Compressors are an essential component in Gasco's plants that transfer enormous energy to process fluids in the form of pressure and velocity. Conventional process and piping design are based on consistent / steady operating parameters and at times designers overlook the possibilities of sudden changes in process parameters. Operating conditions are sometimes altered as part of capacity/performance enhancements. These changes result in difference between design and operating conditions of the compressor which can also impact associated piping systems.
In one of Gasco's plants, abnormal vibrations upstream of anti-surge valve piping at Expander-Compressor, were observed. Sustained vibrations are a threat to critical inline instruments and mechanical components thereby posing integrity risk and potential Loss of Process Containment (LOPC). LOPC can have significant impact not only to process, personal and environment safety but could also lead to production loss with impact on business and reputation.
This paper addresses the study carried out to identify the root cause of vibration, and the remedial measures recommended and implemented to mitigate the risks due to piping system vibration.
An exhaustive multi-discipline investigation was carried out comprising of site visits, data collection, detailed review of existing documentation, as-built conditions and current operating parameters. Process and control system review did not indicate any abnormalities. In addition, piping vibration induced due to compressor or anti-surge valve operation was ruled out as cause based on design review of anti-surge valve and expander compressor/turbine.
Piping system review concluded that inadequate pipe supporting was the primary reason for piping vibration. Comprehensive Stress Analysis (static and dynamic) was conducted to determine the modifications required to mitigate vibrations. The stress analysis accounted for actual operating cases in addition to the design conditions.
Based on stress / vibration analysis, pipe support modifications were proposed to make the piping system adequately rigid and minimize vibration. This resulted in increase of system's natural frequency while ensuring that the stresses are within permissible limits. Proposed modifications were designed for implementation without the need for shutdown.
Post implementation, piping Vibration were reduced drastically and were found to be within acceptable limits specified in Energy Institute guidelines. This has resulted in increased personnel safety, reduced environmental and operating risks, reduced downtime and failures, and preservation of company assets and reputation.
Due to the criticality of the process / equipment and non-availability of near term shut-down window, the main challenge in this exercise was to provide modification solutions which can be implemented while the compressor / plant is online. Innovative approach to design of support system utilizing in-house expertise, and continuous support during implementation are key takeaways from this project.