| Popis: |
Control valves are widely used in modern plants in order to keep process variables within a required operating range and ensure both a quality product and efficient operation. Nonlinearities, such as deadband and high static friction (stiction), are the most common cause of oscillations in process variables. Since plants usually consist of various interacting control loops, even small oscillations propagate and disrupt the process parameters out of their operating range, risking accidents, increasing product rejection rates, and decreasing profitability. Packing friction is the primary source of nonlinearities for rotating and sliding-stem valves, and while it is often tested and defined in room temperature, it is common for packings to undergo one or more thermal cycles during their life cycle. A particular packing style may have low stem friction with little to no stiction in room temperature, but present a completely different behavior either in elevated temperatures or after cycling. Failure to ensure correct packing tightness and select proper packing material can disrupt an otherwise well-designed control loop. This study correlates the resulting valve stem friction for different packing styles when subjected to multiple assembly stresses and temperatures. Finally, those same styles undergo standardized fugitive emissions testing, where packing behavior is studied while undergoing thermal and mechanical cycles. |
