Jianmei Feng

Attenuation of Gas Pulsation in a Reciprocating Compressor Piping System by Using a Volume-Choke-Volume Filter

This paper presents an investigation of the use of a volume-choke-volume low-pass filter to achieve gas pulsation attenuation in a reciprocating compressor piping system, with a focus on its frequency response characteristics and influence on the actual attenuation effects. A three-dimensional acoustic model of the gas pulsation was established for a compressor discharge piping system with and without the volume-choke-volume filter, based on which the gas column natural frequencies of the piping system and the pressure wave profiles were predicted by means of the finite element method. The model was validated by comparing the predicted results with the experimental data. The results showed that the characteristic frequency of the filter was sensitive to both diameter and length of the choke but independent of the parameters of the piping beyond the filter. It is worth noting that the characteristic frequency of the filter constituted one order of the gas column natural frequencies of the piping system with the filter. The pressure pulsation levels in the piping system downstream of the filter could be significantly attenuated especially for the pulsation components at frequencies above the filters characteristic frequency. The measured peak-to-peak pressure pulsation at the outlet of the filter was approximately 61.7% lower than that of the surge bottle with the same volume.

Modeling the valve dynamics in a reciprocating compressor based on two-dimensional computational fluid dynamic numerical simulation

This article presents a numerical simulation of the thermodynamic process in the cylinder and dynamics of the self-acting valves for an air reciprocating compressor. The finite-volume method was employed to solve the compressible turbulent flow in the cylinder and through the valves. A single degree-of-freedom model was adopted to simulate the valve dynamics. The piston’s motion was defined by a user’s preset function, and the fluid-structure interaction between the gas flow and the valve dynamics was solved in a strictly coupled fashion. The technique of non-conformal interfaces was adopted to allow fluxes between adjacent zones with different mesh node locations. Based on the analysis of the characteristics of the flow through valves and the valve movement, sensitivity analysis on the valve impact velocity and the angle of inclination indicates that the valve impact velocity was more sensitive to the variation of the rotational speed and the valve lift while severe inclining motion occurs when the valves are installed in the radial direction. Instantaneous gas flow and force coefficients with the variation of valve displacement are also obtained according to the mass-flow rate across the valve and the gas force acting on the ring plate.

Influence of an Orifice Plate on Gas Pulsation in a Reciprocating Compressor Piping System

This paper presents an investigation of the influence of the orifice plate parameters and installation positions on the attenuation of gas pulsation in a reciprocating compressor piping system. The acoustic wave theory and transfer matrix approach were applied to establish the simulation model, in which the valve chamber was assumed to be the pipe–volume–pipe element. Based on the model, the effects of the size and installation positions of the orifice plate on the gas column natural frequencies and pressure pulsation amplitudes were analyzed for the discharge piping system of a two-stage reciprocating air compressor. A test rig was built to validate the simulation results. The gas column natural frequencies and pressure pulsation amplitudes at different locations of the piping system were measured to verify the model. A favorable agreement was noted, with a maximum error of 2.1% for the natural frequencies and 6.3% for the pulsating amplitudes. The influence of the orifice plate on the gas column natural frequency varied according to its position and parameters. The results showed that all orders of natural frequencies decreased slightly as the inner diameter of the orifice plate decreased when the orifice plate was installed downstream of the vessel. However, the distribution of the gas column natural frequency changed when the orifice plate was installed upstream of the vessel. The pressure fluctuations in the piping system could be attenuated substantially by placing an orifice plate of reasonable parameter downstream of the vessel, within a distance of 0.4u2009m. The degree to which the orifice plate could attenuate the gas pulsation varied under different operating conditions. However, its attenuation effect was more sensitive to the compressor speed than to the discharge pressure.

Attenuation of Gas Pulsation in the Valve Chamber of a Reciprocating Compressor Using the Helmholtz Resonator

This paper presents testing and analysis results associated with a new control method based on the Helmholtz resonator to suppress the pressure pulsations in the valve chamber and cylinder nozzle of a reciprocating compressor. The characteristic response of the designed Helmholtz resonator was analyzed and its attenuation characteristics on the gas pulsation were investigated. A three-dimensional acoustic model of the gas pulsation was established by means of the finite element method (FEM) for a compressor discharge piping system with and without the resonator. The gas column natural frequencies of the piping system and the pressure wave profiles were predicted using the presented model and validated by comparing the simulated results with the experimental data. The results showed that the pressure pulsating amplitude in the valve chamber was reduced by 40.4% when the resonator was installed. If the resonance frequency of the resonator shifted from the cylinder nozzle characteristic frequency by a range of ±13%, the reduction in the pressure fluctuations within the valve chamber was about 24%. The best attenuation effectiveness on the valve chamber, a reduction of 47%, was obtained when two resonators were installed on the valve covers of both the head and crank ends. Two new frequencies of 40.4 Hz and 66.9 Hz appeared to replace the original cylinder nozzle characteristic frequency of 53.9 Hz with the Helmholtz resonator installation, and the corresponding resonance region was transferred from the valve chamber to the resonator.

Vibration analysis and control of a screw compressor outlet piping system

The severe vibration of a screw compressor outlet piping system caused the premature failures of some thermowells and the unplanned shut down of the compressor system. The root causes of the vibration problem were investigated by numerical simulations. An acoustic model was established to predict the pressure pulsation of the gas inside the pipelines, and two finite element models were built to study the vibration characteristics of the overall piping system and of the thermowells. The numerical results indicated that the vibration problem may be attributed to the excessive pressure nonuniformity of the gas inside the pipelines, low overall stiffness of the piping system and the first-order structural resonance occurred on the thermowells. A pulsation attenuator was added and the pipelines were rearranged to reduce the pressure pulsation, some pipe supports were added to improve the overall stiffness of the piping system, and the thermowells were reinforced to avoid the first-order structural resonance. After the modifications, the field measurement data showed that the vibration of the piping system decreased significantly, and the modified piping system has been operating normally for nearly two years.

Numerical Simulation and Experimental Validation of Failure Caused by Vibration of a Fan

This paper presents the root cause analysis of an unexpected fracture occurred on the blades of a motor fan used in a natural gas reciprocating compressor unit. A finite element model was established to investigate the natural frequencies and modal shapes of the fan, and a modal test was performed to verify the numerical results. It was indicated that the numerical results agreed well with experimental data. The third order natural frequency was close to the six times excitation frequency, and the corresponding modal shape was the combination of bending and torsional vibration, which consequently contributed to low-order resonance and fracture failure of the fan. The torsional moment obtained by a torsional vibration analysis of the compressor shaft system was exerted on the numerical model of the fan to evaluate the dynamic stress response of the fan. The results showed that the stress concentration regions on the numerical model were consistent with the location of fractures on the fan. Based on the numerical simulation and experimental validation, some recommendations were given to improve the reliability of the motor fan.

An investigation into oil—gas two-phase leakage flow through micro gaps in oil-injected compressors

Abstract This article presents the investigation on the oil—gas two-phase leakage flow through the micro gaps in oil-injected compressors and provides a new way of investigating the internal leakage process in the compressors. The oil—gas leakage rates were measured through the micro gaps of various gap sizes, the volume ratios of oil to gas, and pressure differences/ratios; and the flow patterns reflecting the flow characteristics were observed by using a high-speed video. The experimental results showed that the leakage flowrate was significantly related to the flow patterns in the gap, which were similar to those found in the existing literature and agreed well with the predicted ones by the Weber number. The gas leakage flowrate through the gap increased rapidly with the increased pressure ratio until the pressure ratio reached the critical pressure ratio, which ranged from 1.8 to 2.7. At the critical pressure ratio, the flow pattern transition from churn flow to annular flow occurred, resulting in gas leakage driven by a different sealing mechanism. As the volume ratio of oil to gas increased by 0.5 per cent, the gas leakage flowrate decreased by 77 per cent.