ATMOSPHERIC AIR POLLUTION
SAMPLING
SOURCE SAMPLING
Stack sampling
An overview of sampling techniques
Prof JC (Koos) Engelbrecht
Associate Professor: Environmental Health
Faculty of Science
TSHWANE UNIVERSITY OF TECHNOLOGY
2013
SOURCE SAMPLING
Stack sampling
An overview of sampling techniques
Main aim of this lecture:
After promulgation of the list of activities which result in atmospheric
emissions which have or may have a significant detrimental effect on the
environment (Government Gazette No. 33064 dated 31 March 2010), the
sampling of point (stack) sources was prioritised. Industries that are holders
of atmospheric emission licenses, are required to conduct stack sampling to
comply to emissions standards as prescribed in the Minimum Emission
Standards. Specific methodology is also prescribed as per Schedule A of the
Standards.
This lecture aims to provide summative information to interested and affected
parties on the basics of stack sampling and to provide guidance on an
overview of sampling techniques and equipment.
ISOKINETIC
ISO - means the same
KINETIC – means moving energy
Misconceptions about
isokinetic sampling
• It is not a quick, simple exercise – fairly involved and complex,
“almost an art”
• Time consuming – setting up of equipment, preliminary checks etc
• It is not related to occupational measurements – totally different
discipline
• Specifically used for determining flue gas particulate / gas
concentrations in a duct or stack
• Purpose designed equipment
ISOKINETIC SAMPLING
DEFINITION
Sampling at such a rate that the velocity and the
direction of the gas entering the sampling nozzle is
the same as that of the gas in the duct / stack at the
same sampling point.
Source sampling Stack (Isokinetic)
Reasons for sampling
• To determine the effectiveness of control equipment
• To demonstrate legal compliance (atmospheric emission license
conditions)
• To determine the specific source contribution to the receptor
environment
• To provide data for dispersion modeling
• To provide data for process efficiency
Particulate
stack sampling
(Isokinetic)
Stack sampling
Isokinetic sampling
Sampling train diagrammatic representation
Stack sampling
Isokinetic sampling equipment
particulates
Stack sampling
Isokinetic sampling
Sampling train for particulates
Stack sampling
Sampling train for gaseous pollutants (SO2) / PM
Stack sampling
Velocity measurement system
Stack sampling
Isokinetic sampling
Criteria for selection of the sampling location
• At the point of greatest interest (e.g. a stack)
• In a straight section op ducting
• 5 or more diameters downstream from any bend or flow disturbance
• 3 or more diameters upstream from any bend or flow disturbance
• Accessible to sampling personnel and equipment
• Availability of electricity, water and air if needed
• Safety of point of sampling
OFTEN AN IDEAL SITE CANNOT BE FIND
AND A COMPRIMISE MUST BE REACHED
Stack sampling
Isokinetic sampling
Exemplified structure of measuring hole
Stack sampling
Isokinetic sampling
Typical sampling point provisions
Stack sampling
Isokinetic sampling
Gas velocity profile
• Velocity of gas entering the sampling probe must be
equal to the gas velocity at the sampling position.
• The gas velocity must be measured continually and
sampling velocity adjusted accordingly
• The sampling needs to be conducted under so called
“Normal conditions” that is prescribed by legislation
as 10% O2, 273 Kelvin and 101,3 kPa
Stack sampling
Isokinetic sampling - Gas velocity profile
Sampling position - theory
Isokinetic sampling - Gas velocity profile
Sampling position - theory
At any point “r” from the center the velocity is found from:
U/Center = (1 – r/R)1/7
Also known as the1/7th law
Samples should therefore not be taken
from the central point of blocks (rectangular)
or rings (round) ducts, but from
representative positions in the duct.
Stack sampling
Isokinetic sampling
Over and under sampling
Stack sampling
Isokinetic sampling
Over and under sampling
Isokinetic sampling
Sampling probe positions
Circular ducting
Annual rings of equal area minimum of 10 sampling points
Isokinetic sampling particulates
Sampling position
Particle distribution in a duct
Isokinetic sampling
Sampling probe positions
Rectangular duct
Minimum of nine measurement in the center
of each envisaged rectangular
Stack sampling
Isokinetic sampling
Sampling position – bends and curves
Sampling must be conducted:
In a straight section of pipe
If bends or flow disturbances:
• 5 or more diameters downstream
• 3 or more diameters upstream
Stack sampling
Isokinetic sampling
Sampling position – bends and curves
Stack sampling
Isokinetic sampling
Angle between inlet tube and stream direction
Stack sampling
Isokinetic sampling
Sampling standards for particulates
Mostly ISO (International Standards Organisation) and EPA (United
States Environmental Protection Agency) are recommended in Schedule
A (Method for sampling and analysis) as methods to be used to confirm
legal compliance. Some examples are:
• ISO 9096 Stationary source emissions – Manual Determination of the
mass concentration particulate matter
• ISO 7935 Stationary source emissions – Determination of the mass
concentration of sulfur dioxide - Performance characteristics of
automated measuring method
• EPA Method 10 Carbon monoxide - NDIR
Other methods that the listed Schedule A may be used with the written
consent of the National Air Quality Officer
Source sampling
Sampling equipment
Isokinetic sampling
Particulates
Sampling nozzle
Isokinetic sampling
Particulates
Sampling head with
filter holder
Isokinetic sampling
Particulates
Filters – in stack fitration
Isokinetic sampling
Particulates
Filters and filter holders
Isokinetic sampling
Stack gas velocity - pitot tubes
Isokinetic sampling
Vacuum pump
Gaseous stack
sampling
GASEOUS SAMPLING PRINCIPLES
• Homogenous nature of gases – therefore not
necessary to sampling isokinetically because:
– Flue gas are presumed mixed thoroughly
– Very small molecules therefore not adversely
affected by gravity
– Properties of gases are such that a sample can be
taken at several predetermined points along the
diameter of the duct and still be representative
without having to sample isokinetically
Stack sampling
Sampling train for gaseous pollutants (SO2) / PM
Gas sampling
Bubblers
Gas washing bottles
Stack sampling
Gases
Sampling train
Associated errors that could
influence sampling accuracy
•
•
•
•
•
•
Moisture
Gas composition
Nozzle, probe direction
Leakages on test equipment
Test equipment not calibrated
Non-compliance to minimum requirements of
relevant standard
• Not collecting enough particulate matter
• Unstable process conditions
CONTINIOUS
MONITORING
(Gases and particulates)
Stack sampling
Opacity monitoring
In line stack
ADDITIONAL REFERENCE MATERIAL
• Methods of Air Sampling and Analysis. Third Edition. James P Lodge.
Lewis Publishers. 1988.
• AIR POLLUTION. Volume III Measuring, monitoring and surveillance of
air pollution. THIRD EDITION. Arthur C Stern. Academic Press. 1976.
• AIR POLLUTION. Volume VII. Supplement to measuring, monitoring and
surveillance and engineering control of air pollution. THIRD EDITION.
Arthur C Stern. Academic Press. 1986.