Industrial Gas Turbines

"Illustration 3.4-1": (Lit. 3.4-6): The long, slender bodies of the turbine blades in the rear stages (lowpressure turbine) require shrouds even in modern engine types. Unlike newer blade types, highpressure turbine blades in older engine types are also outfitted with shrouds. Shrouds on turbine blades improve the seal effect in the region of the blade tips and stiffen/support the blades against high frequent vibrations.

Shrouds on turbine rotor blades are not unproblematic, and exhibit specific types of damage:

Parallelogram-shaped shrouds are usually not braced against one another (right diagram). They only reduce ‘spica field’ vibrations slightly, create gaps under centrifugal loads, and shift against one another when the blades are twisted open (lower sketch, “A“ and “B“). In order to ensure the proper functioning of the shrouds, the blades are braced against one another in a way that places the blade under torsion stress. This gives the shrouds a Z-shape (interlocking shrouds, lower sketch , “C“). The contact surfaces are protected from fretting through the application of hard-facing (e.g., stellite welding;upper frame, right picture). These shrouds do not separate from one another under centrifugal expansion. However, the shapespecific notch promotes fatigue cracking by thermal fatigue and/or high-frequency vibrations, especially if the build-up welding unintentionally reaches the notch radius.Radial overlapping causes the labyrinth fins to be angled, which results in more intensive localized rubbing with the danger of material being transferred between surfaces, and the beginning of a self-increasing damage process. Unlatching („B“) is when the shrouds shift axially to the point that they lose contact. In this case, unallowable creep deformation of the blades is to be expected. The axially shifted labyrinth fins must now deal with all the material removal from rubbing on only one of their own circumferential tracks. Major axial shifting of the blades can cause serious rubbing damage to neighboring blade rows.

Creep deformations (upper frame, left):

• Untwisting is a permanent straightening of the blade due to centrifugal forces. This breaks down the protective vibration-resistant effect.
• Shroud flexure is usually the result of a radial temperature profile that has been shifted outward. In extreme cases, a corner of the shroud can break off.
• Shingling is the overlapping of the shroud corners of neighboring blades. Shingling is promoted by narrow contact faces (lower frame, „b“), flexure of the shrouds and by one-sided fretting of the contact surfaces, which causes them to become very thin and slide over each other
• Fretting wear on neighboring blades promotes shingling. This is generally a combination of dynamic wear and hammering wear. It must be taken account when confirming the suitability of remedies and improvements. Dynamic wear can break down the bracing of Z-shrouds and increase the gaps around parallelogram shrouds. This increases the probability of dangerous blade vibrations.

Fatigue cracking can occur in the corners of the shroud interlocking due to:

• thermal fatigue,
• load cycles under RPM changes,
• high-frequency vibrations,

as well as combinations of these factors. If alterations are made in an area where cracking can potentially occur, then these dynamic loads must be taken into account. Changes that could lower dynamic strength must be considered very carefully. This includes coatings, which can often result in “disimprovements“.

Cracks due to thermal fatigue can form in the blade transition underneath the shrouds. Temperature gradients between the shrouds and blades in areas with notch effects (structural notch, stiffening notch, form notch) promote cracking.

Hot cracks and wear in seal fins caused by rubbing. The seal fins of the individual blades form a labyrinth around the circumference and are susceptible to the typical damage mechanisms of this seal type.

"Illustration 3.4-2": The failure mechanism of sulfidation can be imagined as follows:

• The base material, e.g., Ni base, is normally protected through a compact Cr oxide-layer impervious to sulfur (S).
• Different mechanisms, such as, e.g., the chemical reaction of the oxide layer with molten salt, can, however, lead to the overcoming of these hurdles.
• In the process, the metal ion content in the molten salts loosens, through ion exchange with the Cr, the Cr oxide layer, to such an extent that the sulfur can diffuse unhindered.
• If the sulfur has penetrated the base material, the renewal of the compact protecting Cr-Oxide coating is prevented, so that the molten salts are superfluous.
• Consequently, the base material is destroyed through accelerated oxidation.

Very little sulfur is necessary to trigger this failure mode. It can be accumulated from the fuel or the atmosphere (e.g., industrial by processes, agricultural by fertilizer).

The failure process seems to need only a small effective zone at the transition to the base material and supports the formation of bigger Ni-Oxide amounts. The smallest amounts of nickel sulfide for oxygen transport are sufficient. Correspondingly difficult is the metallographic proof of sulfidation. A non destructive method to prove sulfidation on only outwardly accessible hollow parts, e.g., turbine rotor blades and guide vanes in built in condition is magnetoscope inspection. It reveals from a structural base material change in connection with the sulfidation process. So a nondestructive testing for hollow parts like turbine blades and vanes in the engine is possible. This works even then, if the wall is not perforated to the outside, that means the destruction is not yet visible.

"Illustration 3.4-3": Sulfidation is entirely bound to a certain temperature area in which stable aggressive molten salt exists ( "Ill. 3.4-2"). For this reason, sulfidation is formed at outer areas, even in component typical zones (left frame). Besides having the appropriate temperature, these areas are favorable for deposits.

Sulfidation is especially supported if the oxygen offer is insufficient, so that a thick, protective, oxide layer cannot form. This is apparently the case, especially in one sided, closed, not cooled, hollow blades (weight minimization of aerodynamic advantageous thick airfoil profiles), leading in extreme cases to the breakthrough of both blade walls (‘windows’ , sketch below, right, "Example 3.4-2"). The particular problematic is that the extent of the damage can be first recognized at a later stadium.

In cooled blades (sketch, above right), there seem to be limited regions in the interior, where such aggressive dusts find it convenient to deposit and a suitable temperature reigns.

These are usually to be found on the pressure side of the blade airfoils. In turbine blades with film cooling, such zones are found behind the air exit holes (sketch, above left). Blades with high edge temperatures show small fields parallel to the edges (sketch below, left, "Ill. 3.4-2"). Frequently, the oxides are so worn off, as a result of the sulfidation caused by the erosion of the gas flow and the particles contained in it, that already plain hollows are discernable.

If this is the case, one must conclude that only a short remnant of life, compared to the already expired life, is usable.This is also a problem for a repair by surface polishing/grinding. If hardly detectable micro particles of Ni-sulphide stay in the material, the relatively long incubation time is lost and the life time that can be expected is very short.

"Illustration 3.4-4": (Lit. 3.4-3): Silver is used on hot parts made from Ni-based materials and high-alloy steels in order to prevent galling and fretting and to obtain a controllable coefficient of friction. For this reason, nuts, bolts, and their fitting surfaces are often silver-coated. Silver can dangerously damage parts made from nickel alloys and titanium alloys.

Sketch1: Initiation and promotion of sulfidation through a type of catalytic effect of silver on hot part surfaces..

Sketch 2: At higher part temperatures (probably >700 °C) silver can dangerously diffuse into Ni alloys and high-alloy steels. This is especially likely if these parts are under high tensile stress, as is common in nuts and bolts. This damage occurs primarily in the thread area. It causes embrittlement, strength losses, and fractures. The diffusion is promoted in silver-coated parts by the metallic contact between the silver and the substrate. Damage through diffusion upon contact with silver is less likely in parts with protective oxidized surfaces.

Sketch 3: Incitement of sulfidation on hot parts made from Ni alloys through contact with silvercoated surfaces. This can also unallowably reduce the fatigue resistance.

Sketch 4 describes two compressor disks made from the titanium alloy Ti-7Al-4Mo that burst after cracks occurred in the bolt bores of the rotor shroud. The cracking was attributed to the contact between the titanium alloy and highalloy steel (A-286) bolts that were silver-coated against fretting. Condensation water containing Cl evidently led to the formation of silver chloride at the higher operating temperatures, which resulted in silver deposits occurring in the bolt bores. This type of fouling, especially chlorides, can always be expected in marine environments. The literature mentions that longterm action of Ag has been observed to cause damage to the disk material, Waspalloy. Ag has also been observed to separate from the bolts and damage neighboring parts.

Sketch 5: Pitting corrosion occurred near silver deposits in the Ni-alloy rotor of a low-pressure turbine. The deposits probably occurred due to evaporated condensation water which contained dissolved silver compounds. The aggressive water (marine atmosphere?) apparently desilvered the threaded connectors while standing, and was then thrown outward into the flange sockets when the engine was started up, and later evaporated. Therefore, this is a combination of the damage mechanisms from sketch 1 and 4.

Sketch 6: This HPT disk from a fighter engine is made of an Ni alloy. Cracks occurred in both flanges after longer test runs. They were caused by the silver of the threaded connection.

In order to prevent these types of silver-related damage, the contact surfaces and glide surfaces must not be silver-coated. This has the drawback, which must be accepted, of making detachable connections (bolts, etc.) impossible to open without damaging them to the extent that they cannot be reused.

"Illustration 3.4-5": (Lit. 3.4-5): Seals with a web structure made from oxidation resistant Ni- base metal (honey comb seals ) are used as abradables for labyrinth fins in the hot regions , especially the blade shrouds of the turbine and opposite the labyrinth fins used on the intermediate rings of the turbine rotors. They have also been established by many millimeters of radial and axial cutting in, i.e., the labyrinth fins are not prohibitively damaged. They have a good sealing effectiveness, even by big labyrinth clearances and show a certain damping effect on labyrinth and rotor vibration (Lit. 3.4-2). During very long operation times, typical for use in industrial implementation and/or by special corrosive conditions, it can come to a hefty damage of the web walls through oxidation or hot gas corrosion ( "Ill. 3.4-6"), so that, in an extreme case, the entire web structure brittles and breaks out, correspondingly diminishing the sealing effectiveness. This does not only lead to high clearance loss and deterioration in efficiency ( "Ill. 3.5-2"), but increases the danger of prohibited heating up of the rotor spacer rings through hot gas leakage. For this reason, the manufacturer should credibly demonstrate the long term durability of this sealing system in the hot gas region and for operation specific conditions ( "Ill. 3.4-2" and "Ill. 3.4-6").

"Illustration 3.4-6": (Lit. 3.4-5): If proven engines are used in a new application with different operating conditions it can result in unexpected damage to “proven parts“. If, for example, the operating times between overhauls in civilian operation are increased, or relatively high output levels are demanded over long periods of time, the “wear“ of the blade tip seals in the hot part area can become a deciding factor for the engine‘s operating life span.

The thin metal ridges of honeycomb seals are weakened by oxidation („A“, bottom left diagram) until they break off and increase the tip clearance gap in the turbine („B“) to impermissibly large size (bottom right).