Using hydrogen as a coolant in the power generation industry is common practice: it has an extremely high heat transfer capacity and is more efficient than any other medium. However, this efficiency has its price, as hydrogen is also highly flammable. The higher the moisture level is, the higher the potential for corrosion of the generator internals. High moisture levels lower the thermal conductivity (and therefore efficiency) of the hydrogen. Also, condensed moisture within the system increases the risk of the hydrogen in the system simultaneously igniting – a phenomenon known as ‘flashover’. The risk of this is avoided by ensuring that the dew point of the moisture in the hydrogen is kept to a safe margin below the minimum temperature of the casing, typically around 0°C, to avoid condensation.
As it is not possible to hermetically seal the generator casings, there is potential for moisture to ingress from the surrounding air. The heat exchanger itself will also become more prone to leakage due to corrosion with age, allowing moisture to seep into the hydrogen, so constant measurement is vital to ensure that the dew point remains at a safe level below ambient temperature. There are two options: either continual monitoring or regular spot-checks of the system.
Continuous measurement ensures that the current dew point of the system is always available and alarms can be triggered if it rises above the ambient level. ATEX certified dew-point transmitters, such as Michell’s Easidew PRO I.S. and Easidew TX I.S. are well suited to this application as they can be used to measure the dew-point of flammable gases within the hazardous area. As hydrogen constitutes an explosion risk, a digital indicator, if required, should be installed in a safe location, remote from the generator set. Both transmitters provide a 4-20 mA output signal covering the range -100 to +20°C.
A portable dew-point hygrometer, such as Michell’s MDM300 I.S., may be used for making spot-checks of moisture in the system. This can either provide a fail-safe for the online moisture analyzers, or as the main method of measuring moisture. Portable instruments can be highly cost-effective as they can be used for spot checks in more than one location in a plant, eg also for transformer drying applications.
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