SOURCE TESTING ASSOCIATION I Annual Guide 2019
Quality Control and Assurance
The US EPA Method 5 train relies heavily on having a calibrated orifi ce plate to set the fl ow by setting the ΔH across it when applying a vacuum. It is on the basis of this calibration, and of it being relatively insensitive over a range of normal fl ows which allows a ΔH to be set for sampling. The orifi ce is typically a simple washer welded into the exit pipe of the gas meter. This should be checked periodically to ensure that the pressures are representative of the fl ow rates expected and required by the ΔP readings across the duct. In this way a ΔH@
is determined.
A simple test to do, and as required by the US EPA, is to pull gas through the gas meter at a known differential pressure across the orifi ce (ΔH@
) for 10 minutes and check that a
known amount of gas has passed through the gas meter. This is recommended for every test campaign. The US EPA sets a tolerance on this of 3%. The second most important part of the meter box is the dry gas meter, and this should be checked regularly. There are several ways to test this using another calibrated dry gas meter, a calibrated wet gas meter or a series of calibrated orifi ces. It is recommended that this be performed at least every 200 hours.
UKAS and the MCerts Scheme like everything to be calibrated, but the US EPA Method 5 train is made up of various components some of which affect the result greatly whilst others have little effect. In choosing the nozzle, the key parameters which have an effect are fl ow rate, ΔP (velocity of the gases) and water vapour. These need careful measurement or determination. Barometric pressure, Stack Pressure, Stack Temperature, Gas Meter Temperature and Meter Pressure for most sites have little effect. When setting the isokinetic rate the calculations are relatively insensitive to Stack or Gas Meter Temperatures or Pressures, but are highly dependent on ΔP(Velocity), Dn
(Nozzle diameter) and Water Vapour. Those
of us old enough to use isokinetic slide rules realised this early on, as some parameters have only affect the ΔH setting by a small amount. That is not to say that the other parameters are unimportant, as they most certainly are, but the gas meter, velocity measurements, orifi ce plate calibration and leak checks are paramount to good testing. It is arguable whether the level of external UKAS calibration required for all the components of a Method 5 meter box is really justifi ed.
One of the key ingredients of a successful isokinetic test in the fi eld is good teamwork and communication. A good team gets the job done more easily when everyone knows what needs to be done and gets on with their tasks. However, some tasks take more interaction:
• Leak tests • When to release the vacuum at the nozzle • When to turn off the pump or close the valve. • When to move the probe to the next position • If there is a problem moving the probe etc
All these need to be communicated. Good communication minimises mistakes and can save time. Mobile phones are not good for this even if they are allowed at the site, as they can suffer poor reception, and this is rarely known until you arrive. Walkie talkies often have the same issues. ATEX approved systems are out there but not every site will allow you to use your own radio equipment. The original Universal Andersen Method 5 Control Box had a two/four wired intercom which facilitated operation where the control module was separated from the hotbox/cold box and probe by a long umbilical. This was useful and although we never used it, I think I value the thought that provided it more these days.
In this article I’m not covering the laboratory preparation of fi lters and the recovery of samples. Suffi ce it to say sample recovery is crucial at the lower levels. Probe rinses are key as are blanks and using high quality solvents with negligible evaporative residues and this could be a subject of a whole new article.
To sum up isokinetic testing is complex, so it deserves careful planning and execution. Even when it is a “simple” particulate test, the time in preparation before the test and in the recovery of the sample are as key as the actual test itself. Happy Sampling!
References
1. American Journal of Public Health, L Greenburg p 1077- 1082, 1932
2. Measurement of Solids in Flue Gases: P G W Hawksley, S Badzioch and J H Blackett: Institute of Energy 1995 3. Apex Instruments USA; AER Sampling Inc. USA and Westech Instruments UK
4. United States Environmental Protection Agency
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Annual Guide 2017 I SOURCE TESTING ASSOCIATION
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