Technical Info
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4313 Nebraska Court
Pomfret, MD 20675
Phone:  (301) 934-5605
Fax: (301) 934-5606
Turbine Moisture Drying Probe
Evaporation on hot surfaces is a neglected, but possibly the most important impurity concentration mechanism acting in steam turbines.  It is a part of the generic problem of stress corrosion cracking of turbine disks and pitting and corrosion fatigue of disks and blades.  There is a generic problem of stress corrosion cracking of blade attachments of L-0, L-1, and L-2 blades in both fossil and nuclear turbines.  Stress corrosion cracking of nuclear turbine disk keyways and rims has also been aided by this concentration mechanism.
The evaporation of moisture and the concentration of low volatility impurities occurs when the metal surface temperature is above the saturation temperature in the wet steam regions of the turbine.  The surface temperature can be elevated by heat transfer through the metal from higher temperature areas, such as from the gland steam region to the L-0 disk and by flow stagnation when the kinetic energy of water and other molecules is changed to heat.  Through heat transfer, the interacting surface heats up.
The flow stagnation effects are known by turbine designers, and surface temperature increases as high as 10 C have been measured. In collisions of molecular clusters with surfaces, the temperature increases are much higher and there are high local pressures.
The Drying Probe is installed in the LP turbine as shown in Figure 1.  Droplets carrying dissolved impurities which impact the Probe are instantly converted back to vapor leaving behind the impurities as deposits on the Probe surface.
The Drying Probe can be used to determine:
- whether specific steam generating units produce dry-out deposits
- the chemical character of the deposits
- the type of operation during which deposition occurs
- the rate of deposit accumulation by drying
The Drying Probe is a cartridge heater with an internal Type K thermocouple.  The surface temperature of the heater is controlled by a temperature controller to be approximately 40 C above the saturation temperature of steam at the location where the heater is installed. In most applications, the heater is attached to the manhole cover of the LP turbine at the exit/exhaust of the turbine where the moisture is typically from 8% to 12%.  A high watt density assures operation in the film boiling region and the dryness of the probe surface in wet steam.
Figure 1.  Drying Probe Installation
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Serving the power industry since 1983
Specializing in Sampling and Instrumentation, Corrosion,
Water and Steam Chemistry, and Failure Analysis
Jonas, Inc.