Industrial Gas Turbines

"Illustration 3.6.2-1": (Lit. 3.6-5): Sensors/probes as well as electric cables and connectors are direct or indirect responsible for a high percentage of problems. Not seldom they are in connection with the maintenance. In the following the problems with the main components as there are sensors, connectors and cables are discussed. Naturally plenty potential damaging operation influences affect the components.

Some examples for clarification: („A“) Adaptors, connectors and contacts:

Cables for the electrical power supply and the data transmission can be very different. Equal also the sensivity for operation influences. Generally a connector should be checked before the use for problems like damaged contacts, corrosion, contamination and insufficiend safety.

• Conectors attatched at the outside of the engine can be intensified exposed to corrosion by condensation, especially in marine environment. As a result the oxidation of the pins acts electrically isolating.
• Contaminations with media like oil, hydraulic fluid, cleaning agent.
• High vibration load can cause cable fractures and/or wear when there is a contact to other surfaces.
• Damage during inappropriate handling, e.g., forced plugging.
• Wear of the contacts by vibrations/micro movements (fretting) and frequent use.

(„K“) Cables / lines are conformed to the designated use. Therefore the type can be very different. Normally the signal transferring or energy transfering element is enclosed by a protecting and stabilizing jacket. This can consist of metallic tubes or fabric. Electric and optic conductors use rather plastic/elastomere for the jacket. This is according to the damaging operation influences.

Mechanical actuation and feedback cables as they are used at elder enging types:

• Wear in guidances and force transfering spiral coils (system bowden).
• Fracture as result of force or vibration fatigue.
• Blockage/jamming by insufficiens lubrication, corrosion, freezing and contamination.

Electrical cables to transfer power and/or impulses/measurement data:

• Short circuit, annoyances because of fretting on the insulation at contacts with other components.
• Cable fracture by vibration fatigue or electrical overload (e.g., short circuit).
• Failure of the shielding against electric or magnetic fields.

Optical cables (fiber optic, light cables):

• Fracture due to mechanical overload.
• Fracture because of humidity triggered stress corrosion (stress corrosion cracking = SCC). Particular dangerous is a bending or folding at the entrance into the connector.
• Problems with light guiding contact surfaces.

(„S“) Sensors, probes, measurement devices: Damaging operation conditions are mostly ‘sensor spezific’, that means its effect must be seen in connection with type and principle of the sensor.

Optical sensors: to those belong pyrometers, smoke and flame detectors in fire detector systems and for the monitoring of combustion chambers. Common problem is the unallowable impact of contamination or matting due to erosion of lenses and windows that influence the entering light. Further a damage or change of the light sensitive photo cell is possible.

Electric and magnetic sensors have as common problem, if existing, the failure of coils by mechanical overload (vibrations, thermal expansion) and temperature respectively damaging/aging of the isolation.

Sensors for measuring the flow and its rate: Here especially problematic is the blocking of measuring openings. It can be also about contaminations (e.g., insects) or icing.

Pressure sensor: To those belong especially those to measure the ambient pressure at the engine inlet. They can be blocked by ice, foreigen objects or mechanically damaged.

Systems with electrical coils: To those belong vibration pick-ups and tachometers for shafts. Those probes can, according to the place of installation, encounter extreme vibrations and temperatures. Thereby it must be reckoned with damages of the insulation (e.g., aging/ deterioration of resins). Results are short circuits and mechanical failures. The winding wires can oxidize or corrode. That weakens the cross section up to a failure. So it comes to a lingering change or blackout of the signal. Vibration wear (Fretting) at wires rubbing against each other can lead to decrease of the cross section and fracture.

(„B“) Attatchement of lines serve different purposes. They protect against the contact with other surfaces and wear damages. A further task is the avoidance of vibrations and static mechanic overload, as it occurs with thermal expansion. Used are suited lined clamps or brackets. It’s important that the designated type is attached to the right position. Does this not happen, there exists the danger of a damage of the line by operation loads and/or the attatchment itself. Loosened attatchments, or leaved by mistake, can also produce fretting and/or act as foreign object on other locations.

"Illustration 3.6.2-2": (Lit 3.6-5): Thermocouples (thermal elements) are used in gas turbines not only to measure hot gas temperatures. Further examples of use are

• oil and fuel temperatures,
• temperature at the engine intake,
• temperature in compressors.

There are several constructions to guarantee the reliability over long operation times. Generally in the hot parts region the metal combination nickel („Alumel“) / chromium nickel („Chromel“) is used. Its application temperature lies in the industry at maximal 1200°C. Thermocouples using PT/PtRh have the disadvantage that a catalytic effect with rest hydrocarbons in the hot gas, rises the temperature of the surface. That creates false measurement data.

Thermocouples can be damaged under operation influences in several ways:

A damaged thermocouple lets always expect a drop of the voltage. This can mean that a too low temperature will be shown. For elder engines the temperature may be raised by the control unit or by the operator and so overheating damage can occur over a long operation time. In the most cases the hot junction has visible. 15°C increased component temperature can bisect (see page 2.2-9) the creep life!

Empirical the most frequent problems occur at the junction (joint) of the sensor and at the adjacent links to the line wires. Already during the visual check, if necessary under the binocular, damages can be seen. Typical is cracking with discolorations (e.g., green) and pustule formation (tumor). In some cases the metal can be significant eroded. This is normally to be seen in connection with corrosion/oxidation. Then a repair is no more possible.

A failure mechanism is the diffusion of contaminants from the hot gas and/or a reaction (e.g., hot gas corrosion/sulfidation) with the wires of the element. For example an intensified oxidation can prefer special alloy elemments and so chance the composition of the wires. Also a diffusion of contaminants from insulating ceramic (Fe, Si) into the wires and the region of the junction between the couple materials is possible.

Fracture of the wires can be based on the decrease of the cross section (hot gas corrosion/ sulfidation "Ill. 3.4-2", oxidation, erosion), overload by foreign object damage and/or embrittlement. Diffuse embritteling elements like aluminium (rub in coatings in the compressor) or silicon (dust), cracks can occur. Also a foreign object or own object like coke from the combustion chamber (carbon impact, see also "Ill. 3.3-12") can trigger a fracture.

Deteriorated insulator: in elder engine types thin metallic bridges were observed in the insulator (Mg-oxid?)between the wires of the thermocouple. They showed deviations which affected the temperature measurement unacceptable. Possibly they formed during a faulty, much too high calibration or ‘healing’ temperature in the overhaul. Unfortunately this phenomenon was never satisfactory settled.

During long stand still (weeks) the insulation can reversible absorb condensation / air humidity. Than, by a leak current the voltage declines and the gas temperature seems to drop.

That can result in over heating during the start phase. In such a case normally the OEM prescribes a ‘healing’ of the thermocouples before a start. This can take place in a suitable oven under air at operation temperature.

Short-circuit in the thermocouple or between its wires. At the fracture also a thermocouple develops that produces a signal corresponding to the here acting temperature. Causes for short-circuits in the element itself are

• a broken ceramic insulator,
• metal in the shield tube and
• a cracked bonding.

In the wires itself it is to be about an unprotected, jammed area. The insulation can considerably degrade if the porous insolator is soaked with humidity.

Damage of the protective pipe: enables leakage the entrance of hot gas or molten metal (e.g., abrasion of a severe rub in) to the inner of the element, it is to reckon with pustle formation and unnormal discoloration.

"Illustration 3.6.2-3": (Lit 3.6-5): Against thermocouples, pyrometers have a big advantage. They can measure contact-free the temperature of rotating components like turbine blades. Thereby it becomes also possible to detect over temperatures that rely not on an increase of gas temperatures. This is for example the case when it comes to the blocking of cooling air channels in hot parts. But pyrometers have also weak points that can mean increased maintenance effort. To these belong:

(„1“) Contamination of the front lens (lens fouling) pretends the control unit a lower temperature niveau. That can have a strong influence on the life time of the hot parts. 15°C increase of the material temperature lead at the normal hot parts operation temperatures to bisection of the lifetime ( "Ill. 2.3-2"). Besides of the optical transparency of the lens the calibration of the pyrometer is changed. This shortens the required maintenance interval. Therefore a clean front lens of the pyrometer is an important maintenance task. So the maintenance intervals may not excessed in any case. The resons of the lens contamination are particles in the gas stream which originate from the combustion chamber and end up in the sight tube. To minimize this effect pyrometers are pressurized with cleaning air (purge air) from the high pressure compressor. It acts as sealing air against the hot gas and escapes from the sight tube into the gas stream. But this air can favor the contamination of the lens, just in contrast to the intended effect. This is the case when particles with sufficient high kinetic energy (velocity, size) breakthrough the flow of the purge air and hit the lense by the swirl before.

(„2“) Fractures of glass fibers in the light cable (frame below). If the light is transferred from the photo cell by a glass fiber bundle there is the danger of stress corrosion cracking (SCC) in the fibers. It was observed that apparently during the time in the stand still phases condensation can accumulate in the region of the fiber bundle. Are the glass fibers subject to a certain tensile stress niveau, they can break in humidity with a delayed crack growth. For example dangerous stresses can develop at the fitting of the glass fiber bundle behind the lense and/or before the photo cell. Also a too narrow bending radius of the glass fiber bundle, if it is exposed over a longer time to condensation and/or humid air, can trigger the cracking of fibers. That causes a slow drift of the measurement data to seemingly lower temperatures. Thereby the cosly exchange of the pyrometer system will be inevitable.

(„3“) The problem of the change of the emission behavior of the monitored area on the component is not attributed to the pyrometer itself. However oxidation, contamination, erosion or interactions with foreign objects can change the radiation spectrum and the drift off of the data. Also a falsification by glowing soot particles can not be ruled out.

(„4“) Haze of the lens by erosion was not referred until now, but is a probable damage. It attains actuality with the use of hard particles at the bladetips of turbines and compressors and ceramic thermal barrier coatings in combustion chamber and turbine. Those particles can have quite enough kinetic energy to reach the lens against the cleaning airstream at the lens. Even a very hard sapphire (alumina) lens could be affected by the erosion effect.