TRANSIENT IMPACT OF VALVE CLOSURE TIMES - DISAGREEMENTS BETWEEN DESIGN AND APPLICATION

摘要

Many pressurized liquid systems require emergency shut down procedures in order to prevent damage to the piping and components, environmental contamination and fire hazard. The emergency shutdowns (ESDs) are facilitated by fast closing on-off valves installed at various locations along the piping system. When these valves close they create transient pressure waves traveling through the pipe network. These waves can be reflected at the dead-ends or closed valves. At locations where the pressure decreases below the vapor pressure, liquid column separation followed by a rejoining can cause creation of new transient pressure waves. As these waves travel, they may meet and superpose. These complex surge pressure wave behaviors require modeling of the pipe network and simulation of the transient event as the first step of a transient analysis. The second step of the transient analysis is to pin point the problems such as excessive surge pressures and dynamic loads that may occur in the system. The third step is to provide recommendations to prevent undesired transient consequences. One of the most important components of these recommendations include valve closure times during ESDs. Recent field measurements on the valve closure rates showed that the valve closure times recommended by the transient analysis were not accurately implemented. One reason for this disagreement between the designed closure rates and the applied closure rates is that the actuators of the valves introduce a time lag between the shutdown signal and start of valve closure. Another reason comes from the decision taken by the operator adjusting the actuator timing. Some operators may adjust the actuators such that the valves close within the prescribed time including the lag time which may result in very fast valve closures depending on the lag of the actuators. Other operators may choose to close the valves within the prescribed time including the lag time or even slower than the recommended rates. This may impair the orchestrate of the valve closure events designed in the transient analysis resulting in excessive surge pressures or dynamic loads. This study investigates (i) the discrepancies between the recommendations from transient study made early in the design stage and (ii) the transient impact due to the deviation and/or misinterpretation of those recommendations. Specifically, in this study, these problems are demonstrated in a case study from LNG - Ship loading systems. The results indicated that transient analysis is the essential tool in finding critical components of the system in the field conditions providing a variety of solutions such as valve closure rate adjustments, flow rate reduction at the beginning of ESDs via pump trips and pipe size increase at dead legs. This study showed that the pressure piping systems can deviate from initial design under dynamic field conditions and frequent inspections of the ESD valves are crucial for safe operations of these systems.