Real-time measuring of moisture

Paul Boughton

Careful monitoring of the moisture in oil within large lubrication systems helps to plan servicing and prevent unscheduled downtimethus it can cut maintenance costs substantially. Senja Paasimaa reports.

Several locations along power generation and transmission network have various oil filled systemsfor exampletransformers and turbineswhere oil acts as insulating materiallubricant and/or cooling agent. As water contamination reduces the performance of oilmoisture is an important factor determining the condition of different types of oils. With on-line information on the quality of the oilpreventive actions can be taken and the maintenance costs cut substantially.

Water contamination reduces the performance of lubrication oil. Excess moistureparticularly free waterincreases the risk of corrosionoverheatingmachine malfunction and other problems. Therefore measuring and controlling the moisture in lubrication systems is essential to avoid costly failures and to ensure reliable performance of equipment at all time. Monitoring of the moisture in oil within lubrication systems helps plan servicing and prevent unscheduled downtimecutting maintenance costs substantially.

The determination of moisture in oil is an essential part of a comprehensive transformer maintenance program. Oil immersed transformers rely on the oil for coolingprotection from corrosion and as an important component of their insulation. Excessive moisture in oil causes accelerated ageing of the insulation materials and reduces their dielectric strength. In extreme cases this can result in arcing and short circuits within the windings. If accurate moisture measurements are available leaks in the oil system can be detected in timeas water is absorbed from the surrounding air.

Vaisala has a broad product offering for moisture in oil measurementconsisting of the fixed transmitters the new MMT330 MMT318the intrinsically safe HMT368and the hand-held meter MM70. These instruments are configurable products with several options to meet users’ special needs. They offer a fastreliable and accurate on-line detection for moisture in oil and thus help reducing maintenance costs.

All instruments above are based on the Vaisala HUMICAP capacitive thin film polymer sensorwhich has been further developed for demanding moisture measurements in liquid hydrocarbons. The Vaisala HUMICAP Sensor has excellent chemical tolerance and provides therefore accurate and reliable measurement over the measurement range. The operation is based upon the measurement of changes in capacitance as the film absorbs water molecules from the oilmeasuring moisture in oil in terms of water activity (aw)and temperature (T).

The measurement indicates the margin to water saturation and is therefore independent of oil typeage and temperature.

With Vaisala’s instruments moisture quantity in ppm can be calculated from the measured relative value and temperature. The ppm calculation is available as an option to MMT330MMT318 and MM70 for mineral transformer oils.

Senja Paasimaa is with Vaisala OyjHelsinkiFinland. www.vaisala.com

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