When testing the performance of a Retrofit Emission Control Device (RECD) on a diesel generator set, attention is often focused on the emission analyser, generator load and test duration. However, one important factor can sometimes be overlooked: the location of the exhaust gas sampling port.
Even a good testing instrument can produce questionable results if the sampling point is positioned in an unsuitable section of the exhaust system. Gas flow disturbances, bends, junctions, temperature variations and turbulence can affect the sample reaching the analyser. For this reason, sampling-port position should be considered an important part of RECD emission-test planning.
Why Sampling Location Matters
Exhaust gas does not always flow through a pipe in a perfectly uniform manner. When gases pass through bends, expansions, contractions, silencers or other disturbances, the flow profile can change.
If a sampling probe is installed too close to one of these disturbances, the collected sample may not adequately represent the overall exhaust stream.
This becomes particularly important when evaluating an RECD because the objective is generally to compare emissions under defined operating conditions. A poorly selected sampling point can introduce additional variability into the measurements and make interpretation more difficult.
The Aceget guide on exhaust sampling location and RECD test results explains why sampling-point selection should be considered carefully when preparing an emission-testing arrangement.
Avoid Sampling Too Close to Flow Disturbances
One of the most important considerations is the distance between the sampling location and nearby flow disturbances.
Bends, dampers, junctions, fans, reducers and other changes in the exhaust arrangement can disturb gas flow. Taking a sample immediately after such a feature may result in a non-uniform flow profile.
CPCB's technical guidance for continuous emission monitoring recommends locating sampling positions in suitable straight sections of duct and provides criteria related to flow angle, local reverse flow and velocity distribution. Central Pollution Control Board
This principle highlights why the physical layout of the exhaust system should be reviewed before finalising the sampling port.
Straight Pipe Length Can Improve Sampling Conditions
A sufficiently straight section of exhaust duct can help the gas flow become more suitable for representative sampling.
CPCB technical material on stack sampling describes sampling-port locations using upstream and downstream distances from flow disturbances, with commonly referenced arrangements involving straight lengths of duct. Central Pollution Control Board
However, the exact sampling arrangement should be selected according to the applicable testing method, equipment requirements and project specifications rather than applying a single distance blindly to every DG installation.
The important point is that sampling-port position should be technically justified, not selected simply because it is the easiest location for installation.
Consider the RECD's Position in the Exhaust System
When an RECD is installed, the exhaust arrangement changes. The sampling location may therefore need to be reviewed as part of the overall test configuration.
For example, if the objective is to assess the emissions after the RECD, the sampling location should be representative of the treated exhaust leaving the device. The position should also avoid unnecessary disturbances that could affect the sample.
Where baseline and post-RECD results are being compared, consistency becomes even more important. The testing team should clearly document where the sampling point is located and under what operating conditions each measurement was taken.
Temperature Can Also Influence Measurements
Exhaust temperature is another parameter worth monitoring.
The gas temperature can change as exhaust travels through piping, silencers and emission-control equipment. If the sampling arrangement is exposed to significant temperature differences or condensation risks, the measurement setup may require appropriate provisions according to the applicable test method.
Monitoring temperature alongside emissions can provide useful context when reviewing unusual or inconsistent readings.
A proper test record can therefore include parameters such as:
- DG load
- Exhaust temperature
- Sampling location
- RECD inlet and outlet conditions
- Back-pressure
- Ambient conditions
- Gas analyser readings
- Time of measurement
These details make the final test report easier to review and reproduce.
Sampling Port Accessibility Should Also Be Considered
A technically suitable sampling location should also be accessible to the testing team.
The probe should be capable of being installed securely without creating unnecessary safety risks. Technicians should have sufficient working space around the sampling point, particularly when measurements are being taken on a running DG set.
For permanent installations, access should also be considered for inspection, maintenance and future testing.
This is especially important in basement DG rooms, compact utility areas and installations where exhaust piping is positioned close to walls or other equipment.
Keep Testing Conditions Consistent
When comparing different RECD configurations or conducting repeat tests, consistency in sampling methodology is extremely important.
If the sampling port is moved between tests, or if the distance from bends and other disturbances changes significantly, variations in results may not necessarily reflect the actual performance of the RECD.
A consistent testing arrangement helps reduce unnecessary variation and makes the data more useful for technical evaluation.
CPCB's RECD procedure requires emission testing to follow the applicable procedures and specifications, with data evaluation and result calculations conducted according to the prescribed framework. Central Pollution Control Board
What Should Buyers and Project Teams Check?
Before an RECD emission test begins, project teams can review a simple checklist:
- Where is the sampling port located?
- Is it positioned in a suitable straight section of exhaust duct?
- Are there bends or other flow disturbances nearby?
- Is the sampling point accessible and safe for technicians?
- Is exhaust temperature being monitored?
- Is the same sampling arrangement used consistently for comparison tests?
- Are the sampling location and test conditions documented?
These checks can help identify potential measurement issues before testing begins.
Conclusion
Sampling-port position is more than a minor installation detail. It can influence how representative an exhaust sample is and therefore deserves careful attention during RECD emission-test planning.
A suitable sampling point should take account of exhaust flow, nearby disturbances, straight-pipe sections, temperature, accessibility and the objectives of the test. When these factors are properly considered, the resulting data can provide a more reliable basis for evaluating RECD performance.
For DG set owners, testing agencies and project engineers, the key lesson is simple: accurate emission testing starts with a properly planned sampling arrangement—not just a good analyser.
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