An Ultrasonic Flowmeter for Custody Transfer Measurement of LNG:A Challenge for Design and Calibration

摘要

This paper describes the development project of the 5 beam ultrasonic flowmeter for measuring cryogenic liquids (e.g. LNG or Liquefied Nitrogen) with fiscal accuracy. Tests have shown the importance of the acoustical coupling between the piezo ceramic element and the radiating window of the transducer. A thorough investigation has lead to a special design consisting of a combination of an acoustic coupling and a reliable transducer construction. The patent for this solution is pending. Extensive tests under isolated, static and dynamic conditions have shown that the transducer design is stable and generates reproducible and accurate results. There are two challenges regarding the calibration of any type of flowmeter for use in cryogenic applications. There are no (laboratory) test facilities in the world where flowmeters, with a diameter larger than 4 inch, can be tested on LNG or on liquefied nitrogen against a reliable reference. And, secondly, in most applications it is not possible to calibrate flowmeters on site against a reliable reference. This asked for the development of an alternative calibration method. The transfer of water calibration to application at cryogenic conditions forms the basis of the calibration concept. LNG and liquid nitrogen have a low viscosity, leading to fairly high Reynolds numbers, which, as confirmed by the tests, improves linearity of the 5 beam ultrasonic flowmeter. The calibration procedure for calibrating ultrasonic flowmeters for use of custody transfer of LNG has been developed in close cooperation with NMi, the Dutch Board of Weight and Measurement,