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The B.C.U.R.A. Dewpointmeter APOURS can condense out as investigate the variation of conductivity accuracy in detecting and measuring liquids on to surfaces which are on a quantitative basis, a special type small traces of condensed acid. The below the dewpoint of the gases in of dewpointmeter element was de surface over which condensation takes their immediate vicinity. When these veloped, so that films of standard place is only 1 sq. cm., and it can be liquids are capable of causing cor dimensions could be compared. controlled to any preselected tempera rosion, it is important to know the ture to ±2°F. A reading can be made temperature range over which con in five minutes and the surface can be Rate of acid build-up densation may occur, so that steps can cleaned, reset at a new temperature be taken to limit its effect. The cor Two Pt/Pt-Rh electrodes are pre and a new reading obtained within rosive effects of flue gases are due in formed, jig-mounted and embedded minutes of completing the previous part to the condensation of sulphuric in the glass surface which is lens one. As some measure of th e sensitivity acid, which can seriously affect the polished. One electrode is a point at of the instrument, it has been calculated useful life of the materials used in heat the centre of the other, which is a that with a sensitive microammeter, exchangers and stacks. circle of ¼-in. diameter: the condensing traces of sulphuric acid of the order surface is thus of standard area. A 1 x 10-8g. are detectable; even such The British Coal Utilisation Re constant a.c. potential is applied across small amounts as this can cause cor search Association has developed a these electrodes, the surface tempera rosion over long periods. dewpointmeter which can assist plant ture is controlled at some fixed tem engineers to find the gas and metal A picture of the instrument in use perature below the dewpoint and the temperatures that are likely to lead to on boiler plant was published on page conductivity registered after a given corrosion. Basically a polished glass 184 of our August, 1954, issue. time interval is noted. It has been detector element is brought into con observed that this rate of build-up of tact with hot flue gases and its tempera conductivity is at a maximum at a ture is lowered (by a controlled blast temperature of about 60°F. below the of cool air directed on the underside) LETTER TO THE EDITOR dewpoint and that the build-up until 'dew ' begins to form on the measured at this temperature is related exposed surface. Flush mounted DEAR SIR.—We have now had the to the corrosion of metal specimens pleasure of studying the first three thermocouples measure the dewpoint maintained under the same conditions. issues of this journal with great interest temperature, at the same time as the One method in which the relationship and, on the whole, we think that appearance of a film of dew completes between corrosion and 'rate of build CORROSION TECHNOLOGY is very in a conductivity cell circuit and indicates up ' was checked was to replace the teresting and should have a good a current on a sensitive microammeter. glass element and Pt/Pt-Rh thermo It may be taken that any surfaces future. couples by a steel cap and base metal below the dewpoint temperature may We ourselves are particularly in thermocouples, and compare corrosion become moist. terested in plant materials which are at different surface temperatures with acid-resisting, and we notice that many corresponding 'aci d build-up' figures. plan t manufacturers boldly make Flue gas dew points In another set of experiments, a model claims that their products are acid- While using the instrument in a rotary air preheater was operated in resisting. We would like to suggest survey of different types of boiler synthetic flue gases containing traces that it would be of benefit to the plant plant it was observed that the flue of sulphuric acid, and similar effects manufacturer, as well as the consumer, gases from most coal-fired appliances were observed. if you could persuade them to be more (particularly grate fired) had apparently specific. In our opinion they should The existence of a temperature band two dew points, one in the region of state more clearly which acids their over which corrosion is likely to be 110°F. due to the normal water vapour particular products resist, in what more severe than elsewhere is in content of the gases, the other in the strength and at what temperatures. accord with plant observations that region of 300°F. arising from the For example, many of the new plas corrosion of, say, airheater plates presence of small amounts (about tic pipes are very interesting for appears to be localised over a limited 0.002% by volume) of sulphuric acid hydrochloric acid at normal tempera area. In general terms it would seem introduced by the sulphur in the fuel. tures, but at elevated temperatures that whereas the dewpoint temperature It was also established that from one they completely collapse, and we feel determines the limits of the tempera installation to another and from one it would be much better if the manu ture range over which corrosion may surface temperature to another there facturers could indicate the limitations occur, the additional measurement of were differences in the rates at which of their products more clearly. the 'rate of acid build-up' (as the a conducting film built up on the property is now called) indicates the We put this forward merely as a element. likely severity of the corrosion. This suggestion that you might be able to Although the first observation was can then be confirmed by corrosion persuade people to adopt when word important in establishing a lower limit tests with temperature controlled ing their advertisements. to the operation of heat extraction probes. Yours faithfully, plant, the second was thought to be of the utmost importance in assessing the The advantages of using the dew P. E. KUNZER. corrosive potentialities of the flue gases pointmeter in this manner, as an Fredk. Boehm Ltd., in relation to the surfaces with which instrument for indicating and assessing 19 Bentinck Street, they come into contact. In order to corrosion, are those of speed and London, W.1. 82 CORROSION TECHNOLOGY, March 1955
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Mar 1, 1955
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