Flow meters for gas measurement

Ultrasonic meter : 12 ”Operating flow rate : 90 % QmaxBais Error : 0.5 %$2500/day or $1000000/annumLoss can be avoided by calibration

Cost due to Error of 0.25% is about Rs120 Crore/annum

Initial calibration by manufacturersFor checking of accuracy by the user.After major repairs.Under Statutory ObligationsLegal DisputesTo improve accuracy

Orifice meters Large volumeTurbine meters Large volumeUltrasonic gas meter Large volumeCritical flow Venturi Large volumeCoriolis mass meters CNG

Cheap, commonly used metersReasonable performance, good reliabilityModerate costPressure difference proportional to flow rateSharp square edged cylindrical throat

Flange, corner, D and D/2, pipe tapsFlange tap for Natural gasDiameter ratios ,0.1 – 0.75Orifice diameters > 0.45”Cd depends on , ReD and tap

Depends on Reynolds Number, diameter ratioFor Cd, 0.4 – 1%Expansibility factorGeometrical measurementsFlow parametersOverall uncertainty 1 – 2%

Cheap, good performance and reliabilityModerate cost with Simple metrology Conform to AGA-3 standard No need for wet plate calibrationLow flow range (1:3 Typical)Susceptible to erosion, damage Cost effective metering solutionOverall uncertainty 1 – 2% (0.4-0.6% in Cd)

Flowing gas imparts force on rotorActual rotational speed depend

passage way size shape/ rotor designload due to internal mechanical friction fluid drag external loading gas density

maximum velocity 50 m/s

PassageAnnularInlet

BodyElectrical Readout

Outlet

Houseing andTail Cone

Mechanism

421 3 5

Mechanical or

6connectionEnd

Electronic Pickup

Nose cone

Rotor

1.0% (Qi > Qt)

Maximum peak-to-peak error

1.251.50

0.500.751.00

0.000.25

0.500.25

1.251.00

1.751.50

Percent error

0.75

tmin Q < 0 .2 QQ iFlow rate (Q )

Repeatability +/-0.2%

1.751% for 0.2 Qmax - Qmax1.5% for Qmin - 0.2 Qmax 0.35% for 0.2 Qmax - QmaxRepeatability : 0.2%Effect of swirl should be less than 1/3 of normal specifications

Frequency counters - for instantaneous flow rate Integrator - for totalized flow Transmitters - for control applications Flow computers - for billing purposes

The ability of the meter to duplicate a given output or performance for test runs with an identical set of flowing conditions. Under normal conditions +/-0.1% on successive short duration and +/-0.15-0.2% on day to day basis.

Designed for a maximum flow rate Fixed by rotor speed Normal pressure loss.Same irrespective of operating pressure.

Lowest flow rate at which the meter will operate within specified accuracy limits (+/-1%).Depends on magnitude of non-fluid drag and the density Qmin -0.5

SWIRL EFFECT Rotor designed for axial flow at rotor inlet Swirl affects the rotor speed swirl increases or decreases the speed depending on

the direction of swirl. Tests at 0.25, 0.4, Qmax with atmospheric air. Error shifts 1/3 of maximum permissible error.

Low level perturbation tests Perturbation unit with double elbow out of plane. Resembles perturbations by bends, tees,

convergent and divergent sections.High level perturbation tests

Perturbation unit with double elbow out of plane with half area plate in between

Resembles perturbations by regulators or throttling devices.

Recommended Installation

Less sensitive to upstream disturbances Output digital, linear, electronics Compact with moderate head lossMore expensive (larger sizes)Gradual wear and fouling of surfacesPeriodical recalibrationPerformance of smaller turbine meters

AGA Report No.9 Multipath ultrasonic meters for custody transfer of natural gasDutch Ministry of Economic Affairs PTB, Germany Uncertainty in line with Turbine meterProduction facilities, transmission pipelinesStorage facilities, distribution systems

Bi directional capabilityNo additional pressure dropMeasurement of pulsating flowsWide rangeabilityNo moving partsLess weight and spaceNo drift due to no wear offInsensitive to asymmetrical , pulsating, rotating flows and wet/untreated gas

18 Path USFM for liquids : 0.15 %

Measurement of spool diameterAcoustic path L and XDry calibration accuracy 1% or better Performance depends on signal processing / path configurations Measurement of delays for electronics / transducersMeasurement of sound velocity in pressurized test all.

L2x(tu - tD) Flow Velocity, v = ------------ 2 x tu x tD Lx(tu + tD) Sound Velocity, v = ------------ 2 x tu x tD L : Straight length of acoustic path X : Axial distance tu, tD : Transit times

Precise Geometry of body and transducer locationInherent integration techniquesQuality of profile, pulsation, gas uniformityElectronic clock stabilityConsistent detection of pulseCompensation for signal delaysParallel paths Direct or reflected paths along 2 or more tilted chordsWeighting factors for each path

ACCURACY

ACCURACY

Average ID, Acoustic Path Length, Axial distance between transducer pairAverage ID from 12 measurementsVertical(1), Horizontal(1) and 45o from vertical (2)Near upstream, near downstream and half way between transducerCalculation by trigonometryBody temperature at the time of measurementCorrection to 20oC and to 0.01 mm

Zero Test Meter purged of air with pure test gas or gas mixture Properties of test gas must be knownSound velocities for each acoustic path recorded for 30sMean and standard deviationAdjustments for compliance to performanceTheoretical speed of sound from composition, pressure and temperatureDocument all relevant parametersZero flow offset factor in +/- m/s

Zero Test

Flow Test

qmin, 0.1 qmax, 0.25 qmax, 0.4qmax, 0.7 qmax and qmaxAt expected average operating conditionsSpecific piping configuration and / or flow conditionerDiameters of flanges /connecting pipes to match (±1%)Traceable flow calibration facilityProperties AGA Report No.8 “Detail Characterization Method”

Size : 1/16" up to 10"Flow Range : g/h - 10 t/mRange : 25 : 1 (Typical) and 100 : 1 in some application.Accuracy : ±0.15% - ± 0.25% R+ Zero shift error. ±0.5% R for gasesUncertainty of measurement facilities ±0.3% - ± 0.6% R

Repeatability : +/-0.5% (qt<qi <qmax)Max Peak to peak error : +/-1%(qt<qi <qmax)Max mean error : +/-1% (qt < qi < qmax)

1.75

1.25

0.75

0.250.00

-0.75

-0.25

-1.75

-1.25

Perc

ent

erro

r

Qmin Qt Flow Rate (Qi)

Repeatability ±1.0%

Repeatability ±0.5%

(Qi < Qt)

(Qi > Qt)

(Qi > Qt)Max Peak to Peak spread 1.0%

Qmax

Critical velocity at the throat Mass flow rate maximum for the existing upstream conditions ISO 9300 Throat Reynolds Number 2.1 x 104 - 3.2 x 107 Relative uncertainty in Cd 0.3% - 0.2% Gas pressure and the gas temperature at upstream Pressure ratios of 0.75 - 0.95

c

0.9D to 1.1D

r = 1.8d to 2.2d

Ød_ØD>4d

2.5d ±0.1d

>d _