1、AN AMERICAN NATIONAL STANDARDASME MFC-18M2001MEASUREMENT OF FLUID FLOW USING VARIABLE AREAMETERSOF FLUID FLOWIntentionally left blank MEASUREMENT OF FLUID FLOW USING VARIABLE AREAMETERSASME MFC-18M2001AN AMERICAN NATIONAL STANDARDDate of Issuance: October 22, 2001The next edition of this Standard is
2、 scheduled for publication in 2006. Therewill be no addenda issued to this edition.ASME will issue written replies to inquiries concerning interpretation oftechnical aspects of this Standard.ASME is the registered trademark of The American Society of Mechanical Engineers.This code or standard was de
3、veloped under procedures accredited as meeting the criteria forAmerican National Standards. The Standards Committee that approved the code or standardwas balanced to assure that individuals from competent and concerned interests have had anopportunity to participate. The proposed code or standard wa
4、s made available for public reviewand comment that provides an opportunity for additional public input from industry, academia,regulatory agencies, and the public-at-large.ASME does not “approve,” “rate,” or “endorse” any item, construction, proprietary device,or activity.ASME does not take any posi
5、tion with respect to the validity of any patent rights asserted inconnection with any items mentioned in this document, and does not undertake to insure anyoneutilizing a standard against liability for infringement of any applicable letters patent, nor assumeany such liability. Users of a code or st
6、andard are expressly advised that determination of thevalidity of any such patent rights, and the risk of infringement of such rights, is entirely theirown responsibility.Participation by federal agency representative(s) or person(s) affiliated with industry is not tobe interpreted as government or
7、industry endorsement of this code or standard.ASME accepts responsibility for only those interpretations of this document issued inaccordance with the established ASME procedures and policies, which precludes the issuanceof interpretations by individuals.No part of this document may be reproduced in
8、 any form,in an electronic retrieval system or otherwise,without the prior written permission of the publisher.The American Society of Mechanical EngineersThree Park Avenue, New York, NY 10016-5990Copyright 2001 byTHE AMERICAN SOCIETY OF MECHANICAL ENGINEERSAll Rights ReservedPrinted in U.S.A.FOREWO
9、RDThis Standard is based on current industrial and research practices. It was prepared bythe ASME MFC Subcommittee 10 on Variable Area Meters and approved by the ASMEMFC Standards Committee on Measurement of Fluid Flow In Closed Conduits with anemphasis of definitions and specifications of variable
10、area meters.This Standard was approved as an American National Standard on May 25, 2001.iiiASME STANDARDS COMMITTEE MFCMeasurement of Fluid Flow In Closed Conduits(The following is the roster of the Committee at the time of approval of this Standard.)OFFICERSZ. D. Husain, ChairR. J. DeBoom, Vice Cha
11、irR. L. Crane, SecretaryCOMMITTEE PERSONNELN. A. Alston, Daniel Measurement however, they should not contain proprietary names or information.Requests that are not in this format will be rewritten in this format by the Committeeprior to being answered, which may inadvertently change the intent of th
12、e original request.ASME procedures provide for reconsideration of any interpretation when or if additionalinformation that might affect an interpretation is available. Further, persons aggrieved byan interpretation may appeal to the cognizant ASME Committee or Subcommittee. ASMEdoes not “approve”, “
13、certify”, “rate”, or “endorse” any item, construction, proprietary device,or activity.Attending Committee Meetings. The MFC Standards Committee regularly holds meetings,which are open to the public. Persons wishing to attend any meeting should contact theSecretary of the MFC Standards Committee.vCON
14、TENTSForeword iiiCommittee Roster ivCorrespondence with the MFC Committee vi1 SCOPE . 12 REFERENCES AND RELATED DOCUMENTS . 13 SYMBOLS AND DEFINITIONS . 14 FLOW RATE EQUATIONS 15 VISCOSITY EFFECTS . 26 FLOAT STABILITY 27 DESCRIPTION 27.1 2Float 7.2 2Metering Tube .7.3 2Scale 7.4 2Packing and Seals 7
15、.5 3Upper Body .7.6 3Lower Body .7.7 3Process Connection .7.8 3Accessories 8 UNCERTAINTY . 39 CLASSES 49.1 4Purge Meter .9.2 4Laboratory Meters 9.3 4Process Meter 10 SAFETY 411 VARIABLE AREA METER DEFINITIONS . 511.1 5Scale Length .11.2 5Connections 11.3 5Maximum Working Pressure 11.4 5Maximum Tempe
16、rature .11.5 5Tube Material 11.6 6Float Type and Material vi11.7 6Seal Type and Material .11.8 6Scale 11.9 6Pressure Drop 12 CAVITATION . 6Figures1 Nomenclature 32 Dimensions 43 Metal Tube Meter With Indicator . 54 Purge Meter . 5Table1 Symbols . 2Nonmandatory AppendixA Example, Uncertainty 7viiInte
17、ntionally left blank ASME MFC-18M2001MEASUREMENT OF FLUID FLOWUSING VARIABLE AREA METERS1 SCOPEThis 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 i
18、n a varietyof 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.2 REFERENCE
19、S AND RELATED DOCUMENTSASME MFC-1M Glossary of Terms Used in the Mea-surement of Fluid Flow in PipesASME MFC-2M Measurement Uncertainty for FluidFlow in Closed ConduitsASME Fluid Meters, 6thEdPublisher: The American Society of Mechanical Engi-neers (ASME), Three Park Avenue, New York, NY10016; Order
20、 Department: 22 Law Drive, Box 2300,Fairfield, NJ 07007-23003 SYMBOLS AND DEFINITIONSFor symbols and their definitions, see Table 1.4 FLOW RATE EQUATIONSThe 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
21、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 or
22、iented with flow vertically upfor the analysis to be correct. Orientation substantially1off the vertical will cause errors or a failure to respond.(See ASME Fluid Meters for more complete analysisof the variable area meter).It is not practical to calculate meter capacity fromphysical principles for
23、commercial variable area meters.The manufacturers catalogs do not list the tube crosssection areas, or float volumes, or weights, or inletand exit pressure drops; all of this information isproprietary. The manufacturer supplies all of the capac-ity data in the form of tables. This reduces the equati
24、onfor each meter flow to:Qvp Cr* %Scale 100 (1)The full scale meter flow, Cris defined and tabulatedin the manufacturers catalogs for each specific meteringtube and float. Separate tables are used for liquids andcompressible fluids. The industry often uses the term“normal” typical 1.013 bar and 20C
25、(14.7 psia and70F) conditions for compressible fluid sizing ratherthan “standard”. The user is cautioned to define thereference conditions used. (See the manufacturers litera-ture for guidance on sizing and calibration.) Equation2 shows how to correct for a float material densitydiffering from the b
26、asis density and for a flowing fluiddensity differing from the basis density:Qvp Cr (%Scale 100) H20906(SGf SGl) SGlc(SGfc SGlc) SGl(2)NOTE: Use a consistent basis for SG. For compressible fluids, thenegative terms above become very small and are not significant.Calculate Mass flow as the product of
27、 volumetric flow and upstreammass density.Qmp Qv H9267l(3)MEASUREMENT OF FLUID FLOWUSING VARIABLE AREA METERSASME MFC-18M2001TABLE 1 SYMBOLSSymbol Description Dimensions%Scale Percent of flow full scale NACrSpecific meter full scale flow capacity L3TQvVolumetric flow rate L3TQmMass flow rate MTSGfcS
28、pecific Gravity of float material at calibration conditions NASGISpecific Gravity of fluid, flowing conditions NASGICSpecific Gravity of fluid, at calibration conditions NASGfSpecific Gravity of float material, at flowing conditions NAGENERAL NOTE: SG is the ratio of the fluid density compared to wa
29、ter for liquid applicationsand the ratio of the fluid to air at specified conditions for compressible fluids.5 VISCOSITY EFFECTSFor variable area meters, a fluid viscosity exceedingthe limit value or “viscosity ceiling”,or“viscosityimmunity ceiling” as listed in the catalog tables forthat specific t
30、ube and float, will affect the meter calibra-tion. In general, float designs with a sharp edge onthe maximum diameter part of the float will be lesssensitive to viscosity (See the manufacturers literaturefor guidance). In general, viscosity effects occur withfluids more viscous than water.6 FLOAT ST
31、ABILITYThe float may become unstable and “bob” up anddown even at a constant flow (See the manufacturerscatalogs for warnings and descriptions of this phenome-non). It is normally experienced only in low pressuregas service. Special floats are used to reduce this effect.Smaller flowmeters are more l
32、ikely to be affected bythis problem. These instabilities may be a result of acyclic change between laminar and turbulent flow re-gimes or from fluid mechanical interactions.7 DESCRIPTIONThe variable area flowmeter (see Fig. 1 through Fig.4) as described in this Standard is composed of:(a) float(b) m
33、etering tube(c) scale(d) packing and seals(e) upper body(f) lower body(g) process connections(h) accessories27.1 FloatThe float is the body in the flowing fluid that movesin response to fluid flow. It is typically circular incross section when viewed from the top. From theside, the float geometry ma
34、y be simply a sphere, orit may be much more complex.7.2 Metering TubeThe tube is that part of the body which surroundsand contains the float. It increases in cross section areafrom the bottom to the top. The simplest are circular,but some have vertical guide ribs or a central guide rod.7.3 ScaleThe
35、scale is that part of the meter which shows therelation between the float position and the flow rate.Some have printed or engraved marks and numbers ona transparent metering tube. For metal tube meters, amagnetically coupled indicator is commonly used. Thisis coupled to the float, and an electronic
36、or pneumaticdevice may be attached to develop a signal to betransmitted to another location (See Fig. 1 throughFig. 4).7.4 Packing and SealsFor all but the simplest one-piece purge meters (seeFig. 4), some device is required to seal the meteringtube to the upper and lower bodies. O-Rings are usedin
37、some meters, and packing is common in the largermeters. The selection of packing materials depends onthe process fluid properties, including maximum andminimum pressures; and normal, maximum, and mini-mum temperatures.MEASUREMENT OF FLUID FLOWUSING VARIABLE AREA METERS ASME MFC-18M200110080907060504
38、0302010Exit connection, flangedUpper bodySide memberScaleTubeFloatLower bodyInlet connectionFIG. 1 NOMENCLATURE7.5 Upper BodyThe upper body supports the top or outlet of themetering tube. It usually includes a packing or sealingdevice. It also provides the support for the flow outletprocess connecti
39、on. In the simplest meters, these func-tions are all a part of the top of the meter body.7.6 Lower BodyThe lower body is at the bottom or inlet of theflow tube. It is similar in function and design to theupper body.7.7 Process ConnectionThe process connections are used to install the meterto the ass
40、ociated piping system. Standard connectionsinclude standard inch and millimeter piping threads andflanges.37.8 AccessoriesAccessories include switches controlled by the floatposition; signal-transmitting devices, check valves toprevent reverse flow, needle valves to control flow,and constant differe
41、ntial relays to stabilize flow.8 UNCERTAINTYIn most catalog and technical literature, the uncer-tainty is given as a percent of full scale flow and isdefined only between 10% and 100% of scale. Thevariable area meter is not sensitive to the pipe arrange-ment or the flow profile entering the meter. U
42、ncertaintiescan be minimized with careful application and knowl-edgeable use. If the sizing is based on poorly definedor varying fluid properties and operating conditions,then the accuracy will be compromised. Poor installa-tions with high vibration or excessively non-verticalalignment will reduce a
43、ccuracy. Calibration can reduceMEASUREMENT OF FLUID FLOWUSING VARIABLE AREA METERSASME MFC-18M2001Face to faceFace to faceFlange type, size and Top or Back connectedScalelength100809070605040302010FIG. 2 DIMENSIONSthe uncertainty depending on the quality of the calibra-tion and care in meter transpo
44、rt, installation, and use.Effective accuracy is also a function of the instrumentscale and indicator design. An unstable float positionwill interfere with accurate readings.9 CLASSESVariable area flowmeters are of three general classes:purge or miniature meters, laboratory meters, and pro-cess flowm
45、eters. This grouping scheme is only verygeneral in nature.9.1 Purge meterPurge meters (see Fig. 4) are small and typicallyhave14NPT (6 mm) or smaller connections. Becausethe applications do not justify it, calibration is unlikely.Catalog claims of 2% repeatability and an uncertaintyof 5% of flow rat
46、e may not always be realized inpractice. Tubes vary considerably in design but areoften between 50 mm and 100 mm (2 in. and 4 in.)in length.49.2 Laboratory MetersThe laboratory meters are usually longer 300 mmto 600 mm (12 in. to 24 in.), have longer scales, andinclude more graduations than other me
47、ters of the sameconnection size and capacity. Repeatability is advertisedas12% and a standard accuracy of 1% is promised,which may be improved to12% with calibration.9.3 Process MeterProcess meters with12in. (13 mm), or smallerconnections typically have standard calibration uncer-tainties of 2%. Met
48、ers larger than12in. can often havecertified uncertainty of 1% at the specified conditionsif they are calibrated. Tubes are typically between 150mm and 250 mm long.10 SAFETYMany users limit or prohibit the use of glass tubesin hazardous fluids in industrial service. Shields canbe purchased with most
49、 glass tube meters. The userMEASUREMENT OF FLUID FLOWUSING VARIABLE AREA METERS ASME MFC-18M2001504030201060708090100FIG. 3 METAL TUBE METER WITH INDICATORmust determine if the level of protection provided bythese shields is adequate for the application. The shieldtypically can deflect broken glass and flowing fluid,but is not designed to contain the fluid at the maximumpressure rating of the tube. Glass is brittle and damageto the tube can seriously weaken the tube. Ratingslisted in the manufacturers catalog are for new andundamaged meters.Metal tube me
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