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SOURCE TESTING ASSOCIATION I Annual Guide 2018


solenoid valve which doesn’t close properly or closes too slowly. Sometimes there may be partial blockages of fl ow orifi ces in the analyser throttling back the gases to the detector in the analyser. However, whatever the issue frequently dirt or contamination can be the cause.


Calibration of the sample system and/or the analysers must always be conducted under the same conditions as those experienced during routine analysis (i.e. the calibration gas must be delivered to the detector at the same pressure, fl ow and temperature as the sample gas). Failure to do this will yield mis-calibrated results with most analysers, as they are usually pressure, temperature and fl ow sensitive.


The more sophisticated CEMS system will have an equally sophisticated sample handling system, but lab confi gured systems generally do not.


They are not just tools, they can also affect your results and they become more critical depending on the type of gases being handled and the levels involved. Corrosive and/or reactive gases need to be handled carefully, such that the main cylinder doesn’t become contaminated with air or dust. This means that the regulators may need to be suitably purged to ensure that the gases don’t react in the regulator or the gas bottle head, especially after being exposed to the air and affect the results.


The analysis of NO2 low levels of 0-15 ppm, is a very good case. The calibration gas split between NO and NO2


initially will be nitric oxide (NO) but this will react and form NO2 in the regulator. At relatively high levels this is unimportant as the conversion might be very small. Once we are looking at low NOx NOx


This also assumes that no NO2


applications this becomes all too important as the levels of may be correct but the split between NO & NO2


will not. is lost due to any moisture in


A Hybrid CEMS system using Wet and Dry analysers to achieve optimal analysis and minimise losses


A sample handling system should always consist of a probe, a fi lter, a heated line, a gas conditioner, a sample pump and a sample manifold; where the analysers can take the sample they need from the sample handling system. This means the analyser pump is not taking a sample directly from the stack, rather it is taking what is necessary from the sample handling system and this can be trimmed to be relatively insensitive to fl uctuations due to the stack, or even different stacks. In this way, the delivery of sample gases and calibration gases (span & zero) to the analysers can be ensured.


Regulators are a means of reducing the pressure of a gas and of regulating the pressure and fl ow into an analyser system. These are often mistreated on site and connections not looked after suffi ciently well. They are the means by which very high and hazardous gas pressures of 150-200 barg are reduced to working pressures of 1-2 barg, so perhaps some extra care should be taken of them.


Regulators require regular inspection to ensure they are fi t for purpose. The British Compressed Gases Association (BCGA) recommends that pressure regulators are refurbished or replaced with new ones at least every fi ve years, or more often (perhaps 12-18 months) if used with corrosive species.


20


the regulator, so careful purging of the head is required. The recommendation is that the regulator be purged 3- 5 times, ideally via a purge line with an inert gas, to clear the connection between the regulator and the gas bottle. Ideally, once purged and the main bottle valve is open the regulators should be set such that the low-level regulator is opened into a gas fl owing situation. In this way air cannot become trapped and react with the NO or back diffuse into the calibration bottle and affect the results. This is for special applications but increasingly these are becoming more frequent. In general, for non-reactive gases when a cylinder is changed or a line changed, or disconnected and reconnected it should be purged with the gas of interest at least 3 times to minimise air/water in/on the line or regulator. The regulator can be purged using a T-Piece Valve arrangement, whilst a crossover valve arrangement should be used for an inert gas purge where reactive gases are being used. These recommendations are conducted routinely on fi xed installations but should also be implemented on transportable systems where high precision work is required. Mobile systems need extra care because calibration lines are not fi xed. The lines require special purging when conducting low level work as these can easily become contaminated and are usually PTFE/PFA (which can be semi permeable depending on the wall thickness) rather than stainless steel as in fi xed installations.


A built-in fi lter check on an analyser is always a good check of the operation of the analyser but it is a poor substitute for a calibration gas or mixture. A fi lter check never checks the integrity of the gas transport system to the analyser or indeed within the analyser, but only checks the optics of the analyser and the electronics related to the data output.


A number of years ago in Germany a series of performance trials on a gas turbine were conducted by a German institute. The equipment being installed failed its guarantee trials, resulting in initial retention of the fi nal payment and potentially a legal case. It was decided to send out an independent specialist to investigate, and the same institute appeared with a gas analyser and sample system but no calibration/check gases. The engineer was requested to perform the trials to ISO 11042 but according to “their procedures” no calibration gases were needed at the site. Investigation soon discovered unacceptable oxygen levels being reported for the gas turbine exhaust emissions, leading to investigation of the sample system. The engineer maintained


accurately, and especially looking at the in some combustion processes at


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