Humidity measurement in the power industry
A
Michell Instruments product story
Edited by the Engineeringtalk editorial team
Apr 14, 2005
Humidity measurement instrumentation is now a vital component of the control mechanism operating in every power station in the world.
We all know from an early age that water conducts electricity.
Therefore it is obvious that surplus humidity in air and gases can be catastrophic in an electricity generation plant, with a real risk of expensive equipment damage and even explosion resulting from failure to monitor and control humidity levels.
At a less critical level, humidity is still an important measurement parameter in ensuring highest power plant efficiency and helping to reduce emissions.
For example, noxious gases emitted by gas-fired power stations can be reduced by controlling the humidity of air mixed with the fuel gas.
Energy consumption can be reduced by allowing dew point control of compressed air dryer changeover.
Sulphur hexafluoride (SF6), which is frequently used as an insulating gas within high-voltage switchgear, must be sufficiently dry to prevent arcing, which results in contact erosion and reactions producing corrosive hydrofluoric acid.
In nuclear power plants it is essential that CO2 and steam loops are not counter-contaminated.
This can be assured by verifying the integrity of heat exchangers.
Pipe fracture caused by freezing can be prevented by the proper control of compressed air dryers.
An explosion in a power plant is almost unimaginable; but a few years ago that's exactly what happened at a well known power station in the UK, and it was because of excess humidity.
If H2 gas is not maintained at below 0C dew point in a generator set this can cause the formation of condensation on exposed live metal parts, which may then lead to an explosion.
The science of hygrometry has become increasingly more sophisticated over the years, with new technologies especially in the area of electronics meaning that humidity measurement instrumentation is now a vital component of the control mechanism operating in every power station in the world.
Where once the measurement of moisture was more an art than a science, modern hygrometers can accurately and reliably measure humidity down to parts per billion of moisture.
There are many different methods of measuring humidity, the most accurate being the measurement of dew point.
There are essentially two ways to measure dew point.
Dew-point type impedance sensors are used to measure in absolute units rather than relative humidity.
The sensors are fabricated from a hygroscopic material, the electrical properties of which alter as they absorb water molecules.
Changes in humidity are measured as a change in the sensors' electrical capacitance or resistance, or some combination of the two.
Optical dew-point hygrometers (commonly known as cooled or chilled mirror hygrometers) induce condensate in the form of dew or frost on a small mirror within the instrument.
The onset of condensation is sensed optically, by detecting changes in how the mirror reflects or scatters light.
Optical dew-point measurement is considered the most accurate method of humidity measurement in gases and air, and reference hygrometers used by standards laboratories use this method to maintain traceability between the humidity standard and instruments under test.
Although there are only two main methods of measuring dew point, there are many, literally hundreds, of measurement instruments on the market that use these methods.
Each manufacturer uses different claims for their instruments, and as the market is not regulated these may be more or less accurate, leading to further confusion.
Dew-point measurement instruments using impedance and cooled/chilled mirror technologies can generally be categorised as follows.
A transmitter is a measuring instrument with no display which has an electrical output that can be transmitted over long distances.
An online instrument continually measures and displays a process variable.
A portable hygrometer is for periodically checking, offline, a process variable.
Basically the type of instrument suitable for a certain application depends on the gas to be measured, the environmental conditions and the degree of accuracy required.
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Contact Michell Instruments
Tel +44 1353 658000
Other Michell Instruments stories
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Michell transmitter simplifies oxygen measurement
Michell has introduced the XTP600 self-contained oxygen transmitter, which is claimed to make oxygen measurement in the process industry as simple as a field-pressure or temperature measurement. -
Michell introduces intrinsically safe transmitter
Michell Instruments has added the Easidew Pro IS to its range of Easidew dew-point transmitters. -
Michell sensor speeds up maintenance of RH devices
Michell Instruments has introduced the I7000 Hygrosmart humidity sensor, which is claimed to make the maintenance of the company's relative-humidity (RH) instruments quick and simple with no downtime. -
Michell oxygen transmitter eliminates drift
Michell is offering thermo-paramagnetic technology that eliminates transmitter drift and deterioration by preventing hydrogen molecules from diffusing into the sensor. -
PMA2030 analyser measures moisture in gases
Michell Instruments has introduced the PMA2030, which uses the quartz-crystal microbalance technique for moisture measurement in gases.
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