Maintaining Hydrogen Purity in Turbine Power Generators

The importance of measuring dew point to ensure efficiency and safety

Hydrogen has a wide range of properties that make it an ideal gas for many applications. In particular, it has a low-density – about 14 times lower than that of atmospheric air – high-heat conductance and excellent heat transfer coefficient. These factors have led to the use of hydrogen as an effective cooling mechanism for large turbine power generators.

In simple terms, using hydrogen as a cooling gas helps to reduce frictional heat losses within a turbine generator, enabling a greater proportion of fuel to be converted to power; when compared with an air-based cooling system, hydrogen cooling also means a smaller and more efficient turbine can be used.

Typically, high-purity hydrogen is circulated through non-magnetic ducts that are set end to end in the turbine, removing heat from the stator and rotor windings, bearing and other rotating parts, with the cooling system being maintained at a pressure of at least 30 psig; this minimizes the risk of external air entering the turbine housing and creating a potentially explosive gaseous mix. The hydrogen is circulated by means of fans located at the ends of the generator rotor and is passed through water-filled heat exchangers to remove the latent heat. Pressurized oil-filled seals are used to prevent hydrogen leaking from the generator into the surrounding atmosphere, where it can again create the risk of an explosion or hydrogen fire.

Clearly, maintaining the highest level of hydrogen purity is crucial, both for operating efficiency and safety. Hydrogen explosions in turbine generation plants are extremely rare and, when they do occur, are mostly related to mistakes in handling the gas during purging of the generator, or transfer from tankers to on-site storage vessels.

Moisture contamination in hydrogen

Although allowing hydrogen to mix with air is the greatest risk in turbine generators, there are also potential dangers posed if the moisture content of the hydrogen gas is not carefully controlled. If moisture levels are allowed to increase, they can reduce the thermal conductivity and increase the viscosity of the hydrogen, leading to an increase in wind-resistant losses. Longer term, excessive moisture will also cause corrosion of steel on internal surfaces, such as generator retaining rings. Perhaps of greatest concern, if moisture condenses on live metal parts, is the subsequent danger of arcing or flashover with what could be disastrous consequences. It is worth bearing in mind that the presence of water vapor in the hydrogen can also be an indicator of air leaking into the generator housing.

Moisture ingress can arise from a number of sources, including leaks in the water-cooled heat exchangers, water contamination of the oil used in seals and lubrication systems, and incorrect gas purging after routine generator maintenance. To address these issues, a regenerative desiccant dryer is normally used in the gas circulation loop to remove water vapor from the hydrogen gas.

Dew-point measurement for hydrogen cooling

Dew-point sensors are used both to monitor the operation of the drying system – improving efficiency and helping to reduce energy consumption – and the status of the hydrogen gas as it exits the generator housing. For the latter, it is normal practice for the dew point of the hydrogen gas to be maintained at a safe margin below the minimum casing temperature; typically, an upper limit is set of 0 °C dew point at system pressure. If the dew point rises above this limit, immediate action needs to be taken to shut down the generator and carry out a repair or – more likely – to pump fresh, dry hydrogen into the system to avoid the risk of moisture condensing within the stator environment, which could lead to flashover or explosion.

We offer a range of advanced dew-point measurement systems. These include the QMA401, self-calibrating in-line trace moisture analyzer, capable of producing extremely accurate, fast measurements, and the S8000 Chilled Mirror Hygrometer. Lower-cost options are our Easidew hazardous-area approved transmitters, which are available in intrinsically safe or explosion-proof versions. Click here for more information.

We are the world’s leading experts in moisture monitoring and measurement. We have eight different technologies covering all moisture applications backed by unrivalled technical and customer support. To learn more,speak to one of our application specialists today.

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