The present invention relates to instrumentation for measuring in real time the mass and the energy flow rate of gas through a pipe. In particular, it relates to apparatus for measuring the ratio of the mass flow rate of pipeline gas flowing through a pipeline compared to sample gas flowing through the apparatus. It also relates to apparatus for measuring the energy flow rate of gas through a pipeline.
Mass and energy flow rates of gas through pipelines are normally calculated by flow computers from contemporaneous measurements of several gas parameters. Generally, for measuring mass flow rate, the volumetric flow rate of the pipeline gas is measured and gas temperature, pressure, and composition are measured to enable the gas density and, thus, the mass flow rate to be calculated from the volumetric flow rate. The composition of the gas is normally measured by gas chromatography. When the operating conditions are such that the supercompressibility of the gas in the calculation of density cannot be ignored, supercompressibility properties are estimated from either the virial equations of state for the gas or from precalculated correlations such as NX-19.
Knowledge of the values of the virial coefficients of particular gas compositions is quite limited in the art, so the calculation of gas density from the virial equations of state is not always possible. Furthermore, correlations such as NX-19, for natural gas, are approximate and the accuracy of extrapolations from such correlations is questionable. It is therefore difficult to obtain accurate real time density values for calculating the mass flow rates of gas flowing through a pipeline with present day equipment.
When energy flow rate, in addition to the mass flow rate, is desired, the energy content of the gas must also be determined. The energy content of the gas (energy per unit mass or volume) can be determined either indirectly by measuring the composition of the gas or by direct measurements such as the stoichiometric ratio method. Once the energy content of the gas is determined, the energy flow rate of the gas through the pipeline can be calculated by multiplying the energy content of the gas (e.g. BTU/lb) by the mass flow rate of the gas (e.g. lbs./hr).
Each of the measurements discussed above (volumetric flow, temperature, pressure, and composition) are measured separately and introduce an opportunity for measurement error. The aggregation of these measurement errors can substantially distort mass and energy flow calculations. To minimize measurement errors, each piece of instrumentation must be maintained and calibrated periodically. Moreover, additional errors can be introduced within the flow computer from calculations or inaccurate formulas or correlations.
In U.S. Pat. No. 4,396,299, Clingman discloses a method and apparatus for measuring the rate of energy flow of gas through a pipeline. The Clingman invention, which flows sample gas through a calibrated capillary tube, is able to measure the energy flow of pipeline gas through a pipeline by sampling a constant fraction of the pipeline gas and measuring the mass flow of air which is burned with the sample gas at maximum flame temperature. In the Clingman invention, the mass flow rate of the sample gas varies in direct proportion with the mass flow rate of the gas through the pipeline. This direct variation is the basis for at least two shortcomings of the device disclosed in U.S. Pat. No. 4,396,299.
A first shortcoming is caused by fluctuating sample gas flow. The fluctuations cause difficulties in burning the sample gas to measure its energy content. In the Clingman invention, the sample gas must be burned with air at maximum flame temperature so that the energy content of the sample gas flow is proportional to the air mass flow. If the mass flow of the sample gas varies over a wide range, the flame is not always stable and temperature detection may be corrupted. If this happens, the measured energy content of the sample gas will not be accurate.
A second shortcoming caused by the direct variation of sample gas flow to pipeline gas flow is practical in nature. In the United States, large natural gas pipelines are normally metered using multiple parallel metering runs so that monitoring the flow through large diameter pipes (i.e.&gt;12") is avoided. In Clingman's invention, each of the multiple runs must be treated as an individual independent energy flow rate measurement since the Clingman invention requires that the sample gas flow rate be proportional to the flow rate of the gas flowing through each pipe being monitored.