SPIN-OFFS FROM THE DEVELOPMENT OF ROTARY GAS METERS
J. T. M. Bergervoet
Abstract:
The basic principle of a rotary gas meter consists of two rotors in the form of a figure -8 that rotate inside each other with the precision of a gear wheel. The outer edges of these rotors turn in a very close fitting measurement chamber. The outer edges of the turning rotors transfer fixed quantities of gas from the inlet to the outlet like small buckets. Because the radii of the sealed streams in the middle between the two rotors always vary there is a discontinuous volume of gas passing per angle revolution of the rotors. The shape of variation is near a sinus and the frequency is four times the frequency of the rotor revolution and the amplitude is near 12% of the average flow. These variations give an irregular rotating of the rotor at low flows and pressure and flow pulsation’s at higher flows. These pulsation’s can lead to resonance’s in the installation where they are mounted in. This can give sound problems and mis-indication of the rotary gas meter itself or to other devices in the installation. The by Instromet developed DUO rotary principle (Dutch patent number 1004751) has two pair of rotors which are synchronized in such a way that both sine waves of the transferred volumes per angular movement are in opposite phases to each other. This results in only a small variation with double frequency. To develop a rotary meter with a large measure range it is nece ssary to keep all leak gaps along the rotors as small as possible. The combination of DUO Principle and narrow leak gaps has demonstrated that this type of meter is suitable as high-grade reference meter. This type of meter shows a flat calibration curve o ver a large measuring range. It has a high-grade reproduction and a small pressure dependency over a large pressure range. With rotary meters of the DUO principle as reference meters, Instromet has developed a series of low-pressure test benches (ITF) measure range 0.5 to 10000 m³/h at a very compact construction. For a measure range up to 1000 m³/h only two rotary meters are being used as a reference. For a measure range up to 4000 m³/h more rotary meters are being used parallel as a reference. Above 4000 m³/h also a large turbinemeter will be used as a reference. The total uncertainty in the determination of the measurement error of the meter under test is strongly dependent of the chosen trace to the primary standards. A total uncertainty less than 0.23 % is possible. For small flows, the extent of the enclosed volumes and temperature and pressure stability in the test room also determines highly the final total uncertainty. For high pressure test facilities the “Instromet Rotary Piston Prover” IRPP is developed. The heart of the meter is a cartridge, type DUO rotor meter provided with extra narrowed leak gaps. The cartridge is placed into a cage, which is span with a flexible rubber sleeve. The measurement module, cage and sleeve on itself are put as a assembled unit into a pressure body designed for a design pressure of 100 bar. This assembled unit can be seen as the “real meter”. Now it’s easy and inexpensive to transport the “real meter” without its pressure body, for calibration at specialised high pressure laboratories that may be located far away. These IRPP’s are already in use by the NMi in their new re-calibration system of high pressure test facilities “Trasys” and as a sleeping standard in facility of Trans Canada Calibration in Winnipeg, Canada.
