Expanded Tube Joints in Boiler Drums with Special Reference to the Battersea High-Pressure Boilers

Abstract

Particulars are given of the major pressure parts of two high-pressure boilers installed at Battersea Power Station, each comprising four alloy steel drums and their interconnecting mild steel tubing. Their design and working conditions are described, and a historical review is presented of their erection and of problems which arose whilst rendering the expanded tube joints leakproof. The gradual resolution of these problems is then traced through several stages. It was realized that the leakage was fundamentally due to the elastic strain which occurs in the tube holes of highly stressed drums, which, at Battersea, amounts to about twice that pertaining to mild steel drums. The holding power of the joint is thus relaxed to an abnormal extent, sufficient to permit leakage. The elastic deflexions, or total “spring” of the joint, resulting from the expanding operation, must therefore be correspondingly increased to accommodate the increased drum strain. Various suggested methods of achieving this are reviewed. Insertion ferrules were selected as the most promising, and experimental development tests to investigate its possibilities are described. A full-scale rehearsal of the method carried out on one of the steam receivers is recorded, and this is followed by a description of the final ferruling and testing of both boilers. A number of general observations and conclusions are given, and the subject of leakage and its causes is discussed in some detail. It is concluded that full advantage can be taken of the benefits which result from the use of high-tensile steel boiler drums, with mild steel tubes expanded into the drums. It is, however, essential that in the design stages the expected operating conditions of the expanded joints should be investigated on the lines suggested in this paper. The adoption of high-tensile steel ferrules as a feature of original design is justified by the results obtained at Battersea.