·Table of Contents
·Industrial Plants and Structures
Evaluating the condition & remaining life of older power plantsEyckmans Marc - Product Manager
Laire Charles- Product Manager
D'ambros Laurent - Engineer
LABORELEC - BELGIUM - Failure analysis & Material assessment in plants
As further explained in the paper the Laborelec Remaining Life Time (RLT) methodology; mainly based on corrosion, creep and thermal fatigue principals, is structured in a multi-level approach where the final scientific approach is a mix of design, operation & maintenance history in combination with quantified material characteristics.
The scientific approach results in a theoretical creep & thermal fatigue remaining life estimation extended with recommendations of future predictive inspection intervals or run/repair/replace decisions. These recommendations are determinated by the complementary Risk Based Inspection (RBI) & Reliability Engineering (RE) home developed methodologies. Some practical examples will show the benefit of this "three dimensional" approach.
The availability of a power plant depends on the availability of its non-redundant component. Assurance of a proper operation of these components, so called Key Components, should be, therefore, the main task of a plant Remaining Life Time (RLT) program. The condition of these components can be assessed only by way of a RLT methodology. Based on the RLT results a proper decision can be made as to plant safety and availability :
Taking into account the economic implications of these three alternatives as well as the economic, social and environmental implications of unscheduled outages due to sudden failures the selection of a proper RLT methodology gain in importance. This methodology should provide the best technical solution to ensure a safe operation of the plant.
Unexpected damage may be caused by exceeding thermal, static, and/or dynamic parameters which are used as the basis for calculations.
An important basis for the results is the analysis of the actual operational data (e.g. temperature, pressure, throughput), operational experience and operational tenders.
A systematic integral approach which allows assessment of the plants current operational capability and safety is only possible by correctly drawing a correlation between the operational load and the actual status of the plant or its components, which has been obtained from tests and inspections.
Based on these results, the right measures for future procedures can then be initiated in a reasonable manner.
The Laborelec RLT methodology is structured in a "Three Level Approach" with following sections :
|- superheater / reheater tubes||X||X|
|- superheater headers||X||X|
|- reheater headers,||X||X|
|- desuperheater nozzles||X||X|
|- steam lines||X||X|
|- feedwater lines||X||X|
5.1. NON DESTRUCTIVE TESTING
Non destructive examinations (NDE) are essential constituents of any residual life assessment programme. The objective of such assessment is to compare the current condition of the material of a given component with its original condition to define the amount of deterioration of the component.
Three major questions have to be answered before starting NDE :
The used inspection techniques depend on the given component, the location on the component, the damage modes to be looked at and the material used. Some of the regularly used methods to establish the material condition provide data which can be quantified in analyses whereas others can only indicate whether a defect is present or not. Following table correlates the examination methods with various components of fossil fired power stations.
In underlaying short overview we see the NDE-techniques used on the different boiler components.
|- economiser headers||X||X||X|
|- boilers drums||X||X||X|
|- lower waterwalls and headers||X|
|- junction headers||X||X||X|
|- waterwall risers||X||X||X|
|- waterwall headers||X|
|- superheater headers (welds)||X||X||X||X||X|
|- reheater headers (welds)||X||X||X||X||X|
|- desuperheaters :|
|- HT superheater tubing||X||X||X||X||X||X|
|- steam piping||X||X||X||X||X||X||X|
|- feedwater piping|| ||X||X|
5.2. Destructive testing
The scatter band of material properties (creep strength) is an important source of uncertainty for the calculation of the life expenditure. It may therefore be necessary to determine mechanical property values from specimens of material taken from the actual components. Sampling may however not in any way degrade the integrity of the component. Various sampling methods are used : boot samples, trepanned, core samples, through wall trepanning, ....
For some of them subsequent weld repair may be required. Destructive testing can have following objectives
5.3. Isostress creep testing
Quantitative residual life assessments are performed using the isostress testing technique. Acceleration is obtained by testing at enhanced temperatures, applying the representative service stress.
The decision to choose for increased temperature testing instead of increased stress, is based on the fact that creep is a thermally activated process. For ferritic materials however the maximum temperature is limited to about 720°C (by approaching the AC1 temperature). Test specimens are loaded under tension to a stress equal to the service stress of the component under investigation. Usually 5 to 6 specimens are exposed to mutually different temperatures, chosen such that creep rupture times are invoked in the range of 100 to 1000 hours. The secondary creeprate is deduced from the recorded strain-time evolution. Time to failure and total elongation after failures are determined also. Finally the test results are extrapolated in the co-ordinate system ln (rupture time) versus temperature (°C).
For each of these apporaches we use several home made soft tools like
Degradation mechanisms other than creep and fatigue are less accessible to useful life prediction by calculation
Static load creep degradation can only appear under certain conditions of stress & temperature which has to be superior of 450°C. For each period of time "ti", under a certain stress & temperature, the relative creep life consumption "e" is calculated as follows :
The time to rupture is calculated based on the creep rupture data curves for the different boiler materials. These curves (lower, mean & maximum material resistance) are specified are specified in following standards :
Static load creep life consumptions are calculated by the home made soft programme called "LILCA"; which also allows you eventually to estimate the mean metaltemperature in service.
7.2. High temperature tube life prediction with "LILCA"
To perform assessments and determine the remaining life of high temperature boiler tubing, it is necessary to accurately measure critical tube dimensions and predict the time dependent response of the tube material.
Laborelec measures wall and internal oxide scale thickness to predict remaining tube life with "LILCA" software. Thickness measurements are made using focused UT transducers. Steamside oxide layer thickness measurements allow the evaluation of the average metal temperature of components by using the material oxidation kinetics.
"LILCA", with the possibility for input of external wall loss due to erosion or high temperature corrosion, contains algorithms that allows the user to make deterministic estimates of tube remaining life.
7.3. Fatigue calculations with "Thermstress"
Fatigue only take place under alternating load conditions. During this load alternation 3 types of alternating stresses can be registrated :
There exist "Wöhler"-fatigue curves for these 3 types of stress and the different materials. These curves predict the total acceptable number of stress cycles. The relative life consumption "fi" for one cycle with a constant stress amplitude is calculated as follows :
7.1.E. Linear Damage Summation
Creep-fatigue analysis places an important role in the life assessment activity of power plant equipment. In the U.S.A. an approach to the creep-fatigue design and remaining life calculation was develop in the ASME Boiler and Pressure Vessel Code, Section III, Code Case N-47 which is essentially based on the linear damage summation method given by underlaying equation. This method combines the damage summation for creep and for fatigue as follows :
The damage summation method is very popular because it is easy to use and requires only standard S-N (Wöhler) and creep stress rupture curves.
One of the main damage mechanisms affecting power plant components is the creep damage. such damage may occur in different forms : localized or bulk damage.
Localized damage creep may become manifest in the form of cracks. The cracking process is characterized by a time-dependent growth under an approximately constant load. As in the case of fatigue cracking, creep cracking may be characterized by some fracture mechanics parameters.
Bulk damage it can manifest itself in two forms (fig1):
| Fig 1: (a) Ferrite - Pearlitic structure; |
(b) Carbides Precipitation at the grain boundaries
(c) Spheroidization of carbides from pearlite has begun
(d) Spheroidization of carbides from pearlite is finished.
(e) dispersed carbides (no ferritic - pearlite structure);
(f) carbide coalesence;
8.2. Scope of Application
|Fig 2: Description of the replication principle|
Life assessment techniques based on metallographic methods may be performed destructively by means of sample extraction, preparation and microscopy investigation. Alternatively, metallographic investigations may be done non-destructively by way of replication.
There are two major applications of the replication technique :
Surface oxides as well as decarburized zones must be removed prior to replication of the component surface.
8.3. Remaining Life Assessment and Recommended Inspection
They suggested also corresponding actions for each damage stage :
Because of the high conservatism included into this theory, it is actually used as a monitoring technique, rather than a life prediciton method. Nevertheless, worldwide acceptance with power plant operators because of its simplicity.
Laborelec also suggested a mixted method of assessment where creep degradation and aging proces are taken into account in combination with there repercussions on the component remaining life. Creep damage process is not the only one affecting power plant components operated at elevated temperatures and high pressures. There are many other damage mechanisms that result in an increase of the damage rate of these components and consequently in a decrease remaining life.
The author proposes a parameter to define a correlation between the following variables :
|Fig 4:||Fig 5:|
9.1. The Laborelec RBI methdology
9.2. Identification of probability and consequence
Laborelec uses a quick scan tool in first instance for the item probability ranking. This quick scan is based on a mix of design & proces parameters and operation & maintenance history. Fine tuning is afterwards possible by the implementation of a integrity factor.
The consequence factor is build of 4 subdiaries. These are the costs of security, environment, maintenance and productionloss due to stop time. Only the highest value is taken into account for the consequence ranking
9.3. Trend analysing & inspection intervall modelling
Trend analysing & inspection intervall modelling is only possible fro trending degradation mechanism like uniform corrosion, erosion, creep & fatigue. For each of these degradation mechanisms, the inspection intervall is defined as the remaining lifetime devide by a security factor. At this time Laborelec proceeds for a statistical approach for the erosion & corrosion degradation mechanism. For creep evaluation we prefer a deterministic approach such as a sudden death risk analysis method.
This RLT-methodology in combination with our RBI-methodology, a risk ranking approach also discussed in this paper, allows the client to identify the optimum inspection interval for all proces items that are sensible for trending degradation mechanisms. The proces equipment affected by non trending degradation mechanisms can only be optimised by changes in design, maintenance or operation conditions.
As a mix of trending & non trending degradation mechanisms will always be present in the entire proces, the author is of the opinion that the most effective manner to manage "trending & non-trending" is to implement a progressive but flexible inspection plan worked out by a team build of material- & NDT-specialists + maintenance & operation people of the plant. This plan must be continuously adjusted according to new information and evolving technological + economical factors.
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