| Ultrasonic methods have important advantages in the measurement of gas flow, including the ability to measure almost any gas or gas mixture over a wide pressure range and turndown ratio. Ultrasonic gas flowmeters also are capable of measuring bi-directional flow and may be non-intrusive with no pressure loss. In certain cases the sensors may even be clamped on the outside of the pipe. Traditionally the Transit-time method has been used, but more recently interesting developments in Tag Cross-correlation have yielded promising results, particularly for low-pressure clamp-on applications. Ultrasonic flowmeters for gas have been commercially available since the 1980s, and they are widely accepted today for flaregas and stackgas monitoring and for a wide variety of industrial and fuel gas measurements. Highly accurate multi-path meters are approved and commonly used for custody transfer measurement of natural gas. In addition to the many advantages ultrasonic methods have in the measurement of the gas flowrate, ultrasonic propagation characteristics of the gas may also be measured to determine properties of the gas, such as its molecular weight, density or energy content. Soundspeed, in conjunction with temperature and pressure measurement were first used in the 1980s to determine the molecular weight and derive the mass flowrate of flaregas. Ultrasonic flowmeters for custody transfer natural gas gained acceptance in the 1990s with the publication of the American Gas Association AGA-9 report, which includes guidelines on the measurement and use of the gas soundspeed. More recently gas soundspeed and other properties such as attenuation have been used for the analysis of binary gases and pseudo-binary gases such as breathing gases, biogas, landfill gas, and process gases. Acoustic impedance has been measured and used to determine the density of high pressure gases. This paper reviews the methods to analyze binary, ternary and multi-component gases . |