Gas Passage System Analysis for Modern Reciprocating Compressors, 18-R8120Printer Friendly Version
Inclusive Dates: 12/01/09 04/01/10
Background - Pulsation models of reciprocating compressor systems commonly use a one-dimensional representation with acoustic length modifications to represent the actual three-dimensional system. One-dimensional models are generally accurate for piping systems where the dominant physical length is in the flow direction. The issue of how to appropriately model the gas passage system is particularly important to high-speed reciprocating compressors (750 to 1,200 rpm or higher) with larger gas passage areas and high-energy excitation in the nozzle resonant frequency range (50 to 100 Hz). These modern high-speed compressors can experience severe compressor cylinder nozzle vibrations and pulsations, which can affect performance and reliability. Without accuracy in both frequency and amplitude predictions of the fluid-side pulsations within the gas passage, nozzle and bottle system, a significant acoustic resonance may exist and be overlooked by the compressor OEM, designer and/or operating company.
Approach - The subject research undertook an investigation of a new methodology and its benefits for advanced pulsation analysis of the cylinder gas passage system (which includes the cylinder nozzle and primary volume bottle). The intent was to determine the most effective method of predicting natural acoustic responses within the gas passage system and the potential value to operating companies. Many times the 3-D transient CFD model will be time and cost intensive and cannot be justified for standard design studies (API 618 acoustic analyses). However, the combination of a 3-D acoustic response model and a 1-D fluid representation model can provide accurate predictions of all gas passage system responses. The new SwRI methodology uses a multi-faceted design approach to predict the gas passage and cylinder nozzle responses at optimal frequencies for lower pressure drop compressor manifold systems. Specific conclusions of the research are summarized below.
Accomplishments - This research found that the 1-D representation of the gas passage significantly affects the pulsation model predictions for the gas passage system, nozzles and bottles. A validated 1-D fluid representation model can be used in combination with the larger piping system model. It is important, however, to use a more rigorous time-domain, non-linear fluid solution for the 1-D system, which will provide better accuracy than acoustic wave equation models (either time or frequency domain type).
In addition, results showed that the 3-D acoustic response model (carried through from the cylinder valves to the primary volume of the filter bottle) must be used to calibrate the 1-D fluid model for correct nozzle frequency response and gas passage response frequencies. The 1-D model must also include gas passage volume similarity to the 3-D model and correct damping due to the gas passage diameters and valve boundaries.
The investigation into the absorption of gas passage resonances at higher frequencies (100 to 300 Hz) showed value in the use of a resonator cavity within the gas passage. The miniature "virtual orifice" (VO) warrants further testing experimentally. The model predictions showed a reduction in pulsation amplitudes of approximately 20:1 for a high frequency valve-to-valve response at 270 Hz, evident in Case B. The mini VO would likely be designed to fit on the head or crank end of the cylinder and resemble a small pocket unloader in its overall dimensions. It would be designed to tune out problematic higher frequencies in the gas passage to potentially reduce cylinder vibrations and increase valve performance.