Hoi Yeung, Christos E. Papadopoulos
NATURAL GAS ENERGY FLOW (QUALITY) UNCERTAINTY ESTIMATION USING MONTE CARLO SIMULATION METHOD

In the natural gas market, open access along with gas brokering and marketing has resulted in multiple gas contracts through one physical measuring point. Accurate metering of natural gas has become more important than ever as de-regulation subjects pipeline companies to competition. A more competitive market is driving the need for real-time accurate electronic flow measurement. Modern electronic natural gas metering systems (ENGMS) introduced additional, though necessary, complexity in the estimation and verification of the reported results. Additionally, it becomes more and more important to be able to verify these results. The application of Monte Carlo simulation as a combined energy flow measurement uncertainty estimation method seems to offer specific advantages over the more complex, traditional uncertainty estimation methods while at the same time fully conforms with the method of the ISO/GUM, the authoritative document for uncertainty evaluation. Since Monte Carlo simulation relies on randomness, it seems to capture more naturally and more related to the underlying physics of measurement uncertainty. Typical comparisons of estimated uncertainties by the model and the conventional method (RSS) have been carried out. In general the Monte Carlo method gives slightly higher estimated uncertainties. This is due to the fact that the simplified conventional methods inevitably neglect correlations between the variables.

Josaphat Dias da Mata, Edvaldo A. Carrascosa
AN OPERATIONAL VIEW ON GAS MEASUREMENT USING ORIFICE PLATE DEVICE WITH LOW UNCERTAINTY

The new scenario for the Brazilian energetic policy points out to a steep growth on natural gas demand for diverse applications, such as electric energy generation and industrial, domestic or vehicular use. Therefore, the necessity of an intensified effort toward the improvement of the volume gas balance is increasing.
Recently there was a great technological advance in this area, covering equipments and procedures, which has contributed to the reduction of measurement errors. The modern flow meters present very low uncertainty. Although new equipments have appeared on the market, the orifice place maintains its preponderant status. Recent standards and procedures have reinforced the idea of painstaking work in order to minimize measurement errors.
As an attempt to standardize gas flow rate calculation with orifice plates and to assess the uncertainty of a defined meter run, two softwares were developed in Excel. All the calculations meet the newest standards for gas measurement and uncertainty.
The first software calculates the flow rates, generates tables for instantaneous flow rates and sizes an orifice plate for a given set of process data, using the criterion of minimum uncertainty. This software is useful for operational and design purposes.
The second software was developed in Excel with the objective of calculating the total uncertainty of the meter run, including the equipment and the metering installation. It executes a thorough diagnosis of the installation, identifying points out of the standard. This software is useful for operational and design purposes.
With the use of these tools, some modifications and improvements of gas measurement networks were carried out, based on the results obtained from the softwares. They are able to simulate several application ranges.
The advantages of these softwares are application easiness, standardization of gas calculation, easy process simulation, possibility of printing formatted reports, diagnosis, and personnel training. The softwares and the manuals permit a training update, covering measurement theory, application, errors, calibration, installation, etc.

Harry Dijstelbergen, Harry Bär, Henk Bellinga
REPEATABILITY AND UNCERTAINTY OF TCC’S ISLE DES CHENES TESTING FACILITY

The elements required to design TCC’s new high-pressure calibration facility for the least uncertainty are summarized. Results obtained during commissioning are shown, indicating the results according to the ISO Guide to the expression of Uncertainty of Measurements (GUM).

G. Cignolo, A. Rivetti, G. Martini, F. Alasia, G. Birello, G. La Piana
THE NATIONAL STANDARD GAS PROVERS OF THE IMGC-CNR

A small bell prover and a piston prover of large capacity recently installed in the new Gas Flow Laboratory of the IMGC-CNR (the national primary standards laboratory in Italy for mechanical and thermal quantities) are described. The provers have been completely revised, automated and equipped with state-of-the-art instrumentation for measurement of temperature, pressure, bell or piston displacement and time.
Both provers have been calibrated again soon after installation in the new laboratory and have been used successfully in two EUROMET comparisons carried out in 1998. The paper describes the adopted calibration methods and their results. The uncertainty budgets are given and discussed. The expanded uncertainty (k = 2) of the bell prover can be as low as 0.1% when its full capacity is used.
Thanks to the design criteria selected when constructing the larger prover (that is equipped with a piston of 1000 mm diameter) the mere “volumetric” uncertainty of the piston prover is an order of magnitude lower than that of any bell prover. Therefore, the expanded uncertainty associated with the volumes of gas delivered at the test point ranges between 0.01% and 0.1%, being almost completely dependent upon the stability of temperatures and pressures.

Kazuto Kawakita, Marcos Tadeu Pereira
EXPERIMENTAL INVESTIGATION ON CRITICAL FLOW THROUGH MICROORIFICES AND CAPILLARIES

With the purpose of studying application of critical flows through small orifices in the measurement and control of very low gas flowrates, experimental investigations were carried out on a set of 22 microorifices and capillaries of circular cross sections manufactured out of ruby and stainless steel. Diameters of the orifice and capillary samples ranged from 16 µm to 427 µm, and their lengths from 254 µm to 68 mm. The microorifices were submitted to tests using air, argon, helium and carbon dioxide, and the back to the upstream pressure ratio P_b / P₀ varied from unity down to about 0,1, both under pressure and vacuum operations. Results showed that, when operating under vacuum, the critical flow regime indicates that the discharge coefficient is stable and choking occurs even in flows with very small Reynolds numbers. However, when operating under pressure, the critical flow regime depends on the pressure ratio P_b / P₀ , and other parameters related to the gas properties and the orifice dimensions. Experimental results also showed that compressibility factor is a fundamental parameter to be considered in critical flows through microorifices, mainly when operating under a pressurized condition.