Flow-Induced Vibration Assessment and Mitigation for Compressor Station Expansion

Abstract Compressor station expansion projects present a risk of fatigue failure due to flow-induced vibration. Flow-induced vibration mechanisms relevant to compressor stations include flow-induced turbulence, vortex-induced vibration on barred tees and thermowells, and flow-induced pulsations within deadlegs and strainers. Risk and severity of these vibration mechanisms increase with flow rate. Methodologies for assessing each are presented. The flow-induced turbulence mechanism is excitation due to turbulent kinetic energy in the flow. Screening assessment approach provided by the Energy Institute demonstrates that gas pipelines typically have low turbulent excitation provided the pipework is well supported. Barred (also called pigging or scraper) tees are commonly found in gas pipeline stations adjacent to pig barrels. They can be a vibration and failure risk due to flow-induced vibrations. Flow through the bars can create vortex shedding, similar to other bluff bodies in fluid flow, and excitation forces at the wake frequency. These forces can then excite the lateral modes of the bars. Common bar designs have several different bar lengths, each with their own natural frequency of vibration. This range of vibration frequencies increases the chance of resonance where a vortex shedding frequency coincides with a natural frequency of a bar. Other complicating factors are the non-uniform flow profile as the flow goes from the main header into the branch and the non-symmetrical shape of the bars that have a radius on the header side. Flow-induced pulsations from deadlegs occur when the vortex shedding frequency across a branch opening coincides with an acoustical natural frequency of the deadleg. These pulsations create shaking forces which can lead to high vibration and risk of fatigue failure. Vibration is amplified if the pulsation frequency coincides with a mechanical natural frequency of the pipework. A methodology is presented for identifying deadleg pulsation and assessing the resulting vibratory stress in the piping system. Inline cone strainers can generate very high-amplitude high-frequency vibration and noise local to the tee. Fatigue failure can occur. Strainer noise and vibration is known to occur when vortices are shed through the strainer holes at a frequency that coincides with acoustical cross modes in the pipe. This is further amplified if the acoustic mode coincides with a mechanical shell mode in the pipe wall. Both vibration and noise field data will be presented for the above mechanisms with techniques for identifying and evaluating risk during operation. Simple solutions will be given for reducing the risk of fatigue failure.