物性值法计算天然气压缩因子适应性分析

Applicability analysis on calculating natural gas compression factor by physical property method

目的随着能量计量工作的不断推进,业内出现了一些采用物性关联技术对天然气发热量、相对密度等参数进行在线测试的仪器设备,其应用须结合GB/T 17747.3-2011《天然气压缩因子的计算第3部分:用物性值进行计算》推荐的“物性值法”计算工况压缩因子,物性值法在国内外的应用较少,对其适应性和准确度进行分析探讨,可为相关新技术研发和选用提供参考,有利于保障和促进新兴测试手段在天然气能量计量中的有效应用。方法以GB/T 17747.2-2011《天然气压缩因子的计算第2部分:用摩尔组成进行计算》推荐的“详细组成法”计算结果为主要参考,选用了国内典型的149个天然气组成以及部分模拟天然气组成数据,理论对比分析了详细组成法和物性值法计算天然气压缩因子的相对偏差,探讨了用物性值法计算天然气压缩因子的适用性。结果①对于绝大多数不同类型的商品天然气,采用物性值法与详细组成法计算得到的压缩因子的相对偏差在±0.10%以内;②当输入的高位发热量、相对密度满足±0.50%以内的准确度时,采用物性值法与详细组成法计算得到的压缩因子的相对偏差可满足GB/T 18603-2014《天然气计量系统技术要求》中对A、B级计量系统压缩因子0.30%的准确度要求;③对于GB/T 17747.3-2011适用范围内重烃含量相对较高的天然气,物性值法与详细组成法计算得到的压缩因子可能出现接近0.50%的相对偏差。结论对于绝大多数不同类型的商品天然气,采用物性值法计算压缩因子的方法是适应的,但对等效C 2+摩尔分数高于10%的天然气可能出现较大偏差,建议在能量计量系统方案选择过程中,结合详细组成数据开展不同方法计算压缩因子的差异比对分析,为选用技术经济性更佳的方案提供技术支持。

Objective With continuous progress of energy measurement,some new technologies and equipments for online determination of natural gas calorific value and relative density have emerged in natural gas industry.The physical property method for calculation of natural gas Z-factor specified in GB/T 17747.3-2011 Natural gas-Calculation of compression factor-Part 3:Calculation using physical properties is needed for the application of the before mentioned new measurement method.However,physical property method is seldom used since it was published,evaluating its applicability and accuracy is important for the improvement and effective application of new technologies and equipments in the energy measurement of natural gas.Methods In view of this,this paper assumes that the Z-factor calculated from detailed composition method in GB/T 17747.2-2011 Natural gas-Calculation of compression factor-Part 2:Calculation using molar-composition analysis gives the reference value,then theoretically compared the two algorithms based on 149 natural gases composition data from seven different gas sources and some simulated natural gases to exam the applicability of the physical property calculation method.Results The calculation bias of Z-factors between detailed composition method and physical property method are within±0.10%for most of natural gases and the accuracy of gross calorific value and relative density shall be within±0.50%at least to meet the 0.30%accurate requirements of calculated Z-factor by GB/T 18603-2014 Technical requirements of measuring system for natural gas.The study also discovers that bias of the calculated Z-factors between two algorithms can reach to surprising 0.50%when the content of heavy hydrocarbons is relatively high to reach to the upper limit of GB/T 17747.3-2011.Conclusions For most of the different sorts of commercial natural gases,the physical property calculation method is applicable.However,in some particular cases such as natural gas with equivalent C 2+content higher than 10%,the bias of the calculated Z-factors between two algorithms can reach to 0.50%.It is suggested to theoretically analyze the distribution of Z-factor deviations calculated by two methods before the final decision on energy measurement method is made.