Predictive maintenance

In the context of this Thesis, predictive maintenance and traditional/conventional maintenance are discussed. However, a predictive maintenance culture adopted

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for on-time decision making assesses the health of a component in-service before system failure. The predictive maintenance minimises system life cycle losses and life cycle costs. This predictive maintenance strategy can save cost over a traditional maintenance. It provides appropriate scheduling for a traditional maintenance to prevent unpredicted system failures. The advantages of predictive maintenance include optimisation of spare parts management, enhanced system lifetime, guarantees safety and availability of the system. The predictive maintenance strategy reduces the risk of tragic events, remove unexpected outage and minimise the cost of gas turbine components (Di Maio et al, 2011). Figure 2-8 shows a categorisation and the relationship of predictive maintenance as diagnostics and prognostics.

Figure 2-8: A taxonomy of condition-based maintenance

2.6.1 Component degradation diagnostics

Diagnostics is a process for checking faults and the healthy state of sub-systems and units in an operating environmental condition with the aid of sensors. During maintenance, inspection is required to identify damage on components and provide information on the current performance status (Banjevic, 2009).

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Boyce (2006) analyses the turbine section of the gas turbine, which houses the first-stage vanes and the last-stage blades. In the turbine section, the effect of corrosion on the first-stage vanes of the gas turbine is a severe and preliminary inspection for cracks or bowings can be conducted. However, this Thesis focuses on the first-stage vanes. The first-stage of vanes are typically superficially inspected using a Borescope for gaining entrance into the turbine through the combustion chamber areas or by removing the inspection plates. Degradation mechanisms are also called fouling mechanisms, which affect the turbine section are described in Table 2-1.

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Table 2-1 Failure mechanisms in vanes (Boyce, 2006)

No Failure Mechanisms General Description First Stage Vanes

1 Nozzle vane bowing  Reduction in the passage area  High temperature

 Improper cooling

 High wheel space temperature

 The vanes can suffer from hot corrosion

 Thermal Barrier Coating (TBC) Spallation 2 Burnt nozzle vane Uneven combustion creates various hot

spots, which lead to melting of the vanes

 Trailing edge melted  Damage to vane platforms is

usually due to improper cooling

3 Incomplete

combustion or excess fuel

 During start-up, the fuel is not

combusted and collects in the stationary vanes, which acts as flame holders  Ensures that the control system has a

rate of acceleration and shutdown mode

Vanes totally melted

4 Hot corrosion Type I (over 1500o_F) 

An active form of oxidation, caused by the reaction of the Sodium (Na) in the air of fluid

 Sulphur, which is usually in the fluid and oxygen

 Intergranular attack  Sulphide particles,

 A denuded zone of base metal

 Damage to the leading edge  Erosion of the TBC attack on

the base coating

5 Hot corrosion Type II (between 1100o_{F to} 1450o_F)

 Caused by low melting eutectic compounds resulting from the contamination of Sodium sulphate and some of the alloy constituents such as nickel and cobalt

 Layered type of corrosion

Not applicable

6 Hot gas erosion- oxidation

 Caused by small solids in the air or the fuel

 With common combustor pattern  Excessive engine gas temperature

(EGT) pattern

 Failure of the TBC on the nozzle vane or platforms  Not even around

circumference 7 Blade tip rubs  Due to subtle tip clearance

 High metal temperatures in the blades

Not Applicable

8 Blade fretting erosion  Fretting in the dovetails/fir trees is caused by the rocking action of the blades

 Peaking turbines are highly susceptible to this problem

 Several attacks on trailing edge and leading edge  On the concave side of the

airfoil 9 Blade and wheel

rupture failure

 This failure occurs in high temperature and highly loaded blades (highly stressed) and disks

 Disk failure can be catastrophic  Caused by inadequate cooling due to

blockage cooling passages

Creep distortion usually at trailing edge

10 Foreign object damage (FOD) Domestic object damage (DOD)

 FOD occurs from materials coming from an external source to the gas turbine  DOD occurs from failure of internal

components

Most damage from this point forward

11 Low Cycle Fatigue (LCF)

 Turbine disks

 First stage turbine suffering from low steady state stress due to thermos- mechanical fatigue problem  Peaking turbines more susceptible

 Cracks in the Vanes  Single vane segments suffer

less than multiple vane segments

12 High Cycle Fatigue (HCF)

 Can Occur in any blades or vanes due to the blade resonance frequency being excited

 Occurs in blades where there are no tip or mid-span shrouds

 Not applicable to most designs

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2.6.2 Component degradation prognostics

Prognostics defined as "analysis of the symptoms of faults to predict future condition and residual life within design parameters" (International Standard Organisation, 2015). Medjaher et al (2012) argue that Prognostics is the estimated-time-to-failure (ETTF) based on the risk of existence or subsequent appearance of one or more failure modes. Prognostics estimate the time after which a component can no longer perform its intended or expected functionality to improve system safety. Physics-based and data-driven approaches can support this process. The data-driven approach engages a collection of maintenance and monitoring data to deduce failure modes, while physics-based model utilises mathematical models to estimate lifespan of components (Chen et

al, 2012; Daigle and Goebel, 2010). The next section discusses visualisation of

events.