Z/Z-METER, ON-LINE MEASUREMENT OF COMPRESSIBILITY-RATIOS FOR REFERENCE VALUES OF VOLUME AT OPERATIONAL CONDITIONS
Abstract
One of the methods to realize reference values for Volume of e.g. Natural Gas at increased pressures is via a calibration under a pressure difference (i.e. with expansion). In such a process, applied since the sixties in the Netherlands, the "un-known" meter or Meter-under-Test (MuT) is compared through expansion with the "known" meter or Reference Meter (RM) at low-pressure. In a "normal" calibration-process the difference in pressure between the two compared meters is relative small and the resulting difference between the compressibility at the Reference Meter <i>Z</i><sub>RM</sub> and the compressibility at the Meter-under-Test <i>Z</i><sub>MuT</sub> has hardly any effect on the ratio <i>Z</i><sub>RM</sub> / <i>Z</i><sub>MuT</sub> and can be neglected (the ratio is thus considered to have a value of 1). However, in a calibration under expansion a comparison is made of a gas-flow under significant differential pressure-conditions and one of the most important contributions of uncertainty stems from the real gas constant <i>Z</i>. So, the ratio <i>Z</i><sub>RM</sub> / <i>Z</i><sub>MuT</sub> is of importance, rather than the absolute values of the compressibility factor. The <i>Z</i> / <i>Z</i>-meter is one of the current technical developments at NMi VSL-Flow. The uncertainty contribution in the conventional method of calculation, described e.g. in M-GERG of 1991, is rather high (0,1% for the compressibility factor) and the resulting uncertainty of the <i>Z</i><sub>RM</sub> / <i>Z</i><sub>MuT</sub> ratio is 0,14%.<BR />Therefore, a method leading to a lower contribution to this source of uncertainty offers advantages. In the presented approach is the ratio <i>Z</i><sub>RM</sub> / <i>Z</i><sub>MuT</sub> is considered to be the measurand. The ratio should then be measured more accurately than the one resulting from calculation of the absolute values of the compressibility factor. As a consequence, the impact of this source of uncertainty is reduced. NMi has developed a device that is able to measure the ratio <i>Z</i><sub>RM</sub> / <i>Z</i><sub>MuT</sub> directly. <BR />In this paper, the development of a new measuring instrument is described that measures the Z<sub>1</sub> / Z<sub>2</sub> ratio of gases (at pressures P<sub>1</sub> and P<sub>2</sub> between 1 and 70 bar) with a maximum uncertainty of 0,03%. The measuring principle of the <i>Z</i> / <i>Z</i> ratio meter is based upon the accurate measurement of a piston-displacement. The piston is displaced by a controlled gas expansion from high-pressure to a low-pressure condition (or for compression, vice versa) in a measurement cell. The measurement cell with an adjustable volume is mounted inside a high-pressure vessel for an adequate control of temperature and pressure. The cell is constructed as a cylinder-piston combination together with small ball-valve actuators that can be opened and closed to connect or disconnect the chamber of the cell to the surrounding vessel and to fill or empty the system.<BR />During its operation, measurements are taken of the piston-displacement, together with measurements of pressures and temperatures. For the calculation of the ratio <i>Z</i><sub>1</sub> / <i>Z</i> <sub>2</sub> basically only the ratio of the cell volumes at pressures <i>P</i><sub>1</sub> and <i>P</i><sub>2</sub> needs to be established.
Event details
- Event
- FLOMEKO 2003
- Technical Committee
- TC9
- Place
- Groningen, NETHERLANDS
- Time
- 12 May 2003 - 14 May 2003
- Website
- www.flomeko2003.nl