Lessons Learned in Pressure and Flow Measurement of SCO2 Systems From the STEP Demo Facility

Abstract Measurement of supercritical CO2 pressure and flow can pose challenges not seen with other fluids and, especially, ideal gases. This paper describes several of the flow measurement challenges experienced at the STEP facility as well as the solutions implemented. Many flow meters within the facility rely on differential pressure measurements across an orifice plate flow meter. These differential pressure transducers utilize relatively long static sensing lines to the orifice flanges. Because of the specific operating pressure and temperature of some flow meters, CO2 within the sensing lines is believed to condense to a liquid or a dense phase if no mitigation steps are taken. If the high- and low-pressure lines are not routed identically, this densification can lead to significant hydrostatic head differences and negatively affect the differential pressure measurement, and, therefore, the flow measurements. One effective mitigation measure was adding heat trace and insulation to maintain a sensing line temperature well above the critical point of CO2. The presence of fluids other than CO2 can also lead to these hydrostatic head differences due to density differences at pressure, especially if the tube routing creates low or high point “traps”. Care must be taken to purge other fluids from each sensing line prior to operation. This paper presents operating data before and after several mitigation measures were implemented, demonstrating the effectiveness of the measures in improving flow measurements. Flow tests were performed with multiple inline flow meters to confirm measurement improvement through relative mass flow rate agreement of the upstream and downstream meters. CO2 was pumped from a liquid storage tank and vaporized before passing through one of two legs of the test setup. Various pressures, temperatures, and flow rates were investigated to help determine when the flow discrepancies were most prevalent, and to verify that the mitigation measures implemented were effective across a range of conditions.