Wilson Zone affects on turbine performance 2

[ComponentCooling]

Could someone explain how the Wilson Zone affects turbine performance. Could the result of operating in the Wilson Zone cause turbine blade failure?

 

[Inthevalley]

Firstly define what the Wilson Zone is - From EPRI TR-108943:
Spontaneous condensation to relieve supersaturation (historically referred to as the Wilson line or
zone) has not been seen in operating turbines, although it can be and has been observed in Laval and other nozzle tests. As a result it is suggested that in operating turbines, this region should be renamed the Phase Transition Zone (PTZ) and that it encompass the region from before the saturation line to about 4-5% moisture on the
Mollier diagram.

Now for my comments...
Typically on a fossil fired unit, the L-2 stage (last row being designated L-0) of the LP turbine is in the PTZ. This is load dependent, and at low load the PTZ moves to the L-1 row.

The key issue in this zone is the change in sollubility between the vapour and liquid phases. As an example, if you have 10ppb Cl in the steam, you can get around 80ppb Cl in the very fine moisture droplets, which gets into every small hole it can find. Thus corrosion is a major problem in the root areas of L-2 blades. It also becomes an issue for cycling machines on the L-1 row. Once the corrosion pit has grown sufficiently (a hole 0.2mm deep or thereabouts), a crack WILL form.

These cracks then grow, and depending on the relative level of stress and contaminents, the cracking could be high cycle fatigue (HCF), Corrosion Fatigue (CF) or Stress Corrosion Cracking (SCC). Mostly high chromium steels are used (no-one can afford titanium anymore), thus the material is generally sensitive to SCC. Once the crack has formed, it will eventually grow to failure!

For fir-tree design roots, phased array ultrasonic inspections is generally the only way to find cracks - mostly the blades are really difficult to remove due to all the corrosion present.

Cracks can generally be repaired by localised grinding and the use of notch blocks. It is also recommended to do a thin skim cut (0.2 mm or less), to remove any corrosion pits. Generally the old blades can be re-used. Sometimes a short term solution till the next outage is to use scotch keys to take the load off the blade area (typically less than 2-3 years). For severe cracking, long shank titanium blades or cutting off the disk and re-forming the root with weld-build up seems to be the only viable options. At this stage one wonders whether a totally new rotor is not cheaper

On L-1 and L-0 stages, droplet erosion is a problem, but blades are generally protected at the root. If this is an issue, it can generally be fixed using laser welding, using a 12Cr material.

Some of the latter stage blades are also subject to blade flutter failure at either low vacuum or low load conditions, but this is more a function of blade design.

 

[poetix99]

I think that "inthevalley" has offered some useful, general comments. I don't think that any of his comments relate in any particular way to "operation in the Wilson Zone".

Condensation of supersaturated steam is a non-equilibrium phenomenon; not well (or easily) understood, but not inherently dangerous. The rapid change of state in a turbine expansion is typical of a steam turbine.

Potentially corrosive components (e.g. chlorides) condensing out of the vapor phase and existing in much higher concentrations in the liquid phase is a fact of life - irrespective of the Wilson line.