High Speed Reciprocating Compressors – The Importance of Interactive Modeling

Cost-effective, reliable operation of reciprocating compressors requires a balance between pulsation control and performance. With high-speed units, acoustic excitation frequencies more severely overlap the acoustic and mechanical response frequencies of the passages and the piping system. To achieve good cylinder performance and pulsation control, the designer must more than ever understand the capabilities and limitations of the tools used for design. Internal gas passage acoustics, cylinder-to-cylinder and cylinder-to-piping interactions, and non-linear flow losses can have a significant effect on the accuracy of the design predictions. Industry standard digital models typically solve the linear frequency domain piping response equations coupled to a piston face velocity source. Typically, these models are non-interactive with the piping and, therefore, the predicted cylinder performance does not accurately reflect the acoustics of the cylinder passages or the attached piping. Analog models (which also solve the linear acoustics of the piping system) do, in a limited fashion, allow interaction of the piping and the cylinder. However, this interaction is controlled by the very nature of the analog electrical components and, for high-speed units, the analog can under predict valve and pulsation effects. The Gas Machinery Research Council (GMRC) has funded an effort to develop a digital design system with full interaction between cylinder and piping components, enhanced to meet the demanding requirements of today’s high speed compressor packages. This paper presents the initial results of this effort. This enhanced model accurately reflects the effect of the internal gas passages and piping acoustics on the performance of the cylinder. A comparison is made between field data, predictions from the enhanced digital model, predictions from a non-interactive cylinder and piping digital model, and predictions from an analog simulation. Differences in the capabilities of the various design tools investigated are summarized