CORRECTION FOR K FACTOR OF GAS TURBINE FLOW METER

Xue Chunling, Lv Deyue, Sun Yanzuo

Abstract

According to the International Standard ISO 9951 all gas turbine flow meters should have a maximum permissible relative error of ± 1% over the higher flow range (from 0.2 <i>Q</i>_max to <i>Q</i>_max), in the lower flow range (<i>Q</i>_min to 0.2 <i>Q</i>_min) the maximum permissible relative error is ± 2%. Usually the linearity of <i>k</i> factors, in percent, is used to express the basic relative error of a gas turbine meter. That means in the higher flow range the gas turbine meters shall have maximum permissible linearity of ± 1%, and in the lower flow range the gas turbine meters shall have maximum permissible linearity of ± 2%. The method commonly used to determine the mean <i>k</i> factor, <i>K</i>₀, is: At first, the values of <i>K</i><sub>max</sub> and <i>K</i><sub>min</sub> are found out in the higher flow range, and then the mean <i>K</i> factor, <i>K</i>₀, is calculated. Finally, the maximum deviation of <i>K</i> factors from <i>K</i>₀ in the higher and lower flow ranges are evaluated to make an error judgment. In some cases this method might introduce a bias error into the <i>K</i>₀, because the shapes of <i>K</i> factor signature curves ( the different modes of <i>K</i> factors distribution in different flow range) were not considered. Therefore, it is necessary to make different corrections for <i>K</i>₀ in different cases. Our approaches are as follows: When <i>K</i><sub>min</sub> appears in the lower flow range and the <i>K</i><sub>max</sub> appears in the higher flow range, <i>K</i>₀ = (<i>A</i> <i>K</i><sub>max</sub> + <i>K</i><sub>min</sub>) / 2, here <i>A</i> = 1.0099; When the <i>K</i> max appears in the lower flow range and the <i>K</i><sub>min</sub> appears in the higher flow range, <i>K</i>₀ = (<i>B</i> <i>K</i><sub>max</sub> + <i>K</i><sub>min</sub>) / 2, here <i>B</i> = 0.9902; When both the <i>K</i><sub>max</sub> and the <i>K</i><sub>min</sub> appear simultaneously in the same lower or higher flow range, <i>K</i>₀ =(<i>C</i> <i>K</i><sub>max</sub> + <i>K</i><sub>min</sub>) / 2, here <i>C</i> = 1.0. During the individual calibration of each turbine meter this method has shown clear positive effects. While consideration was given to meter’s performance in the higher flow range, the performance of turbine meter in the lower flow range has been improved, therefore, more gas turbine meters are ensured to be up to standard. Based on the European Standard PrEN12261:1998 Turbine Gas Meter, the WME (Weighted Mean Errors) were calculated. The WME show that this is an easy method to obtain the proper value of mean K factor for gas turbine meter.

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DOI

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IMEKO TC

TC9 - Flow Measurement

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