Tuning of the Acoustical Analysis Model for Hypercompressors Through Strain Gage Pulsation Measurements

Reciprocating compressor plants are typically exposed to vibrations, resulting from the discontinuity of the flow (pressure pulsations) and from external mechanical loads (compressor motion). To minimize the impact of pressure pulsations on vibration issues, a robust acoustical study is needed in the early design stage. Reliable standards and guidelines concerning vibration and pulsation are available for common applications (up to 350 bar) [1][2]. Within this pressure range it is possible to meet the standards by introducing pulsation suppression devices such as volume bottles or orifices. The Low-Density Polyethylene (LDPE) process requires very high pressures obtained with dedicated machines (Hypercompressors). There are no specific pulsation guidelines for these extreme pressures; in addiction reactive pulsation suppression devices like volume bottles are ineffective due to the high speed of sound in real gas (and related long wavelengths) at such pressures. This paper describes the case history of a plant which exhibited high piping vibration from the first machine start-up. A survey was made to measure vibrations and pressure pulsations by means of strain gages: internal pressure was derived from external deformation through pressure vessel theory. Strain gages were chosen because they can be easily installed without positioning restrictions, while dynamic pressure sensors require pressure taps and must be limited to specific points. Pulsation measurements were compared with an acoustic analysis, showing some discrepancy, especially at relatively high harmonics, mainly due to incorrect evaluation of the thermodynamic properties of high-pressure ethylene. In fact, in the LDPE process pressure range, isentropic exponent kv and compressibility factor Z can reach very high values, consequently affecting the calculation of the speed of sound. A new acoustic modelling was developed to achieve better consistency with the measurements. Pressure-dependent kv and Z were taken into account and a sensitivity analysis of the most relevant valve parameters was performed. Comparing the vibrations and pulsations spectra, once the MNF, compressor manifold arrangement and external loads are known, is fundamental to detect the origin of vibrations and how they are affected by pulsations.