天然气的流量计算在智能孔板流量积算仪中的实现

对于天然气计量用的智能孔板流量积算仪，流量的计算方法对计量结果有非常重要的意义。而天然气的流量计算涉及参数多，计算过程复杂，选择合理的计算方法，不仅能够满足实时计算的要求，同时又能实现计算的精确性。从天然气流量计算的数学模型入手，充分利用单片机的现有资源，提出一种适用于智能孔板流量积算仪的天然气流量计算方法，并成功运用于所开发的积算仪中。经过实践检验，该方法满足了标准孔板流量计计量系统对准确度的要求。     

Natural gas flow computer with open architecture using intelligent instrumentation and field bus

A new approach to natural gas flow computer design is presented in this paper. The developed system runs on a personal computer and employs the state-of-the-art mathematical models for corrections of some aspects of fluid flow dynamics, as well as for compressible behavior of gaseous fluid considerations. Orifice plates were used as primary elements. Measurements were performed through intelligent sensors. Results of the system metrological tests are also presented.     

Examination of disturbed pipe flow and its effects on flow measurement using orifice plates

In a great number of measurements the influence of a disturbed flow on the flow coefficient of a standard orifice plate was investigated. Single bends and double bends out of plane with and without spacer tubes were used as typical disturbances. Experiments were also performed using a combination with a star-shaped flow straightener. The necessary correction factors of the flow coefficient were determined for upstream straight length shorter than detailed in ISO 5167. The flow velocity profiles produced by the disturbances were examined and on this basis profile numbers were calculated. The examinations presented here show that the existing standard should be revised as regards the definition of the fully developed turbulent flow profile and the selection of the required upstream straight lengths.     

The fractal flow conditioner for orifice plate flow meters

The sensitivity of orifice plate flow meters to the quality of the approaching flow continues to be a cause for concern in flow metering. The distortions caused by pipe fittings such as valves, bends, compressors and other devices located upstream of the orifice plate can lead to non-standard velocity profiles and give errors in measurement. The design of orifice plate meters that are independent of the initial flow conditions of the upstream is a major goal in flow metering. Either using a long straight pipe, or a flow conditioner upstream of an orifice plate, usually achieves this goal.The effect of a fractal flow conditioner for both standard and non-standard flow conditions was obtained in experimental work and also using simulations. The measurement of mass flow rate under different conditions and different Reynolds numbers was used to establish a change in discharge coefficient relative to a standard one. The experimental results using the fractal flow conditioner show that the combination of an orifice plate and a fractal flow conditioner is broadly insensitive to upstream disturbances.The simulation results also show that the device can be used as a part of a flow metering package that will considerably reduce installation lengths. Previous work with orifice plates has shown that a combination of flow conditioner and orifice plate was promising. The results of using a combination of the fractal flow conditioner and orifice plate for non-standard flow conditions including swirling flow and asymmetric flow show that this package can preserve the accuracy of metering up to the level required in the Standards.     

Flow rate measurement by an orifice in a slowly reciprocating gas flow

It is shown that small density changes can give rise to misinterpretation of flow rate signals in unsteady (reciprocating) flows. Basically a flow rate measured at some point A cannot simply be assigned to a remote point B. Depending on the way of plotting a hysteresis appears which, in fact, does not exist. Unsteady conservation of mass is applied to a volume and orifice flow system to obtain an equation which explains and predicts the apparent hysteresis. The equation in dimensionless form contains a key parameter β which holds the flow determining quantities. Experiments are conducted with respect to a wide spread of β. It is shown that the equation predicts reality quite well.     

An orifice meter for bidirectional air flow measurements: Influence of gas thermo-hygrometric content on static response and bidirectionality

This paper presents the design and calibration of an ISO non-compliant orifice plate flowmeter whose intended use is for respiratory function measurements in the bidirectional air flow range ±9 L/min.The novelty of the proposed sensor consists of a plate beveled in both upstream and downstream sides: a symmetrical geometry is adopted in order to perform bidirectional measurements of flow rate. A mathematical model is introduced to quantify the influence of temperature on the sensor output. Four different positions of the pressure static taps are evaluated in order to maximize bidirectionality. An index is also introduced in order to quantitatively estimate the anti-symmetry of the sensor's response curve.Trials are carried out to evaluate the influence on sensor output of air temperatures (22 °C, 30 °C and 37 °C) at different values of relative humidity (5%, 55% and 85%). Experimental data show a quite good agreement with the theoretical model (R²>0.98 in each condition).The influence of air temperature on the sensor output is minimized by introducing a correction factor based on the theoretical model leading to measurement repeatability better than 2% in overall range of calibration. The mean sensitivity in the calibration range is about 2 kPa L⁻¹·min allowing to obtain a sensor discrimination threshold lower than 0.2 L/min in both directions. The time constant of the whole measurement system, equal to 2.40±0.03 ms, leads to a bandwidth up to 80 Hz making the sensor suitable for respiratory function measurements.     

A study of ultrasonic propagation for ultrasonic flow rate measurement

For the purpose of accurate flow measurement, an automatic three-dimensional (3D) sound field measurement system has been developed, and an experimental study has been conducted on ultrasonic properties. By using this system, ultrasonic sound pressure distributions and radiation angles in water have been measured. According to Snell’s law, the ultrasonic transmission properties can be obtained on the basis of incidence angle, acoustic impedance, basic frequency of ultrasound, and material and thickness of the metallic plate. However, this law cannot be applied to certain cases where an ultrasonic incident wave passes through a metallic plate and turns into a longitudinal wave, a shear wave and a Lamb wave. Consequently, the ultrasonic propagation paths have been investigated experimentally at various angles of incidence. From the experiments, it was confirmed that the ultrasonic beam paths change with incidence angles. Hence, the most suitable incidence angle has been determined from the result of measurements. Velocity measurements using an ultrasonic velocity profiler were made at various incidence angles. The accuracy of measuring flow rates changed with the incidence angles. The optimal incidence angle determined from 3D field measurements was found to yield the most accurate flow rates.     

Uncertainty analysis of energy measurements in natural gas transmission networks

The value of natural gas depends on the energy obtainable from its combustion. Despite this, natural gas measurement and billing are normally performed using volume measurements subsequently converted into base conditions. Thus, to correctly achieve network balancing and accurate billings, both civil and industrial natural gas consumptions should be measured in energy. Unfortunately, energy measurements for natural gas are actually possible only in an indirect way by means of complex measurement chains with a flow-meter, a volume conversion device and a gas chromatograph or gas analyzer. Moreover, for technical and economic reasons, gas quality is often considered as constant and known despite the unavoidable variations due to the mixing of gases from different origins and type (i.e. importations, national productions, liquefied, biogas).In this paper the authors present the results of a detailed metrological experimental analysis of the typical energy measurement plants installed in natural gas networks. Modern networks are characterized by a wide variety of flow-rate measurement principles, constructive technologies and plant configurations. Therefore, flow regimes, thermodynamic conditions and chemical properties of the gas play a crucial role in determining metrological performance of natural gas energy measurements and uncertainties can become critical for inaccurate billing and unaccounted for gas.     

A neural network developed in a Foundation Fieldbus environment to calculate flow rates for compressible fluid

This paper proposes the development of an artificial neural network multilayer perceptron, implemented in a Foundation Fieldbus environment, to calculate the flow rate of natural gas by using an orifice plate in a closed pipe. The principal benefit of using neural networks lies in their low computational cost and simplicity of implementation, which allows just standard blocks to be used, making the technology independent of the Foundation Fieldbus system manufacturer. To perform the calculation, the proposed methodology relies on static pressure, temperature and differential pressure measurements, which are typically available in industrial plants. The developed methodology generates highly accurate results, and this approach can be implemented at a relatively low cost for Foundation Fieldbus system users.     

Theory of errors in Coriolis flowmeter readings due to compressibility of the fluid being metered

The compressibility of fluids in a Coriolis mass flowmeter can cause errors in the meter’s measurements of density and mass flow rate. These errors may be better described as errors due to the finite speed of sound in the fluid being metered, or due to the finite wavelength of sound at the operating frequency of the meter. In this paper, they are investigated theoretically and calculated to a first approximation (small degree of compressibility). The investigation is limited to straight beam-type (and does not consider shell-type) Coriolis meters. A lumped-parameter (coupled oscillator) model is used to explain the process causing the errors, and a simple 2-D continuum mechanics model is used to derive expressions for the magnitudes of the errors. Applications might be to Coriolis metering of gases, or to two-phase mixtures in the form of aerated liquids.     

色谱分析仪在天然气计量中的应用

天然气各组分含量是天然气计量的基础参数,天然气气质的准确分析对天然气计量至关重要。气相色谱法是测量天然气组成最常用的方法,气相色谱法具有选择性高、分离效率高、灵敏度高、分析速度快等特点。本文介绍了Pro GC-2000天然气色谱仪在绍兴天然气门站输气管道处的成功应用,通过在线色谱仪实时取样分析,得到天然气组成、密度、发热量、沃泊指数、压缩因子等物性参数,并将结果及时输入流量计算机中,实现对天然气流量和能量的及时修正,确保天然气计量准确可靠。本文阐述了色谱分析原理、结果分析方法、天然气计量系统组成和工作原理等。     

Time resolved measurement of pulsating flow using orifices

This article reviews the theoretical background of the measurement of pulsating flow using orifices as flow to pressure transducers, providing a synopsis of work done in this field. Special attention is paid to the temporal inertia and the applicability of expressions thereof given in the literature. Other factors influencing the measurement, such as changing flow profiles and the effect of connection tubes between the pressure sensor and the orifice are discussed. An experiment was performed to investigate the applicability of an equation taking reverse flow and temporal inertia into account for the measurement of pulsating flow with relative pulsation amplitudes around 1 and frequencies up to 50 Hz. It was found that the suggested equation may give tolerable results if the ratio of the pulsating part of the velocity to the angular frequency times orifice diameter is not too high. For high ratios, however, the results could not be explained by the suggested equation.