Measurement of the pre-explosive concentrations of hydrocarbons in the air of the working zone of the processing plants
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PHYSICOCHEMICAL MEASUREMENTS MEASUREMENT OF THE PRE-EXPLOSIVE CONCENTRATIONS OF HYDROCARBONS IN THE AIR OF THE WORKING ZONE OF THE PROCESSING PLANTS OF OIL-REFINING ENTERPRISES
A. P. Beloshitskii and L. A. Gavrilov
UDC 543.27.08+504.3.064
Questions related to metrological assurance of the thermomechanical indicators of pre-explosive concentrations of hydrocarbons in the air using as an example the STM-30-50 plant are considered. Causes for the appearance of an additional error in these measuring instruments and methods of eliminating (decreasing) this error are analyzed. Keywords: control gas mixture, measured component, volume fraction of component, lower concentration limit of flame propagation, calibration.
Monitoring the air of the working zone of processing plants is conducted in order to prevent dangerous explosive situations. Thermomechanical indicators, for example, the STM-30-50 and other devices in this series produced by the Smolensk Anatpribor production association are widely used for this purpose. These measuring instruments are simple to use and possess a comparatively high degree of reliability. Nevertheless, they suffer from a number of drawbacks, in particular, those relating to metrological assurance. In accordance with the operating documentation, calibration and adjustment of the indicators at the second alarm signaling threshold (11% of the lower concentration limit of flame propagation, LCL) are performed with respect to a methane control gas mixture (CGM) (calibration component). An additional error arises in measurements by such instrument of air mixtures with other components, and alarm signaling is triggered, for example, for benzene vapor once 20% of the LCL is reached. Such an assertion is not in accord with the requirements of the regulatory documents [1, sec. 3.2, 3.10; 2, sec. 2.7, 3.2, 2.7, 3.9], where it is determined that the limits of the permissible absolute error of the particular measuring instrument employed must be ±5% and alarm signaling must be triggered once 50% of the LCL is reached. A halt to the production process due to premature (less than 50% of the LCL) triggering of alarm signaling that the permissible level of concentration of hot gases and vapors in the air has been exceeded leads to unavoidable losses. The operating manual of the STM-30-50 [3] presents the nominal static characteristics of the transformation of the indicator due to an output current signal together with specification of the proportionality factor (scaling ratios) for methane, hexane, and methyl acrylate. With the use of the conversion factors, the concentration of any component may be found (with defined error) by means of an indicator that has been calibrated with respect to a methane CGM and its value at which alarm signaling is triggered is determined. Precisely how to measure the concentration of other components, determine the levels of response of signaling to the established thresholds, and what is the error of the indicator are not specified in [3]. The operating principle of thermo
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