Modeling of Pressure Pulsations for LDPE Reciprocating Compressors and Interaction With Mechanical System

Abstract

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.

Preliminary Piping Sizing and Pressure Pulsation Evaluation

“…The unsteady gas flow of reciprocating compressors generates pressure pulsations that might be increased by acoustical resonance. Depending on plant configuration and fluid characteristics, this can cause high vibrations, high noise levels and reduced performance [6,7,8]. Therefore, it is necessary to make very detailed analyses -particularly for high-pressure piping -to prevent failures from cyclic stresses that result from: The studies to be performed are:…”

“…-A bid-stage evaluation of residual pressure pulsation -A preliminary-acoustical analysis at the beginning of the project -A final acoustical study based on final piping data • A mechanical analysis of the complete system, to avoid mechanical resonance and prevent dangerous vibrations and stresses [3,4] In hyper applications ( Figure 1) acoustic filters (volume bottles or dampeners) are generally too expensive to be used because of the high walls thickness and auto-frettage treatment that would be needed to manufacture them [6].…”

“…In all complex and critical plants, the entire system subjected to pressure pulsation (i.e., to the proper boundary point) must be analyzed in all operating conditions [6]. This includes the piping, cylinders, valves, orifices, intercooler, pre-heater and tubular reactor.…”

“…The standing wave field is the sum of several harmonic components, each with a certain frequency, module and phase. The ideal position will be identified by fine-tuning trials, with the aim of damping as many harmonic components as possible [6,9]. If the first two steps are correctly executed, this last check will only require orifice adjustments for location and pressure drops that usually remain within previously estimated ranges.…”

“…These guidelines prevent significant modifications that otherwise would emerge from the final mechanical analysis. Typically the guidelines set the allowable values of [6]:…”