Always growing capacities, size of machines and performance requirements and often changing plant layout arrangement, determine a continuous challenge for designers of machines and plants. For hyper compressors and connected piping system, the goals of safety and reliability can be achieved only with a deep analysis of all operating conditions to determine pulsations and vibration effects. Innovative methods of simulation, modeling and technologies improve the possibility to evaluate piping and compressor behavior, allowing to obtain the efficiency and a smooth and safe plant operation.

The combination of booster-primary and hyper compressors reaches very high pressures in Low Density Polyethylene Plants. Pressure and flow pulsations due to the unsteady flow generate an acoustic energy that interacts with the mechanical system of the plant, causing mechanical vibrations. Pressure pulsation and vibration control on relatively high pressure piping is necessary to prevent failures caused by cyclic stresses due to both internal pressure fluctuations and vibrations generated by the compressor. To increase the compressor performance, safety and reliability designers require advanced methods of simulation to evaluate piping and compressor behavior during the initial stage of the project. The paper will investigate the modeling of the pulsation with the influence of fluid-dynamic aspect related to the gas and the relevant system interaction.

LDPE production developments require processing equipment with increased capacities and existing applications uprating. They include large reciprocating compressors with several cylinders operating at very high pressures. Selection of piping size, as these special components have very long delivery time, including reactors and intercoolers, requires a careful analysis to be performed at an early stage of a project to properly estimate the costs. A preliminary evaluation should be made during the bid stage using compressor data sheet and P&I diagram. A specific pre-study linked to compressor sizing program can be used for preliminary cost estimation. Prior to piping purchase order issue, based on the first issue of real layout around compressor, the above evaluation has to be confirmed by a preliminary pulsation study. Once the plant arrangement will be finalized, the final acoustic study will have to be performed. Usually such study will only result in minor adjustments to the design not involving the piping size.

The system consisting of the reciprocating compressor and associated bottles, known as the “Cylinder manifold” may potentially be the source and location of high vibration problems. Consequently special attention must be paid to the complete simulation of the system to assure smooth and safe operation. Applicable standards specify the items to be included in the study (crosshead guides, distance pieces, cylinder flanges, joints, supports, etc.). However only a model built using manufacturing drawings and validated by site measurements can provide a sufficient accurate description of the characteristics of these critical components and therefore realistic results. Knowledge of the frequencies and amplitudes of pulsation induced shaking forces defined by acoustical simulation, internal gas forces in the cylinder, and unbalanced mechanical forces and moments allows a proper forced response analysis of the cylinder manifold system to be performed. These forces are applied to the finite element model to calculate the relevant vibrations and stress amplitudes by performing a harmonic analysis. When the dynamic stresses are out of the limits it is necessary to go back to the cylinder manifold system analysis or to the acoustical study to find a solution using different supports, with lower shaking forces, or by modifying the volume bottle design. This enables an iterative analysis of the system until all requirements have been satisfied. Additional results of a forced response analysis are the reaction forces on the cylinder and discharge volume bottle supports. When the application requires a large and heavy acoustic damping system with consequently a low mechanical natural frequency, or the compressor speed is significantly high, the possibility of mechanical resonance in the first design is very high. Therefore the execution of these studies at a very early stage of the project is fundamental. The proper solution can be found only by close cooperation between the compressor manufacturer, end user, engineering contractor and vibration specialist.

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