SOURCE TESTING ASSOCIATION I Annual Guide 2019


pitots come from the US. The temperature thermocouple can also be mis-aligned. Although this is less likely to be an issue, it should be positioned correctly to allow temperature to be measured as close to the sampling point as is possible.


Again, care must be taken if the plant has a wide variation in conditions during these pre-test pitot tube measurements. If there is a rapid fl uctuation in fl ow and if this cannot be predicted, then a second pitot tube should be used to measure the variation and the measurement traverse normalised to this.


In the US Method 5 the isokinicity of the test must be +/- 10% yet in Europe we use the -5% to +15% isokinicity requirement. Again, where we use US EPA Equipment care needs to be taken because most calculators set the isokinicity at 100%, yet apparently the optimum isokinicity seems to be 105%. Why is this? Back to aerodynamics!


In the UK people traditionally mix up US EPA Method 5 and its variants (out-of-stack fi ltration) with US EPA 17 (in-stack fi ltration). The EU standards allow both methodologies. Which is better? If you are trying to determine the particulates in a stack and the temperature is above the dewpoint, in-stack fi ltration will be easier and quicker and does not suffer the drawbacks of the Method 5 train, which can only be inserted into a stack horizontally. However, in-stack fi lter housings typically have upper limits with respect to temperature because they have more components and seals and often they can’t be used above 300o


C.


It is easier to determine the dust level using the in-stack method. There is less likelihood of losses of particulate in the nozzle whilst in the out-of-stack confi guration one has a long probe that is ideal for losing particulates. There is also a probe wash to contend with which adds uncertainty to low level measurements. With the in-stack confi guration there is also neither probe heater nor hot box to worry about or fail. Nonetheless in-stack fi ltration has been relatively unfashionable to use in the UK since the demise of the old BCURA and CEGB MKIII Samplers.


The effect of anisokinetic sampling on different particle sizes The effect of anisokinetic sampling on different particle sizes


r the effect that sampling faster than the gas stream has on sampling different sizes, then we can see that sampling too fast has a less deleterious effect than


g too slow. Compare the gradient of the lines where the Sampling Velocity /Free Velocity is <1 to that where it is >1. The gradient is much flatter when the sampling is is better to sample slightly faster than the gas stream because it affects the particle ion less.


r, is size distribution that important when we are simply conducting an isokinetic test mical species? Yes, when we are sampling for species adsorbed onto the


Consider the effect that sampling faster than the gas stream has on sampling different particle sizes: we can see that sampling too fast has a less deleterious effect than sampling too slow. Compare the gradient of the lines where the Sampling Velocity / Free Stream Velocity is <1 to that where it is >1. The gradient is much fl atter when the sampling is >1, so it is better to sample slightly faster than the gas stream because it affects the particle distribution less.


ates, such as dioxins/furans or heavy metals, the smaller particulates have relatively surface areas compared to their volume. This makes the smaller particles more


ent/adsorbent to chemical species. In other words, the smaller the particulate the kely it is to be carrying chemical species adsorbed onto it. Measuring anisokinetically n give the wrong amount of the chemical species.


k versus Out of Stack Sampling okinetic sampling methodologies have in-stack and out-of-stack methods, i.e. where


ation of the particulates occurs either in the stack or the filter is held in an oven and he gas brought to it.


However, is size distribution that important when we are simply conducting an isokinetic test for chemical species? Yes, when we are sampling for species adsorbed onto the particulates, such as dioxins/furans or heavy metals, the smaller particulates have relatively greater surface areas compared to their volume. This makes the smaller particles more absorbent/adsorbent to chemical species. In other words, the smaller the particulate the more likely it is to be carrying chemical species adsorbed onto it. Measuring anisokinetically will often give the wrong amount of the chemical species.


UK people traditionally mix up US EPA Method 5 and its variants (out-of-stack


n) with US EPA 17 (in-stack filtration). The EU standards allow both methodologies. is better? If you are trying to determine the particulates in a stack and the


In-Stack versus Out-of-Stack Sampling


ntally. However, in-stack filter housings typically have upper limits with respect to ature because they have more components and seals and often they can’t be used 300oC.


Most isokinetic sampling methodologies have in-stack and out- of-stack methods, i.e. where the fi ltration of the particulates either occurs in the stack or the fi lter is held in an oven and fi lters the gas brought to it.


6


ature is above the dewpoint, in-stack filtration will be easier and quicker and does not the drawbacks of the Method 5 train, which can only be inserted into a stack


If there is the possibility of condensation on the fi lter, or if the gases are too hot to use an in-stack unit due to the seal properties of the housing, then the only option is out-of-stack fi ltration. Out-of-stack fi ltration adds new complexities. At which temperature should the fi lter be held at and at what temperature should the probe be operated? The most diffi cult case is where actual droplets are in the stack as these must not be impacted onto the fi lter. The amount of energy needed to turn droplets of water (and worse still acid vapour) back into vapour is considerable due to the latent heat of vaporisation, (which for water is relatively high and for acids even higher). The residence time in the probe is probably insuffi cient to allow for this.


We must also minimise the effect of the presence of SO2 present) on particulates, because the SO2


the fi lter as artefacts. As a result, temperatures of 160o even 180o


(if will form salts on C or C are required to minimise artefact formation from


acids. Once a fi lter becomes damp then the fi lter will “blind” and the vacuum pressure will increase rapidly. This becomes a major problem when the day is very cold and/or there is a very cold wind, and the probe and hot box have not had suffi cient time to heat the fi lter assembly thoroughly and allow it to reach the full oven temperature. Reaching full temperature usually takes a minimum of 15-30 minutes. Early hot box heaters were insuffi ciently hot to reach these higher temperatures because Method 5 equipment was designed to operate at 120o than the higher temperatures now required.


C rather Particulate Sampling Probes


Testing is frequently conducted with inappropriately sized probes for the duct. This is due to the wide range of duct sizes encountered and the lack of a wide enough selection of probes by the contractor.


sier to determine the dust level using the in-stack method. There is less likelihood of of particulate in the nozzle whilst in the out of stack configuration one has a long that Is ideal for losing particulates. There is also a probe wash to contend with which


uncertainty to low level measurements. With the in-stack configuration there is also probe heater nor hot box to worry about or fail. Nonetheless in-stack filtration has unfashionable to use in the UK since the demise of the old BCURA and


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