PRACTICAL EXPERIENCE ON INSPECTIONS OF ORIFICE PLATES MEASUREMENT SYSTEMS FOR NATURAL GAS
Kazuto Kawakita
Abstract:
Everyday, in many countries producers and importers supply larg e volumes of natural gas produced from domestic fields or purchased abroad. To attend to this need, a whole host of interlinked and co- ordinated activities and technical installations are required both on the purchase and the sales side. Besides the production and processing facilities, compression stations, transmission and distribution pipelines, operation and control stations, many measuring systems are necessary to quantify the huge volumes of gas commercialized among parts. Natural gas measurement sys tems can be found in gas production plants, custody transfer stations, citygates, gas processing units and industrial gas consumers. Due to the commercial aspects involved, in most of these cases, gas deliverers and gas receivers operate its own independent measurement systems, always checking if the differences between both measurements are under the tolerances agreed through detailed contracts. The main objective of gas measuring stations is to perform accurate and reliable measurements of gas volumes, by use of controlled equipment and recognized metering techniques, which can assure an acceptable level of uncertainty for the measurement and that comply with the national regulations for the gas industry. More complete metering systems are able to measure gas flow in terms of energy flow, the effective product under trade. Though the thousands of turbine meters in operation and the increasing acceptance of multipath ultrasonic flowmeters in the gas industry, one of the most common measurement principle fo r natural gas still used throughout the world is the orifice plate. It is a traditional and well-known technology which origin goes back to 1779 when Giovanni B. Venturi, an Italian physicist, first conceived a differential pressure type flowmeter. Some of the main advantages of an orifice plate is that it has no moving parts exposed to wear, and it doesn’t require a calibration of the primary element in a laboratory since the discharge coefficient of an orifice plate can be calculated by empiric relations like the Reader-Harris-Gallagher (RG) equation. Two of the main disadvantages of the orifice plate is the significant pressure drop and the relatively low rangeability, making it suitable especially for metering stations operating under a relatively low flowrate variation regime and installed in pipelines where a permanent pressure loss is not a critical factor.
