels and piping that are installed either initially or as replacements without any welding, and to the installation of such nozzles without welding the nozzle to the vessel. (A "nozzle" may also be, or include as part thereof, a sleeve and/or piping. A "vessel" may also be large bore piping.) The invention more particularly relates to nozzles and procedures which replace nozzles that are attached to the vessel on the inside diameter of the vessel with a J groove structural weld, and has particular application to nozzles and procedures which replace or initially install nozzles in pressure vessels and large bore piping of pressurized water reactor nuclear power facilities which have failed due to phenomena known as Primary Water Stress Corrosion Cracking, PWSCC. The invention also particularly relates to nozzles and procedures for installing nozzles without welding as replacements or initial installations in large bore piping. A typical nuclear power generating facility includes in part a reactor vessel, steam generator, pressurizer vessel, and a reactor coolant piping system, all of which operate under high pressure. Nozzles are attached to the vessels and/or piping for a number of purposes, e.g., for connecting piping and instrumentation, vents, and to secure control element drive mechanisms and heater elements. A typical pressurizer vessel 20 is shown in FIG. 1 with nozzles 22 for vents, nozzles 24 for liquid level, nozzles 25 for pressure sensing, a nozzle 26 for temperature measuring, and a number of nozzles 27 for heating elements. All of those nozzles were heretofore welded to the pressurizer vessel at the time of original manufacture. As shown in FIG. 2, cladding 29 is welded to the interior of the pressurizer vessel which is made of carbon steel. The temperature nozzle 26 shown in cross section in FIG. 2, which is exemplary of the welded nozzles 22-27, passes through a hole or bore 30 in the pressurizer vessel 20 and is structurally welded at its interior end 32 to the vessel 20 with a J-weld 34 along the interior opening to the bore 30. The diameter of nozzle 32 is slightly less than the diameter of bore 30, so that there is a small annular space 36 between the nozzle exterior and the wall of bore 30. The J-weld 34 also functions as a seal weld to seal the annular space 36. A reactor vessel (not shown) similarly has nozzles represented by nozzle 26 in FIG. 2 welded thereto. The piping of the reactor coolant system (not shown) also includes similar nozzles welded thereto. Further details of pressurizer vessels, reactor vessels and coolant system piping, in particular, and nuclear power facilities, in general, are known to those of skill in the art. As mentioned, the invention has particular application to the prevention of nozzle failures in nuclear power facilities due to PWSCC phenomena, which occurs on components having a susceptible material, high tensile stresses, and which are in a corrosive environment, conditions which primarily exist on nozzle penetrations in the pressurizer vessel, reactor coolant piping, and the reactor vessel. Such failures are manifested by cracking, which the applicant recognized resulted from high tensile stresses introduced by welds which structurally attach and/or seal the nozzle to the vessel and the corrosive effect of the coolant within the vessel. Such cracking occurs at the grain boundaries on the inside diameter of the nozzle material (Alloy 600) at or near the heat affected zone of the weld and propagates radially outward through the thickness of the nozzle which eventually leads to small leakage of the reactor coolant supply. As indicated, nozzles of these types have failed over time and have had to be replaced, either because of a failure in the nozzle or the weld attaching and/or sealing the nozzle to the vessel. A typical replacement procedure in a nuclear power plant environment requires shutting down the nuclear power plant, removing the nozzle, which typically requires machining operations, and welding a replacement nozzle to the vessel or piping. The welded replacement nozzles currently in use closely duplicate the original welded nozzle they replace, except that they may be made of a different alloy, e.g., Alloy 690 (less susceptible to PWSCC) instead of Alloy 600, and may also be represented by the nozzle shown in FIG. 2. Other weld repair methods involve installing a thick weld pad on the outside of the vessel and structurally welding the nozzle to the pad, and seal welding the interior end of the nozzle to the vessel. Replacements employing the above-described procedures in a nuclear power plant currently require a minimum of approximately fourteen days for some types of nozzles and are extremely expensive. Including the lost revenue resulting from plant shut-down, which may be as high as $750,000 per day, the total cost of each repair is millions of dollars. The above-described nozzle replacement procedures and any other replacement procedure that requires welding the replacement nozzle to the vessel not only is time consuming and therefore expensive, but also exposes repair personnel to radiation hazards, particularly where the nozzle replacement method involves personnel entering inside the vessel to perform the replacement. Also, both the original welded nozzle and the welded replacement nozzle and method subject the nozzle to high residual stresses imposed by weld shrinkage. These high residual stresses increase the susceptibility to PWSCC. Thus, the welded replacement nozzle offers no improvement over the original nozzle in terms of expected life and reduction of failures, other than any improvement that may result from use of a superior nozzle material. Although, Alloy 690 material is less susceptible to PWSCC than Alloy 600, it is not known at this time whether the change in nozzle material alone will eliminate the possibility of nozzle failures. Furthermore, one utility that has replaced nozzles using the original design criteria and Alloy 690 material experienced failures in the weld material itself. Based on this information, an improved nozzle replacement method is needed. U.S. Pat. Nos. 5,149,490 and 5,202,082 (both of Brown et al.) describe methods and apparatus for replacing a nozzle for a pressurizer vessel in which the replacement nozzle is threaded to the bore. Although the replacement nozzle of the '490 patent is mechanically attached to the pressurizer vessel, according to the '490 patent, welding is still required to provide the seal between the nozzle and the pressurizer vessel. Therefore, the residual stresses discussed above are imposed on the nozzle by the weld whether it be a structural weld or a seal weld. In the replacement procedure and nozzle described in the '082 patent, the original welded nozzle is not fully removed, and a mechanical seal is made between the remaining cracked nozzle portion and the end of the replacement nozzle. Leaving part of the existing nozzle at the interior welded end of the nozzle may lead to a future failure because the existing failed portion of Alloy 600 nozzle which was not removed from the vessel has cracks near the existing J weld that may propagate out to the base material of the vessel and cause further cracking in the failed portion of the nozzle. Further cracking in the remaining portion of the failed nozzle would not likely result in reactor coolant leakage, and therefore might be justified for the life of the plant; however, a better design practice would be to remove the cracked nozzle to eliminate further degradation of the vessel. The procedure described in the '882 patent thus has the drawback that a portion of the failed nozzle remains structurally welded to the vessel and therefore continues to subject the vessel to the same stresses as the original nozzle. In any event, the remaining nozzle portion and the vessel portion surrounding the bore opening are subject to further degradation. As far as the applicant is aware, the nozzles and replacement procedures disclosed in the '490 and '882 patents have not been used in a nuclear power facility. The following U.S. patents disclose other procedures for replacing or repairing nozzles, sleeves or tubes which include welding: 4,255,840 (Loch et al.); 4,440,339 (Tamai et al.); 4,615,477 (Spada et al.); 5,091,140 (Dixon et al.); 5,094,801 (Dixon et al.); 5,196,160 (Porowski); 5,209,895 (Wivagg); 5,271,048 (Behake et al.); and 5,274,683 (Broda et al.). U.S. Pat. No. 4,826,217 (Guerrero) discloses a mechanical tube clamp for boiling water reactors. U.S. Pat. No. 5,278,878 (Porowski) discloses a method for reducing tensile stresses in the welded nozzles. Also a method previously used in steam generator tube repairs has been proposed with certain modifications to the Nuclear Regulatory Committee for repairing a leaking nozzle. According to the proposal, a sleeve is rolled into an existing nozzle and deformed against the ID of the existing nozzle such that a seal is created between the nozzle and vessel. A similar design was also proposed for a plug. However, the Nuclear Regulatory Committee declined the proposals because that rolling technique causes high tensile stresses at the rolled transition region which promotes PWSCC, and because that repair method was only leak limiting which could allow the boric acid in the reactor coolant to erode a portion of the carbon steel vessel. Nozzles are currently being replaced in pressurized water reactor (PWR) nuclear power facilities both because they have failed and as a preventive measure where a statistical analysis has indicated a high probability of a future failure. Nozzle failures and such statistically indicated failures have been occurring frequently enough to be a major concern for nuclear power plant operators (and owners) for a number of reasons including the high cost of repairs and the millions of dollars in lost revenue due to plant shut down. Therefore, there is a need for a replacement nozzle and a method for replacing nozzles that have failed or may fail in the future, that (a) reduce the time and expense required to make the replacement and (b) do not require confined entry into a pressure vessel, which reduce radiation exposure to the personnel performing the replacement, and (c) reduce the susceptibility to PWSCC and do not result in further degradation of the vessel, and accordingly reduce the risk of future failures. A similar need also exists for a nozzle for initial installation applications and a method of initially installing such a nozzle in a vessel. The invention disclosed herein addresses the above-described needs and avoids the problems discussed above, and provides nozzles and procedures for installing nozzles mechanically in pressure vessels in nuclear power facilities (and in other fields) that do not employ (a) a structural weld or a seal weld, and (b) any part of an existing nozzle which is being replaced. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention disclosed herein to reduce the susceptibility to PWSCC of replacement nozzles and initial installation nozzles in nuclear power facilities as much as reasonably achievable, and thereby reduce the possibility of future nozzle failures. It is another object of the invention to reduce the time and expense involved with installing a replacement or initial installation nozzle in a vessel, particularly in a nuclear power facility. It is another object of the invention to not require confined entry into a vessel in order to install a replacement or initial installation nozzle in the vessel, particularly in a nuclear power facility. It is another object of the invention to reduce the exposure to radiation of repair personnel in a nuclear power facility while installing a replacement or initial installation nozzle in a vessel. It is another object of the invention to install a replacement or initial installation nozzle in a vessel without structurally welding the nozzle to the vessel, particularly in a nuclear power facility. It is another object of the invention to install a replacement or initial installation nozzle in a vessel without seal welding the nozzle to the vessel, particularly in a nuclear power facility. It is another object of the invention to install a replacement or initial installation nozzle in a vessel without structurally welding and without seal welding the nozzle to the vessel, particularly in a nuclear power facility. It is another object of the invention when replacing a nozzle in a vessel to remove the entire existing nozzle and to install a complete (whole) mechanical nozzle replacement, thereby removing the defective portion of the existing nozzle and avoiding further degradation to the vessel. It is another object of the invention to provide nozzles which may be installed in vessels while achieving the objects set forth above. The invention in achieving the above and other objects provides a full replacement or initial installation nozzle for vessels and a method for mechanically attaching the full nozzle to the vessel without any welding at all, i.e.,without using a structural or seal weld. The full nozzle is clamped or bolted to the vessel or attached to the vessel with an interference fit, and a seal is obtained using an interference fit between metal surfaces of the nozzle and the vessel (which may be polished), and/or by use of gasket materials. In the case of replacement, the entire existing nozzle is removed and the full nozzle is mechanically attached and mechanically sealed. The invention departs significantly from the prior art of nozzle replacement and initial installation by not utilizing a weld of any kind, which eliminates the stresses imposed by welding and significantly reduces the risk of a PWSCC type failure. The method for replacing a nozzle attached to and sealed against a vessel comprises removing the entire existing nozzle from the vessel, mechanically attaching the full replacement nozzle to the vessel with the replacement nozzle passing through the bore in the vessel from which the existing nozzle was removed, and mechanically sealing the replacement nozzle in or at the bore. Where a nozzle is initially installed (e.g., in a new vessel or as an additional nozzle on an existing vessel), a bore of the desired configuration is made in the vessel, and a new nozzle is installed generally as described for installing a replacement nozzle. In some embodiments of the invention, the existing bore is modified (e.g., configured to conform to the configuration of the nozzle). Depending upon the pressure to be encountered in the vessel, a mechanical seal is obtained by contacting the metal surfaces of the nozzle and the bore, which may or may not be polished, and/or by the use of gasket material. Polishing contacting metal to metal surfaces permits the surfaces to make intimate contact over a substantial area or areas thereof when forced together (e.g. in an interference fit or simply interfering parts forced together), and thereby create a seal of the contacting surfaces without in some applications requiring gasket material between the contacting surfaces. A gasket material employed for mechanically sealing the nozzle against the vessel may be a nickel alloy or other alloy plated or sprayed on the nozzle and/or possibly on the wall of the bore which upon compression forms a seal, or any suitable seal material which when positioned between two surfaces and compressed therebetween seals the two surfaces in the particular application of interest. As used herein, "positioning" or "placing" gasket material on or between the nozzle (and/or a flange thereof) and the vessel is meant to encompass plating or spraying the gasket material on a surface or surfaces of the nozzle and/or vessel as well as mechanically providing a material between two surfaces of the nozzle and vessel. As mentioned above, mechanically attaching the nozzle to the vessel may be accomplished by clamping, bolting or an interference fit of the replacement nozzle to the vessel. Some clamping embodiments comprise a nozzle which has a threaded portion projecting exteriorly of the vessel, means associated at least with the nozzle for engaging the vessel and preventing the nozzle from being withdrawn through the bore, and a nut threaded and tightened on the nozzle which bears against the exterior of the vessel and causes the engaging means to firmly engage the vessel. A spacer may be provided between the nut and the exterior of the vessel, in which case the nut bears against the exterior of the vessel through the spacer. The engaging means referred to above may comprise an interior flange attached to the nozzle surrounding the bore on the interior of the vessel, or interfering portions of the nozzle and the bore. Where the engaging means comprises interfering portions of the nozzle and bore, the nozzle and the bore may have circular cross sections, and a portion of the nozzle within the bore has a larger diameter than the largest diameter of the bore thereby providing the interfering portions. For example, the bore and nozzle may both be tapered. Gasket material is preferably positioned between the nozzle and the vessel to ensure a mechanical seal therebetween. In another clamping embodiment, the means for mechanically attaching the nozzle to the vessel comprises an exterior flange bolted to the vessel and means associated at least with the nozzle for engaging the vessel and preventing the nozzle from passing through the bore to the interior of the vessel. In still another clamping embodiment, the nozzle includes a separate sleeve and a nozzle body. The sleeve is externally threaded and the bore includes a threaded portion which receives the sleeve. The nozzle body and the bore are configured so that tightening the sleeve in the bore forces the nozzle body into engagement with the bore to clamp the nozzle body to the vessel. A mechanical seal is obtained from contacting metal surfaces of the nozzle body and the bore and/or gasket material as described above. Two specific embodiments of bolting the nozzle to the vessel include (1) threading the nozzle in the bore and (2) attaching an exterior flange (attached to or engaging the nozzle) to the vessel with bolts. In the first of those embodiments, the means for mechanically attaching the nozzle to the vessel comprises threads on a portion of the nozzle, a threaded portion of the bore, and means associated at least with the nozzle for engaging the vessel and preventing the nozzle from being withdrawn or pushed through the bore of the vessel. Where the engaging means comprises interfering portions of the nozzle and bore, the nozzle and the bore may have circular cross sections, and a portion of the nozzle within the bore has a larger diameter than the largest diameter of the bore thereby providing the interfering portions. For example, the bore and the nozzle may both be tapered. A tapered nozzle which is threaded in the vessel's bore may be inserted into the bore either from the interior or the exterior of the vessel depending upon the direction of the taper. In the clamping and bolting embodiments described herein, mechanically sealing the nozzle in or at the bore may comprise positioning gasket material between the nozzle (and/or a flange thereof) and the vessel and pressing that material against the vessel in the bore or at an end of the bore sufficiently to create a seal between the nozzle and the vessel, or mechanically attaching the nozzle to the vessel so as to force at least a part of the nozzle against the vessel to mechanically seal the nozzle to the vessel, or both. A mechanical seal may be obtained by pressing the flange against the vessel, or configuring the nozzle and bore to interfere and forcing the nozzle into engagement with the bore, e.g., by means of a tapered bore and nozzle as described above. Where a mechanical seal is created by forcing the nozzle against the vessel, to obtain a seal it may be necessary to machine polish the contacting surfaces as discussed above. However, it may be necessary to position gasket material between such contacting surfaces if high pressure and thermal transient conditions exist in the vessel. In such a case, the nozzle and vessel surfaces may not require polishing. For example, the gasket material may be positioned between the interior and exterior flanges referred to above and the interior and exterior, respectively, of the vessel, or along the tapered regions of the nozzle and bore. In another embodiment of mechanically sealing the nozzle to the vessel, the mechanically sealing means comprises at least one O-ring type gasket material between the nozzle and the interior of the bore. In yet another embodiment, a sleeve is mechanically attached in the bore by a shrink fit, rolling, etc., the nozzle is inserted into the sleeve, thereby providing a corrosion barrier for the vessel, and gasket material is positioned between a flange attached to the nozzle and the vessel. The bore, sleeve and nozzle may all be cylindrical or they may all be tapered. In one method of mechanically attaching the nozzle to the bore with an interference fit, the nozzle has a larger outer diameter than the inner diameter of the bore at a given temperature of the nozzle and the vessel adjacent the bore. A temperature gradient is provided between the vessel adjacent the bore and the nozzle sufficient to enlarge the diameter of the bore, reduce the diameter of the nozzle, or both to allow the nozzle to be inserted into the bore, and sufficient when the temperature gradient is substantially reduced to mechanically attach the nozzle in the bore in the temperature range of interest. After the nozzle has been inserted into the bore, the temperature gradient is reduced effective to produce the mechanical attachment. A nozzle attached as described above without an interior flange may be provided with an anti-ejection feature which prevents the nozzle from being ejected from the vessel should the mechanical attachment of the nozzle thereto fail. The anti-ejection feature may be embodied by at least one flange which is formed on the interior end of the nozzle and mechanically engages the interior of the vessel surrounding the opening to the bore. Inventive nozzles, and related attachment methods, have so far been described with respect to "vessels" in general, which term broadly encompasses piping. Specifically, all of the clamping, bolting and interference fit attachment embodiments described above are applicable to pressure vessels such as a pressurizer vessel or reactor vessel and to large bore piping. However, the invention also provides a method and apparatus specifically for attaching a nozzle to a large bore pipe in which the thickness of the pipe only allows minimum diameter changes in the bore or no installation of bolts in the pipe. A hole or bore through the circumference of the pipe is either made or an existing hole or bore in the pipe's circumference is used. A nozzle is provided having a first portion with a diameter therealong smaller than the diameter of the bore, a second portion with a diameter therealong larger than the diameter of the bore, and a flange therebetween. The nozzle portion with the smaller diameter is inserted into the bore with the flange bearing against the exterior circumference of the pipe. The flange is attached to the pipe with a suitable clamping device, and the exterior of the nozzle is mechanically sealed to the pipe. Sealing means for mechanically sealing the nozzle may comprise gasket material positioned between the flange and the outer circumference of the pipe and/or between the nozzle and the pipe which is compressed by the clamping device. The particular configurations of the smaller diameter portion of the nozzle and the ID of the bore may be conformed, for example as described above for certain vessel embodiments. Also, the mechanical sealing mechanism may be one described above for the vessel embodiments.