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New high-pressure natural gas test-rig needs a new primary standard


To ensure the traceability of high-pressure natural gas measurements, PTB has been collaborating with Opens external link in new windowpigsar, which has been PTB's long-standing partner and part of the test-rig infrastructure of the OpenGrid Europe (OGE) gas transmission company for more than 25 years. At the beginning of 2021, pigsar will commission a new facility for gas meter calibration with high-pressure natural gas – the Closed Loop pigsar [1]. Thanks to this new facility, the maximum flowrate will increase from 6500 m³/h to 22000 m³/h, and the maximum pressure from 50 bar to 65 bar, compared to the previous facility. To keep the measurement uncertainties in the required range despite the increased measuring range, PTB has developed a new primary standard – the high-pressure comparator.

PTB's current high-pressure primary standard is a stainless-steel piston prover whose piston is displaced by the flowing gas. The pipe was honed – a mechanical process ensuring that the inner diameter of the pipe varies by less than 20 µm over its entire length, which is less than the thickness of a human hair. The piston prover allows a maximum flowrate of 480 m³/h to be measured with a measurement uncertainty of 0.065 %. By using several calibrated measuring instruments (so-called transfer standards) simultaneously in a multiple-step process, it is possible to guarantee a measurement uncertainty of 0.16 % at 6500 m³/h [1]. By extending pigsar's range from 6500 m³/h to 22000 m³/h, measurement uncertainties will initially increase by a few hundredths of a percent. To further reduce these uncertainties and to shorten the traceability chain, a primary standard with a higher flowrate has been developed.


 Figure 1: Piston plate with valves on the linear translation table. Source: manufacturer

The new high-pressure comparator was designed for flowrates up to 1850 m³/h; it is equipped with an active drive. The piston of the comparator is disk-shaped. This disk is actively driven through a pipe with an inner diameter of 600 mm by a linear translation table (see Figure 1). The drive and controller of the linear translation table enable the piston to move at the velocity of the gas flow without the build-up of a pressure difference between the front and the rear side of the piston. The projected measurement uncertainty of 0.10 % may be slightly higher than the uncertainty of the piston prover. However, due to the greater flowrate range of the primary standard and the associated shorter traceability chain for the maximum flowrate of 22000 m³/h, this represents an improvement for the Closed Loop pigsar [1]. The respective constructions of the piston prover and the high-pressure comparator are described in detail in [2]. The info box below also illustrates the need for small measurement uncertainties in the natural gas sector.


Measurement uncertainty and the natural gas sector
The measurement uncertainty is the range of values that can be reasonably attributed to the measured quantity. Thus, when considering a length of 1000 mm ± 1 mm, all values between 999 mm and 1001 mm are possible. In this case, the 1 mm uncertainty equals 0.1 %.
In Germany, approximately 85 billion cubic meters of natural gas are consumed every year. In addition, the volume transmitted through Germany amounts to approx. 100 billion cubic meters. These volumes are usually measured and billed more than once. Assuming that a cubic meter of natural gas is billed at approx. 0.5 € to the end user, this rough calculation would already result in a trading volume of 92 billion euros if measurement/billing occurs only once.
The gas transmission companies and the gas utilities aim to limit the difference between the volume of gas purchased and the volume delivered to 0.1 %.  


The working principle of the high-pressure comparator will be explained using the example of a turbine gas meter calibration process (see Figure 2). At the start of the calibration process, the set flowrate flows through the open bypass. Next, the closed piston is accelerated to the velocity of the gas flow, from its start position in the direction of the flow, and the bypass is closed. If the velocities are in ideal agreement, the differential pressure across the piston amounts to ΔpK zero. The measured volume is determined via the piston velocity and the known cross section of the inner pipe. The measurement stroke of the comparator is 4.5 m at a total piston stroke of 6 m. At the end of the measurement process, the bypass is opened again, the piston is slowed down, the piston valves are opened, and the piston is brought back to its original position. The measurement error of the turbine gas meter with reference to the comparator is based on mass conservation. Instationary effects due to the gas compressibility are compensated for by means of a correction; possible leakage currents along the piston and the dynamic behavior of the turbine gas meter are also corrected [3] in order to minimize measurement errors.

Figure 2: Calibration of a turbine gas meter using the high-pressure comparator.

In the future, the high-pressure comparator will be an integral part of the German national traceability chain and thus contribute to reducing the measurement uncertainty values when disseminating the units [1]. For this purpose, both primary standards will still be operated simultaneously in the future. Furthermore, participation in international interlaboratory comparisons for primary standards for high-pressure natural gas measurement [4] is planned. Besides operation with natural gas, metrologically validated operation with hydrogen/natural gas mixtures is also envisaged.



[1] Jos van der Grinten, Detlef Vieth and Bodo Mickan, The new Closed Loop pigsar high-pressure gas flow calibration facility and its projected CMC, 36th International North Sea Flow Measurement Workshop, Aberdeen, 22 – 24 October 2018. Opens external link in new windowLink

[2] B. Mickan, Th. Kappes, S. Singh, Hochdruck-Gas: Neue Technologien für Rückführung auf die SI-Einheiten bei großen Durchflüssen, 10. Workshop Gasmengenmessung - Gasanlagen - Gastechnik, 4. / 5. März 2020, Kötter, Rheine, Germany, pp 7-40.

[3] Böckler, H.-B., Messrichtigkeit von mechanischen Gasmessgeräten bei Verwendung von unterschiedlichen Gasbeschaffenheiten, Dissertation, Universität Duisburg-Essen, 2019, ISBN 978-3-95606-493-7.

[4] Jos G.M. van der Grinten, Arnthor Gunnarsson, Mijndert van der Beek and Bodo Mickan, An intercomparison between primary high-pressure gas flow standards with sub-permille uncertainties, 35th International North Sea Flow Measurement Workshop, Tønsberg, Norway, 22 - 24 October 2019. Opens external link in new windowLink


Jos van der Grinten, FB 1.4, AG 1.43, E-Mail: Opens window for sending emailjos.v.grinten(at)ptb.de