Gas Void Fraction Monitoring with Speed of Sound Measurements for Hydraulic System Aeration Monitoring

Knowledge of the level of entrained air in pump inlets is important for hydraulic system design. Entrained air levels, often quantified as gas void fractions, can change with operation of hydraulic systems, so monitoring entrained air during operations can provide substantial design insight to a hydraulic system. The performance of a SONAR-based entrained air measurement system was investigated on a test hydraulic system utilizing a Coriolis meter as a reference. Two instances of the measurement system were installed on the inlet line of a hydraulic pump in series with a Coriolis meter. One instance of the SONAR-based entrained air measurement system was installed to measure the gas void fraction (GVF) within the flow tubes of a Coriolis flow meter, and a second instance of the SONAR-based entrained air measurement system was installed to measure the GVF within in a section of hydraulic hose in series with the Coriolis meter. Data were recorded to evaluate the SONAR-based entrained air measurement systems both upstream and downstream, as well as collocated with, the Coriolis meter. Speed of sound (SOS) measurements were acquired within the aerated hydraulic oil in the inlet line as the GVF was varied utilizing a variable area flow restriction installed within the pump inlet line. Increasing the restriction reduced the pressure and increased the GVF within the hydraulic fluid due to (1) the existing gas expanding and (2) additional out-gassing from the hydraulic oil. The measured SOS was utilized to determine a gas volume fraction (GVF) within each instance of the SONAR-based GVF measurement system. Additionally, GVF was also calculated from the density measurements of a Coriolis flow meter. GVF measurements from the SOS measurement test section across the Coriolis meter were highly correlated with GVF calculated from Coriolis meter density measurements. The SOS GVFs were shifted up by up to 0.126 % GVF and matched with <0.2% RMSE. For the upstream or downstream SOS GVF measurements, there was variation from those calculated from Coriolis meter density measurements. These differences were anticipated due to pressure differences in the pipe section from that in the Coriolis meter. These pressure differences resulted in differences in the air released from the hydraulic fluid and differing GVFs in the two sections. These results show that the SOS measurement technique can accurately measure the entrained air status of hydraulic systems.