Velocity and Pressure Characterization of Pulsations in Elevated Pressure Natural Gas and Effects on Ultrasonic Flow Meter Accuracy
Ultrasonic flow meters (USMs) are common tools in the natural gas industry used to measure gas flow rate with precision. They are highly advanced devices most frequently used in custody transfer, an application in which just a few tenths of a percent error can account for tens of thousands of dollars per day. Because USMs are relative newcomers to the field of natural gas flow measurement, sources of error are still being discovered and studied. One of the most poorly understood sources is pulsation, or a traveling pressure and velocity fluctuation in the flow. In this thesis, a USM was subjected to pulsations at frequencies between 5 Hz and 45 Hz in elevated pressure natural gas flow (Re = 470,000 and 1,540,000). A pulsation generator was used to create and control the pulsation frequency and amplitude. The meter error was measured at each frequency for two flow rates. The pulsations themselves were measured directly using high-speed piezoelectric pressure transducers and a traversing hotwire probe that was specially adapted to operate in natural gas. The hot-wire data was used to examine the direct effects of pulsations on flow profile. Due to the pulsations, USM measurement errors exceeding 1% were observed. Results suggest that the error cannot be predicted using maximum pulsation amplitude. The error was found to correlate with pulsation frequency, flow rate, and most significantly the amplitude of pulsation at the meter location. A regression fit using these variables predicts USM measurement error with an R2 of 95%.