Patent Number: 051046113
Section: summary

BACKGROUND OF THE INVENTION Nuclear seam generators which have an economizer or preheater section include a physical barrier or flow divider plate to separate the hot inlet fluid from the colder outlet flow areas of the heat exchanger tube bundle on the secondary side of the tubesheet. A typical nuclear steam generator is a heat exchanger vessel containing a tube sheet which separates a primary side cavity and a secondary side cavity. A bundle of U-shaped tubes is provided with each tube end mounted to extend through the tubesheet in fluid communication with the primary side cavity the to extend into the secondary side cavity to provide a heat exchanging relationship between fluid in the tubes and fluid in the secondary side cavity. The secondary side cavity flow divider plate extents across the vessel and perpendicularly from the tubesheet. Together with other flow directing baffle plates it makes up the economizer or preheater section of the heat exchanger. The economizer improves heat transfer by preheating the incoming feedwater using the low temperature heat transfer portion of the U-tubes, i.e., adjacent where they pass fluid outwardly into the primary cavity outlet side which is separated from the primary cavity inlet side by a divider plate. The steam generator includes a handhole through the vessel wall adjacent to the tubesheet and divider plate for maintenance and cleaning n the secondary side cavity. Use of the handhole is typically to remove loose parts and debris from be annulus between the vessel inside wall and the tube bundle and to remove sludge on the tubesheet between he tubes, as by lancing. When an economizer divider plate is present in the steam generator, it obstructs physical access for inspection and maintenance by effectively creating two distinct secondary side cavity portions which would normally require separate handholes for access. A solution to this problem has been to provide the handholes 80.degree. apart at each end of the divider plate in alignment with the plate. A portion of the plate called a "notch" has been removed from the plate end opposite both of the handholes such that the 360.degree. circumference around the tube bundle becomes accessible. During operation, the divider plate notches must be blocked to prevent cross flow from one side of the flow divider plate to the other. The purpose of the flow blocker of the improved heat exchanger of the invention is to seal the notches while the steam generator is in operation yet to be removable for maintenance or inspection. In providing the flow blocker for use in a nuclear steam generator having an economizer, various design requirements were considered. One design requirement was that the flow blocker must be easy to install and disassemble. Another was that it has to be stable during operation of the steam generator to eliminate problems from vibration and flow loads. A further requirement was the accomplishment of its primary purpose of effectively blocking substantial bypass flow from one side of the divider plate to the other during operation. No prior are device accomplished these objectives. SUMMARY OF THE INVENTION The invention is for use in a nuclear heat exchanger of the type which includes an economizer having a flow divider plate and a pair of handholes arranged 80.degree. from each other, at the ends of the divider plate, opposite notches provided in the plate for inspection and maintenance in the secondary side cavity. The essence of the invention is a novel flow blocker for closing the passages between opposite sides of the economizer or secondary side divider plate created by the notches. The flow blocker structure includes a hollow inner cylindrical member with a recessed end portion and a hollow outer cylindrical member into which sail inner member is telescopically assembler. A spring biases the inner member and the outer member in opposite directions. A central longitudinal member with a hex head in the outer cylindrical member's recessed end portion is threadedly or otherwise fastener to the outer cylindrical member to limit the spring bias's longitudinal telescopic extension of the two members. The recessed portion is of lesser transverse dimension than the divider plate thickness to prevent passage of fluid therethrough and around the plate. A flange on the other cylindrical member has both holes which permit the flow blocker to be attached to a handhole closure plate, as by means of bolts. The flow blocker's spring biased structure insures stability of the device from vibration and flow loads and ensures a good fit in the notch against the divider plate. To enhance the fit, the plate's opposed notch edges adjacent the vessel are spaced by the diameter of the flow blocker and are arcuate and concentric with the flow blocker and the handhole. In the preferred embodiment, the spring is preloaded before installation and the flow blocker device is then bolted to the access opening or handhole's removable cover or closure plate. The cover and blocker assembly is then installed such that the flow blocker is in contact with the divider plate notch and centered in the notch and the handhole in alignment herewith so that the flow lane is closed. The access cover closure bolts are then tightened, applying additional load to the spring so that the total spring load is several times greater than the maximum postulated operational loads on the assembly. The spring is designed for a life equal to the steam generator without reducing the preload below the level which guarantees stability against predicted loads. Installation of the flow blocker device is simplified by the bolted construction. As an added precaution against failure, the material for fabrication is a non-corrosion alloy such as ferritic stainless steel.