MRF Primary Flow Calibration
The primary mass flow standard for each Metering Research Facility (MRF) test system is a gravimetric system that weighs the gas collected during a precisely measured time interval. Each of the two flow facilities, High-Pressure Loop (HPL) and Low-Pressure Loop (LPL), has a primary calibration system specifically sized for its range of operation. Both systems are located on-site at the MRF and operated from a single control center.
The primary calibration systems of the HPL and LPL are used to calibrate the secondary flow standards routinely used for flow meter calibration and flow meter research. The primary calibration system performance specifications of the flow facilities are summarized in Table 1.
The weigh scales are gyroscopic balance systems believed to be the most sensitive weighing systems available for MRF applications. The weigh tank scales are calibrated using mass standards traceable to U.S. National Institute of Standards and Technology (NIST).
|Pressure (psig)||Tank Volume (cubic feet)||Scale Mass Range (kilograms)||Scale Resolution (grams)||Total Measurement Uncertainty of the Scale (kilograms)|
|HPL||1,440||550||0 - 1,000||10||± 0.021|
|LPL||200||314||0 - 155||3||± 0.010|
Both the HPL and LPL are recirculating flow loops. When the primary calibration system is used on either of these flow loops, it is necessary to divert flow from the recirculating portion of the loop to the weigh tank.
To maintain constant flow conditions in the test section during a primary calibration (i.e., weigh tank) run, gas must be added to the recirculating portion of the loop at the same rate it is drawn off to the weigh tank. Therefore, the HPL and the LPL are equipped with gas storage bottles, which are used to supply makeup gas to the test loop at the same rate the gas is diverted to the weigh tank. Special, fast-acting hydraulically powered diverter valves are installed in both the HPL and LPL to control the flow of gas to the weigh tank and from the makeup bottles. These valves are computer controlled and independently actuated, with a full-stroke response time of less than 50 milliseconds.
MRF Secondary Flow Calibration
The primary calibration systems in the two flow facilities are used to routinely calibrate secondary (transfer) flow calibration standards. The HPL and LPL secondary standards consist of ASME/ANSI MFC-7M critical flow nozzles. The dimensions and capacities of the nozzles are presented for the HPL in Table 2 and for the LPL in Table 3.
|Nominal Plenum Diameter (Inches)||Nominal Natural Gas Flow Rate (ACFM)|
|Nominal Plenum Diameter* (inches)||Nominal Natural Gas Flow Rate (ACFM)|
|2, 3, 4, 6||1.56|
|2, 3, 4, 6||3.12|
|2, 3, 4, 6||6.25|
|2, 3, 4, 6||12.5|
|2, 3, 4, 6||25|
*Four plenums—2, 3, 4, and 6—are available simultaneously.
The secondary flow calibration standards are highly repeatable and provide excellent test accuracies when calibrated using the gravimetric primary standards. Because the secondary standards allow many data points to be collected in a relatively short amount of time, their use ensures quick turnaround time for routine meter calibrations.