🌿 Water vapour in waste gases — condensation-adsorption method (reference)
In short
A known volume of gas is sampled with a heated probe through a filter into a collection system (adsorption, or condensation + adsorption) and the water vapour content is determined from the increase in the mass of the system; with saturated gases — from the temperature of the gas; the standard reference method for 4–40 % by volume (29–250 g/m³). The test is performed according to PN-EN 14790:2017-04 (EN 14790:2017); Range: 4–40 % by volume; 29–250 g/m³ of wet gas; validation 7–26 % by volume, sampling for 30 min. The procedure comprises 6 steps; it is used for: Measurement of the moisture of waste gases accompanying every emission measurement — conversion of the concentrations to dry gas, Calibration and control (QAL2, AST) of peripheral water vapour AMS — the reference method under the regulation on measurements of emissions, Waste incineration and co-incineration plants, large combustion plants, plants with wet desulfurisation (saturated gases — the temperature method).
At a glance
- Standard: PN-EN 14790:2017-04 (EN 14790:2017)
- Category: Environment
- Procedure steps: 6
- Range: 4–40 % by volume; 29–250 g/m³ of wet gas; validation 7–26 % by volume, sampling for 30 min
- Temperature of the probe: ≥ 120 °C and ≥ 20 °C above the (acid) dew point; without heating it is permissible with a gas of > 200 °C, if the temperature downstream of the filter does not fall below the acid dew point
- Gas meter: expanded uncertainty ≤ 5 % of the volume; temperature and pressure ≤ 2 %; a dry gas meter requires drying to < 10 g/m³ (dew point 10.5 °C, 1.25 % by volume)
Overview
EN 14790:2017 specifies a standard reference method (SRM) based on a sampling system with a condensation/adsorption technique for determining the concentration of water vapour in waste gases from ducts and stacks. It gives the performance characteristics and the criteria for measuring systems, is applied to periodic monitoring and to the calibration and the control of automated measuring systems (AMS) and specifies the criteria of equivalence for alternative methods (EN 14793). Range of application: 4–40 % by volume of water vapour, that is 29–250 g/m³ of wet gas (the upper limit depends on the maximum vapour pressure at the given temperature); all the concentrations are expressed under standard conditions (273 K, 101.3 kPa). For saturated gases the condensation-adsorption method is not applicable — the standard gives guidance for gases with droplets. It was validated in field tests at incineration plants, co-incineration plants and large combustion plants for periods of 30 min in the range of 7–26 % by volume. The 2017 edition replaced EN 14790:2005. The measurement of water vapour serves to convert the emission results to dry gas; the regulation on the measurement of the quantity of emissions indicates PN-EN 14790 (or an equivalent instrumental method) as the reference method for the water vapour content in the control of continuous measurements.
Method principle
It is assumed that the gas in the stack is approximately in adiabatic equilibrium — droplets can arise only in a saturated gas, so the absence of droplets means an unsaturated gas. The gas sample is taken at a constant flow; the water vapour is collected by adsorption or by condensation with adsorption, and the mass of the vapour is determined from the increase in the mass of the collection system. In the presence of droplets the method overestimates the result: if the measured value is equal to or higher than the value for the saturated gas at the given temperature and pressure (the table in annex B), the result is rejected and the gas is considered saturated — the temperature method is then applied: a temperature probe in the stream until equilibrium is reached, and the vapour content is read from the liquid-gas equilibrium table. All the parts upstream of the collection system are heated and made of materials which do not react with water vapour (stainless steel, borosilicate glass, quartz, PTFE, titanium); the probe is in a heating jacket at a temperature of at least 120 °C and 20 °C above the (acid) dew point. The collection system: at least one cartridge/gas-washing bottle with a desiccant (e.g. coloured silica gel) or a condensation stage (with optional cooling) and an adsorption stage; the condensation of water in parts which are not weighed is not permissible, and the effectiveness of the system is checked in accordance with the standard.
Applications
- Measurement of the moisture of waste gases accompanying every emission measurement — conversion of the concentrations to dry gas
- Calibration and control (QAL2, AST) of peripheral water vapour AMS — the reference method under the regulation on measurements of emissions
- Waste incineration and co-incineration plants, large combustion plants, plants with wet desulfurisation (saturated gases — the temperature method)
- Simultaneous sampling with SO₂ (EN 14791) or HCl (EN 1911) through the same probe, where there are no droplets
- Determination of the volumetric flow rate of dry gas (EN ISO 16911-1)
Key parameters
| Parameter | Value |
|---|---|
| Range | 4–40 % by volume; 29–250 g/m³ of wet gas; validation 7–26 % by volume, sampling for 30 min |
| Temperature of the probe | ≥ 120 °C and ≥ 20 °C above the (acid) dew point; without heating it is permissible with a gas of > 200 °C, if the temperature downstream of the filter does not fall below the acid dew point |
| Gas meter | expanded uncertainty ≤ 5 % of the volume; temperature and pressure ≤ 2 %; a dry gas meter requires drying to < 10 g/m³ (dew point 10.5 °C, 1.25 % by volume) |
| Balance | resolution ≤ 0.1 g or 2 % of the mass of the water |
| Barometer and thermometer | barometer ≤ 1 kPa; gas thermometer 273–373 K, uncertainty ≤ 2.5 K, low inertia |
| Uncertainty of the method | relative expanded ≤ 20 % at the measured value; the criteria of table 1 (volume ≤ 2.5 %, T ≤ 2 %, p ≤ 2 %, leakage ≤ 2 % of the flow) |
Standard
- Standard number
- PN-EN 14790:2017-04 (EN 14790:2017)
- Title (PL)
- Emisja ze źródeł stacjonarnych — Oznaczanie pary wodnej w przewodach — Standardowa metoda odniesienia
- Title (EN)
- Stationary source emissions — Determination of the water vapour in ducts — Standard reference method
Step-by-step procedure
-
Planning
Plan the measurement in accordance with EN 15259 together with the measurements of the components; agree the operating conditions with the plant. A measurement at a single representative point is permissible where the requirements of EN 15259 as to the distribution of the concentration are met; otherwise — a grid measurement.
-
Assembly of the system
Weigh the collection system before sampling; fill the condensation gas-washing bottles with water (less than half the volume) or with the absorption solution of another component, and the last cartridge — with the desiccant. Connect the heated probe, the filter, the collection system, the pump and the gas meter; carry out a leak test (leakage ≤ 2 % of the flow).
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Sampling
Introduce the probe in its heating jacket (≥ 120 °C, ≥ 20 °C above the dew point) into the measurement point and sample the gas at a constant flow for the established time (e.g. 30 min), recording the volume, the temperature and the pressure at the gas meter and the atmospheric pressure. Check the effectiveness of the collection in accordance with the standard.
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Weighing
Weigh the collection system after sampling (resolution ≤ 0.1 g or 2 % of the mass of the water); check the repeatability of the weighing; in the field protect the balance against fluctuations of temperature, draughts and vibrations.
-
Check of saturation
Compare the result with the saturation value for the temperature and the pressure of the gas (annex B); if it is equal or higher — reject the result, consider the gas saturated and determine the vapour content by the temperature method (a temperature probe until equilibrium, reading from the table).
-
Calculations
Calculate the mass of the water, the volume of dry gas under standard conditions (with a correction for water vapour with a wet gas meter), the mass concentration of water vapour in g/m³ and the volume fraction; draw up an uncertainty budget (annex D) and a test report.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Heated sampling probe with a heating jacket and a protective tube | stainless steel, borosilicate glass, quartz, PTFE or titanium; marked so as to reach the measurement points | — |
| Filter housing with a particle filter | in-stack (downstream of the nozzle) or out-stack, connected to the probe without cold sections | — |
| Collection system | cartridges/gas-washing bottles with a desiccant (coloured silica gel) or a condensation stage with optional cooling (ice, cryogenic system) + an adsorption stage; examples in annex C | — |
| Leak-tight pump, rotameter, buffer vessel, control valve | constant sampling flow | — |
| Dry or wet gas meter with a thermometer and a manometer | uncertainty ≤ 5 %; with a dry gas meter a condenser/drier (gel 1–3 mm dried at 110 °C for ≥ 2 h) | — |
| Balance, barometer, gas thermometer | balance ≤ 0.1 g; barometer ≤ 1 kPa; thermometer 273–373 K ≤ 2.5 K | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Desiccant (e.g. coloured silica gel) | — | adsorption stage for the water vapour; dried before use |
| Water (or an absorption solution in accordance with EN 14791 / EN 1911) | — | filling of the gas-washing bottles of the condensation stage (less than half the volume of the bottle) with joint sampling with SO₂ or HCl |
Health and safety (OHS)
- The heated probe and the filter housing are liable to cause burns; a cryogenic system (dry ice) — protective gloves, ventilation
- Work on a stack platform — protection against falling; with acid gases protection of the respiratory tract when opening the ports
- Absorption solutions of other components (H₂O₂, acids) — in accordance with their safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN 14790:2017-04 (EN 14790:2017) — “Stationary source emissions — Determination of the water vapour in ducts — Standard reference method”.
How does this method work?
It is assumed that the gas in the stack is approximately in adiabatic equilibrium — droplets can arise only in a saturated gas, so the absence of droplets means an unsaturated gas. The gas sample is taken at a constant flow; the water vapour is collected by adsorption or by condensation with adsorption, and the mass of the vapour is determined from the increase in the mass of the collection system.
What is the measuring range and accuracy?
Range: 4–40 % by volume; 29–250 g/m³ of wet gas; validation 7–26 % by volume, sampling for 30 min; Temperature of the probe: ≥ 120 °C and ≥ 20 °C above the (acid) dew point; without heating it is permissible with a gas of > 200 °C, if the temperature downstream of the filter does not fall below the acid dew point; Gas meter: expanded uncertainty ≤ 5 % of the volume; temperature and pressure ≤ 2 %; a dry gas meter requires drying to < 10 g/m³ (dew point 10.5 °C, 1.25 % by volume); Balance: resolution ≤ 0.1 g or 2 % of the mass of the water.
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?
Heated sampling probe with a heating jacket and a protective tube, Filter housing with a particle filter, Collection system, Leak-tight pump, rotameter, buffer vessel, control valve, Dry or wet gas meter with a thermometer and a manometer, Balance, barometer, gas thermometer.
Where is this test used?
Measurement of the moisture of waste gases accompanying every emission measurement — conversion of the concentrations to dry gas; Calibration and control (QAL2, AST) of peripheral water vapour AMS — the reference method under the regulation on measurements of emissions; Waste incineration and co-incineration plants, large combustion plants, plants with wet desulfurisation (saturated gases — the temperature method); Simultaneous sampling with SO₂ (EN 14791) or HCl (EN 1911) through the same probe, where there are no droplets; Determination of the volumetric flow rate of dry gas (EN ISO 16911-1).
What safety precautions apply?
The heated probe and the filter housing are liable to cause burns; a cryogenic system (dry ice) — protective gloves, ventilation; Work on a stack platform — protection against falling; with acid gases protection of the respiratory tract when opening the ports; Absorption solutions of other components (H₂O₂, acids) — 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 14790.