Moisture Analyzer, Piezoelectric Sensor, Smart Transmitter, Internal Verification, Quartz-Crystal Microbalance
Piezoelectric moisture analyzers have been used in the process industry for over two decades. While this is an older technology for measuring moisture in gas phase samples, it is still one of the most reliable in the industry. The evolution of the quartz-crystal microbalance (QCM) has produced a new and substantially improved moisture analyzer, but the needs of industry continue to evolve. This paper describes modifications performed on an existing process moisture analyzer, which resulted in the lowering of the detection limit from 0.1 parts per million by volume (ppmv) to less than 0.01 ppmv.
The performance requirements for process moisture analyzers are pushing the existing measurement technologies to their limits. Not only do these analyzers need to survive under a wide range of environmental conditions, but the need for lower detection limits is increasing. Detection limits of less than 10 parts per billion by volume (ppbv) water vapor – a value that was previously only achievable by laboratory grade instruments – are now being requested for process measurements. One example of these new requirements is in the natural gas processing industry. Specifically, there is a need to measure water vapor concentration below 100 ppbv in a natural gas stream that is entering a turbo-expander plant for the separation of the ethane and propane fractions. Because of the cryogenic conditions present in the turbo-expander separation process, even trace levels of water vapor will lead to ice formation on expansion valves, and on turbine blades. If the plant operators catch the problem early, the result is down-time to remove the ice from the critical components. In the event that the problem goes undetected, ice formation on the turbine blades can lead to permanent damage, causing down time and a very expensive repair. At present, there are well in excess of 300 turbo expander facilities operating around the globe (1); as such, there is a large demand in the gas processing industry to detect water vapor at ultra-trace levels.