Problems with Problems with PitotsPitotsMeasurement Issues in IndustrialMeasurement Issues in Industrial

Emissions Flow MonitoringEmissions Flow MonitoringRod RobinsonRod Robinson

National Physical LaboratoryNational Physical LaboratoryNPL, UKNPL, UK

David Butterfield, NPLDavid Butterfield, NPLDave Curtis, STADave Curtis, STA

Tommy Thompson, Littlebrook CalibrationTommy Thompson, Littlebrook Calibration

OverviewOverview

Project BackgroundProject BackgroundFlow measurement theoryFlow measurement theoryPotential issues in emission monitoringPotential issues in emission monitoringSurvey resultsSurvey resultsProposed future workProposed future work

Project BackgroundProject Background

Project in the UK Department of Trade andProject in the UK Department of Trade andIndustry funded programme to underpin flowIndustry funded programme to underpin flowmeasurementmeasurementAim of the project is to review potentialAim of the project is to review potentialmeasurement issues relating to air flowmeasurement issues relating to air flowmeasurement in industrial emissionsmeasurement in industrial emissionsmonitoringmonitoringDevelop and propose future work programmesDevelop and propose future work programmesto address key measurement issuesto address key measurement issues

Potential Issues in EmissionsPotential Issues in EmissionsMonitoringMonitoring

The project was initiated because of concernsThe project was initiated because of concernsover the measurements of flow carried out inover the measurements of flow carried out instacksstacksIn particular there were concerns over theIn particular there were concerns over theperformance of performance of Pitot Pitot Tubes at highTubes at hightemperaturestemperaturesFlow measurement often considered a solvedFlow measurement often considered a solvedproblem, problem, PitotsPitots are readily available, and they are readily available, and theywork – don’t they?work – don’t they?

Importance of FlowImportance of FlowMeasurement for EmissionsMeasurement for Emissions

MonitoringMonitoringStack gas flowStack gas flow– Emissions fluxes– Possibly emissions trading– Stack flow characterisation – meeting the monitoring

requirements of sampling methods– Enabling and demonstrating isokinetic sampling

Sample flow – gas metersSample flow – gas meters– Concentration measurement

PitotPitot-Static Tubes-Static Tubes

PitotPitot tubes are still routinely used for flow tubes are still routinely used for flowmeasurementsmeasurementsBased on differential pressure measurementBased on differential pressure measurementProvide the reference for flow measurementsProvide the reference for flow measurementsTwo general typesTwo general types– L type (also the ellipsoidal tipped NPL

or E-Type)– S type

L TypeL Type

Defined standardDefined standardCriteria on its constructionCriteria on its construction

Differential Manometer

Flow

Sta tic pressureports

Impa ct pressureport

S TypeS Type

Larger bore pressure orificesLarger bore pressure orificesLess susceptible to clogging by moisture/dustLess susceptible to clogging by moisture/dustCalibrated against the standard tubeCalibrated against the standard tubeEasier to fit into stackEasier to fit into stack

Differentia l Manometer

Flow

StandardsStandards

ISO 3966-1977, ‘Measurement of fluid flow in closed conduits,ISO 3966-1977, ‘Measurement of fluid flow in closed conduits,Part 2 Velocity area methods, Section 2.1 Method using PitotPart 2 Velocity area methods, Section 2.1 Method using Pitotstatic tubes’static tubes’Standard withdrawn, though national versions still exist (egStandard withdrawn, though national versions still exist (egBS1042 part 2.1)BS1042 part 2.1)ISO 10780 ISO 10780 ““Stationary Source emissions Stationary Source emissions –– Measurement of Measurement ofvelocity and volume flow-rate of gas streams in ductsvelocity and volume flow-rate of gas streams in ducts”” [6] [6]describes the construction and use of S-Type describes the construction and use of S-Type PitotPitot tubes tubes– references ISO 3966-1977 to provide the standard or

reference L Type tube - but hold on this has beenwithdrawn!

CEN TC264 WG 23 are developing new flow standardCEN TC264 WG 23 are developing new flow standard– Automatic and manual– Currently on hold due to funding issues

US EPA method 2 – multiple parts, recently updated toUS EPA method 2 – multiple parts, recently updated toaccount for issues encountered in some situations(Parts G,H)account for issues encountered in some situations(Parts G,H)CEN low dust standard references ISO 3966CEN low dust standard references ISO 3966

A Little TheoryA Little Theory

First described by Daniel Bernoulli in the 18First described by Daniel Bernoulli in the 18thth Century CenturyBernoulli equation states that, given certain conditions,Bernoulli equation states that, given certain conditions,there is conservation of energy along a flow linethere is conservation of energy along a flow lineThere are different forms of BernoulliThere are different forms of Bernoulli’’s equations equationdepending on the conditions (and assumptions) madedepending on the conditions (and assumptions) madeOne of the simplest forms, and that most often used inOne of the simplest forms, and that most often used inemissions monitoring, isemissions monitoring, is

Constgp

gV

=+ρ2

2

In effect two terms, one due to the kinematic energy and one due to the potential energy

Differential Pressure EquationDifferential Pressure Equation

P1 is a flow line, brought to a halt at P2P1 is a flow line, brought to a halt at P2Conservation of energy between point P1 and P2Conservation of energy between point P1 and P2We actually measure the differential pressure We actually measure the differential pressure ∆∆p p between P1between P1and P3, taking P3 to be representative ofand P3, taking P3 to be representative of P1P1

Stagnation point

P1 P2P3

ρpV ∆

=2

Why Go Back to Basics?Why Go Back to Basics?

Because there are a number of assumptions implicit in thisBecause there are a number of assumptions implicit in thisderivationderivation– Flow is brought to a halt at the stagnation point– Two pressures are representative of the same flow line –

need tube in correct orientation, with no swirl or turbulence– Conditions on the stack flow, (Reynolds number)– may be met theoretically, based on stack diameter and

average flow.– Actual conditions in the stack, such as obstructions,

complex flow conditions, pulsed flow from fans, and thesample probe itself, may well cause the flow to becomehighly unstable and non-laminar.

– Remember the standard Pitot was developed for niceclean duct flows, not stack conditions

Further Conditions RequiredFurther Conditions Required

Negligible air viscosity and compressibilityNegligible air viscosity and compressibility– There are corrections in the standard, but are

they used, and are they valid for stacks?Steady state conditionsSteady state conditions– The tube must be in thermal equilibrium,

temperature differentials will have an effect– Steady state flow – not always true

The ISO standards provide a large number ofThe ISO standards provide a large number ofconditions that have to be met to enableconditions that have to be met to enablemeasurements to be carried outmeasurements to be carried out– How many of these are met in real conditions?

Calibration and TraceabilityCalibration and Traceability

Measurement methods require flow measurements toMeasurement methods require flow measurements tobe traceable to standard L-Type be traceable to standard L-Type PitotPitotThe S Type The S Type PitotsPitots require calibration to the standard require calibration to the standardPitotPitotSingle point calibration, carried out in a laboratory, in aSingle point calibration, carried out in a laboratory, in adry, clean well characterised duct.dry, clean well characterised duct.ISO 10780 states that tubes calibrated according to theISO 10780 states that tubes calibrated according to theprocedure given will have an accuracy of 3% forprocedure given will have an accuracy of 3% forvelocities in the region 5 m/s to 50 m/svelocities in the region 5 m/s to 50 m/s– Based on US studies

No quantification of the effect of conditions that were notNo quantification of the effect of conditions that were notcovered by the calibration, covered by the calibration, eg eg temperaturetemperature

Review of Previous StudiesReview of Previous Studies

There have been a number of studies on differential pressure tubeThere have been a number of studies on differential pressure tubeperformanceperformance– US EPA development of Method 2, F,G and H

• Issues with wall effects, yaw, and highlighted calibrationissues

• S-type and 3D flow measurement• EPRI study- mainly on misalignment and flow direction• EPA collected papers – includes work which supports the

single calibration point for S type tubes– Various studies in Europe

• Swedish study• Dutch study on performance of ISO10780• Repeatability and reproducibility

– Early work , including the original NPL paper on the ‘new’ellipsoidal tip tube

• Air ducts not stacks

Review- continuedReview- continued

The review of published research highlighted theThe review of published research highlighted thefollowing issues inter-alia:following issues inter-alia:– Wall effects– Manual pressure reading bias– Effects of pulsating pressure readings– Use of assumed calibration factors for S Type Pitot

tubes– Calibration being performed at unrepresentative

Reynolds numbers– Displacement effect – transverse flow across tube– Experimentally determined repeatability and

reproducibility range from few percent to factors offour.

Potential ProblemsPotential Problems

Pitot Pitot tubes are capable of giving adequate flowtubes are capable of giving adequate flowmeasurements, under certain stack conditions, providedmeasurements, under certain stack conditions, providedcare is taken in their usecare is taken in their useEarly papers, even when discussing measurements inEarly papers, even when discussing measurements inclean ducts, stress the need for good quality control toclean ducts, stress the need for good quality control toensure reliable resultsensure reliable resultsCorrect positioning, angle into stack, closeness to wallCorrect positioning, angle into stack, closeness to wallInstrument condition, no blocked holes, undamaged andInstrument condition, no blocked holes, undamaged anda note to certain test teams, don’t put it in stack gas soa note to certain test teams, don’t put it in stack gas sohot that it melts!hot that it melts!

Pressure readingPressure reading

In general either an inclined manometer orIn general either an inclined manometer ordigital meter are used to measure thedigital meter are used to measure thedifferential pressuredifferential pressureNearly all digital pressure meters used in theNearly all digital pressure meters used in theUK are used at the extreme low end of theirUK are used at the extreme low end of theirrange, leading to the potential for largerange, leading to the potential for largeuncertaintiesuncertaintiesManometers are often calibrated under veryManometers are often calibrated under verydifferent conditions from those found in use,different conditions from those found in use,and there are potential issues with user biasand there are potential issues with user bias

UK SurveyUK Survey

Carried out a survey of STA membersCarried out a survey of STA members29 organisations replied29 organisations replied360 360 PitotPitot Tubes, 41% L type, 56% S type (3% other Tubes, 41% L type, 56% S type (3% othertypes)types)59% were routinely measuring stacks that have flows59% were routinely measuring stacks that have flowslower than stated in ISO 10780, and 38% madelower than stated in ISO 10780, and 38% mademeasurements in ducts smaller than covered inmeasurements in ducts smaller than covered inISO10780ISO10780All respondents who used digital pressure sensors wereAll respondents who used digital pressure sensors wereusing them at the extreme low end, with many usingusing them at the extreme low end, with many usingthem below 0.1% of their full scale rangethem below 0.1% of their full scale range

Summary of Key IssuesSummary of Key Issues

There are very defined limitations on the rangeThere are very defined limitations on the rangeof conditions in which differential pressureof conditions in which differential pressuredevices may be used with confidence – realdevices may be used with confidence – realconditions often exceed theseconditions often exceed theseThere is very little research work quantifyingThere is very little research work quantifyingeffects outside these conditionseffects outside these conditionsHow representative are calibrations of S-TypeHow representative are calibrations of S-Typetubes for real stack conditions?tubes for real stack conditions?A further area not covered in this talk are theA further area not covered in this talk are thenewer technologies such as Laser Dopplernewer technologies such as Laser DopplerAnemometry, and continuous systems.Anemometry, and continuous systems.

Future Work?Future Work?

Work programmes are proposed toWork programmes are proposed to– Assess performance under stack conditions.

Quantify the effects of non-standard conditions.– Provide training and workshops on the issues

related to the measurement of flow in stacks– Improve the calibration and traceability of flow

measurement, including use of newtechnologies

““It is regrettably an inescapable conclusion that in theIt is regrettably an inescapable conclusion that in thepresent state of knowledge calibration factors of pitot-present state of knowledge calibration factors of pitot-static tubes once conditions depart appreciably fromstatic tubes once conditions depart appreciably fromsuch as may be called very good are by no means wellsuch as may be called very good are by no means welldefined or are easily determined”defined or are easily determined” – original NPL paper – original NPL paperdescribing the development of the L-Type ellipsoidaldescribing the development of the L-Type ellipsoidaltube, 1962tube, 1962

‘they are rarely used in process streams but are‘they are rarely used in process streams but areused occasionally in utility streams where highused occasionally in utility streams where highaccuracy is not necessary’accuracy is not necessary’Applied Instrumentation in the Process industries, 1980