Engine Systems Symposium 15-16-17 March 2011

Gas turbine usage monitoringStefano RIGOSIa

Cranfield University, Cranfield, Bedfordshire MK43 0AL, United Kingdom, s.rigosi@cranfield.ac.uk

Abstract

The growth of gas turbine application in recent decades has been mainly due to high levels of reliability and availability, both in shaft power generation and aircraft. However, this perception is incorrect, since component degradation and wear occurr inevitably. The engine operates in a combination of critical conditions and aggressive environments, leading to different mechanisms of failure. During the development of this technology, several modes of breakdown have been observed. In particular turbine blades are considered the main life-limiting parts of the engine and the most common mechanisms of damage are discussed and illustrated in this paper.

The health state is determined by a rigid control procedure carried out during periodic inspections. Several non-destructive testing techniques are applied in order to avoid an excessive degree of deterioration and ensure that cracks are detected at the earliest stage.

Condition monitoring is therefore considered an important asset in usage and maintenance of gas turbines. Moreover, an adequate maintenance management can be achieved by supporting preventive maintenance and on-condition monitoring, leading to cost savings and high reliability. A comparison between different strategies is also carried out and the benefits of considerable reduction in in-service failure and the cost associated are highlighted.

Keywords: Usage Monitoring; Non-Destructive Test; Preventive maintenance; Gas Turbine

Abbreviations

TET Turbine entry temperature

TMF Thermo-mechanical fatigue

HTHC High temperature hot corrosion

LTHC Low temperature hot corrosion

HCF High cycle fatigue

LCF Low cycle fatigue

FOD Foreign object damage

NDT Non-destructive test

LPI Liquid penetrant inspection

MPI Magnetic particles inspection

1. INTRODUCTION

The gas turbine’s industry main target is to maintain high efficiency and reliability during the whole cycle gas turbine life. Considering the critical operating conditions and the complexity of the engine it is widely accepted that a perfection system design cannot be achieved. Therefore, gas turbines design is the result of a compromise between design requirements, economic and manufacturing issues, reliability, and maintenance. In addition, gas turbines operate at a wide range of power setting and jet engines in particular are subject to rigid weight restrictions.

For these reasons maintenance is considered a mandatory, complex and on-going process in order to ensure the proper behaviour of the system. Great efforts have been made to avoid damage mechanisms, although in-service failure can sometimes occur during the engine life. In the case of power generation engine, site overhauls and downtime costs have been estimated to be higher than the initial cost of installation. Indeed, a sudden interruption of the process due to an unexpected failure may cause a relevant increase in energy production cost. In the case of aircraft, the improvements in flight safety, availability, and airworthiness are considered as key issues by all worldwide companies.

2. HYSTORY

In 1980s the gas turbine engine could achieve turbine entry temperature of 1300 K and provide power output of 120 MW with thermal efficiency of approximately 32%. Since this high value of temperature inside the cycle, the parameter that controlled the life of the blades was the hot corrosion. After the introduction of protective coatings the situation changed and creep became the dominant factor on the component life.

The development of gas turbine in 1990s enabled to double the size (about 250 MW) and TET raised up to 1600 K with a maximum efficiency around 38%. For these generations of gas turbine the main issues were faster heating and transient process in aircraft engines, in particular for military application. Hence, the thermo-mechanical fatigue led to failure of the rotor blades. This type of damage had as results the introduction of directional solidified and single-crystal structure in the manufacturing process of blades. In addition, the combination of critical conditions and aggressive environment, which contains corrosive elements, such as sodium, potassium, vanadium and lead, led gas turbine to be limited by corrosion processes. Even if the concentration of particles was few parts per million, deterioration and wear from continuous use caused very frequently failures in the hot section of the engine. The development of technologies to prevent hot corrosion focused on high

corrosion strength materials and protective surface coating. Moreover, the currently available technology involves the use of single-crystal superalloys, nickel-based superalloys from directionally solidified or obtained by oxide dispersion strengthened.

As Professor Legrani showed [14], the key issues driving the performance of gas turbine, in terms of fuel consumption, manufacturing cost and technology development, concern how a proportional increase in efficiency is directly related to a rise in turbine entry temperature. Hence, the latter affects the material improvement, thermal barrier coating development and all the cooling technologies in today's products. Therefore the importance of increasing temperature is emphasized, as shown in Figure 1, and its trend will asymptotically reach a stoichiometric limit (2850 K).

Figure 1 - TET trend and bucket material capacity [8]

3. FAILURES

Even if the gas turbine can be considered as high reliable engine, its components may suffer various mechanism of damage. The most common failures are caused by a combination of several breakdown modes. The figure below shows that failure modes are almost equally distributed and they might be expected to occur at the same time.

Turbine blades in particular operate at the edge of alloy resistance and they are usually affected by several mechanical damages, such as creep, thermal stress and

Figure 2 - Distribution of failure modes in jet engine [Fatigue and Fracture course note, February 2011]

fatigue. It is worth nothing that the rotor part of blade is particularly exposed to both thermal and mechanical stress cycle. This failure mechanism is often referred to as thermo-mechanical fatigue. Moreover, the turbine entry temperature has to be considered the major parameter which determines the life of these components and a prolonged overheating is the most common cause of engine breakdown, as shown in Figure 3. For this reason an adequate analysis of the health state of the engine is strictly mandatory and a general overview of the failure modes is described below.

Figure 3 - Overheated turbine blade [15]

3.1. Creep

As the engine operates at high temperature the strenght of components material is considerably reduced due to a costant tensile load undergoing time-dependet deformation. This failure, the so-called creep, is usually a life-limiting process for turbine blades and its mechanism is illustrated in a simple way in diagram (see Figure 4).

Figure 4 - The general creep curve [Mechanical Design of Turbomachinery course note, October 2010]

As the temperature is higher than 0.4 of melting point temperature, the phenomenon of creep becomes the dominant damage and it can be expected either for continuously exceeding of operating temperatures or for a failure in inspection procedure. Furthermore, the creep damage in turbine blades appears along the directionally solidified material, as observed in Figure 5.

Figure 5 - Creep observed during inspection [5]

3.2. Hot corrosion

Although the phenomenon of hot corrosion is well known since 1990s, a corrosive attack in various components is inevitable and many different parameters, including alloy composition, thermo-mechanical stress and atmospheric contaminants, will influence the surface composition both in compressor and turbine blades. This form of oxidation can consume the material with an unpredictable corrosive velocity, reducing the loading ability of the components and finally leading to a breakdown of the engine. Hot corrosion process can be classified in two main categories depending on the range of operating temperatures [3]:

High temperature hot corrosion = 1100 – 1200 K

Low temperature hot corrosion = 900 – 1100 K

It is well known that the hot corrosion must be detected at the incipient stage to avoid severe damage. As an example, a turbine blade failure was inspected and an advanced hot corrosion attack was observed through an optical micrograph (see Figure 6), with a secondary crack propagating from the scale into the base material.

Figure 6 – LTHC investigation with optical micrograph [6]

Military aircrafts are designed to operate in the most corrosive conditions and need to refuel anywhere. Thus fuel range available for this engine is considerably wide and usually with higher harmful ingredients. In order to prevent hot corrosion failures the turbine buckets are always protected with sophisticated coatings and the situation is monitored by frequently inspections.

Ground-based engine commonly operate in environments where the atmospheric contaminants are extremely aggressive. The turbine blade are exposed to strongly oxidation due to marine environments, pollution from industry and combustion products, leading to high corrosive attack by means of active elements, such as sulphur and sodium, as shown in Figure 7.

Figure 7 - Sulphuric corrosive attack [5]

3.3. High cycle and low cycle fatigue

The fatigue failure can be conveniently divided into two different categories: high cycle fatigue and low cycle fatigue. The former type occurs when a large number of strain or stress cycles are required to cause breakdown in gas turbine. HCF is usually detected and overcome at the design stage. LCF is related to low number of cycles leading to fatigue failure and it occurs any time the engine is accelerating or decelerating. It is well known that LCF is the most limiting factor on the whole engine life, since the

heavy rotating parts, such as shaft and disc, experience large changes in load and hence much higher stress cycle.

Nowadays the main efforts are concentrated in material development. The introduction of directional solidified alloy enables to increase the high-temperature resistance of components, by controlling the orientation of grains. A further improvement is offered by single-crystal alloy due to the presence of a single-crystal structure. Thus the associated grain boundary stress is reduced and the melting point temperature rises, compared to equiaxed or directionally solidified structures. A considerable benefit in LCF life can therefore be achieved using the new alloys (see Figure 8).

Figure 8 - LCF life in different bucket alloy [8]

An example of turbine blade failure due to HCF is illustrated in Figure 9. After a macroscopic inspection loss of material was found near the leading edge and a crack propagated from this region into the material. The breakdown of turbine blade may result from a low stress fatigue, such as vibrations.

3.4. Fretting fatigue

Turbomachinery engine are rotating at elevated velocity in presence of high relative surface motion between rotary and stationary parts, like blade root and disk, and hence they are more prone to fretting failure. Since the surface wear is induced by slight contact, loud noise and vibration can be produced, leading eventually to an unexpected breakdown.

A 32 MW ground-power engine was analysed as an example of fretting fatigue due to a severe failure

occurred after 70000 hours. When the engine was shut down the shaft vibration increased suddenly near the second natural frequency and it caused a high degree of damage at the last stages of compressor. As shown in Figure 10, the compressor blades were almost broken in stages 14-18 and they were twisted in the earlier stages. Moreover, the compressor disks were deformed in shape similar to a horseshoe as the result of failure.

Figure 10 – Fretting fatigue in industrial gas turbine [7]

3.5. Mechanical damage

The gas turbine engine ingests a huge amount of air and any material entered inside the engine with the air will affect the compressor life, limiting the integrity of its components. In some applications, high-strength alloy is introduced in the first row of compressor blade in order to reduce Foreign Object Damage. However, this system seems to be inefficient, since the ingested sand during landing or taking off is the most common mechanical damage. The deterioration of blade airfoil occurs by material erosion or abrasive debris deposit (Figure 11).

Analysing a high-bypass ratio turbofan engine the core stream tends to experience low FOD, since the fan throws out the foreign material, whereas the fan blade are exposed higher damage by abrasive wear.

Furthermore, in jet engine other ingested materials have to be taken into account at higher altitude, including ice particles, volcanic ashes and birds.

Figure 11 - FOD on the leading edge of compressor bade [5]

4. NON-DESTRUCTIVE TESTING TECHNIQUES

In the last few decades, numerous techniques have been developed both for maintenance and condition monitoring and they can be divided into two groups: non-destructive test, focused on undamaged component inspections, and destructive test, based on broken products. In particular, non-destructive testing is relevant in the inspection of gas turbine engine, in order to avoid excessive degree of deterioration before a maintenance activity. The most common types of NDT techniques are analysed below.

4.1. Visual inspection

The visual inspection was the original NDT and can be the first step of non-destructive examination, although this method should be neglected in an accurate evaluation of the engine reliability.

4.2. Liquid penetrant inspection

Liquid penetrant inspection is largely applied to detect surface defects, such as cracks, porosity and laps, and it is accepted as a low cost inspection. This method is usually performed through the following steps [10]:

Cleaning of the surface;

Application of coloured liquid penetrant;

Removing the excess of penetrant, after application;

Application of developer;

Inspection.

Figure 9 - Losses of material near the leading edge and a detail of propagating crack into the material [6]

There are several types of liquid penetrants available in the market and the cost of dye is proportional to its sensitivity. They are divided into different categories depending on whether they can be removed by water washing (water washable), by application of special solvent (solvent removable) or by using a special emulsifier (post-emulsification). As an example of LPI, Figure 12 illustrates a series of parallel cracks on the airfoil of a turbine blade, without any indication of severe coating deterioration.

Figure 12 - Blade crack revealed under ultraviolet light after LPI [2]

4.3. Magnetic particle inspection

In magnetic particle inspection, a magnetic field is applied in order to detect any defects on the surface of the ferromagnetic material. During the test, the particles are attracted by the magnetic field and the breaking crack shows up as a distortion of the magnetic flux (see Figure 13). This non-destructive testing provides a careful visualization of the crack and it is one of the most extensively applied electromagnetic methods in industry due to its low cost and instrumentation portability. The main drawback of MPI is that this method cannot be applied to non-magnetic materials, for example Aluminium. Besides, the magnetic particle inspection is strictly limited to the inspection of surface flaws and its accuracy is related to many factors, such as the magnetic properties of the materials or the orientation and the size of cracks.

Nowadays the magnetic particle inspection is often employed in gas turbine engine. However, it can be used only for small components, typically blade and disc, otherwise it would be very expensive.

Figure 13 - Magnetic field and magnetic particles in the vicinity of the crank [10]

4.4. Ultrasonic testing

The ultrasonic testing method uses the transmission of high frequency sound waves into or through inspected object. The process is described as follow:

Transmitting unit is placed in contact with the surface;

The receiving unit can be the same transducer or a different one,

The sound wave propagates inside the component and its reflection from the far surface is measured by the receiver (backwall echo);

If there is an internal flaw the signal is reflected back to the receiver, known as flaw echo.

Cracks can be accurately inspected by means of differences in time and amplitude between flaw echo and backwall echo (Figure 14). This method investigates the internal defects without damaging the components. Thus ultrasonic testing is largely applied in gas turbine inspection either in quality control or in preventive maintenance. In case of condition monitoring, the cracks can be detected before a catastrophic failure occurs and a scheduled maintenance is frequently employed.

Figure 14 - Ultrasonic pulse echo system [10]

4.5. X-Ray Diffraction analysis

The X-ray diffraction analysis is performed to detect the subsurface defects and to measure the degree of corrosion in the engine. The component is exposed to a radiation source, the unabsorbed rays go trough the thickness of material and they are impressed on the opposite side by a photographic film, as shown in Figure 15.

Figure 15 - X-ray inspection [10]

The radiography analysis can be performed in two different ways:

Ex situ, for a post oxidation detection;

In situ, during oxidation process.

The in situ inspection is often applied for the hot corrosion investigations in gas turbine, giving information about the oxide scale as a variation of grey level and measuring velocity of each oxidation process. The main advantage is the possibility to determine the behaviour of any corrosion process after a certain period of time. This method provides also visual information and it allows to generates a 3-D imagine of the internal structure. Thus it is largely used for flaws and defects inspection of turbine blade. The main drawback is that the production of radiation is time-consuming and costly, limiting applications and portability.

4.6. Vibration analysis

Gas turbine is a turbomachinery engine producing a large amount of vibration due to the high rotational speed. Any change from the health state is accompanied by an increase in noise and vibration and thus the engine can be monitored instantaneously by means of a meter. In addition, the variations of vibration can be analyses in terms of frequency and amplitude and the latter parameter is usually measured in three different ways (displacement, velocity and accelerator), depending on the magnitude of vibration. Furthermore this inspection allows to determine the origin of vibrations, such as unbalance, defective bearings or misalignment, and it enables to repair the components before a breakdown occurs.

4.7. Oil analysis

This inspection analyses the oil from the lubrication system of gas turbine. By monitoring the amount of wear per unit oil it is possible to determine the risk of engine failure. An increase in contaminants in the fluid causes a rapid degradation of all mechanical components, leading to a faster wear in the system.

The inspection consists in two parts. Firstly, the average amount and the size are measured by means of a particle counter and sizer. Secondly, a spectrochemical analysis is carried out in order to determine if the contaminants are dirt, wear material or particular additive and hence from which components the particles originate.

5. MAINTENANCE

The gas turbines are considered as a complex device where both a high degree of technology and a high level of reliability are achieved. The development of a more efficient engine always brought to an increase in pressure

and temperature. Since the operating conditions have become more and more critical, the maintenance cost during the lifetime may exceed the manufacturing cost (see Figure 16). It is therefore accepted that an adequate health monitoring is required to ensure that the engine has not only high level of reliability and long life but also it operates at maximum efficiency. In addition, engine monitoring is an important asset in usage, maintenance and overhaul of all the components, because their lifetime is dependent on a variety of operational factors that cannot be controlled entirely, such as ambient conditions, type of fuel and loading.

Figure 16 - Life cycle cost for industrial gas turbine [3]

5.1. Preventive maintenance

As Kinnison reported [12], the general meaning of maintenance is “the process of ensuring that a system continually performs its intended function at its designed-in level of reliability and safety”. This definition includes all the procedures and efforts, such as inspection, replacement and overhaul, carried out in order to ensure the proper behaviour of the system or equipment. However, the company can follow various strategies and techniques to determine when the overhaul should be performed and this will affect significantly the operating and maintenance costs. The maintenance types can be categorized in three different groups: corrective, preventive and predictive.

Corrective maintenance is essentially required when a component has already failed. It is worth nothing the high risk of damaging other components and the downtime in production related to the application of this technique. Preventive maintenance tends to evaluate the health state of the engine by performing periodically inspections and preventive actions. The main advantage of preventive maintenance is that any unsafe condition can be detected in time to prevent a failure. This technique is emerging as the most widespread maintenance technique with a large reduction in maintenance costs, as shown in histogram below.

According to Electric Power Research Institute [3], predictive maintenance may become more cost-effective than a traditional preventive maintenance. Even though the introduction of this strategy is particularly difficult, the benefits in terms of operation and maintenance costs are considerably high.

Figure 17 - Comparison between various maintenance techniques [3]

Preventive maintenance it is usually adopted for a health state monitoring. This strategy includes also scheduled maintenance, which consists in periodic inspections once the schedule has been set. In addition, recurring checks are carried out providing a continuous ‘not-exceed-limit maintenance’ in terms of time limits or detectable wear of engine parts. Thus, a reduction in traditional scheduled inspections is obtained as a result of a progressive introduction of very close monitoring, leading to high efficient and reliable maintenance. The total maintenance system is performed by means of composite condition monitoring systems, which are a combination of mechanical properties, corrosion monitoring and performance analysis. Finally, a considerable effort has been made in preventive maintenance optimization leading to a significant reduction in expected total cost. Although this strategy offers a considerable reduction in failure and its associated cost, any unnecessary inspection will result in higher maintenance cost. Since overhaul and replacement restore fully the engine reliability, a scheduled detection next to the repair has been considered unsuitable (see Figure 18). As an example, turbine blades of an aircraft engine usually require inspection before 2000 hours and overhaul before 5000 hours to avoid in-service failure. Moreover, for ground power and marine application a longer period before the first inspection may be expected [6].

5.2. Usage monitoring

The major benefit of a performance-based usage monitoring is recognized as diminishing maintenance cost in both in ground and aircraft engine. On the one hand the objective of industrial gas turbine is to operate with minor disruption of the power plant and avoid non-schedule maintenance due to unforeseen faults. The engine is always monitored in order to reduce excessive downtime and maintain high availability. On the other hand the aircrafts are performed to operate with safety and airworthiness in any circumstance and to avoid flame out and component failure during the flight. An effective monitoring can also be successful in extending life, increasing time between inspections and diagnosing problems.

A performance monitoring requires an improvement in instrumentations and sensors quality for the measurement of the main parameters, such as power or thrust, TET, rotational speed oil pressure and vibrations. Furthermore gas turbine should be routinely monitored and the real-time data gathered are used to a close assessment and evaluation of the mechanical properties and thermodynamic cycle. Additionally, engine monitoring requires an accurate investigation of component deterioration with non-destructive test techniques. An implementation of the data acquired is performed within advanced degradation models.

The maintenance program currently in use in gas turbine industry consists in three different techniques: hard time, on-condition and condition monitoring.

Firstly, Hard time process requires the component to be removed from the engine and overhauled, restored or discarded, depending on the degradation. This procedure is usually applied to those components that fail after a certain amount of time of operation and affect safety and reliability of engine without any possibility of maintenance check. As shown in Figure 19, engine intake, fuel filter and replacement of life-limited components are controlled by hard time.

Secondly, On-condition method requires the engine parts to be periodically inspected by means of an appropriate parameter (wear or degradation) in order to detect whether or not the component has to be restored. This process is restricted to system where the measure of reliability can be done without a downtime inspection. This process consists in a priori evaluation of the deterioration and a prediction of imminent failure by means of specific data acquisition (i.e. oil and fuel consumption) and a comparison with the

standard values of a clean engine. The on-condition process is employed by many aircraft company because it allows to achieve close to a maximum lifetime of each component. However, it requires ad hoc investigation of components and up-to-date knowledge of engine health state in real time.

Figure 19 - Typical maintenance schedule for an aircraft [15]

Finally the condition monitoring process consists in the monitoring of the failure rates of the individual component when neither lifetime limits nor detectable wear can be introduced to evaluate the life expectancy. This process is based on the detection of any engine deterioration at earliest stage. In particular, to ensure continue airworthiness in aircraft system, the data acquisition is related to in flight deck indicators and recorders and ground indicators.

To sum up, the total performance monitoring system is used in order to achieve the following targets:

Maintaining high availability;

Reducing maintenance costs;

Minimizing degradation of components;

Maintaining operation condition near design point;

Diagnosing problems, and avoiding catastrophic failure;

Extending time between inspections.

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Figure 18 – Inspections reschedule [1]

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