The NIST Cryogenic Flow Measurement Facility, located in Boulder, Colorado, is used for calibrations and tests of flow meters, using a closed loop liquid-nitrogen flow system. It uses a dynamic weighing system to measure liquid mass and calculate totalized mass and volume flow rates through a meter under test conditions.
All measurements are traceable to national standards. Upon completion of meter calibration, the customer is provided with final reports, tabulated data, and plots summarizing the results. See http://www.nist.gov/calibrations/cryogenic_flow.cfm#18800S for information on this service.
The facility has been in operation at NIST for nearly fifty years, and operates under the NIST Quality System (in conformance with ISO/TEC 17025). The calibration of cryogenic flow meters is listed among the NIST Calibration and Measurement Capabilities (CMC) within the key comparison database (KCDB) of the Bureau International des Poids et Mesures (BIPM). This listing was published in December, 2011. See http://kcdb.bipm.org/ for the complete list of capabilities recognized under the international Mutual Recognition Arrangement.
The facility has been instrumental in evaluating various types of flow metering devices to determine their capabilities for measurement of cryogenic flows, including metering of liquid nitrogen, liquefied natural gas, and other cryogens. Types of flow meters examined include:
The calibration flow parameters approximate the metering of commercial cryogenic fluids that might be encountered on truck-trailer type deliveries and provide a nearly full scale test of a 50 mm (2 inch) flowmeter.
Components and Instrumentation
Figure 1 is a schematic of the liquid flow facility. Liquid nitrogen, the process fluid, is circulated throughout the closed loop by a variable-speed centrifugal pump. The liquid flows through the subcooler, a heat exchanger consisting of a finned tube submerged in a nitrogen bath, where the thermal energy due to pumping and ambient heat leak is removed. The temperature of the liquid nitrogen in the flow loop can be changed or controlled by adjusting the liquid level in the subcoooler tank. Some of the test fluid can also be diverted around the subcooler, if necessary.
After leaving the subcooler and bypass, the fluid passes through a vacuum-jacketed loop containing an in-line heater, through the test section, and into the weigh tank/catch tank system (described below) which represents the heart of the measurement system. The liquid flows into the bottom of the weigh tank through a pipe and diffuser. The diffuser removes the vertical component of the flow. The liquid then flows through the weigh tank valve and into the catch tank, and is drawn back into the circulation pump.
The flow system is pressurized with helium gas to prevent the liquid nitrogen from boiling. The nitrogen is subcooled by 10 to 15 K, and the helium absorbed in the liquid nitrogen does not exceed 0.5 mole percent. No test points are taken if evidence of bubbles is detected visually through a sapphire window located in the flow path near the meter being tested.
The weigh tank, with a capacity of 0.378 cubic meters (100 gal) and a tare weight of approximately 70 kg (145 lb), is suspended from the load cell apparatus located in the dome above the catch tank. A mechanism in the load cell apparatus allows the sequential suspension of four weights from the load cell. Each weighs approximately 113 kg (250 lb), and the weights are used to establish the sensitivity of the load cell. The load cell sensitivity is determined before installation, and using the calibration weights, the load cell sensitivity is evaluated every day that the system is operated, to ensure its operation has not shifted.
Periodically, these in situ data and recorded pressure information are used to update the load cell sensitivity equation. The catch tank is a stainless steel pressure vessel with a 0.443 cubic meter (117 gal) usable capacity. Both the weigh tank and the catch tank are contained in a vacuum-jacketed chamber.
The flow loop is particularly well-instrumented in the test section, and the weigh tank/catch tank assembly. This provides information needed to determine mass flow rate as well as fluid property information used to determine volume flow rate and weigh tank buoyancy effects.
Operation and Control
When the temperature and pressure in the system have reached steady state, a test point is taken. A test point is one mass measurement by the weigh tank and may include any or all of the following measurements; elapsed time, flowmeter output, temperatures, and pressures. To begin a test point, the weigh tank valve is closed and liquid nitrogen accumulates in the tank. The output voltage from the load cell is monitored by a voltage comparator, and as it reaches a preset level, data acquisition begins. A timer is initiated and a computer stores digitized information about the system and any flow measurement devices installed. For example, if a flow meter with a frequency output is installed in the test section, a counter counts the meter pulses while the computer records digitized outputs from pressure transducers, thermometers, and initial load cell signals.
As the weigh tank fills, the output voltage from the load cell is monitored. As it reaches a preset level, the test point ends, the weigh tank valve opens, and complete circulation resumes. Data acquisition stops and information from the timer, counters, and load cell voltmeter is sent to the main minicomputer. The load cell output is recorded at the beginning and at the end of the test point, and the difference in voltage is a measure of the accumulated mass. The mass accumulated for each data point is varied, but the average is approximately 181 kg (400 lb).
The flow rate through the test section is controlled by the combination of a control valve downstream from the test section and the circulation pump. Gross variations in pressure are controlled by addition of helium gas, with finer adjustments made by a combination of the flow control valve and pump. Temperature is controlled by the depth and vapor pressure of the subcooler bath, in-line heaters, and bypass flow.
The measurement of accumulated mass in the weigh tank and elapsed time allow computation of the mass flow rate during a test point; however, many of the meters tested measure volume flow rates. The pressure and temperature of the fluid is measured in close proximity to the flowmeters being tested. Using the standard equation of state formulation presented in REFPROP, the NIST thermophysical property software, the density is calculated and the volume rate through the meter can be derived. See http://www.nist.gov/srd/nist23.cfm for information about NIST REFPROP.
Flow Measurement Uncertainty
NIST guidelines are used to classify the types of uncertainties and determine their values. See http://www.nist.gov/pml/pubs/tn1297/index.cfm for information on the expression of uncertainties.
Throughout many years of operation, the facility has been well used in evaluating various types of flow metering devices to determine their application in measuring cryogenic flows. Types of flowmeters examined include: positive displacement, turbine, head-type (orifice), angular momentum, Coriolis force, and vortex shedding flowmeters. The facility has been used to calibrate transfer standard meters for state certifications. It is anticipated that LNG (liquefied natural gas) will have an expanding role as an alternative fuel, and NIST is able to provide assistance in developing accurate measurement methods for LNG flows. NIST is committed to maintaining the quality of this facility and to continuing to provide this unique flow measurement capability to its customers.