Why Steam Turbine Blades Suffer Water Droplet Erosion in Wet-Steam Stages

Water droplet erosion is commonly found on steam turbine blades operating in wet-steam stages, especially around exposed leading-edge areas.

During operation, condensed water droplets move through the steam path and repeatedly strike the rotating blades at high relative velocity. Small pits and roughened areas may appear first. With continued exposure, these areas expand and the original leading-edge profile gradually loses material.

For turbines equipped with erosion shields, the same type of wear can eventually become visible on the protection strip.

How Water Droplets Form in Wet-Steam Stages

Steam pressure and temperature decrease as the steam expands through the turbine.

In later low-pressure stages, part of the steam enters the wet region and condenses into liquid. The resulting droplets travel through the steam path with different sizes and trajectories.

Blade speed creates a high relative velocity between the rotating blade and the incoming droplets. The actual erosion exposure varies with steam conditions, droplet size, blade geometry, rotational speed and operating conditions.

This is also why erosion patterns can differ between turbine designs even when both operate in wet-steam environments.

How Repeated Droplet Impact Damages the Blade

A single water droplet produces only a very small impact area, but the effect changes when the same region is struck repeatedly over a long operating period.

Repeated impact creates localized deformation and small surface pits. As the pits increase in number and begin to overlap, the blade surface becomes rougher and small quantities of material are gradually removed.

The damaged surface then presents more exposed edges and irregular areas to later droplets, so the erosion becomes increasingly visible.

Condensate chemistry can also influence the appearance and progression of the damaged area. Wet steam may contain dissolved oxygen, chlorides, sulfates and other constituents that contribute to corrosion-assisted surface damage.

In actual turbine service, mechanical droplet impact and the wet-steam environment can therefore appear together on the same blade area.

Typical Water Droplet Erosion Areas on Steam Turbine Blades

Water droplet erosion is frequently seen on the leading edges of low-pressure turbine blades, particularly in later-stage regions with significant wet-steam exposure.

Common surface features include localized pitting, roughened areas, edge recession and gradual loss of the original profile.

The damage pattern varies with blade geometry and steam flow. Some blades show a narrow eroded band, while others develop a wider roughened area extending along part of the leading edge.

Cobalt alloy erosion shields installed in these regions are exposed directly to the droplet impact. After long-term service, pitting, thinning or profile loss may also appear on the shield surface.

Why Cobalt Alloys Are Used for Turbine Blade Protection Strips

Cobalt alloys have a long history in wear- and erosion-resistant applications because they combine good resistance to mechanical surface loss with corrosion resistance.

For steam turbine blades, the cobalt alloy strip forms the exposed protection surface along the area subjected to concentrated droplet impact.

Cobalt Alloy 6 and Cobalt Alloy 6B are both found in turbine blade protection applications.

Cobalt Alloy 6 belongs to the cobalt-chromium-tungsten alloy family and is available through different manufacturing forms.

Cobalt Alloy 6B is a wrought cobalt alloy commonly supplied as sheet, plate and bar. Its wrought product form is particularly relevant to thin, curved erosion shields manufactured from sheet or plate.

The alloy designation used on a replacement component usually comes from the turbine drawing, historical material documentation or the existing component specification.

What Happens as the Protection Strip Wears

A turbine blade protection strip is itself a working surface.

After prolonged exposure to wet-steam erosion, the strip may develop pits, surface roughness, thinning or recession along the leading edge. The amount and position of material loss depend on the local droplet impact conditions and the geometry of the blade.

During turbine overhaul, worn protection strips are often inspected together with the surrounding blade area.

For replacement projects, the existing strip geometry provides important manufacturing information. Curvature, thickness, cross-section, length and edge shape can vary between turbine stages even when the same cobalt alloy grade is specified.

This makes steam turbine blade protection strips highly drawing-specific components rather than standard straight cobalt alloy strips.

Custom Cobalt Alloy Steam Turbine Blade Protection Strips

SYTOP supplies finished steam turbine blade protection strips in Cobalt Alloy 6 and Cobalt Alloy 6B according to customer drawings.

Custom manufacturing can cover different:

• Curvatures

• Strip lengths

• Thicknesses

• Cross-section profiles

• Edge geometries

• Surface requirements

• Material specifications

Material certificates and dimensional inspection reports can also be supplied with the finished components. Additional testing can be included when specified in the project documentation.

For replacement projects, customers commonly provide the existing component drawing, alloy designation, quantity and available inspection or material records.

Send your steam turbine blade strip requirements to WhatsApp / WeChat: +86 130 0924 9727 or sales@cocralloy.com for quotation.