Guidance

Monitoring stack emissions: measurement locations

Updated 14 December 2022

Applies to England

This guidance is for:

• industrial plant designers who need to know the standard to meet when designing sample locations for stack emission monitoring

• industrial permit holders and stack emissions monitoring companies who need to know about sample locations for stack emissions monitoring

1. Overview

This guidance has information on sample locations for monitoring stack emissions to air.

It provides guidance on ‘European standard EN 15259: Requirements for measurement sections and sites and for the measurement objective, plan and report’. This guidance focuses on the parts of the standard that deal with measurement sections and sites.

EN 15259:

• specifies measurement location requirements for stack emission monitoring

• applies to both short term (periodic measurements) and to continuous emission measurement systems (CEMS)

• outlines the sampling strategy you will need to get a representative sample

This guidance does not repeat text, tables or diagrams from EN 15259. Instead, it refers to the relevant sections of this standard.

2. Designing a representative measurement location

To get reliable and comparable emission measurement results you must have suitable measurement sections and sites. You should plan these when designing a plant.

You must install appropriate measurement ports and working platforms so you can carry out emissions measurements. You should consider their installation when planning a measurement section.

3. Principles of representative sampling

This section describes how the sample location affects whether the sampling is representative of what you are measuring.

Particulate sampling

For gases carrying particulates, the inertial effects introduced by gravity and the duct geometry lead to the uneven distribution of particles in the stack.

You must collect samples from multiple sample points (a grid measurement) across the sampling plane to give an overall average of the particulate emission. EN 15259 specifies where you should locate these sampling points. It also defines the flow criteria a cross section must comply with to carry out a grid measurement – these criteria are based on having an even flow profile.

For extractive particulate methods, you must collect the sample isokinetically using a grid measurement approach.

You must achieve isokinetic sampling by making sure the gas that enters the sampling nozzle is at the same velocity and direction as the gas travelling in the stack. If you do not sample isokinetically, it can lead to an overestimate or underestimate of the concentration.

To allow for the non-uniformity of particulate distribution, you must extract gas at a pre-selected number of points across the sample plane.

Gas sampling

Stack gases may be heterogeneous (not well mixed) because of differences in chemical composition, temperature, and velocity. This may lead to stratification and swirling. However, you can simplify gas sampling if the concentration does not change over the cross section of the stack.

Where you are only measuring the concentrations of gaseous species, you can select a sampling location where you do not have to also measure flow and particulates.

This means designing a measurement section for gas concentrations alone can have less stringent requirements than for measuring flow or particulates. Also, if the gases are well mixed, the sampling approach is more straightforward because you can carry out single point sampling, rather than grid sampling.

However, you can only simplify gas sampling in this way if you can prove that the stack gas is homogeneous (well mixed).

If the gases are not well mixed, you will have to sample the gases using a grid measurement technique. When using a grid measurement technique, you must be able to measure the velocity at several points across the sample plane.

This means you should consider at the design stage the possibility that the gases are heterogeneous.

If you need to calculate the mass emission rate of a gas, you must measure the gas volumetric flow rate. You must take velocity measurements at several points across the sampling plane using a grid measurement approach.

Multiphase and aerosol sampling

Some pollutants, for example, metals and dioxins, are present in both particulate and vapour phases. Other pollutants, for example, hydrogen chloride, may be present in an aerosol phase and vapour phase. Aerosols are normally treated as particulates. In all these cases you must use grid measurement sampling.

4. Periodic sampling using grid measurements

This section describes the information in EN 15259 on grid measurements.

Positional requirements

EN 15259 details the positional requirements for carrying out a grid measurement. You must use this approach for:

• sampling particulates

• sampling multiphase pollutants

• sampling using wet chemistry when droplets are present

• velocity measurements for reporting mass emission

• sampling gases that are distributed heterogeneously

• finding a representative sample location for CEMS

Criteria for locating the sampling plane

You must carry out sampling at a suitable location on the stack.

Bends, branches, obstructions and fans can all cause undesirable variations in the velocity profiles. These may make the location unsuitable for measuring velocity and carrying out isokinetic sampling.

Section 6 of EN 15259 gives more information on locating a measurement plane in a stack.

Meeting the required flow criteria for grid measurements

You must carry out a velocity traverse to find that a sampling location is suitable.

Section 6 of EN 15259 gives more information on the required flow criteria.

Number and position of sampling points

The number and position of sampling points is determined by the size of the stack. You must divide the sampling plane into equal areas and sample from points in the centres of these areas.

Section 8 of EN 15259 gives more information.

5. Representative sampling of gases

As gases are often well mixed, you may be able to prove that a grid measurement is not required, and that you can carry out sampling from a single point.

To prove this, you must carry out a homogeneity test to assess the spatial and temporal variations in the gas concentration.

If the distribution is heterogeneous, you may still be able to sample from a single point, provided you can find a single representative point. This approach is based on assessing whether the variations in concentration across the stack are within the allowable measurement uncertainty.

If you have not carried out a homogeneity test, or you cannot find a representative point, you must carry out grid measurements.

Where you need to make grid measurements, you must measure the velocity at each sample point so you can achieve representative sampling.

Once you have completed the homogeneity test you do not need to repeat it, unless there is a significant change in the:

• fuel type

• structure of the stack

• process abatement systems

There’s more information on the homogeneity test in section 8 of EN 15259 and the Environment Agency’s Method Implementation Document (MID) for EN 15259.

6. Sampling requirements: CEMS

You must calibrate CEMS by comparing simultaneous measurements taken using a periodic standard reference method.

You should locate CEMS and periodic monitoring locations close to each other. However, CEMS must not obstruct, or be affected by, sampling probes used to perform periodic measurements.

For CEMS that extract stack gas from a single point in the stack, you must find a representative measurement point. Section 8 of EN 15259 and MID 15259 explains the procedure.

7. Sampling facilities for stack emission monitoring

This section describes the facilities you need to install so you can carry out sampling.

Platforms

The Environment Agency recommends that you use permanent platforms wherever possible.

You may decide to use a temporary platform. For example, when limited access prevents you from installing a permanent platform.

This guidance gives a summary of the information on platforms and sampling facilities in BS EN 15259.

EN 15259 includes diagrams of platform designs.

Measurement ports

Measurement ports allow you to access into the stack. You must make sure they are big enough so you can insert and remove the equipment used and reach the measurement points.

You may need to design measurement ports specifically for periodic monitoring of PM₁₀ and PM_2.5 because the sampling equipment is larger than that used for measuring particulates only. The dimensions of the measurement ports should allow you to insert the sampling equipment so that the orientation of the PM₁₀ and PM_2.5 cascade impactor sampling device is parallel to the longitudinal axis of the stack.

EN 15259 includes diagrams of measurement ports designs.

Sampling facilities for periodic monitoring using multiphase sampling techniques

Multiphase sampling uses a combination of isokinetic and gas phase sampling techniques. It measures pollutants that may be in both the particulate and gas phase, such as metals, and dioxins and furans. You cannot use multiphase sampling equipment, without deviations from standard methods, on horizontal circular ducts with an internal diameter of greater than 0.35m. This is because the orientation of the measurement ports prevents access to the measurement points. If you use a horizontal duct, it should be rectangular, rather than circular, as this will allow access to the required measurement points.

Sampling facilities for periodic monitoring of gases

Access and facilities requirements for gases alone are usually less demanding than for particulates. Meeting the requirements for particulates will also satisfy the requirements for gases. When you are only measuring gas concentrations, it may be possible to have smaller measurement ports than those required for particulate sampling.

Sampling facilities for CEMS

When you design a platform for CEMS, you must have access and facilities to allow:

• calibration by periodic monitoring

• routine maintenance and operational functional checks, such as span and zero checks, to be carried out

Essential services

You will need to provide an electricity supply. You may also need to provide lifting equipment, ventilation and lighting.

8. Safety

The sample location must be safely accessible at all times required during the monitoring process.

There are many hazards associated with stack emissions monitoring. The Source Testing Association’s booklet Risk Assessment Guide: Industrial-emission monitoring gives detailed information.

9. Requirements and recommendations for plant designers and operators

This is a list of the requirements and recommendations for plant designers and operators.

Measurement section

The measurement section must be clearly identified and labelled.

Sample plane location for CEMS and periodic monitoring that requires a grid measurement approach

The information in this section applies to locating CEMS and to periodic monitoring using a grid measurement approach (for example measuring velocity and particulates).

The Environment Agency recommends that, where possible, you carry out an exploratory velocity traverse before installing a permanent sample platform.

Generally, you will meet the flow criteria requirement specified in EN 15259 when following these recommended locations:

• as far downstream or upstream from any disturbance which could change the flow direction (for example, bends or fans)

• in a section of duct with a constant shape and cross-sectional area

• 5 or more hydraulic diameters upstream and 2 or more hydraulic diameters downstream (or 5 or more hydraulic diameters from the top of the stack)

The following applies to the orientation of the sample plane:

• EN 15259 recommends vertical stacks

• particulates may collect in horizontal ducts and interfere with sampling

• horizontal ducts should be rectangular

• horizontal ducts may be circular if they have a diameter of less than 0.35m

The mandatory flow criteria specified in EN 15259 that apply to locating CEMS, and to periodic monitoring using a grid measurement approach (for example measuring velocity and particulates), are:

• an angle of gas flow that is less than 15° to the duct axis

• no local negative flow

• minimum velocity (a differential pressure of 5Pa, which equates to 3m/s)

• a ratio of the highest to lowest gas velocity of less than 3:1

Sample plane location for periodic measurement of gas concentrations

This section applies to the measurement of gas concentrations only.

Single point sampling is appropriate where you have proven that gas concentrations are homogeneous or where you have determined a representative point.

You must carry out grid sampling if you have not proven that the gases are homogeneous or you cannot determine a representative point.

EN 15259 recommends locating the sample plane in a vertical stack because gas concentration stratification may occur in large horizontal ducts.

Horizontal circular ducts with a diameter of 1.13m or more may require access from above.

Sample ports

The Environment Agency recommends that you install access ports before the plant is operational. Retrofitting can be expensive and requires plant shutdown.

For rectangular ducts you must install the ports on the longer side.

The port socket must not project into the gas stream.

The Environment Agency recommends you consider installing sampling monorails above the sampling ports. This is so you can suspend certain designs of sampling equipment.

The following applies to access port size and access:

• access ports should have a minimum internal diameter of 125mm – you may need a smaller socket (for example 75mm internal diameter) for small stacks (less than 0.7m diameter)

• where you may need to measure PM₁₀ or PM_2.5 in the future, the Environment Agency recommends you install rectangular measurement ports with minimum internal dimensions of 100mm wide by 400mm high

• you may need extra ports to allow access at the same time to measure other parameters (for example velocity and water vapour)

• you may need extra ports for CEMS

• for large ducts you may need 4 main ports

• the Environment Agency recommends that ports in vertical ducts are 1.2 to 1.5m above the floor of the platform

• you should have a minimum clearance of 45cm between the floor of the sample platform and the ports – this is to make sure there is enough space to manoeuvre sample equipment connected to the sample probe (particularly relevant at horizontal rectangular ducts where the lowest port is close to the floor of the sample platform)

The Environment Agency recommends that you clearly label access ports.

Platforms

Permanent and temporary platforms must have a load bearing capacity that is large enough to meet the measurement objective.

Some measurement objectives may require platforms that support up to 6 people plus up to 300kg weight of equipment.

The Environment Agency recommends that the work area is large enough so you can move monitoring equipment without it overhanging platform guardrails.

Guard rails must not prevent you inserting fully assembled sample equipment.

You can work out the recommended work area required on a platform from the internal diameter or depth of the duct and the wall thickness, plus 1.5m.

If you install measurement ports opposite each other, a correspondingly smaller work area is required.

You may need to provide a platform at two levels. You must do this if both of these apply:

• the selected sample plane is in a horizontal section of a large rectangular duct

• some of the sample points are positioned above a convenient and safe working height (nominally 1.5m maximum for sample probe handling)

You must install removable chains or self-closing gates at the platform to prevent falls through access ladders.

You must prevent free-standing water building up and, if necessary, provide drainage.

You must use toe boards.

The Environment Agency recommendeds access is by stairs or lift.

You should consider installing personal protection systems on vertical ladders.

Safe working area

You must consider the health and safety at work act.

You must be able to safely access the sample location throughout the monitoring process.

The Environment Agency recommends that the sample location is in a building (this has both health and safety, and technical benefits).

You must consider how you will tell stack emission monitoring staff about any issues that may put them at risk.

You must locate your work area away from:

• sources which emit unexpectedly, for example rupture discs, overpressure valves and steam discharges

• areas that require access by machinery or vehicles

You should consider reducing exposure to stack gas by avoiding areas of significant positive pressure.

The work area must be well ventilated.

You should protect the work area from heat and dust.

You should consider providing protective measures (for example, weather protection and heating) to make sure conditions are appropriate for staff and equipment.

You may need to provide artificial lighting or facilities for temporary lighting.

Power supply

You must provide a single phase 110V electrical power of a suitable current from a suitable number of waterproof sockets.

Lifting equipment

You must consider lifting operations and lifting equipment regulations. This includes installing:

• lifting systems for raising and lowering equipment, where access to a sampling platform is by vertical, or steeply inclined, ladders or stairs

• a support structure for securing portable lifting systems (handrails are not usually suitable for supporting lifting systems)

You must inspect lifting systems (for example, hoists) and attachments (for example, eyes) and they must be maintained by a competent person.

Contact us

You can contact the Environment Agency if you need any help.

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