Metal Orifice Assemblies - O'Keefe Controls Inc

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Calibrated OrificesWATER OR AIR FLOW37Air FlowFor flow standards or other critical uses,O'Keefe Controls Co. can provide measuredair flow data for any of the standard precisionorifice products. This optional serviceis available at a nominal charge and can beprovided for pressure or vacuum flow conditionsin the standard ranges listed below.A calibration sheet with measured data isprovided for each orifice (see sample below).Data is obtained using a digital mass flowmetertraceable to the National Institute ofStandards Technology (NIST). Standardconditions are 70ºF and 14.7 psia(See photo page 34).Air flow can be measured for any of thefollowing conditions:Pressure 0-5, 0-10, 0-20, 0-50, 0-100 psigVacuum 0-30" HgFlow 0-.002, 0-.01, 0-.02, 0-.05,0-.1, 0-.2, 0-.5, 0-1, 0-2, 0-5,0-10, 0-20, 0-50, 0-200 SLPMFlow 0-.004, 0-.02, 0-.04, 0-.10,0-.2, 0-.4, 0-1, 0-2, 0-4, 0-10,0-20, 0-40, 0-100, 0-400 SCFHC v MeasurementWater at a constant height equivalent to 1 psig isused to determine the amount of water that passesthrough a test orifice in a measured time period.The C v is calculated from the measured data.Water FlowCalibration of standard orifices using waterflow is accomplished by measuring thequantity of water flowing through an orificein a given time period at a specific inletpressure.The calibration procedure determines thewater flow rate through the orifice and thecorresponding C v of the orifice is calculated.The C v can then be used to predictthe water flow rate at varying differentialpressure conditions using the equation:Q = C v∆PFor liquids other than water flow rate canalso be predicted using the C v method. Seepage 36 for details on liquid flow - C vmethod.A calibration sheet with measured data isprovided for each orifice. The calculated C vis the average value computed from fiveseparate tests (see sample below).P.O. BOX Q • TRUMBULL, CT 06611 • CT PHONE (203) 261-6711 • TOLL FREE PHONE (800) 533-3285 • FAX (203) 261-8331© O'KEEFE CONTROLS CO. • 2000 ALL RIGHTS RESERVED e-mail ca@okcc.com • website www.okcc.com
38The Basic ConceptA greatly misunderstood and misappliednotion is that of “choked flow”, also referredto as “critical flow”.In gas flow through an orifice there is an occasionwhere the gas velocity reaches sonicconditions. This occurs for air flow when theabsolute pressure ratio is .528, i.e. when thedownstream absolute pressure (P 2) is 52.8%of the upstream absolute pressure (P 1).P 1P 2Sonic velocity occurs for air flow whenP 2 /P 1 ≤ .528.AirVelocity➔The air flow velocity is limited once the absolutepressure ratio is ≤ .528.For air flow through an orifice with aninlet air temperature of 68 o F the choked(sonic) velocity is 1129 ft/sec.The Misconception!Once sonic velocity is achieved in orifice airflow (P 2/P 1= .528), it is easy to "assume" thatthe mass flow rate is constant for all pressureratios less than .528; i.e. P 2/P 1≤ .528. Forexample, when P 2is 14.7 psia and P 1is 27.84psia, sonic velocity occurs through the orifice.As P 1further increases there is no furtherincrease in the velocity of the air flowingthrough the orifice.P 1 = 27.84 psia= 13.14 psigSonic VelocityConditions for the onset of sonic velocity in orifice air flow.Consider all the Factors!The mass flow rate through an orifice is a functionof three basic parameters.Q (flow) is a function of• Velocity• Density• Orifice Area1129 ft/sec @68°FAir velocity isconstant inthis region01 .528➔ 0P 2 /P 1 – Absolute Pressure RatioSonic VelocityThrough OrificeP 2 = 14.7 psia= 0 psigChoked Flow of GasesWhen the air velocity reaches sonic velocity(P 2/P 1≤ .528) further increases in P 1(upstreampressure) do not cause any further increase inthe air velocity through the orifice. Consequentlyit is wrongly concluded that the massflow rate also does not increase.As the air pressure (P 1) increases, the densityof the air also increases; and since the massflow rate is also a function of density, the massflow rate increases linearly with pressure (P 1).MassFlow Rate➔Sonic VelocityAir Velocity01 .528Mass FlowRateP 2 = 14.7 psia= 0 psigEven though the air velocity through the orifice is limitedto the speed of sound, the mass flow rate continues toincrease as the absolute pressure (P 1) increases.What is Choked?The parameter that becomes “choked” or“limited” is the velocity of the air. It is moreaccurate to use the term “choked velocity”rather than “choked flow” when the absolutepressure ratio of air through an orifice is≤ .528.Vacuum ConditionsP 1 = 14.7 psia= 0 psigStandardAtmosphericConditionsAir at atmospheric pressure enters the orifice and flowsto a downstream vacuum pump.In the case of vacuum conditions on the outletof an orifice and where the inlet is at ambientatmospheric pressure, both the air velocityand the mass flow rate become choked(limited) when sonic velocity is achievedthrough the orifice.➔MassFlow RateTUTORIALFor atmospheric inlet pressure and downstream vacuum,both the air velocity and mass flow rate are limited.The reason for the mass flow rate limitation isthe fixed inlet density combined with the fixedvelocity. The flow charts on pages 16-18 showthe choked mass flow effect for vacuum conditions.At vacuum levels between 15-30” Hgthe mass flow rate is fixed.Choked Flow for PositivePressure ConditionsAs in the case of the above vacuum conditionsthere are certain situations in which chokedflow does occur for positive (above atmospheric)pressure. By maintaining a fixed inletpressure to the orifice and allowing theoutlet pressure (back pressure) to vary, thereis a range of outlet pressures over which themass flow rate is fixed.For example (see chart below).• With an inlet pressure of 80 psig, the massflow rate is choked (limited) for all outletpressures less than 35.30 psig (includingvacuum conditions).• The actual flow rate is constant for theoutlet pressure range of 35.30 psig to aslow as a complete vacuum. The flow ratecan be obtained from the charts on pages16-18; e.g. for an orifice of .010" diameterand with 80 psig inlet pressure, thechoked flow rate is 8.4 scfh (page 16). Thisflow rate will be constant for all outletpressure conditions between 35.30 psigand full vacuum.P.O. BOX Q • TRUMBULL, CT 06611 • CT PHONE (203) 261-6711 • TOLL FREE PHONE (800) 533-3285 • FAX (203) 261-8331© O'KEEFE CONTROLS CO. • 2000 ALL RIGHTS RESERVED e-mail ca@okcc.com • website www.okcc.com➔0➔ 0P 2 /P 1 – Absolute Pressure RatioSonic Velocity1129 ft/sec @68°FP Choked Flow1 = 14.7 psiaChoked Velocity01 .528➔ 0P 2 /P 1 – Absolute Pressure RatioAirVelocityP 2 < P 1Vacuum➔AirVelocitySonic Velocity Conditions – Air FlowInlet PressureOutlet PressureFor Sonic VelocityGage Absolute Absolute GagePressure Pressure Pressure Pressurepsig psia psia psig100 114.7 ≤ 60.56 ≤ 45.8690 104.7 ≤ 55.28 ≤ 40.5880 94.7 ≤ 50.00 ≤ 35.3070 84.7 ≤ 44.72 ≤ 30.0260 74.7 ≤ 39.44 ≤ 24.7450 64.7 ≤ 34.16 ≤ 19.4640 54.7 ≤ 28.88 ≤ 14.1830 44.7 ≤ 23.60 ≤ 8.9020 34.7 ≤ 18.32 ≤ 3.6215 29.7 ≤ 15.68 ≤ .9814.7 29.4 ≤ 15.52 ≤ .8210 24.7 ≤ 13.08 ≤ -1.625 19.7 ≤ 10.40 ≤ -4.301 15.7 ≤ 8.29 ≤ -6.470 14.7 ≤ 7.76 ≤ -6.94Temperature 68°F
Page 1 and 2: 2Table of Contents®®PagePicture I
Page 3 and 4: 4Sapphire Orifice AssembliesDescrip
Page 5 and 6: 6DescriptionOne-piece construction
Page 7 and 8: 8DescriptionOne-piece construction
Page 9 and 10: 10DescriptionThe inserts shown on t
Page 11 and 12: 12Metal Orifice AssembliesDescripti
Page 13: 14DescriptionPrecision orifice asse
Page 17 and 18: 18Sapphire Orifice Air Flow - SLPMO
Page 19 and 20: 20Supply Pressure — psigPrecision
Page 21 and 22: 22DescriptionStainless steel wire s
Page 27 and 28: 28DescriptionFine screens are used
Page 29 and 30: 30BreathersSS WIRE SCREENSDescripti
Page 31 and 32: 32Orifice KitsMETALDescriptionFor r
Page 33 and 34: 34Fundamentals of Flow MeasurementG
Page 35: 36Technical ConsiderationsMISCELLAN
Page 39 and 40: 40Water Flow TotalizersREMOTE READO
Page 41 and 42: 42AccessoriesFLOW METERSIn-Line Flo
Page 43 and 44: 44Check Valves - Fixed Flow Control
Page 51 and 52: 38Pneumatic Pressure SwitchesNON-EL
Page 53 and 54: AIR PROXIMITYSENSORGENERAL DESCRIPT
Page 55 and 56: Air/Electric SwitchesDESCRIPTIONThe
Page 57 and 58: PNEUMATICFLOAT SWITCH[Non-Electric]
Page 59 and 60: COMPRESSEDAIR DRYERSAND ACCESSORIES
Page 61 and 62: Low Dew Point Air Dryers3SELF-REGEN
Page 63 and 64: Compact Air Dryer5LOW DEW POINT8"Al
Page 65 and 66: Standard Air Dryer7INSERT MODULEDes
Page 67 and 68: Insert Module1/4"12-7/8"12-1/4"Stan
Page 69 and 70: Standard Air Dryer System11PNEUMATI
Page 71 and 72: Compressed Air Accumulators13AIR RE
Page 73 and 74: High Flow Air Dryer System15PULSE F
Page 75 and 76: Point-of-Use Air Dryer System17DEW
Page 77 and 78: Air Dryer Accessories19MOISTURE IND
Page 79 and 80: Automatic Drain Valves21ADJUSTABLE
Page 81 and 82: Pneumatic Controls23SENSORSPneumati
Page 83 and 84: MiniatureStainless SteelFittings an
Page 85 and 86: DescriptionBARB ADAPTERMiniature St
Page 87 and 88:
Description10-32 x 1/8" NPTMiniatur
Page 89 and 90:
BREATHERMiniature Stainless Steel F
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Metal Orifice Assemblies - O'Keefe Controls Inc

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