The results of an analysis of a CA6NM plug and cage control valve which seized in hydroprocessing service are presented. The valve was found to have seized due to the accumulation of corrosion deposits resulting from a relatively brief exposure to a high temperature hydrogen / hydrogen sulfide environment. Wear surfaces on the valve were nitrided to prevent galling. Nitrided surfaces were found to have a significantly lower corrosion resistance than predicted by the Couper-Gorman curves for 12 Cr stainless steels. This high corrosion rate is explained by depletion of chromium in solution due to the formation of chromium nitrides.
Following the commissioning of a new hydroprocessing unit at a Middle Eastern refinery in July 2001, seizures of control valves for the recycle gas heater were reported in January, April, and June of 2002. On stream time between the January, April, and June failures was about 60 days in each case. No abnormal operating conditions were reported during these periods. Figure 1 is a diagram showing the general arrangement of the unit and the location of the valve bank. The normal recycle gas stream is composed of 90% hydrogen, 3 ? 4% hydrogen sulfide and 6 ? 7% light hydrocarbon. The normal operating pressure and temperature of the recycle gas stream at the location of the valve bank were 17.25 MPa (2500 psi) and 400o C (752o F) respectively.
The valve bank, shown in Figure 2, consisted of ten - 10 cm (4 inch), balanced, plug and cage valves, mounted in parallel. The purpose of the valves was to ensure uniform flow distribution to the ten heater passes. Seizure of the valves resulted in the inability of the refinery to balance the flow between heater passes and created the potential for maldistribution of flow within the furnace. The refinery was, however, able to operate the unit for some time at full capacity following the seizures. The valves were designed for automatic control but were often controlled manually. Figure 3 is a cross section of the valves showing the valve body, the trim (plug and cage) and the seat. The recycle stream enters the valve from the left, passes through the holes in the cage into the center of the cage and then exits the valve to the right. Throttling of the flow is accomplished through up and down movement of the plug which covers or exposes the holes. The plug has holes in its upper surface which allow gases to pass to and from a dead space under the valve bonnet. This ensures that pressures on both sides of the plug are equal and minimizes the forces required to move the plug.