Table 1: Recommended location for meeting sample location requirements


General location of sampling plane


The sampling plane should be situated in a length of straight duct (preferably vertical) with constant shape and constant cross- sectional area.


Where possible, the sampling plane should be as far downstream and upstream from any disturbance, which could produce a change in direction of flow (e.g. bend, fan or a partially closed damper).


Location of sampling plane in straight section


The sample plane criteria are usually met in sections of duct with five hydraulic diameters of straight duct upstream of the sampling plane and two hydraulic diameters downstream.


If the sampling plane is to be located near the top of the stack outlet then the distance from the top should be five hydraulic diameters (making a straight length of 10 hydraulic diameters).


Table 2: Flow-stability criteria


Angle of gas flow <±15° from stack longitudinal axis Flow direction


No local negative flow


Minimum velocity


Gas velocities variations


Dependant on the method used (for Pitot tubes a differential pressure larger than 5 Pa)


Ratio of highest to lowest local gas velocities less than 3:1


The sampling plane should be situated in a length of straight duct with constant shape and cross-sectional area


available, pollutant sampling cannot be carried out in compliance with the required sampling methods. This means that the uncertainty of the results is greatly increased. In these circumstances meaningful results from stack emissions monitoring cannot be achieved. Stack emissions monitoring requires defined flow conditions at the sample location. These flow conditions are required to measure pollut- ants that exist in a particulate phase and are also required to measure velocity, which is used to calculate pollutant mass emissions. As well as selecting the correct location, it is also important to ensure that sampling facilities are suitable. This includes appropriate access to the sample location and a large enough work area for the sampling equipment and sampling personnel.


If a suitable location and/or sampling facilities are not available, pollutant sampling cannot be carried out in compliance with the required sampling methods. This means that the uncertainty of the results is greatly increased. In these circumstances meaningful results from stack emissions monitoring cannot be achieved.


Designing new plants


It is essential that designers of new plants remember to take account of stack emission monitoring at the plant’s design stage. Once a plant is built it is extremely difficult, if not impossible, to retrofit appropriate sampling facilities. It is extremely frustrating for all concerned if the sampling location fails to comply with the sampling requirements because it is either located in the wrong place or the sampling platforms are too small and do not allow access to the stack. In England and Wales, permit applications and variations require the operator to assess the sampling arrangements against the requirements


of Environment Agency Technical Guidance Note M1. It is essential that this process is carried out at the plant design stage. Once this has been done, it is strongly recommended that advice and confirmation of the sampling arrangements is sought from the Environment Agency.


Installing a suitable sample location


The sample plane must be positioned such that there are homogenous flow conditions, which allow representative samples of the stack gas emission to be taken. For newly built plant the sampling plane should be located according to the recommendations contained in Table 1, and by meeting the recommendations in Table 1 the flow stability require- ments summarised in Table 2 should be met.


Sampling facilities for gas concentrations The location requirements for measuring gas concentrations are less exacting than for particulates, as variations in velocity profiles tend not


Source Testing Association Annual Guide 2011 5


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