Air Monitoring 31
The S-type Pitot, shown in Figure 1b, is commonly used to establish iso-kinetic sampling conditions when measuring dust concentrations. This is normally inserted into the flow so that the ‘impact’ orifice faces into the flow and the ‘wake’ orifice is then positioned at 180° to this. Operation as a 2D Pitot is described in detail in US EPA Method 2G. Note that, if a Pitot tube is used in a configuration with a closely coupled gas-sampling probe, then the device must be calibrated in this configuration.
For determining the average velocity, the traverse points are located at centres of equal area so that a simple average of the point readings gives an area weighted average in a duct of circular cross-section. The procedures for determining the required number and location of points are specified in EN15259, noting that the ‘tangential method’ is required by EN ISO 16911, i.e., the centre-line of the duct cannot be included. Twenty measurement points are normally sufficient in large ducts.
according to US EPA Method 2H. However, the correction is usually very small and it is normally sufficient to multiply the measured average velocity by a Wall Adjustment Factor of 0.995 for a smooth duct or 0.99 for a rough (brick-lined) duct of circular cross-section. This is a requirement when calibrating a flow monitor.
Tracer transit time methods determine the bulk (average) velocity directly by recording the time taken for a tracer material to travel between two measurements stations (Δt). The distance between these two stations, situated in duct work of constant cross section, is divided by the measured time-of-flight to obtain the average velocity. The example in the standard is based on the injection of a radioactive tracer, upstream of the flue. Two sets of clamp-on detectors are then used to detect the arrival of the tracer at two different heights within the flue. The medians of the recorded tracer concentration peaks are extracted so that the shape of the detector response is taken into account to obtain an accurate Δt.
In order to obtain the volumetric flow rate the average velocity must be multiplied by the duct’s cross-sectional area. EN ISO 16911 requires the Test Laboratory to measure the duct dimensions, across at least two axes, rather than simply relying on plant drawings.
Figure 2 EN15259 Traverse Points
The field trial validation indicated that lack of uniformity of the flow profile (Figure 3) caused by a poor measurement location did not significantly affect the average velocity determination. That is, a 20 point average from a poor flow profile gave the same result as a 20 point average from a uniform flow profile.
Performance requirements and quality assurance requirements are specified for each technique. For Pitot tubes, a pre-test leak check is required and, when using an electronic pressure reading device, a daily calibration check is required using a liquid manometer device (temperature corrected) or a calibrated pressure sensor with an uncertainty better than the test device. The repeatability also needs to be determined at a single measurement point (the standard deviation of five consecutive one minute velocity readings). Each point velocity measurement must be obtained from a one minute average ΔP based on a continuous measurement or at least three separate readings.
A velocity traverse to EN 15259 does not have sufficient resolution to capture the very low velocity boundary layer at the duct wall. For a large duct, this can optionally be measured
The tracer dilution method directly determines the flue gas flow rate and does not, therefore, require the cross-sectional area to be known. A tracer is injected into the flue gas, for a short period of time, well upstream of the flue, so that the tracer is intimately mixed with the flue gas. The concentration of tracer in the flue gas is then measured. A one-off EN 15259 concentration traverse must be performed to demonstrate that the tracer is well mixed for the given injection configuration. Simple dilution relationships are then used to calculate the flue gas flow rate from the tracer injection flow rate and concentration.
If all of the above techniques are regarded as different implementations of the same method, the ensemble average uncertainty, based on validation field trials, is estimated to be ± 5% at 95% confidence, assuming that the flow is non-swirling. However, it is anticipated that a lower uncertainty can be obtained using a specific technique in a given application. The Test Laboratory must calculate the uncertainty of the method, using the approaches described in the standard, and ensure that this complies with the requirements of the Test Objective.
Part 2: Automated measuring systems
Part 2 of the standard is also performance based, that is, provided that the specified performance requirements are satisfied, any continuous monitoring technique can be employed, e.g., single point or averaging Pitot tubes, hot 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.
Figure 4: Cross-duct ultra-sonic flow meter configuration
The 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. The 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. The 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.
The Quality Assurance (QA) approach defined in the standard is based on EN 14181 which defines three Quality Assurance Levels (QALs). QAL1 requires that the instrument 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 accredited Test Laboratory using Standard Reference Methods (SRMs) defined in Part 1. The calibration must also be checked annually by the Test Laboratory by means of an Annual Surveillance Test (AST). QAL3 requires the ongoing 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 ΔP measurement combined with procedures to ensure that the pressure tappings remain blockage free. The instrument 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 calibration of the flow monitor. EN14181 employs 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 standard for the ELV (120% of the maximum measured value) and the uncertainty (Δo = 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 between the test results and continuous monitoring results must be good (R2 > 0.9).
Calculation of the flue gas flow rate from fuel consumption can be also employed for continuous monitoring purposes (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.
Concluding remarks Figure 3: Velocity profiles from a validation field trial
Applying this standard to existing combustion plant poses a number of challenges relating to a) sample port provision and access, b) choice of manual test method and c) implementation of the QA requirements in a consistent and meaningful way.
www.envirotech-online.com IET September / October 2019
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