Abstract
At several natural gas compressor stations control valves have experienced low-frequency vibrations when placed directly downstream of pipe bends. It is know that at pipe bends a pair of counter-rotating vortexes form and extend several diameters downstream. These vortexes are not symmetric and instead oscillate back and forth at a low-frequency phenomenon referred to as swirl-switching. It is suspected that these upstream flow oscillations enter the valve and are the cause of the valve vibrations. Large-eddy simulations have been performed on pipe bends with two different bend radius at a Reynolds number of 80,000. The simulations showed that the frequency of the vortexes decreased as the bend radius increased. When non-dimensionalized the frequency obtained from the simulation is comparable to the field measurement from the compressor station. This consistency provides support to the argument that upstream flow-induced oscillations are the cause of the low-frequency valve vibrations. The velocity fluctuations due to swirl-switching dissipate downstream of the bend and by placing several diameter lengths of straight pipe between a control valve and a pipe bend, the issue of low-frequency vibrations induced at the valve could be avoided. The objective of this study is to provide evidence that low-frequency valve vibrations can be linked to upstream pipe geometry and put forward a plausible explanation of why.
