Stiction : a hidden menace.

• Main Author: Leo, Evelyn Wan Ling.
• Other Authors: Vinay Kumar Kariwala
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering::Chemical engineering::Chemical processes
• Online Access: http://hdl.handle.net/10356/16503
• Institution: Nanyang Technological University

Valves are essential elements of a control loop. Their behavior determines the performance of control loops. A major cause of non-linearity in valves is stiction, which occurs due to overly tight packing surrounding the stems of valves. The presence of stiction causes abrupt valve movement and oscillations in process variables. These oscillations can propagate throughout the plant, resulting in plant-wide oscillations. Stiction is, therefore, a menace. Conventionally, valve stiction models are simulated in Matlab Simulink. The project, however, aimed to simulate valve stiction in Hysys because in Hysys, the thermodynamic properties are taken into consideration. Therefore, it is a closer replica of the plant process compared to Simulink. A Visual Basic script in Excel was written to introduce the valve stiction model into Hysys. The model was tested using the case studies of a simple flash separator and hydrodealkyalation of toluene (HDA) process. Results showed that valve stiction was successfully simulated, indicated by the oscillations in process variables. However, for a controller tuned to have a large integral time, the oscillations in the sticky valve cannot be observed clearly. Stiction is also observed to cause plant-wide oscillations. The script written can be easily incorporated into existing Hysys plant models, thereby aiding industrial professionals in the study of valve stiction. Future work can be done to combat the data storage limit in Excel by using Matlab to communicate with Hysys instead.