Design implications of tests on a large 90° smooth bend

Abstract

The paper details the design implications of a site strain gauge test carried out on the 6 ft 6 in diameter reactor outlet ducting of a nuclear power station. The test subjected a portion of the ducting to a horizontal load of 10 ton, and the strain gauge survey was taken on a smooth bend of the ducting which was inherently flexible and did not incorporate the usual bellows units to absorb thermal expansions. Theoretical stresses in the bend of the test portion were calculated under the horizontal load, and comparison made between experimental and theoretical values. At the design stage of the station a detailed analysis was carried out for the complete system under thermal expansions, and this analysis is briefly described. Using the comparison obtained from the test portion, an estimate is made of the probable stress levels that will exist in the complete duct system under reactor operating conditions. During fabrication of the smooth bends a dimensional survey was carried out on a complete component, and the tolerances achieved are discussed and compared with tolerances obtained on the more usual smaller diameter welding elbow. The effect of end restraints on the flexibility of smooth bends is also discussed, and the lack of design information on this aspect is emphasized. It is concluded that the stresses in the bends may be accurately predicted, but a larger error is likely to occur in the prediction of end reactions and the stresses induced in vessels subjected to these reactions.