Wear reduction between disc edge and seat in a butterfly valve
Abstract: This thesis was written on behalf of SOMAS instruments AB. SOMAS develops, manufactures and markets valves. The mission was to reduce the wear between the disc edge and the seat in a butterfly valve that are used at high temperatures. This study investigated the possibility of using a surface treatment or coating that can reduce wear or if a new base material is better to use than the current steel 316 material. Possible materials, surface treatments and coatings were investigated and compared in a material study. In discussion with the company, four test pairs were chosen; • Steel 316 - Steel 316 • Steel 316 - Nitronic 60 • Nitronic 60 - Nitronic 60 • Steel 316 - Steel 316 with a surface coating of Tribaloy T-400 The wear behavior of the selected materials was investigated using a test rig at Karlstad University based on the method of block-on-ring. The machine spins a cylinder that corresponds to the seat against a block which in this case corresponds to the disc edge. The tests were carried out at room temperature, 250 ˚C and 500 ˚C. The cylinder was spinning at a speed of 100 rpm while the block pressed against with a load of 50 N. A profilometer, micro-hardness tester and scanning electron microscope (SEM) were used to investigate the wear. A comparison was made based on the volume of material removed from the blocks, the maximum wear depth of the blocks and the maximum wear depth of the cylinder. Wear mechanisms, chemical composition and hardness profiles were used to explain differences in results. The results were compared with the steel 316 to steel 316 solution. The Tribaloy T-400 showed good properties to reduce the wear at room temperature. Block made of Tribaloy T-400 showed no wear, instead steel from the cylinder had been adhered to the blocks. At higher temperatures, the amount of adhered material on the tribaloy T-400 increased, resulting in more wear at the surface of the cylinder than at room temperature. The wear on the cylinder was comparable to the wear of a steel cylinder that slid against a steel block. Nitronic 60 against nitronic 60 exhibited a significant reduction in wear compared with steel to steel at room temperature. At higher temperatures, nitronic 60 against nitronic 60 was the test pair that exhibited least wear. Nitronic 60 is recommended for further examination as a material in both the disc edge and the seat in a butterfly valve to be used at both room temperature and elevated temperatures.
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