Flow Control - Jan 2012
case study Doing Diagnostics Real World Applications of Digital Valve Technology C ontrol valves may have a direct and significant impact on plant efficiency product quality and safety Valve performance is therefore a key focus as plant personnel face global competition and growing pressure to increase profits boost plant performance and improve process reliability These business realities have made valve diagnostics more important than ever To keep a plants control valves performing optimally plant personnel must be able to monitor valve performance and gather actionable information to use in diagnosing valve problems and planning maintenance activities And they need to have this information in hand before valve problems impact process performance Presented here are two case studies that illustrate how valve diagnostics can help operations personnel make important maintenance decisions that can potentially improve process integrity reduce maintenance costs and increase output Case Study 1 Mapping Out Maintenance Strategies with Offline Diagnostics A North American power plant was having difficulty controlling its turbine The 20 inch steam conditioning valve was moving erratically and the plant could not ramp up the power on the grid resulting in approximately 1 million in lost production each day Removing such a large valve requires a crane and careful logistical planning Offline valve diagnostics helped plant personnel determine whether this step was necessary The signature Figure 1 clearly showed abnormally high and constant friction throughout the full stroke of the valve Further the seating profile depicted a nonsharp transition as the plug was moving in and out of the seat The dotted lines in Figure 1 indicate what would have been a normal level of friction and good seating profile These diagnostics indicated that there was a serious issue with the internal parts of the control valve Because of this information the crew removed and disassembled the valve and found that surfaces of the plug and cage had been damaged by metal debris that had flown through the valve Figures 3 4 The debris had become lodged in the holes of the cage preventing the plug from sliding easily within the cage In addition the seat was damaged because the plug would slam into the seat Figure 1 The dotted lines in this graph show the expected signature for the steam conditioning valve described in Case Study 1 and the blue lines are the actual signature recorded Friction was twice expected levels preventing the valve from moving smoothly The valves seating profile also was abnormal with the opening line showing a shallow angle rather than a sharp transition from the closed position Figure 2 3 Metal debris was lodged in the cage of the steam conditioning valve severely damaging the guiding surfaces This caused the high levels of friction identified in the valve signature 22 January 2012 Flow Control
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