PRESSURE AND TEMPERATURE EFFECTS FOR ORMEN LANGE ULTRASONIC GAS FLOW METERS

摘要

Ultrasonic gas flow meters (USMs) may be influenced by pressure and temperature in several ways. Change of the meter body's cross-sectional area (the "pipe bore") influences directly on the amount of gas flowing through the meter. Change of the ultrasonic path geometry (i.e. change of the inclination angles and lateral chord positions, caused by e.g. meter body diameter change and change of the orientation of the ultrasonic transducer ports) influences on the transit times and the numerical integration method of the meter. Change of the Reynolds number influences on the integration method. Change of the length of the ultrasonic transducer ports influences on the acoustic path lengths, and thus on the transit times. Likewise, change of the length of the ultrasonic transducers influences on the acoustic path lengths, and thus on the transit times. In addition, changes of the transducer properties such as the directivity, influences on the diffraction correction, and thus on the transit times. Some of these issues are addressed to some extent in current draft standards for such meters, such as the AGA-9 (1998) report, and the ISO/CD 17089-1 (August 2007). Other of these effects have not been described or treated in the literature. In the present paper, pressure and temperature effects have been investigated for 18" Elster-Instromet Q-Sonic 5 ultrasonic flow meters (USMs) to be operated in the Ormen Lange fiscal metering system at Nyhamna in Mere and Romsdal, Norway, from October 2007. Pressure and temperature changes from flow calibration (Westerbork, at 63 barg and 7 °C) to field operation (Ormen Lange, nominally at 230 barg and 40 °C) conditions are evaluated. The effects addressed are changes related to (a) the meter's cross-sectional area, (b) the ultrasonic path geometry (inclination angles and lateral chord positions), (c) length expansion of the ultrasonic transducer ports, (d) length expansion/compression of the ultrasonic transducers, and (e) Reynolds number correction. The various effects (a)-(e) contributing to the measurement error are discussed and quantified. Investigations are made using a combination of analytical modeling and finite element numerical modeling of the meter body and the ultrasonic transducers, combined with a model for USM numerical integration relevant for the Q-Sonic 5 multipath ultrasonic flow meter in question. It is shown that for the Ormen Lange application, investigation and evaluation of all of the factors (a)-(e) mentioned above have been necessary to evaluate the effect of pressure and temperature on the meter. Expressions for pressure and temperature effects on ultrasonic flow meters proposed in ISO/CD 17089-1 do not appear to be preferred for the Ormen Lange fiscal metering system. The study shows that pressure and temperature affects the Q-Sonic 5 by about 0.26 % in the Ormen Lange application. If this systematic measurement error is not corrected for, the Q-Sonic 5 will underestimate the volumetric flow rate by the same amount. Significant economic values are involved. Two correction factors are thus proposed for the Q-Sonic 5 in this application: (1) one "nominal P&T correction factor" (accounting for by far the largest part of the correction, about 0.26 %), and (2) an "instantaneous P&T correction factor" (accounting for small deviations in pressure and temperature from nominal to actual Ormen Lange conditions), which is typically an order of magnitude smaller than the nominal P&T correction factor. The correction factors and the individual contributors to these are discussed and quantified.