ASME MFC-18M pdf download MEASUREMENT OF FLUID FLOW USING VARIABLE AREA METERS
This Standard describes the common variable areaflowmeter. This Standard does not attempt to standardizedimensions because the commercial products differ toowidely.
The variable area meter is manufactured in a varictyof designs.This Standard addresses only those metersbased on a vertical tapered tube of round or a modifiedround cross section. Specifically not addressed are thevarious vane type meters，meters with horizontal flow,or meters which use a spring deflection to oppose flowforces.
2REFERENCES AND RELATED DOcUMENTSASME MFC-1M Glossary of Terms Used in the Mea-surement of Fluid Flow in Pipes
ASME MFC-2M Measurement Uncertainty for FluidFlow in Closed Conduits
ASME Fluid Meters，6t Ed
Publisher: The American Society of Mechanical Engi-neers (ASME),Three Park Avenue，New York,NY10016; Order Department: 22 Law Drive, Box 2300,Fairfield,NJ 07007-2300
3 SYMBOLS AND DEFINITIONS
For symbols and their definitions，see Table 1.
4FLoW RATE EQUATIONS
The variable area flowmeter is composed of a bodycontaining the fluid and a“float,” which is free tomove in the body to a position related to the flowrate. The balance of forces positions the float. Gravitypulls the float downward. The buoyancy of the floatplus the velocity related dynamic fluid forces lift thefloat. The float rises to increase the flow area until thefluid forces lifting the float match the downward force.The meter must be oriented with flow vertically upfor the analysis to be correct. Orientation substantially off the vertical will cause errors or a failure to respond. (See ASME Fluid Meters for more complete analysis of the variable area meter). It is not practical to calculate meter capacity from physical principles for commercial variable area meters. The manufacturer’s catalogs do not list the tube cross section areas, or float volumes, or weights, or inlet and exit pressure drops; all of this information is proprietary. The manufacturer supplies all of the capac- ity data in the form of tables. This reduces the equation for each meter flow to:Q v p C r * %Scale ⁄100 (1)
The full scale meter flow, C r is defined and tabulated in the manufacturer’s catalogs for each specific metering tube and float. Separate tables are used for liquids and compressible fluids. The industry often uses the term “normal” [typical 1.013 bar and 20°C (14.7 psia and 70°F)] conditions for compressible fluid sizing rather than “standard”. The user is cautioned to define the reference conditions used. (See the manufacturer’s litera- ture for guidance on sizing and calibration.) Equation 2 shows how to correct for a float material density differing from the basis density and for a flowing fluid density differing from the basis density: