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Source TeSTing ASSociATion | Annual Guide 2014 6 wire or hot film sensors, point or cross-duct ultrasonic devices (Figure 4) or correlation (pattern matching) devices. However, it is recognised that the uniformity of the velocity profile at the monitoring location, and the stability of this profile with regard to plant operations, may affect the choice of flow monitor and how this is configured. Annual Surveillance Test (AST). QAL3 requires the ongo- ing monitoring of instrument zero and span drift. QAL1 defines additional certification requirements and emphasises the need to have an appropriate reference material, or surrogate approach, for checking the zero (or low level) and span (high level) instrument capability. For example, a Pitot tube would require the capability to check the ∆P measurement combined with procedures to ensure that the pressure tappings remain blockage free. e instru- ment configuration, and sensitivity to changes in flue gas conditions and velocity profile shape, must also be audited by the Test Laboratory during the certification field trial. QAL2 defines the approach to be taken for in-situ cali- bration of the flow monitor. EN14181 employs the Emission Limit Value (ELV) and an uncertainty level specified in the relevant European Directive when assessing the quality of the calibration. Since these parameters are not defined for flue gas flow rate, surrogate values are defined in the stan dard for the ELV (120% of the maximum measured value) and the uncertainty (σ o = 4%). Testing does not have to meet any particular time constraints, e.g., a QAL2 can potentially be conducted in one day, and the number and range of the measurement points can be reduced if a pre- investigation of the flow profile is conducted, as noted above. In addition to the usual variability (QAL2) and bias (AST) assessments, the quality of the linear regression be- tween the test results and continuous monitoring results must be good (R 2 > 0.9). Calculation of the flue gas flow rate from fuel consump- tion can be also employed for continuous monitoring pur- poses (according to Part 1 Annex E) subject to QAL2/AST verification. QAL3 requires the usual control chart approach for the assessment of instrument drift using the internal reference points established under the QAL1 certification. Applying this new standard to existing combustion plant poses a number of challenges relating to a) sample port pro- vision and access, b) choice of manual test method and c) implementation of the QA requirements in a consistent and meaningful way. However, the standard provides a, long over- due, framework for improving the quality of flue gas flow rate monitoring for emissions reporting and other purposes. n David Graham Technical Consultant E.ON New Build & Technology Reproducibility Crest factor Skewness Monitoring approach Comments of normalised profile < 5% < 1.3 < 1.2 Single probe point measurement Flow profile unlikely to change (or limited length) > 5% < 1.3 < 1.2 One cross-duct monitoring path Flow profile is expected to change with flow rate > 1.3 < 1.2 One cross-duct monitoring path in Flow profile is expected to change considerably the plane with the highest Skewness with flow rate > 1.3 > 1.2 Two cross-duct monitoring paths A skewed flow profile, possibly due to swirl, i.e., the point in the profile with the maximum flow rate is rotating and the best way to secure a representative average is to monitor in a cross or across two chords e standard therefore encourages a pre-investigation of the velocity profiles at the proposed monitoring location, based on point velocity measurements (see Part 1). For a new plant, this can be conducted using Computational Fluid Dynamics. e survey needs to be performed at the normal base load operating condition and the minimum stable operating condition. Table 2 presents informative guidance to assist in the selection of a flow monitor. e profile is assessed by means of three parameters: • Reproducibility: the deviation in the normalised velocity profile shape between the minimum and maximum plant flow rates. • Crest factor: the ratio of the maximum to average velocity • Skewness: the ratio of the average velocities either side of the duct centre-line. If a pre-investigation (velocity survey) is performed, the plant does not then have to operate at minimum load when the monitor is calibrated. e Quality Assurance (QA) approach defined in the standard is based on EN 14181 which defines three Quality Assurance Levels (QALs). QAL1 requires that the instru- ment is fit for purpose and this is satisfied by an appropriate instrument certification. QAL2 requires in-situ calibration of the CEM using parallel test data obtained by an accre- dited Test Laboratory using Standard Reference Methods (SRM) defined in Part 1. e calibration must also be checked annually by the Test Laboratory by means of an Table 2: Informative guidance on monitoring arrangements Figure 4: Cross-duct ultra-sonic flow meter configuration STA Article 2 WP.indd 6 05/02/2014 19:47 Cleaner emissions for the future? Naturally. Whether it is for a power plant, an incinerator or an industrial process, choosing our proven range of gas analysers means that you just can't lose. Increasingly strict environmental legislation and penalties controlling the emissions from your plant means that ensuring compliance is no longer a choice. Our rugged, compact systems help you to meet the demands of the CEN standards for continuous emissions monitoring EN14181. MCerts and TüV approved, we offer integrated packages that include reporting and remote maintenance capabilities. All backed by industry-leading service and support. www.abb.com /measurement ABB Limited Tel: 01480 475321 Fax: 01480 217948 moreinstrumentation@gb.abb.com STA_MP_A4_clean_emissions_Dec2013.indd 1 12/12/2013 09:34 STA p07.indd 7 05/02/2014 19:48

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