Failure analysis of pressure letdown valve from the Synthoil Process Development Unit

Several processes are being developed to convert coal with high S and ash contents to fuel oil with significantly lower concentrations of these constituents. One such process is Synthoil, a one-step hydrodesulfurization process that accomplishes over 94% conversion to oil in a short time. The reaction is carried out at temperatures around 450/sup 0/C and pressures of 2000 and 4000 psig. One area subject to excessive wear is the pressure letdown valve on the product oil line. At 900 kg/day PDU at PERC, these valves, which operate at a pressure differential of 4000 psi and a temperature of 125/sup 0/C must be replaced after about 1000 hr. One of these valves that failed in a typical manner after 915 hr was examined. It contained a replaceable trim set made of cemented WC inserts brazed to the valve plug and press-fitted to the valve seat. Failure of the valve was due to localized erosion of the WC inserts. The failure mode was preferential erosion-corrosion of the Co-Cr binder, allowing the hard WC particles to be washed away in the product oil. A full-scale Plexiglas model of the valve was made and it was found that slight misalignments of the plug with respect to the seat cause large changes in flow patterns through the valve. Many factors can change the local rate of erosion at a plug-to-seat region. In this case eccentricity of the trim set probably contributed to uneven flow aroundthe plug, and progressively higher local flow rates with increasing wear caused failure. The eccentricity may have arisen during manufacture or during service as a result of cracking of the stem-to-tip braze.