Study On Measurement Accuracy Of Standard Orifice Flowmeter For Urban Gas

Standard orifice plate flowmeter has been widely applied on natural gas measurement for its simple structure, easy and fast operation and highly-developed technology without calibration. Therefore, improvement of measuring precision has profound significance on control of urban natural gas pipeline networks transmission error and assurance of fair gas measurement. Analysis of data provided by flow field simulation with FLUENT can be both theoretically and practically instructive in enhancement of orifice plate flowmeter measuring precision.Firstly, we have analyzed influence of flow rate, Reynolds number, structural factors of orifice (aperture ratio, oblique angle and orifice thickness) on discharge coefficient with caliber of DN80and DN150. The simulation results show that:(1) Relative error of discharge coefficient is small while flow rate is between1-15m/s, but bigger while flow rate is beyond15m/s. Therefore, flow rate should be lower than15m/s.(2) There exists small relative error of discharge coefficient with a moderate aperture ratio β (0.45-0.65), so β should be controlled in this range.(3) It is recommended to install the flowmeter with an oblique angle F of45°, at which relative error of discharge coefficient is the lowest.(4) Discharge coefficient relative error of flowmeter with caliber of DN80is small while orifice thickness E is between1mm and3mm and that increase as E>4mm, so E should be under4mm on flowmeter with caliber of DN80.Based on the analysis of flowmeter structure, we carried out further research on influence of measuring system on precision of flowmeter while combining measuring procedure and rectification apparatus of conventional orifice flowmeter at home and abroad. Influence of diffuser and manifold upstream of flowmeter on flow field at measuring section has been discussed and length of straight pipe has been determined under different situations. The results show that:(1) Length of straight pipe before flowmeter with19rectifier tubes is6times of caliber, which is11times shorter than that without19rectifier tubes.(2) Length of straight pipe before flowmeter with19rectifier tubes installed in the manifold opposite to inlet is8times of caliber, which is17times shorter than that without19rectifier tubes. Length of straight pipe before flowmeter with19rectifier tubes installed in the manifold not opposite to the inlet is10times of caliber, which is20times shorter than that without19rectifier tubes installed at the inlet.Recommendation obtained from the research on precision of orifice plate flowmeter simulated by Fluent can be applied to improve the measuring precision of orifice plate flowmeter, reduce length of straight tube before flowmeter, which avoids reduction of flowmeter precision resulting from non-standard configuration of sites. This can strictly control the transmission error of urban natural gas pipeline networks caused by flowmeter system and ensure the veracity and fairness of natural gas measuring at the connection.