Methodologies to mitigate attenuating effects in flare gas flow measurement by ultrasonic technology at low pressure and high carbon dioxide concentration

Name: Ligia Gaigher Franco
Type: MSc dissertation
Publication date: 29/05/2020

Namesort descending Role
Rogério Ramos Advisor *

Examining board:

Namesort descending Role
Márcio Coelho de Mattos External Examiner *
Marcio Ferreira Martins Internal Examiner *
Oscar Mauricio Hernandez Rodriguez External Examiner *
Rogério Ramos Advisor *

Summary: Worldwide, hydrocarbon flow measurement is governed by regulations. It is well known in literature that CO2 strongly attenuates wave generated by ultrasonic flow meters, which is the technology most used in flare gas application. In this context, this work proposes and tests methodologies to mitigate ultrasonic flow measurement failures due to signal attenuation in low pressure gas with high CO2 concentration. First, the flow meter is examined in zero-flow condition in order to isolate gas composition effect, establishing a reference state for further analysis. Although the meter didn’t fail, it is noticeable loss of signal power. Then, flow metering performance is evaluated in wind tunnel at several flow levels. For this purpose, a closed-circuit wind tunnel is projected, with hydrodynamic similarity between internal flow and natural gas flow. The velocity profile in test section is validated using laser doppler velocimetry technique. Failure criteria are applied to detect and account failure in ultrasonic flow metering readings. With transducers in reference position, flow measurement fault begins in 70% of CO2 at Qmin = 2,500 Nm³/h and in 45% of CO2 at Qmax = 9,300 Nm³/h. Two methods are tested in order to mitigate CO2 attenuation: i) transducers approximation and ii) recovery angle. With transducers approximation, there are no permanent failures up to 100% of CO2. Recovery angle strategy increased CO2 limit without failure to 84% at Qmin = 2,500 Nm³/h and 54% at Qmax = 9,300 Nm³/h.

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