Industrial Gas Turbines

"Illustration 3.6.1-1": (Lit. 3.6.1-3): Although auxiliary gearboxes account for a relatively small portion of severe problems in aero engines, they are susceptible to many different potential damage mechanisms due to their wide variety of elements (gears, housings, bearings, shaft connections, seals, oil supply). Due to the similarity, this experience can also be interesting for derivates. These damages can be causally influenced by different phases of the part life. This is shown in the following with the aid of selected examples:

Production: Light metal cast alloys, and especially Mg sandcasting (typically used in older engines), often have continuous porosity. This porosity can be sealed through infiltration, coatings/paints and compacting (peening) of the surface during production. However, overhaul procedures (degreasing, cleaning, stripping) can reopen and increase the porosity. This occurs through initiation of synthetic material infiltration and/or chemical corrosion of the substrate. The result are leaks through which oil or fuel “sweats“ out. Material flaws and weak points such as fields of porosity or oxide skins can crack under dynamic loads. Improperly brazed or welded injection nozzles can fracture due to dynamic fatigue and fall into the gears. If the sides of narrow teeth are case-hardened, internal flaws may develop. They are the result of hydrogen embrittlement and can cause the tooth to fracture during operation.

Overhaul: Penetrative crack detection requires stripping and degreasing, as well as disassembly of all steel parts (bearing seatings, threaded inserts). This can prevent chemical attacks and corrosion due to local cell action. Because these fastening elements must be reinstalled later, larger inserts or coatings are used on the fitting surfaces of the housings to compensate for wear and material removal (operation, disassembly). This can limit the number of possible overhauls. Gear shafts wear at the seal surfaces of the seal rings. Circumferential grooves are created on the shafts. In these zones, the parts must be reworked (ground) and coated to restore the original diameter (e.g., chroming). The usual blackening is also removed in the course of this repair process. It is later reapplied in a hot leachate. This repair process can cause cracking and serious damages. These cracks can not always be safely detected afterward through magnetic crack detection. During operation, these cracks can lead to an unpreventable failure of the part.

Operation: There is a high probability that design-specific weak points in gearboxes will make themselves known during development and testing. This makes it possible to prevent them before serial production begins. This is the most likely reason for the many reports of gearbox problems during development of new engines, but relatively few damages are recorded during operation. Gearboxes are subjected to considerable dynamic loads through the fastenings, gears, and power input and takeoff. For this reason, special attention must be paid to flaws and weak points that could promote dynamic cracks. This is especially true for gears. A special problem is presented by the oil conditions in the gearbox. If gearboxes have “dead“ zones in which larger amounts of oil can collected it is called oil hiding. In extreme cases, this can cause oil shortages in the main bearings and even engine failure.

Nozzles, which frequently take the shape of pipes attached to the gearbox wall with clamps, can be excited to vibrations and suffer dynamic fatigue fractures. This can result in spontaneous damage if fragments become trapped between gears. If there is merely a localized lack of lubricant, then bearings and gears may be damaged over time. This type of damage usually makes itself known early with metal shavings on magnetic plugs ( "Ill. 3.5-5"). Experience has shown that dynamic fatigue fractures in oil nozzles occur in notch areas such as transitions to distributer parts, brazing and weld seams. For this reason, these notch areas should not be located in regions that are potentially subjected to high dynamic loads, i.e., near the clamps.

Gear failures are covered in the chapter ‘stationary gears’ ( "Ill. 3.7.2-5.1", - 5.2 and - 5.3). Failures of antifriction bearings are in "Ill. 3.5-11" assembled.

Long standing times can be a part of operation. Standing times can have damaging effects on gearbox components such as bearings (corrosion, brinelling) and seals. In O-rings, reported problems are aging with embrittlement and lasting deformations ( "Ill. 4.2.3.2-1"). It is also possible that shaft seal rings ( "Ill. 4.2.3.2-6") may leak and/or run hot if they encounter dry surfaces where the oil film has run off.

"Illustration 3.6.1-2": (Lit. 3.6.1-5): Leakages and failures on pipelines of gas turbines can have different causes.

„A“ Cracks and fractures: In the most cases may be about vibration fatigue. Thereby especially the pipe zones near the fixing point are at risk. This may have several causes:

• High bending stresses as result of the lever during vibrations and tensioning ( "Ill. 3.6.1-3").
• Changes in stiffness (flange, bolting).
• Unfavorable position of weld seams.
• Contamination with aggressive media like deteriorated hydraulic fluid can release spontaneous crack initiation (stress corrosion cracking =SCC). E.g., titanium alloys react under tension stresses during exposure to halogens like chlorine sensitive to this failure mode.

„B“ Chafes mean a particular danger. This shows the complex course and the alignment of the lines on a gas turbine. With tis. With this it becomes understandable that there is quite a potenzial for the contact of the lines. Naturally the designer will already care that this doesn’t happen. The experience shows however that different effects can lead to breed this.

• Using similar (not equal!) components. To those belong variants of the same engine type.
• Problems with the fixing, e.g., failure or missing of a attachment (clamp).
• Loose clamps, wearing the tube wall.

During contact of a pipeline with an other component like a casing, an other pipeline, cable, cover, locking wire etc a damage can occur in several ways:

• Weakening of the cross section until failure (force, vibrations).
• Notch effect, triggered respectively favored by a fatigue crack.
• Damage of the tube material by fretting (vibration wear). This danger is expecially high on titanium alloys.

„C“ Fused surface: The danger that a tube line is fused up to perforation exists under exposure to concentrated flames or electric sparks/arcs. Both can happen in connection with pipeline failures. If a pipeline is not sufficient cooled at the inner surface, favors this under strong heat addition the danger of overheating. Problematic is a low flow rate to the empty line. This can be the result of a failure (e.g., malfunction of a pump).

„D“ Damage by fragments: do rotor fragments escape from the casing (uncontained failure), then instantaneous exists the danger of a leak from a damaged pipeline. The design engineer tries to minimize this risk even during the planning of the line.

„E“ Loose connection: it may be that during maintenance/assembly the connection was not enough tightened ( "Ill. 4.2.3.1-1"). In this case especially the accessibility and the visual check plays a role (human factors). Those factors are to consider even at the design phase. There can also be a problem with elastomeric elements like O-rings. Thinkable are damages ( "Ill. 4.2.3.2-1"). Further possibilities are the loosening of the nut and/or a damage of the sealing element during operation. A loosening is experienced as likely result of an assembly fault. In such cases vibrations and heat expansion can trigger the leak. During test runs after an assembly such leaks will not always be found. This is especially the case if not the full engine performance existed. Are the pressures (fuel, oil) not high enough no observable leak may occur.

„F“ Loose flanges : Studs or flange bolts as well as T-head- bolts from V-bands can come loose or fail ( "Ill. 4.2.3.1-3"). A possible cause are deficiencies in repair and/ or assembly. Bolts made of high-strength steels can fracture due to faults in production like heat treatment and hydrogen embrittlement. A further possibility of a leak in the region of a flange is a failing elastomeric sealing between the seal faces. Also here the whole line-up of causes like material problems (aging, "Ill. 4.2.3.2-1") and assembly can be relevant.

"Illustration 3.6.1-3": (Lit. 3.6.1-5): Fatigue cracks in pipe lines are not seldom attributed to mechanical tensioning. The dynamic load can occur during vibrations and/or low frequent cyclic strains by temperature or changes of the inner pressure. A tensioning respectively pre stressing lowers the fatigue strength of the tube material (diagram above right) because it increases the mean stress. Thereby an exzess of the dynamic design conform operation stresses gets more likely.

Causes for the tensioning of a pipe line (sketch above left): Dimension inaccuracy from the production can as well at the end connections as in the region of the clamps require an elastic bending. During tightening of the connection especially in the region of the ends, high stresses must be expected. The frame shows the sequence of the attachment procedure at a pipe line for a as low as possible controllable tensioning. Generally however count the instructions in the specifications respectively manuals of the OEM

Damages before the assembly can result in a similar situation like dimension deviations. Its also possible that an already assembled line got damaged, i.e., plastically deformed. In this case the spring-back leads to a tensioning.

Of course the crack position is influenced by notches and deformations like buckles. A fatigue crack can be expected in the region of the end connections on the side of the tube that is located opposite to the deformation.

Operational tension can be a consequence of different thermal expansions between pipe and attachments. An example is a fuel cooled pipe which is connected to a hot casing, respectively accessories. Similar conditions develop if mechanical operation forces deflect connections and/or clamps.

Also a high internal pressure can elastically deform a pipe between the connections and so setup stresses.

Internal stresses promote as tensile stresses the vibration fatigue. Such stresses are inserted by the manufacturing.They are generated during welding, mechanical machining and especially during dressing. Thereby process deviations are particularly problematic. As result of a plastic deformation (damage) develop by spring back tensile stresses too. They can according to the size of the deformated zone; concentrate rather at the deformation or distribute over the whole pipe length.

"Illustration 3.6.1-4": (Lit. 3.6.1-4): Especially in older engine types, many parts were made from Al and Mg sand casting. Examples include:

• Compressor inlet housings with bearing chambers,
• front compressor housings,
• housings of the auxiliary components such as gearboxes and pumps.

These cast parts are prone to the typical flaws of this casting technology. A certain amount of cavity formation or flaws, originating in fragments of the casting skins from the casting flow, were unavoidable. The usable dynamic strength of these materials was minimal, even without considering the influence of their structures and the missing fatigue strength. The most important flaws that occurred during operation are:

Shrinkage cavities that locally spread through the entire cross-section (top left detail) are a typical problem in larger sand cast parts, especially gearbox housings. These leakages can be sealed through infiltration with organic (synthetic resins) and inorganic (sodium silicate) media in the new part. This sealing effect can be reinforced through shot peening of the surface (e.g. Al shot). Experience has shown that multiple overhauls carry the risk of reopening cavities and causing leakages. If oil has entered the cavities, it makes resealing difficult and can end the life of the part.

Larger oxide skins (bottom left detail): These represent a crack-like separation and are not easy to detect with X-rays and penetrant testing. In some cases, their size can cause spontaneous part fractures even after short operating times.

Bubbles and gas pores (right detail): These are gas inclusions that either originate in the casting process or are related to common subsequent reparatory welding of new parts in the area around cavity fields. These flaws should be sufficiently reliably detectable with X-rays.

"Illustration 3.6.1-5": (Lit. 3.6.1-1): Corrosive attack on the edge of a coated/painted gearing casing made from a cast Mg alloy. The pitting corrosion under the influence of marine atmosphere is typical (Cl ions, "Ill. 3.1.2.3-1"). Repair is possible in areas subject to low mechanical loads by filling them with synthetic material.

Proper corrosion protection in accordance with regulations on magnesium parts has been proven to ensure long damage-free operating times. Therefore, if corrosion occurs, the cause is most likely defects or damage in the coating.

Corrosion of edges usually occurs at mechanical damage to the paint/coating, insufficient coating thickness, or coating defects. Corrosion in the flange area is promoted by corrosion cracking conditions and cell action in contact with the mating surface.

Corrosion pittings on coated surfaces are often related to improper preparation of the surface to be coated, such as corrosive remnants of cleaning baths and blasting/shot- peening materials.

This results in typical blistering due to corrosion between the coating and the base material.

"Illustration 3.6.1-6": (Lit. 3.6.1-6 and Lit. 3.6.1-7): The curved teeth coupling serves the transfer of high performances like from a power turbine ( "Ill. 2.1-2"). Such couplings should primarily possess a circulatory oil lubricant. There exists the danger of leakage with grease or oilfillings (O-rings, "Ill. 4.2.3.2-1"). The lubricant shortage is then observed not until a visual check. Heavy wear up to galling and friction welding are the results. Thereby non tolerable axial forces can affect the shafts.

Cardan shafts/universal joints are applied when there are large angles between the shafts. Axial misalignment needs in addition a second universal joint with a intermediate shaft (right sketch). This concerns rather accessories like smaller pumps and generators. For cardan shafts it must be considered that during one revolution the angular velocity changes periodically. This can cause severe torsion vibrations and fatigue failures at the connected aggregates ( "Example 3.6.1-1"). With a malfunction affected control units can critical influence the operation behavior.

"Illustration 3.6.1-7": (Lit. 3.6.1-5): Shaft couplings with multi splines are frequently used and are proven in gas turbines, especially in derivates. A big advantage is the easy assembly by pushing together. That benefits the assembly /disassembly of accessories like generators, control units and pumps. In spite of those advantages multi splines are under unfavorable operation conditions also a potential weak point. They are primarily exposed to fretting (sliding wear). This leads to an abrasion of the loaded flank. In an extreme case it comes to the fracture of the remaining tooth cross section and so to the failing of the coupling.

To prevent failures an adequate lubrication is of great importance. It should be adjusted to the application.

• Lubrication oil is usual for components of the oil system.
• Leakage oil from neigboring seals (e.g., rotary shaft seals, Chapter 4.2.3). This is a usual lubrication. Often this is not aware. For this reason exists the danger of ‘disimprovement’. Endeavours to cut also the last leakage can lead to unexpected failures of multi splines. Do such failures occurnot until longer operation times the problem will be aggravated. Than the extent of the affected components/gas turbines is already big.
• Dry lubrication coatings (e.g., containing graphite or MoS2 ) are used for couplings that can’t be reached by oil. They ere adjusted for operation temperatures that are too high for an oil lubrication. This is frequently the case for couplings of main shafts.
• Fuel servs in shaft connections to control units and pumps as lubrication. The comparatively poor lubrication efficiency of the fuel requires an extra careful selection of the triboligical system.

The lubrication additionally influences the wear with consistency and transport of the wear products. Mostly these are oxides, we speak about fretting remainders. Do those stay in the splining their bigger volume compared to steel produces a ‘blasting effect’. This can favor fatigue cracks and fractures. Become the wear products transported out of the splining, widens the play and so the relative movements. Shock like loads occur (hammer wear). That boosts the wear further.

According to the lubrication additional the materials combination of the contacting coupling teeth can be of essential significance for the wear behavior. This is to keep in mind especially for repair coatings. If the coating in any way (e.g., pre-treatment product, deposit process) differs from specifications in the manuals, the OEM must be consulted. In case of doubt appropriate proofs are inevitable. Those require, as typical for wear processes, the relevant operation.

The existent type of wear is not without cause labeled as wear corrosion (fretting corrosion). The corrosion influence has great importance for the formation of the wear (abrasion) oxides. As electrolyte it comes into consideration primarily in marine atmosphere respectively in the stand still with sea salt contaminated condensate. The fine metallic abrasion is during formation chemically very active. It responses especially to oxidizing and corrosive influences.

Besides the lubrication conditions the micro movements between the contacting tooth flanks play an important roll. They can have different causes that can also act in combination:

• Alignment failure of the shaft ends,
• Vibrations of the shaft system.
• Start shocks,
• different elasticity of the coupling components (dimensioning/design). Causes for the crack formation in multi spline couplings are probably:
• Condition of manufacturing by grinding and/ or blackening.
• Cracks by vibration fatigue can develop at wear grooves in the splining, also at the concave/bushing side of the coupling. Under dynamic torsion overload axial cracks in the tooth root can be expected.
• High bending loads can trigger cracks in a predetermined breaking point of the neighbored shaft.