Industrial Gas Turbines

"Illustration 4.2.3.2-1": (Lit. 4.2-4): To prevent failures of O-rings or define remedies it’s necessary to identify failure causes and mechanisms. Precondition is the assessment of the failure mode. In the following specific problems should be discussed:

Primarily chemical deterioration is destined by the operation temperature. It schould lie under the long time temperature for elastomers, mostly between 135 and 150°C. Thereby it must be indicated that this deals with a permanent tempeature and not a short term maximum temperature (see paragraph „thermal damage“). At a too high temperature it comes to a change in the molecule structure (aging). The thermal durability of an elastomere is even for the same type markedly depending from the quality. Thereby contaminations and crosslinking of the molecules play an important role. A deterioration can, e.g., also triggerd by trace metals. That means, that the quality insurance finally must rely on the producer and the source of supply.That can be of importance under the aspect of suspect umapproved parts SUPs

Typical consequences of an aging are:

• The embrittlement is the main problem and can be seen as result of a temperature induced aging process. This makes especially susceptible for failures by crack formation or shock loads. To embrittlement tend „nitriles“ with:
• Drop of strength,
• Material change / deterioration,
• Crack formation and crack growth. A special type is the inner crack development as result of diffused gases.
• Bubble formation. These damages can be also traced back at gas diffusion.
• Permanent deformation, creep in this connection the decrease of spring back (resilience) must be seen.
• Carbon dioxide can be totally solved in some elastomeres, swelling them with a sponge like structure.

Thermal damage is here understood as the result of an overheating. It starts if the temperature is exceeded at which the elastomer deteriorates in a short time. Such a situation exists if the air or oil cooling lacks after shut down of the gas turbine. That results in a heavy heating from the neighboring hot parts („heat soaking“, "Ill. 3.5-8").

Compression deformation of the O-ring under impression forces is plastic (enduring). Leakage occur in connection with temperature changes and thereby the volume change of the ring. Expecially fluor-elastomers (fluor rubbers) tend at the typical hight of service temperatures to a creep deformation.

Explosive pressure drop: Into the O-ring diffused medium, expecially gas, accumulates beneath the surface at flaws and micro pores. During a sudden pressure drop the gas expands or a vapor formation occures. The result are defects like blisters, cracks and holes.

Extrusion can be traced to overload of the Oring by a pressure difference at the sealing gap.

Assembling damage: Different types are found. Most frequent are cuts and sheared regions that happen during careless pushing together of the sealing surfaces. To this can also added so called spiral failures. They can be associated with a twisting of the Oring. This happens when the pushing together happens with a turning.

Outgassing and/or leaching of components shows itself by shrinking due to a volume loss. At higher temperatures a chemical deterioration can occur. This renders components of the sealing material to liquid or gas. Especially thermoplastic urethans are susceptible for such damages.

Ingresses water vapor or overheated water into the pores that form during outgassing, a sponge like structure will be the result. Is the O-ring not enough supported, with a short term sealing failure must be reckoned.

"Illustration 4.2.3.2-2": (Lit. 24.2-4): The layman seems the mounting of an O-ring easy and without problems. This misleads. As well the inserting of the O-ring into the flute, respectively at the seat, demands attention and skill. Has the ring to be hauled over edges of the flute, landings or threads ( "Ill. 4.2.3.2-5"), it can be damaged by a cutting act. This promotes the usual elastic expanding and equivalent high cutting forces. A particularly crucial assembly phase is the push together of the sealing faces.

If the O-ring already sits in the flute of the shaft („A“), a delicate moment of the insertion can be observed.

Sits the O-ring in the flute inside a bore („B“), an observation is no more possible. Improper handling and/or a too sharp edge can unnoticed shear the O-ring. To notice this experience and feeling is needed.

Shifted O-rings („D“) are unable to achieve the sealing function. Thereby the O-ring can as a whole or partly slip out of the flute. It can, even during the assembly, slip as far that a missing will not be noticed.

As an earnest, hardly to expect problem forgotten O-rings are highlighted („E“). Such a situation gets more probably with two O-rings in one sealing. Thereby not seldom the lacking of ‘human factors’ is concurrently causative.

"Illustration 4.2.3.2-3": Apparently deviations from the aligning, centric position of the shaft against the sealing ring can have very different effects.

• Axial runout/shaft run-out/ rotary bending up to a magnitude of tenth millimeters can against expectation improve the sealing behavior. The consequence of such a run-out (excentricity) of the sealing lip differs from a displacement. A run-out only occurs during rotation. This stresses the sealing lip over the whole circumference.
• Radial displacement, often only called displacement (sketch in the middle), contray to the run-out also exists in the stand still phase. The sealing lip will be deflected locally at the circumference as well during stand still as also during rotation. This can result in damages by thermal overload and/or plastic deformations (creep).
• Inclination of the shaft (lower sketch): In the region of an angular degree the sealing effects seems not to be declined. That is also true for grinding scores that slightly deviate from the circumferencial orientation. The axial movement of the shaft is enabled by the bearing clearance and supports the hauling. So this improves the sealing effect.

"Illustration 4.2.3.2-4": The topografie of the shaft surface can be of considerable importance for the sealing effect. Normally this is a matter of grinding surfaces, produced by a cut in process (plunge cut method, lower sketch). Anyway there is always to reckon with machining marks. The twist effect of those microscopic lanes often was overestimated. That changed with the awareness, that the hauling effect is based on the structure of the elastomer sealing face which acts as actual sealing mechanism.

There is the chance to take advantage of the hauling effect of the twist of the grinding marks. For this purpose the direction of the lead must be adjusted to the rotation and sealing ring direction (upper sketch). In such a case the twist direction has to be marked with an unmistakable sign on the part.

"Illustration 4.2.3.2-5": The assembling method (lower sketch right) depends from the insertion direction. It is important if the seal first is moved onto the shaft and then into the casing (seat). The other possibility is first to insert the seal into the seat and then to introduce the shaft. If possible the first method schould be preferred. The introducing face side of the shaft should have a chamfer (upper detail) or a radius. For repaired sliding faces especially attention must be payed that they are free of burrs and have soft edges. That is also to apply to small break outs at the edges of a cromium coating. For the safety, a sleeve with a favorable edge can be slided on the shaft (right sketch). This is especially advisable if the shaft has flutes. A slight rotation movement can ease the inserting.

The symmetric and gentle forced fitting with this the alignment of the seal in the casing/seat can be assured with a bell shaped tool (sketch below left). Thereby the outer seat of the seal can be lubricated, as specified. Also the entrance edge of the locating bore in the casing should have a chamfer (detail above left). A continuous forced fitting process can be executed without overload. Thereby the sticking friction is avoided. Under unfavorable assembling conditions, it can be necessary to remove at first the tension spring of the seal and to attach it afterwards after the positioning at the seal again. That is recommended if the seal must be inserted reverse side.

"Illustration 4.2.3.2-6": This chart should support the practitioner. However it can not replace informations in manuals and specifications. That must be applied for remedies for seal failures.

Realizing (features) the problem is premise for a constructive remedy. For this the failure mechanism with the specific causes must be identified.

Evidence of a maintenance problem: At this point begins the difficulty with the identification of the failure attribute. Not always it attracts our attention. If its occurence lies in special distance (oil drops , oil track) to the leak, sometimes this features can not be connected.

Next arises the question if the observation can be seen as normal or if it is unusual. A desicion needs satisfying informations in manuals, together with expertise and experience. For example it must be decided if bleeding oil is to be seen as a lekage. The details in the manual need an interpretation if the instruction allows a discretion. In such a case enough technical qualification is necessary. We must know concerned functions and likely consequences.

For the attendant or supervisor detectable outside features of a failures of a radial sealring are:

• Alarming high leakage rate. For this classification there should be sufficient detailed information in the manual.
• Observable damaged seals: crack formation, mechanical damage.
• High friction forces, that arise in the sluggish behavior of the shaft. Here also experience is needed for an evaluation.

Is the indication recognized respectively classified as a failure, the cause must be suggested from the relevant features of the finding.

"Illustration 4.2.3.2-7": This picture shall make aware the many damaging effects at an axial face seal and give so a support for the treatment.

Occurs a failure or problem at an axial face seal, the failure mode and symptoms can give the expert important hints at causative influences. For a statement as sure as possible some elements of the findings have to be combined. Wear patterns at the contact faces of the axial adjustment can be connected with track/contact patterns ( "Ill. 4.2.3.2-9").

A survey about the consequences of typical influences follows: Corrosion and other chemical reactions: It must be considered that wear processes produce fresh reactive metal surfaces which are unexpected susceptible to corrosion.

Oil contaminations ( "Ill. 3.5-3" and "Ill. 3.5-5") and aging products can attack seal elements ( "Ill. 4.2.3.2-1" and "Ill. 4.2.3.2-8"). In all cases a specific sensitivity of the material is needed. This counts, e.g., for stress corrosion cracking (SCC) or hydrogen embrittlement of springs made from high strength steels. The formation of a corrosion cell develops during contact of different metals in an lubrication oil acting as electrolyte. This can be traced back to not suitable additives, aging or contaminations.

Hard thermal spray coatings and sinter layers at the rotating sealing face („2“) can be damaged by corrosion. From tungsten carbid (TC) in a CoNi-matrix, the matrix gets dissolved. So break out of the hard TC particles occures. As result a high wear rate is to be expected.

Elastomers of the static sealing („4“, e.g. Orings) of the face sealring can be damaged in different ways ( "Ill. 4.2.3.2-1"). Is the axial move of the sealring hampered a lekage occures.

Wear can arise in different types.

• Vibration wear (fretting) develops on contact surfaces of the axial adjustment. This can be alternately reinforced with corrosion.
• Sliding /abrasion wear at the sealing faces is caused by dry run after a breakdown of the lubrication film.

A further problem, are abrasive particles/oil contaminations or

• an unsuitable tribological system in the lubrication gap.
• Erosion at the surfaces which are impinged by an intensive oil stream with hard contaminations.

Cavitation ( "Ill. 3.5-19") depends from flow conditions in the oil. It is influenced by pressure, temperature and velocity. Also characteristics of the sealing faces like planarity have a crucial effect. So it comes to the formation of gas bubbles and vapor bubbles in the lubrication gap with break outs and abrasion at the sliding faces.

New parts production and repair: Primarily the sealing faces of the seal ring and the shaft are affected by aberrations:

• Axial run-out /distortion,
• corrugation,
• topografy/roughness,
• No sufficient bonding strength in a slide coating,
• Discrepancy of the slide ring material (carbon/graphite): porosity, weak bonding.
• The cartidge type promotes the possibility of the use of unlicensed products (SUPs).

Assembling - and handling problems: An appropriate storage must already guarantee the defect free condition of the seal. That is especially applied for the sensitive carbon sealring. Typical assembly relevant problems are

• alingnment of the sliding faces.
• Damage of the slide ring edges.
• Hampering of the axial adjustment.
• Unsuitable axial contact pressure.
• Contaminations (dust, abrasive particles).

Operational mechanical loads: Occur failures at axial face seals they are probably secondary. Thereby it must be checked if operation conditions existed which explain unnormal high mechanical/physical loads:

• Severe vibrations (unbalances, air oscillations or gas fluctuations in the combustion chamber, "Ill. 3.2.2-5").
• Large axial and radial movements of the shafts (bearing play, compressor surge, "Ill. 3.1.1-6", thermal expansion), labyrinth failures.
• High pressures/pressure differences.
• Oilfire.
• Pressure shocks (compressor surge, "Ill. 3.1.1-6").

"Illustration 4.2.3.2-8": In this picture it is attempted to display schematic details of typical failure modes with typical features of sliding ring tracks from axial face seals.

„A“ Nonuniform sliding track, possible causes:

• Distortion of the sliding ring by too high joining/holding forces.
• Distortion due to production induced internal stresses.
• Insufficient/uneven supporting contact surface of the sliding ring rear side.
• Misalignment of a segmented seal.
• Assembly mistake.
• Quality deficiency.

„B“ Scratched sliding track, „C“ Erosion, possible causes:

• Contamination during assembly.
• Lifting of the sliding surface by Vibrations, short term dry contact (flashing), distortion by temperature influence, too high pressure in the sealing gap.
• Lubrication starvation on the sliding track.

„D“ Coke formation, possible causes:

• Too high operation loads (temperature, pressure, circumferential speed),
• Contaminated, aged/deteriorated oil,
• insufficient heat dissipation,
• Evaporation of the oil in the sealing gap,
• insufficient monitoring of the operation data.

„E“ Thermal crack formation, possible causes:

• Too high operation loads (temperature, pressure, circumferential speed),
• Lubrication shortage at the sliding track, dry run.
• Insufficient monitoring of the operation data.

„F“ Break outs at the edges of the sliding race, possible causes:

• Lifting of the sliding surface.
• Operation in the evaporation region,
• vibrations,
• cavitation,
• production/quality,
• insufficient installation conditions,
• too high operation loads (pressure, temperature, circumferential speed),
• Lubrication shortage on the sliding surface.

„G“ Break outs in the race track, blistering. Possible causes:

• Insufficient material/tribological system,
• Too high operation loads (pressure, temperature, circumferential speed),
• short term dry run (flashing).

Comment: blistering at carbon seal rings will be dealt with in "Ill. 4.2.3.2-10". It occurs during contact with oil.

"Illustration 4.2.3.2-9": The sliding face of an axial face seal can give important hints at problems and failure causes. The pictures show typical schematic features for the tribological system carbon slide ring - steel race. Typical symptoms of the contact pattern are (see also "Ill. 4.2.3.2-8"):

Rotating sliding face

• intensity, existence, absence(d),
• width (c,b),
• radial displacement (f,g),
• the race track exists only partly (f),
• discontinuity and it’s distribution (e).

Failures of the race track at the rotating sliding face:

• Penetrative wear (k),
• bigger out breaks of a hard facing (i),
• smaller break outs (pits) at a hard facing (h),
• crack formation (I).

Static sliding surface (carbon ring)

• break outs at the edges (m),
• pittings on the race track (n),
• blistering on the race track (n).

"Illustration 4.2.3.2-10": Carbon blistering at carbon sealing rings for the sealing of oil apparently is the most important cause of leakages. The failure mode are bright areas. Sometimes they show small radial cracks and/or pits at a broken up blister (detail below right). The blistering in the race track disturbs the evenness, needed for the sealing effect of the sliding face. Three features are distinguished:

• Type I: Bright spot.
• Type II: Bright spot with origins of radial micro cracks.
• Type III: Pitting shaped break out with radial micro cracks.

Formation mechanism of the blister formation (middle frame): This failure forms even at a few damaging cycles. This is a question of a selfenergizing cyclic damage mechanism. In the initial phase (type I) a polishing effect creates the bright surface. The hydrodynamic oil film penetrates the pores near the surface in the carbon slidering („1“ and „2“). During a fast temperature rise, due to high oil temperatures or friction heat, the oil expands in the pores („3“). Is the oil not able to escape fast enough, a zone under the sliding face bursts open (upper detail). Thereby the affected area (‘cover’) will be slightly lifted. Even about ten damaging temperature cycles can trigger blistering. This shows that the temperature cycles are crucial. In the region of a blister the contact pressure and friction heat increases. So thermal stresses and mechanical bending stresses in the ‘blister cover’ develop. Further loadcycles produce cracks (Type II) until the friction forces lift the blister cover (Typ III).

The blistering will be intensified

• markedly by the oil pressure to be sealed, which governs the pressure in the pores,
• markedly by the oil viscosity. It influences the pressure release in the pores,
• lesser pronounced by the sliding velocity.

This behavior is material dependend for all three parameters (cycles, oil pressure and oil viscosity). For example the damaging blister formation on the sliding ring is at a ceramic counter face generally weaker than at a steel face.