LAWR

5 Annual Guide 2014 | SOURCE TESTING ASSOCIATION to establish iso-kinetic sampling conditions when measur- ing dust concentrations. is is normally inserted into the ow so that the 'impact' ori ce faces into the ow and the 'wake' ori ce 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 con guration with a closely coupled gas-sampling probe, then the device shall be calibrated in this con guration. 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. e procedures for deter- mining the required number and location of points are speci ed 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 su cient in large ducts. e eld trial validation indicated that lack of uniformity of the ow pro le (Figure 3) caused by a poor measurement location did not signi cantly a ect the average velocity deter mination. at is, a 20 point average form a poor ow pro le gave the same result as a 20 point average from a uniform ow pro le. Performance requirements and quality assurance require- ments are speci ed for each technique. For Pitot tubes, a pre-test leak check is required and, when using an elec tronic pressure reading device, a daily calibration check is required using a liquid manometer (temperature corrected) or a cali- brated pressure sensor with an uncertainty better than the test device. e repeatability also needs to be determined at a single measurement point (the standard deviation of ve consecutive 1 minute velocity readings). Each point velocity measurement must be obtained from a one minute average ∆P based on a continuous measurement or at least three separate readings. A velocity traverse to EN 15259 does not have su cient resolution to capture the very low velocity boundary layer at the duct wall. For a large duct, this can optionally be measured according to US EPA Method 2H. However, the correction is usually very small and it is normally su cient to multiply the measured average velocity by a Wall Adjust- ment Factor of 0.995 for a smooth duct or 0.99 for a rough (brick-lined) duct of circular cross-section. is is a require- ment when calibrating a ow 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 (∆t). e distance between these two stations, situated in duct work of constant cross section, is divided by the measured time-of- ight to obtain the average velocity. e example in the standard is based on the injection of a radioactive tracer, upstream of the ue. Two sets of clamp-on detectors are then used to detect the arrival of the tracer at two di erent heights within the ue. e 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 ∆t. In order to obtain the volumetric ow rate, the average velocity must be multiplied by the duct 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. e tracer dilution method directly determines the ue gas ow rate and does not, therefore, require the cross- sectional area to be known. A tracer is injected into the ue gas, for a short period of time, well upstream of the ue, so that the tracer is intimately mixed with the ue gas. e concentration of tracer in the ue gas is then measured. A one-o EN 15259 concentration traverse must be per- formed to demonstrate that the tracer is well mixed for the given injection con guration. Simple dilution relationships are then used to calculate the ue gas ow rate from the tracer injection ow rate and concentration. If all of the above techniques are regarded as di erent implementations of the same method, the ensemble average uncertainty, based on validation eld trials, is estimated to be ± 5% at 95% con dence, assuming that the ow is non- swirling. However, it is anticipated that a lower un certainty can be obtained using a speci c technique in a given appli- cation. e Test Laboratory must calculate the uncertainty of the method, using the approaches described in the stand- ard, 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 speci ed performance requirements are satis ed, any continuous monitoring technique can be employed, e.g., single point or averaging Pitot tubes, hot * © British Standards Institution (BSI – www.bsigroup.com). Extract reproduced with permission. Source: BS EN 15259:2007 Air quality. Measurement of stationary source emissions. Requirements for measurement sections and sites and for the measurement objective, plan and report. FD- points FD 25 20 15 10 5 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 D C A FD+ points Velocity proles (20m) L1 & L2 U = 19.5 m/s AE-CG Traverses 5-6-9-10-12-13-16-17-19-20 Radial position (m) Velocity (m/s) F G E FD- H B Figure 3 Velocity proles from a validation eld trial 1 2 x d 3 4 i Figure 2: EN15259 Traverse Points* STA Article 2 WP.indd 5 05/02/2014 19:47

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