📏 Velocity and volumetric flow rate of gases in ducts — Pitot tube (manual reference method)
In short
Periodic determination of the axial velocity and the volumetric flow rate of gas in ducts and stacks from point measurements (Pitot tube or vane anemometer) in an equal-area grid in accordance with EN 15259, with alternative tracer and calculation methods; uncertainties of 1–10 % at 20 m/s. The test is performed according to PN-EN ISO 16911-1:2013-07 (ISO 16911-1:2013); Quantities: point velocity [m/s], mean axial velocity, volumetric flow rate [m³/s or m³/h] — stack or standard conditions (273.15 K, 101.325 kPa), wet or dry gas. The procedure comprises 6 steps; it is used for: Determination of the volumetric flow rate of waste gases for calculating the mass emission (kg/h) from stacks and ducts, Calibration of automatic flow monitoring systems (AMS) in accordance with part 2 of the standard and EN 14181, Establishment of the isokinetic conditions in the sampling of dust (EN 13284-1), metals (EN 14385), HCl and other components.
At a glance
- Standard: PN-EN ISO 16911-1:2013-07 (ISO 16911-1:2013)
- Category: Metrology
- Procedure steps: 6
- Quantities: point velocity [m/s], mean axial velocity, volumetric flow rate [m³/s or m³/h] — stack or standard conditions (273.15 K, 101.325 kPa), wet or dry gas
- Uncertainty: 1–10 % (depending on the method) at 20 m/s; confidence level 95 %, k = 2
- Point techniques: Pitot tubes L, S, 2D, 3D (ISO 3966, ISO 10780, EPA 2/2G); vane anemometer
Overview
EN ISO 16911-1 specifies a method for the periodic determination of the axial velocity and the volumetric flow rate of gas in ducts and emission stacks and for the calibration of automatic flow monitoring systems installed on a stack. It is applied to circular and rectangular ducts with measurement sites meeting EN 15259; the limits of the size of the ducts follow from the practical possibilities of the instruments described. All the flow measurements must have a demonstrated metrological traceability to national or international standards. For the method to serve as a standard reference method, the user demonstrates that its performance characteristics are no worse than the criteria of the standard and states the total uncertainty at a confidence level of 95 %; the individual methods have uncertainties of 1–10 % at a velocity of 20 m/s. The standard describes the velocity profile from point measurements and the volumetric flow rate from a grid of points, and also the alternative methods: tracer dilution (annex C), tracer transit time (annex D) and calculation from the energy consumption (annex E), together with guidance on when to apply them. Part 2 deals with automated measuring systems. In the Polish regulation on measurements of emissions the velocity of the flow of the gases is measured "by any method guaranteeing" the required quality, and the volumetric flow rate is determined in order to calculate the emission; PN-EN 13284-1 requires that the measurement of the point velocity for isokineticity meet EN ISO 16911-1.
Method principle
The axial velocity at a point is determined by one of two techniques: by measuring the difference of pressures with a Pitot tube (the principle in accordance with ISO 3966 — one pressure opening under the impact of the gas, the others under the static pressure; the difference is measured by an external transducer) in the standard L design, the S type or the multi-axis 2D/3D design (methods based on ISO 10780, ISO 3966 and US EPA Method 2; 2D tubes in accordance with the QA/QC of EPA Method 2G), or with a calibrated vane anemometer. The volumetric flow rate is determined from a series of point velocity measurements in the sampling plane: the points are distributed according to the equal-area principle (EN 15259; for circular ducts the tangential method, because the central point of the general method gives the maximum, not the mean), the mean velocity is taken with a good approximation as the mean of the point measurements, and the flow rate = the mean axial velocity × the area of the cross-section, where necessary with a correction for the wall effect (the WAF factor). The result is given at stack conditions or under standard conditions (273.15 K, 101.325 kPa) for wet or dry gas — the conversions require O₂ (EN 14789) and water vapour (EN 14790). The standard also specifies the swirl (the tangential component of the flow) and the quality controls of the measurement conditions.
Applications
- Determination of the volumetric flow rate of waste gases for calculating the mass emission (kg/h) from stacks and ducts
- Calibration of automatic flow monitoring systems (AMS) in accordance with part 2 of the standard and EN 14181
- Establishment of the isokinetic conditions in the sampling of dust (EN 13284-1), metals (EN 14385), HCl and other components
- Checking of the homogeneity and the velocity profile in the sampling plane (EN 15259), detection of swirl
- Energy balances and control of flue gas fans — the alternative methods (tracer, energy)
Key parameters
| Parameter | Value |
|---|---|
| Quantities | point velocity [m/s], mean axial velocity, volumetric flow rate [m³/s or m³/h] — stack or standard conditions (273.15 K, 101.325 kPa), wet or dry gas |
| Uncertainty | 1–10 % (depending on the method) at 20 m/s; confidence level 95 %, k = 2 |
| Point techniques | Pitot tubes L, S, 2D, 3D (ISO 3966, ISO 10780, EPA 2/2G); vane anemometer |
| Grid of points | equal areas in accordance with EN 15259; circular ducts — the tangential method |
| Alternative methods | tracer dilution (annex C), tracer transit time (annex D), calculation from the energy consumption (annex E) |
| Metrological traceability | required to national/international standards |
Standard
- Standard number
- PN-EN ISO 16911-1:2013-07 (ISO 16911-1:2013)
- Title (PL)
- Emisja ze źródeł stacjonarnych — Manualne i automatyczne wyznaczanie prędkości i strumienia objętości w przewodach — Część 1: Manualna metoda odniesienia
- Title (EN)
- Stationary source emissions — Manual and automatic determination of velocity and volume flow rate in ducts — Part 1: Manual reference method
Step-by-step procedure
-
Measurement site and measurement points
Check that the section and the sampling plane meet EN 15259; establish the measurement lines and points according to the equal-area principle (circular ducts — the tangential method); measure the dimensions of the duct (the area A).
-
Preparation of the instruments
Use a Pitot tube (or an anemometer) with a current calibration and a known factor; check the leak-tightness of the pressure lines and the zero of the transducer; prepare the measurement of the temperature, the static pressure and the composition of the gas (O₂, H₂O).
-
Point measurements
At each point of the grid set the tube axially (with 2D/3D tubes determine the angle of the flow and the swirl), read the dynamic pressure (the mean of a series of readings, the standard deviation), the temperature and the static pressure.
-
Velocities and the mean
Calculate the velocity at each point from Δp, the factor of the tube and the density of the gas (from the temperature, the pressure and the composition); determine the mean axial velocity as the mean of the point measurements, where necessary with a correction for the wall effect (WAF).
-
Volumetric flow rate
Calculate the flow rate as the product of the mean axial velocity and the area of the cross-section; convert it to standard conditions and to dry gas (O₂, water vapour) and — where required — to reference O₂.
-
Quality control and test report
Carry out the quality controls of the measurement conditions (profile, swirl); estimate the uncertainty (ISO 20988, GUM) at the 95 % level and demonstrate the metrological traceability; in the alternative methods (tracer, energy) proceed in accordance with annexes C–E.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Pitot tube (L, S, 2D or 3D) with a calibration certificate | the K factor of the tube; the designs described in annex A | — |
| Differential pressure transducer (micromanometer) | measurement of the dynamic pressure Δp; the mean of m readings at the point | — |
| Calibrated vane anemometer | an alternative to the Pitot tube (annex B) | — |
| Gas thermometer, manometer for the static and the atmospheric pressure | for determining the density of the gas | — |
| O₂ analyser (EN 14789) and measurement of water vapour (EN 14790) | the composition of the gas for calculating the density and the conversions to dry gas | — |
| Tracer dosing system (the alternative methods) | a known mass flow of the tracer and measurement of its concentration or of its transit time | — |
Health and safety (OHS)
- Work on a stack platform at ports with hot gases — protection against falling, heat-resistant gloves
- Long Pitot tubes in ducts under pressure or under negative pressure — secure them against being torn out or drawn in
- Use tracer gases (the alternative methods) in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 16911-1:2013-07 (ISO 16911-1:2013) — “Stationary source emissions — Manual and automatic determination of velocity and volume flow rate in ducts — Part 1: Manual reference method”.
How does this method work?
The axial velocity at a point is determined by one of two techniques: by measuring the difference of pressures with a Pitot tube (the principle in accordance with ISO 3966 — one pressure opening under the impact of the gas, the others under the static pressure; the difference is measured by an external transducer) in the standard L design, the S type or the multi-axis 2D/3D design (methods based on ISO 10780, ISO 3966 and US EPA Method 2; 2D tubes in accordance with the QA/QC of EPA Method 2G), or with a calibrated vane anemometer. The volumetric flow rate is determined from a series of point velocity measurements in the sampling plane: the points are distributed according to the equal-area principle (EN 15259; for circular ducts the tangential method, because the central point of the general method gives the maximum, not the mean), the mean velocity is taken with a good approximation as the mean of the point measurements, and the flow rate = the mean axial velocity × the area of the cross-section, where necessary with a correction for the wall effect (the WAF factor).
What is the measuring range and accuracy?
Quantities: point velocity [m/s], mean axial velocity, volumetric flow rate [m³/s or m³/h] — stack or standard conditions (273.15 K, 101.325 kPa), wet or dry gas; Uncertainty: 1–10 % (depending on the method) at 20 m/s; confidence level 95 %, k = 2; Point techniques: Pitot tubes L, S, 2D, 3D (ISO 3966, ISO 10780, EPA 2/2G); vane anemometer; Grid of points: equal areas in accordance with EN 15259; circular ducts — the tangential method.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Pitot tube (L, S, 2D or 3D) with a calibration certificate, Differential pressure transducer (micromanometer), Calibrated vane anemometer, Gas thermometer, manometer for the static and the atmospheric pressure, O₂ analyser (EN 14789) and measurement of water vapour (EN 14790), Tracer dosing system (the alternative methods).
Where is this test used?
Determination of the volumetric flow rate of waste gases for calculating the mass emission (kg/h) from stacks and ducts; Calibration of automatic flow monitoring systems (AMS) in accordance with part 2 of the standard and EN 14181; Establishment of the isokinetic conditions in the sampling of dust (EN 13284-1), metals (EN 14385), HCl and other components; Checking of the homogeneity and the velocity profile in the sampling plane (EN 15259), detection of swirl; Energy balances and control of flue gas fans — the alternative methods (tracer, energy).
What safety precautions apply?
Work on a stack platform at ports with hot gases — protection against falling, heat-resistant gloves; Long Pitot tubes in ducts under pressure or under negative pressure — secure them against being torn out or drawn in; Use tracer gases (the alternative methods) in accordance with their safety data sheets.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 16911-1.