Examination of a 15. 24-cm ball valve from the Morgantown Energy Technology Center Lock Hopper Valve Testing and Development Project: failure analysis report

Abstract

A 15.24-cm ball valve failed while undergoing tests in simulated lock hopper service. The valve stem developed severe torsional deformation after 3134 cycles while handling limestone fines, and a replacement stem also exhibited deformation after an additional 124 cycles. The test was terminated at that point due to excessive valve leakage. Inspection of the valve at METC revealed large quantities of limestone fines packed between the valve ball and the body, and also revealed chipping of the chromium oxide coating around the valve stem slot. The valve seats, ball, and stem were sent to the Materials Science Division of Argonne National Laboratory for further examination. A visual examination at Argonne confirmed the general features seen at METC, and also revealed chipping of the coating on the overspray region of one of the valve seats. It was determined that the torque required to cause gross plastic deformation of the valve stem was well below that which the actuator was capable of applying in the valve test at METC. A microstructural examination of sections from each of the two valve seats indicated no significant debonding of the coating, and an independent analysis by the Linde Division of Union Carbide Corp. found the coating to meet the manufacturer's specifications. Variations in coating thickness on the valve ball of the order of 50% were observed using eddy-current techniques, but absolute values for the coating thicknesses were not obtained.Hardness readings on the valve-seat coating material were found to be slightly below the specified values, but the difference did not appear to be significant.