Characterizing The Degradation Mechanism Of Rotor Blade In Gas Turbine: A Critical Appraisal

Section 1: Brief overview of the case study (in your own words) and your critique (i.e. key points that you agree with and key points that you think need improvement or elaboration)

Section 2: Detailed appraisal: i.e. critically appraise and evaluate

1.Appropriateness and sufficiency of methods (methodology)

2.Quality and sufficiency of evidence

3.Soundness of conclusions and judgement

You may justify your critique through one or more of following,
Review of similar case studies from literature,

Personal analysis

Expert opinions

In each case, do not just criticise, suggest alternates. For example, if you feel that the methodology was not appropriate, not sufficient or in need of improvement, then discuss which alternate or additional method should be used. This recommendation needs to be supported with some evidence and justification.

Section 3: Legislation and regulations

Discuss that which legislation, regulations and/or technical standards will apply to the selected problem. What will be the consequences legal and/or ethical of not following the recommendations? This discussion should be based on appropriate literature review of relevant legislation, regulations and standards. It may also discuss this in context of the consequences of a past documented experience of failure in similar situation.

Overview of Case Study

The given case study deals with characterising the degradation mechanism of rotor blade in gas turbine. The given analysis describes the failure analysis of gas turbine, the capacity of this gas turbine is 6.5 MW, which in operation around 6500 hours and failed occurred in air foils. The examination carries out was fractography, microstructural characterisation, the equipment’s used in this analysis is Scanning electron microscope, and energy dispersive X-ray (EDX)

As we know that blades of the rotors are one of the important components in gas turbine. The degradation is due to the reason that, gas stream which is of high temperature passes through the blades and continuously distortion is being occurred in root of blade as well as other part like main bearing, rotor, stator etc. After degradation mechanism starts in any kind of gas turbine, the countdown of service life. The studies about failure of turbines shows that, the most common failure in gas turbine is due to creep, creep, damage, fatigue, corrosion and erosion attack. Especially hot cracking is due to creep and fatigue mechanism. Most common failure of component is due to reduced fatigue strength which caused due to creep damage. The failure of gas turbine resulted in shutdown of power plant. In last few decades, the material used in any industries has been improved significantly, the material used in rotor blade also considered for improvement, The currently used material is nickel and cobalt based superalloy, which can be able to withstand high stress and temperature (650 – 1100 ^o C), with paint and coating on it, it can withstand even more temperature and strength. At the same time, it has high resistance to corrosion and oxidising environment. This material is also known as supper alloy IN738LC.

In general, the chemical composition found in Nickel based supper alloy IN738LC is tabulated below.

Table 1- Chemical composition of rotor blade material, Source - (Jiri Zyka, 2014)

The composition shown in table above is a standard composition analysis in various journals and specially by (Jiri Zyka, 2014). Now we will look into the microstructure study given in failure blade analysis, to analyse the chemical components in the structure, the test pieces were prepared from aerofoil and root of the blade, first it was ground on different grit paper, polished with diamond paste, followed by etching and studies under the optical microscope and scan electron microscope, at the end EDX probe were used to analyse the local chemistry of the carbides present in test samples, The distribution of carbides and their microstructure is presented below.

The further analysis about micro structure will be discussed in the section-2.

The structural analysis of the super alloy indicates that, in general IN738LC has multiphase microstructure, the strength to overcome the temperature is gaining through γ’ intermetallic compound phase (Ni₃Al), which is nothing, but FCC solid solution based on nickel. The formation of multiphase carbide takes place inside the grain, which is surrounded by M₂₃C₆ grain boundaries. There is possibility of some element present in metallic carbide such as W, Ta, and small amount of Cr and Ni. It all precipitates during cooling of alloys. The role played by M₂₃C₆ on grain boundary performs an important role in withstanding the temperature of about (760-980 ^o C). This happen due to highly saturated carbon present in the matrix of MC carbide. The equation suggests that, in M₂₃C₆, the M can be Cr, W and Mo. The structure shown in the film signifies that, there is discontinuity in the film of grain boundaries. The interior side of metallic carbide grain is pinned by M₂₃C₆ along the boundaries, which try to stop the sliding of boundaries, this helps to improve the creep resistance. The rate of propagation for crack is also delayed by presence of M₂₃C₆. But due to operation performed in high temperature can cause to change in distribution system of Metallic carbide and M₂₃C₆.

Appraisal of Methodology and Evidence

The figure above clearly shows that the effect on distribution system before and after operation. The similar affect can also be seen in pinning of the grain boundaries, which are interlinked to each other. From structural point of view, the change in microstructure affects the tensile and creep properties of the material. The structure distribution abnormality is due to high temperature, which shows precipitated phase of γ’ in irregular structure weak the material as compared to new blade. The figure given below is comparison of pinning affect shown in blade microstructure which is before and after.

The microstructural study taken in from samples of aerofoil also dictates the change in distribution of microstructural phase. There is one significant observation in aerofoil is that, there is change in size of γ’ phases in sample. This was observed with the help of scan electron microscope. The size of γ’ present in aerofoil is much smaller than γ’ present in root. The gradual decomposition of metallic carbide is also seen in samples. The chemical composition present in γ’ of aerofoil consists of Ti, Ta, and Nb carbide, which is determined as metallic carbide.

The fractography analysis reveals that there are two zones in fracture surface, one of these zones show dendrites which grown from edge to centre and another one clearly shows that, it is locally melted and solidified after temperature getting down. Near the failure region, there are many cracks generated and propagated through grain boundaries. The optical analysis of aerofoil shows that, there is occurrence of dissolving the γ’ in the matrix during high temperature and again redeposited during shutdown of turbine. The dissolving activity of γ’ occurs above 1160 ^o C.  The local melting also indicates that, the temperature should be more than 1000 ^o C for several times. The conclusion also depicts the same thing as above discussed.

As the case study reviewed above, initially it was declared that, this case study is about characterising the degradation mechanism of rotor blade in gas turbine. With the help of fractography, microstructural characterisation using optical microscope, scan electron microscope and X-ray diffraction techniques. The most important point is that writer is stick to their guideline declared initially, and provided the very specific result, which is related to metallurgical point of view. The study of micrograph shows that, the reason given for failure of gas turbine with high accuracy of visual analysis of the microstructure of nickel based super alloy. There is little more interest they have shown on chemical composition of the blade what they found in fractography and microstructure study.

My suggestion in this regard is that, this chemical composition should align with standard chemical composition generally found in material of rotor blade. The standard chemical composition given by (Jiri Zyka, 2014), (Lakshay, Vineet and Krunal, 2018), (Ali, Seyed and Seyed, 2013), clearly dictates the standard chemical composition found in rotor of the blades. A comparative study is necessary for chemical composition found in failure blades. If we compare the chemical composition from literature given above stated journal, it can be summarised that, there is lack of at least three elements, and these elements are Cr, Nb, and nickel it. In all three elements, there is slight deviation in presence of Chromium (Cr), and Nickel (Ni), but the presence of Niobium is nil. The presence of niobium plays a very important role in super alloy material, due to its high melting point (2468 ^o C) and light weight as compared to titanium and nickel, provide great property of withstanding high temperature and lower the overall weight of material, which further support in minimising centrifugal force acting upon the rotor blade. The absence of this element may be one of the causes of getting high temperature by the blade (MC, M and G, 2016).

Discussion of Legislation and Regulations

From the review of the journal, there is significant cause of failure is high temperature absorbed by the rotor blade and its component, in other word we can say that heat generation in rotor blade is the main culprit behind the failure of Gas turbine. The further investigation should be done related with gas turbine auxiliary system, the bleed system is primarily used for cooling the internal part like, rotor blade in all stages, the good function of bleed system (cooling), always reduces the risk of temperature getting high in critical components of gas turbine. Theses cooling system equipped with temperature sensors, which records temperature throughout lifetime of gas turbine. In a Gas turbine like 9HA.01/.02, which is quite heavy, has at least, twelve different stages of cooling and bleed system, each of them equipped with temperature sensors, records the temperature with very high automation system. The further analysis of these records answers some question of high rise in temperature. The quality of air supply in bleed system, auxiliary cooling system, can be checked from logged data during operation. The inadequate air supply, malfunctioning of cooling system, and improper exhaust can lead to rise in temperature (Kazempour, S and Akbari, 2011).

There is various scope of analysis can be done for finding the root cause analysis of failure of turbine blade. It may be metallurgical analysis, which is described above. The other scope is mechanical analysis, in this approach, focuses mainly on structural and thermal analysis of turbine blade using finite element analysis method. The software can be used for finite element analysis is ANSYS with the aid of Catia or Solid works. One of the analyses described here which is adopted by Kumar and Rao, ( (V, IN and N, 2014). It describes that 3D modelling of rotor blade and simulating the operation as the original one, will provide the data in two stages. The results of this two-stage analysis are combined with the help of union Boolean operation. The results obtained from Boolean operation simulation dictates that, there are mechanical stresses, with elongation experienced by Gas turbine rotor blades. The stress generated on blades not only due to centrifugal forces, on blades, but also stress due to gas forces applicable on it (S et al., 2013).

The results obtained from finite element analysis show that, the deformation produce in the blade is due to the forces acting on it. The stress due to gas and centrifugal forces, and temperature is also taken into consideration in simulation.

The results of the FEA analysis also indicates that, there is deformation in length of the blade which is maximum the tip of the blade (0.01237 m) and minimum at the root of the blade which is. From both review, one thing is clear that that, there is not rubbing action taken place at the tip of the blade with casing of the gas turbine. From the manual available on GE power, the deformation due to forces is under acceptable limit. The maximum stress generated during simulation is about 1.958 G Pa, and this occurs at the root section also. From the above figure, we can see the temperature distribution at the tip and root of the blade. The maximum temperature observed during simulation is 1246.8 ^o C. The temperature observed in the simulation is par above the specified temperature range of acceptable limit, which is (650 – 1100 ^o C), but not outside the melting temperature of the blade. This suggests that there should be combined effect of temperature as well as induced stress, results in failure of the blade (V, IN and N, 2014).

The numerical approach to finding the root cause also one the important approach. There are various literatures available which describes the numerical approach for rotor blade failure, one them is given by (Ali, Seyed and Seyed, 2013). This mainly deals about numerical approach related with angle of blade mounted on root. The performance of nay gas turbines is directly proportional with angle given to blade with base. Generally, the acceptable limit of angle for gas turbines varies between 14^o to 19 ^o; the angle given in GE power gas turbine is about 15.5^o. The numerical approach for 14^o angle was simulated at 3000 rpm speed. The stress generated due to rotation on blades area found within acceptable limit, and yet temperature is getting higher. But when the angle of blade is changed, the and again simulated for same rpm, it was found that temperature is rising. But in our case GE power gas turbine angle is not a problem, because it is less than 19^o. In both cases, the vibration making and important cause to giving stress on blade, the modelling and harmonic analysis indicates that the natural frequency is too close to the operation frequency, which is 50 Hz. It clearly means that any change in blade installation cause the natural frequency to be like the operational frequency and consequently causes resonance phenomena. The periodic force applied on blades that, sometime exceeds the endurance limit of material, will reduce the life of the blade and lead to failure after sometimes (bishoy, 2018).

The guideline provided by Department of environment, food and Rural affairs (DEFRA), suggest that, Emissions from gas turbines and combustion should in normal operation be free from visible smoke. The smoke generation during start up and shutdown should be equivalent to Ring Lemann Shade 1 as described in British standard BS 2742: 2009.In case of odour, it is suggested that, the emission is free from offensive odour from outside the site boundaries.

As discussed in reviews about recording the data, the legislative requirement suggests that, continuous monitoring should be done. Either it is quantitative or qualitative. The discharge of emission should be measure in quantitative measure and it should in milligrams per cubic meter. The acceptable limit of emission is 600 mg/m³ liquid fuels gas turbine. The variation or reduction in monitoring frequency should not be permitted, where continuous quantitative or indicative minoring is necessary. All operative time data should be kept as an evidence for further analysing the problem. The place where non-continuous monitoring is required, the monitoring variation can be done to some extent. The results which are ranges from 15 -45 mg/m³ might not qualify for reduction in monitoring.

The calibration of gas turbine is necessary using continuous monitor, or specific extractive test carried-out at definite frequency agreed with the regulator. The extractive test should be carried out at least 70% of operated efficiency of gas turbine.

The health and safety of operator of the process is prime concern for the environmentalist as well as management. The requirement of authorisation permit should be approved before operation and maintenance. The permit of authorisation must contain safety of operator as well as people around that machinery; this is the prime responsibility of health and safety enforcing authority.

The new guiltiness which is going to be enforced, is suggested by gas turbine association consists of 1200 lbs CO₂ / MWh emission standard. From now onward, simple cycle gas turbine is excluded and sliding capacity scale-based gas turbine efficiency is adopted. The start-up, shutdown sampling should be done at 50% operating efficiency. The averaging in compliance is excluded and new peak and low value data is adopted for sampling. The cut-off pint for capacity for gas turbine will be 150MW, the lower that this capacity emits more smoke and damage environment. The rules and regulation should be aligning to encourage the adaptation of combined cycle gas operation. The method of computation of average should be simplified to 12 month rolling average. Prohibition of NSPS CO₂ limit for simple cycle turbine and Co₂ BACT level should be adopted. There should be option for utilising new NSPS subpart TTTT versus including language subparts Da or KKKK. In order to adjust within PM_2.5 challenge the law is being modified to accept the emission limit as low as 10-ton per year for PM_2.5 and 15-ton per year for PM₁₀.

Based on the literature suggested, the multidimensional approach for finding root cause is best approach suitable for Gas turbine. Because doing multidimensional analysis we found the results in different perspective.

References

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