Dark spot shows gas dew point

Tom Shelley reports on a novel accurate and fast automatic technique for measuring the dew point of natural gas and similar process gases An automatic dew point meter for natural gas uses the formation of a dark spot, in light reflected by a film of liquid, to detect the onset of condensation. It is essential to avoid the formation of natural gas condensates in pipelines especially if they include water, because of the possible formation of solid hydrates capable of clogging everything up. Even if no water is present, condensate droplets can cause severe problems in power station combustor nozzles and other mechanical items. Customers generally want hydrocarbon dew point to be -3 deg C or less at 27bar. Contracts usually specify no higher than -2 deg C. For this reason, Michell Instruments in Cambridge have for some time been supplying equipment to the gas industries to accurately and quickly measure natural gas dew points. Their latest instrument, the Condumax II, makes use of a patented optical measurement technique that is sensitive to 5mg/m3 and avoids the need for human operators to try to see almost invisible films of condensate. Mists of condensed hydrocarbons are much harder to detect on chilled mirrors than water, because of their low surface tension. The crucial part of the sensor is an acid etched, semi matt stainless steel reflector, with a central conical depression that is cooled during the measurement cycle. Collimated visible red light is focused onto the central region of the optical surface. When the reflector is dry, the incident light is dispersed over an angle. But when the condensate forms on the surface, it becomes a specular reflector, because of the low surface tension of the hydrocarbon liquid. The light is then reflected around the detector in an annulus, leaving a dark area at the detector's position, so that its output falls. When this occurs, the instrument electronics record the temperature of the optical surface as the hydrocarbon dew point and initiate a recovery cycle whereby the optical surface is heated to evaporate the condensates back into the flowing gas sample. The process is fully automatic and takes less than ten minutes. The optical detection system, sensor, thermocouple, and three stage Peltier cooler are all mounted in a stainless steel cell that can withstand operating pressures up to 100bar and still achieve a depression capability (lowest measurable hydrocarbon dew point) of almost -35 deg C at room temperature. Accuracy is {{plus minus}}0.5 deg C and a complete system costs around £30,000. Michell Instruments