Standards for Cryogenic Flow

J.L. Scott, M.A. Lewis, and J.D. Siegwarth

Objective: To maintain the national standard for cryogenic flow measurement and to advance our services by improving data acquisition, piping configuration, process control, and by reducing measurement uncertainty.

Problem: The cryogenic flow calibration facility is the only independent facility of its kind in the world. It provides the measurement standard for liquefied air gases, and it can be used to evaluate metering methods for liquefied natural gas (LNG), as it becomes a viable alternative fuel. A dynamic weighing system is used to measure the total mass and, with the use of thermophysical property data for density, the volume. Calibrations are typically performed using liquid nitrogen over a flow range of 0.95 kg/s to 9.5 kg/s, a pressure range of 0.4 MPa to 0.76 MPa, and a temperature range of 80 K to 90 K. The precise measurement of any fluid is difficult due to the variability in numerous process conditions; with roughly 570 L of liquid nitrogen at 80 K, the complexity of the measurement is substantially increased. The flow measurement uncertainty is a combination of uncertainties in mass, time, temperature, pressure, and, if volume flow is required, an equation of state for density calculations. While the uncertainty of mass flow measurement is 0.17 %, our uncertainty in volume flow measurement has been three times larger. The new equation of state for nitrogen will make the volume flow measurement uncertainty virtually the same as that of mass flow. Our attention is also focused on mechanical aspects; we must anticipate and prevent component failures due to age and temperature extremes. We want the flow entering the meter under test to be as reproducible as possible, and we must keep the operation and control software on a current platform and in a form that is adaptable to future system operations.

Approach: Improvements are made on a continuing basis to the operation and reliability of the cryogenic flow facility. All components of the weighing system must be in thermal equilibrium with the liquid nitrogen, and, for that reason, measurements are made dynamically. We have upgraded our load cell, the primary measurement instrument in the process, and acquired a high-speed, high-accuracy digital voltmeter to read it. The result is improved reproducibility. The most difficult process control is temperature control. Almost the entire apparatus is vacuum-jacketed with soldered copper piping. As age and thermal cycling compromise these lines, they are replaced by standard stainless steel vacuum piping with o-ring joints. This makes access and modifications simpler. To minimize the effect of flow disturbances upstream of the test section, we have positioned as much straight pipe upstream of the meter installation, as our laboratory space will allow; however, this enhances the problems of pipe contraction at cryogenic temperatures. The inside lines contract, but the vacuum jacket does not. A robust bracing system and a combination of bellows were designed to compensate for these contractions. In the 1980’s a minicomputer was installed for data acquisition of the many sensors throughout the system. Though the best choice at the time, operation was cumbersome and difficult. A PC with a graphical programming language now performs the data acquisition. This not only enhances programming flexibility, but it makes it easier to train additional facility operators. This software incorporates a new equation of state for nitrogen as well as the equation that is currently in NIST 12. The new equation will become the NIST standard when the next version of NIST 12 is released (FY00).

Results and Future Plans: We calibrate and/or test various types of flowmeters (turbine, Coriolis, positive displacement) for customers that include meter manufacturers, state regulatory agencies, and aerospace companies. These meters may be used for transfer standards or in test stands. Our independent cryogenic flow calibration facility can help meter manufacturers compete in the international marketplace by providing interlaboratory comparisons with privately held foreign facilities. Continuing improvements to the facility include reducing heat leaks, expanding the flow range of the facility, enhancing automation, and evaluating our processes to ensure that the national cryogenic flow standard is state of the art.