Patent Number: 051006103
Section: summary

BACKGROUND This invention relates to apparatus for inspecting tubular members and more particularly relates to a system for inspecting hollow tube plugs installed in nuclear steam generators. Before discussing the current state-of-the-art, it is beneficial first to briefly describe the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator comprises a vertically oriented shell and a plurality of U-shaped heat transfer tubes disposed in the shell. Pressurized radioactive primary fluid (e.g., water), which is heated by a nuclear reactor core, flows through the tubes as non-radioactive secondary fluid (i.e., water) of lower temperature circulates around the tubes. Heat is transferred from the primary fluid to the secondary fluid for producing steam in a manner well known in the art of nuclear-powered steam production. Such a nuclear steam generator is disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al. It is important that the radioactive primary fluid remains separated from the non-radioactive secondary fluid so that the secondary fluid is not radioactively contaminated by the primary fluid. Contamination of the secondary fluid by the primary fluid may lead to excessive release of radioactivity to the environment. Excessive release of radioactivity to the environment is undesirable from a health and safety standpoint. Therefore, it is important that the tubes disposed in the steam generator remain leak-tight so that the radioactive primary fluid will not commingle with and contaminate the non-radioactive secondary fluid. Occasionally, however, some of the tubes may experience degradation and may develop through-wall cracks due, for example, to stress corrosion cracking. These through-wall cracks may lead to commingling of the radioactive primary fluid with the non-radioactive secondary fluid. Therefore, the tubes are periodically inspected to identify those tubes suspected of being degraded or of having through-wall cracks. Heat transfer tubes suspected of being degraded or of having cracks are typically either sleeved or plugged, in a manner well known in the art, so that the primary fluid will not commingle with the secondary fluid. When a degraded tube is sleeved, a tubular sleeve is inserted into the tube and secured thereto, such as by welding, so that the degraded portion of the tube wall is covered. On the other hand, when a degraded tube is plugged, a tubularly-shaped hollow tube plug is inserted into one or both open ends of the tube so that radioactive primary fluid will not enter the degraded tube. Thus, in the case either of sleeving or plugging, the primary fluid is prevented from commingling with the secondary fluid even though the tube is degraded or cracked. With respect to tube plugs, one type of tube plug suitable for use in nuclear steam generator heat transfer tubes is disclosed in U.S. Pat. No. 4,390,042 entitled "Tube Plug" issued June 28, 1983 to Harvey D. Kucherer et al. This type of tube plug comprises a shell having a closed top end and an open bottom end and having a tapered inner surface against which an externally tapered expander member moves by application of a pulling force. The expander member is drawn from the closed end to the open end of the shell. The motion of the expander member relative to the shell causes the shell to expand into contact With the heat transfer tube thereby plugging the tube. However, the tube plug itself may become degraded and develop through-wall cracks. In this regard, it has been observed that the upper wall section of the tube plug is susceptible to stress corrosion cracking as a result of the substantial tensile stresses applied to the upper region of the walls of the tube plug by the expander member when it is drawn from the closed end to the open end of the tube plug. Such substantial stresses may eventually lead to through-wall cracks. A tube plug that is cracked may allow the primary fluid to enter the degraded tube in which the tube plug is disposed. If this occurs, there will be an increased risk that the radioactive primary fluid will commingle with the non-radioactive secondary fluid. In addition, if through-wall cracks develop in the upper region of the walls of the tube plug, there is a risk that the top of the tube plug will separate from the remainder of the tube plug and become a high-velocity projectile in the heat transfer tube. That is, the pressure of the primary fluid acting against the tube plug could under certain conditions separate and propel the degraded top portion of the tube plug upwardly within the heat transfer tube and damage the heat transfer tube, particularly in the U-bend region of the tube. Such severe damage to the tube may cause a breach of the tube wall. This is undesirable from a safety standpoint because such a breach could lead to commingling of the radioactive primary fluid with the non-radioactive secondary fluid. Consequently, it is desirable to inspect the upper wall section of the tube plug between the top of the tube plug and the top of the expander member to determine if this region is degraded or cracked. However, the expander member presents an obstacle to inspecting the upper interior region of the tube plug located between the top of the tube plug and the expander member. Therefore, a problem in the art is to inspect the upper interior region of the tube plug located between the top of the tube plug and the expander member even though the expander member may hamper access to that region of the tube plug. Probe devices for inspecting tubularly-shaped members are known. For example, an electro-mechanical eddy current probe having a rotatable sensing head for sensing the wall thickness of and locating local defects in a tube or conduit through which it is passed is disclosed in U.S. Pat. No. 4,625,165 entitled "Tube Inspection Probe With Rotating Eddy Current Coil" issued Nov. 25, 1986 to Samuel Rothstein. Although this patent may disclose an electro-mechanical eddy current probe, this patent does not appear to disclose a system capable of inspecting the upper interior region of a tube plug having an expander member disposed therein. A probe for longitudinally traversing and circumferentially inspecting the interior of a tube is disclosed in U.S. Pat. No. 4,772,849 entitled "Rotating Probe Head For Tube Inspection". issued Sept. 20, 1988 to Joseph A. Tedder. Although this patent may disclose a probe for inspecting the interior of a tube, this patent does not appear to disclose a system capable of inspecting the upper interior region of a tube plug having an expander member disposed therein. Thus, although the above recited patents may disclose apparatus for inspecting tubularly-shape members, these patents do not appear to disclose a system for inspecting the upper interior region of a tube plug having an expander member disposed therein. Consequently, what is needed is a system for inspecting a tube plug having an expander member disposed therein, wherein the system is capable of inspecting the upper interior region of the tube plug between the top of the tube plug and the top of the expander member. SUMMARY Disclosed herein is a system for inspecting the upper interior region of tube plugs, particularly hollow tube plugs of the type having a tapering inner wall defining a tapered cavity and having an exteriorly tapered expander member disposed in the cavity. The expander member of such a tube plug has a narrow threaded bore therethrough for receiving a threaded push-rod tool for longitudinally moving the expander member such that the expander member slidably engages the tapered inner wall. As the expander member is moved, the outer walls of the tube plug expand radially outwardly to engage the inner surface of the heat transfer tube for plugging the tube. In general, the inspection system of the present invention comprises probe means connectable to the tube plug for extending an inspection sensor probe therefrom through the narrow bore of the expander member for inspecting the upper interior region of the tube plug located between the top of the expander member and the top of the tube plug. The system further comprises rotatable hose means connected to the probe means for transversely rotating the sensor probe and for longitudinally translating the sensor probe as the sensor probe inspects the tube plug. Moreover, the system further comprises drive means engaging the hose means for rotating the hose means and for extending the sensor probe into sensing contact with the inner wall of the tube plug. More particularly, the probe means of the invention comprises a probe carrier housing for housing the sensor probe and for carrying the sensor probe into the tube plug. The probe carrier housing has a slot through the top portion thereof and also has external threads therearound for engaging internal threads belonging to an elongated extension member which surrounds the probe carrier housing. The probe carrier housing is sized to pass through the narrow bore of the expander member. The extension member is sized not to pass through the narrow bore of the expander, but rather to abut against the bottom of the expander member for reasons provided hereinbelow. The probe means further comprises a first collar surrounding the extension member, the first collar having a depending shoulder for abutting the open end of the tube plug. Also surrounding the extension member and spaced-apart from the first collar is a second collar. Connecting the first collar and the second collar are a pair of guides each having an end thereof anchored in the first collar and each having the other end thereof slidably received through the second collar. Moreover, surrounding the extension member and interposed between the first collar and the second collar is a compressible spring member for maintaining the first collar and the second collar in a spaced-apart relationship and for biasing the first collar into abutment against the open end of the tube plug. Surrounding the extension member and attached thereto is a rotator for rotating the probe carrier housing. The rotator has a bore therein for slidably receiving an actuator. Attached to the actuator is a resilient leaf spring, which extends from the actuator, through the probe carrier housing to adjacent the slot formed in the probe carrier housing. The top portion of the leaf spring is bent at a predetermined angle and has a cam surface thereon for reasons disclosed hereinbelow. Attached to the bent portion of the leaf spring is the sensor probe. Moreover, attached to the interior of the probe carrier housing adjacent the bent portion of the leaf spring is a cam for slidably engaging the cam surface belonging to the leaf spring. The hose means of the invention comprises a flexible, segmented, and hollow hose having a flexible cable extending therethrough. Connecting adjacent segments of the segmented hose is a connector configured for maintaining the hose in a tangle-free state. The hose has one end connected to the rotator and the other end connected to the drive means. Moreover, one end of the cable is attached to the actuator and the other end of the cable is connected to the drive means. During operation of the inspection system the probe means is coaxially aligned with the tube plug. The probe means is axially translated such that the shoulder belonging to the first collar abuts the open end of the tube plug. As the probe means continues to be axially translated, the distance between the first collar and the second collar decreases as the spring member therebetween compresses. Moreover, as the probe means continues to be axially translated, the extension member enters the tube plug and abuts the bottom of the expander member. As the system operates, the drive means rotates the cable which in turn rotates the rotator. The rotator rotates the probe carrier housing to threadably engage the external threads of the probe carrier housing with the internal threads of the extension member. The probe carrier housing thus threadably advances through the extension member as the probe carrier housing is rotated. As the probe carrier housing advances through the extension member it also advances through the narrow bore formed in the expander member because the probe carrier housing is sized to pass through the narrow bore of the extension member. The drive means is also operated to pull the cable connected to the actuator. As the cable is pulled, the actuator is moved which also moves or pulls the leaf spring. As the leaf spring is pulled, the cam surface belonging to the bent portion thereof slides across the cam disposed in the probe carrier housing. As the cam surface slides across the cam, the top portion of the leaf spring is deflected flexibly radially outwardly through the slot formed through the top portion of the probe carrier housing. Of course, the sensor probe is connected to the top portion of the leaf spring; therefore, the sensor probe is also radially outwardly moved as the top portion of the leaf spring is deflected flexibly radially outwardly. After the tube plug is inspected, the top portion of the leaf spring is deflected flexibly radially inwardly through the slot formed through the top portion of the probe carrier housing in a manner substantially the reverse of its outward movement. As the top portion of the leaf spring is deflected inwardly, the sensor probe is thereby withdrawn into the probe carrier housing for protecting the sensor probe from damage. According to the present invention, the sensor probe is both transversely rotated and longitudinally translated to inspect the inner wall of the tube plug along a helical path having a pitch equal to the pitch of the external threads of the probe carrier housing or the internal threads of the extension member. Moreover, in the manner summarized hereinabove, the inspection system extends the sensor probe radially outwardly from the probe carrier housing to inspect the inner wall of the tube plug and retracts the sensor probe radially inwardly into the probe carrier housing to protect the sensor probe from damage. An object of the invention is to provide a system for inspecting the upper interior portion of a hollow tube plug of the type having an expander member disposed therein, the expander member having a narrow bore therethrough, wherein the expander member hampers access to the upper interior portion of the tube plug. A feature of the invention is the provision of a drive means for simultaneously rotating and translating a sensor probe through the narrow bore of the expander member for helically scanning the upper portion of the tube plug. An advantage of the invention is that the inspection system will inspect the upper interior portion of a hollow tube plug having an expander member disposed therein.