This International Standard specifies the geometry and method of use (installation in a system and operating conditions) of critical flow nozzles (CFNs) used to determine the mass flow-rate of a gas flowing through a system basically without calibrating the CFN. It also gives the information necessary for calculating the flow-rate and its associated uncertainty.
If the CFN is to be flow calibrated, all the installation, operating conditions, and calculations shall comply with the instructions by the calibration facility, therefore, this International Standard does not cover it.
For some conditions, e.g., small CFNs, gas with significant vibrational relaxation effect, and so on, the flow calibration is recommended.
This International Standard is applicable to nozzles in which the gas flow accelerates to the critical velocity at the minimum flowing section, and only where there is steady flow of single-phase gas. When the critical velocity is attained in the nozzle, the mass flow-rate of the gas flowing through the nozzle is the maximum possible for the existing inlet condition, while the CFN can only be used within specified limits, e.g. the CFN throat to inlet diameter ratio and Reynolds number. This International Standard deals with the toroidal- and cylindrical-throat CFNs for which direct calibration experiments have been made in sufficient number to enable the resulting coefficients to be used with certain predictable limits of uncertainty.
Information is given for cases where the pipe upstream of the CFN is of circular cross-section, or when there is a large volume (chamber) upstream of the CFN or a set of CFNs mounted in a cluster. The chamber configuration offers the possibility of installing CFNs in parallel, thereby achieving high flow-rates and/or variable volume flow-rate.
For information, the diameter correction method (DCM) is described that uses the result of a flow calibration at a single Reynolds number to match the discharge coefficient curve on a reference one by modifying the throat diameter without affecting the calculation result of the flow-rate. The DCM replaces the role of throat diameter by the calibration result; therefore, it is not necessary to measure the throat diameter accurately when applying the DCM.
The CFNs are especially suited as transfer standards, reference flow meters for calibration and testing, and for precise flow-rate control applications. Provided the upstream condition is stable, a CFN
immediately produces a stable gas flow of known flow-rate without any adjustment except for lowering the downstream pressure sufficiently. The CFNs should be associated with a precise statement of uncertainty for the measured flow-rate.