From Piping Deformation to Pressure Pulsation Measurements to Solve LDPE Plants Vibration Issues
Authors: Carcasci Cosimo , Sacco Marco , Landucci Marco , Fiaschi Marco 
Published: October 2018

Damages and failures in industrial plants are often related to vibration issues. Reciprocating compressors are typically affected by vibration phenomena due to the very nature of reciprocating motion as alternating forces and pressure pulsations are direct and inevitable consequences of reciprocation. Many preventive technical measures are undertaken in the detailed engineering design to avoid high levels of vibration, e.g. properly designed foundations, mass balancing, volume bottles, restriction orifices and piping supports. Nevertheless, vibration problems may still arise after a machine is installed in a plant and is started up, and often the vibration is not the result of a poor detailed design of the compressor itself but may depends on the piping and supports layout in the plant. Considering the extremely high pressures involved in the LDPE process (discharge pressure is generally between 160 and 350 Mpa), especially in tubular reactor plants, safety is a key consideration, and avoiding vibrations and consequential piping ruptures is essential for optimal and safe plant operation.
In a tubular reactor polyethylene plant, high piping vibrations were present on the Hypercompressor piping from the first machine start-up. Despite immediate analysis and small modifications to a few pipe supports, some areas continued to be subject to this phenomenon, potentially leading to ruptures, welding failures and hazardous gas leakages. Therefore, the end user decided to involve an independent third party consultant. During the site survey, the piping system was fully analysed to investigate the nature and the causes of the high vibrations, and it was decided that both vibration and pulsation measurements had to be performed, to obtain a complete and realistic picture of the phenomenon. While vibration measurement could be performed as a standard procedure on this kind of machine, pulsation measurement was a challenging operation since dynamic pressure transducers could not be used at such high pressures (above 100 MPa). Thus, an experimental technique was used. The pulsation measurements were performed using strain gage sensors that dynamically detected the circumferential deformation of the pipes. Information about the internal pressure was derived from the pipe deformation through the well-known theory of cylinders under internal pressure and in this way the pulsation measurements could be compared to the acoustical analysis performed during the detailed engineering phase. The analysis highlighted acoustic resonances that were not present in the project analysis, mainly due to an incorrect evaluation of the thermodynamic properties of ethylene gas, which changes significantly when the gas is subject to the high pressures at which the Hypercompressor works. Moreover, the vibration measurements were compared to the pulsations at some key points and to the mechanical natural frequency of the relevant piping segment, identifying also areas subject to mechanical resonance. After understanding the root cause of these vibrations, effective and low-impact countermeasures were recommended and implemented in a few key points, leading to a drastic reduction of the vibrations, below the limit values recommended by the standards.