Problems of Ageing Equipment Residual Life Assessment
Dr., Professor A.A. Dubov
Based on the analysis of existing approaches to the ageing equipment
residual life assessment formed in various branches of industry, general
problems were revealed that are caused by low effectiveness of conventional
methods and means of non-destructive testing and by imperfection of calibration
calculations of strength. It was shown that equipment and structures reliability
and life are determined by stress concentration zones (SCZ), being the main
sources of damaging development. Application of "passive" diagnostics methods
using radiant energy of structures (acoustic emission and metal magnetic memory
methods) is valid for SCZ detecting in proper time.
The problem of providing the reliable operation of equipment, vessels, gas
and oil pipelines and various structures becomes more and more relevant every
year as the equipment ageing in many branches of industry significantly
surpasses the rates of technical re-equipment. For instance, in power
engineering as of September, 2002, about 90% of thermoelectric power stations
equipment had exhausted its park life and the significant part of it had
achieved physical wear. The above-mentioned problem is aggravated by the lack of
scientifically grounded concept of technical diagnostics and life determination
and by insufficient effectiveness of conventional methods and means of metal
Based on the analysis of existing approaches to the ageing equipment residual
life assessment formed in various branches of industry, the following general
trends can be marked out.
Firstly, many specialists in the sphere of equipment
reliability pass from probabilistic methods of life assessment based on failure
statistics to assessment of individual life of the ageing equipment base on the
complex approach combining the results of destructive and non-destructive
testing with calibrating calculations of strength.
Secondly, at life assessment a tendency has been noticed of
shifting from crack detection to technical diagnostics methods based on
combination of fracture mechanics, physical metallurgy and NDT. Equipment and
structures stress-strained state NDT methods come to the forefront.
Thirdly, the necessity for a 100% examination of the ageing
equipment aimed at determination of potentially dangerous zones has been
At the same time the following drawbacks and defects existing at realization
of these approaches should be noted.
At a complex application of various methods and means of non-destructive and
destructive testing there is no strictly specified order and sequence of their
application for a specific test object.
As it is known, the order, the scope and the frequency of equipment
inspection is determined, on the one hand, by the park (design) life,
damageability, overhaul life and, on the other hand, by availability of
inspection methods and means and their capabilities.
Special instructions on the order and frequency of inspection and
prolongation of equipment life are available only in certain most critical
branches of industry (for example, nuclear-power engineering and heat-power
engineering) [1, 2, 3]. And even in these advanced fields (from the viewpoint of
arrangement of the equipment metal state control) there is a problem of metal
limiting state determination and the equipment individual life assessment
The suggested methods of strength calibration calculation can be
conditionally divided in four groups:
methods of calculation by metal corrosion rate;
metal crack resistance calculation methods;
metal fatigue calculation methods;
calculation methods for equipment units operating in conditions of
The main defect of well-known methods here is that they suggest a low level
of permissible stresses [σ]. As a rule, the level [σ]≤σ0,2/2, where σ0,2 - is the conditional metal yield strength.
There is a requirement in level calculation for critical structures
[σ]<0,3σ0,2. It is known that these
requirements are governed by the equipment metal work in conditions of glide and
by shear strain. As the practice shows, these conditions of metal work are
determining for the structure reliability. However, it is impossible to predict
in advance the zone of metal glide areas on the equipment using calculation
Besides, the existing strength calculation methods assume, as a rule,
independent flow of corrosion, fatigue and creep processes, though in practice
these processes flow simultaneously in various combinations.
The tendency of shifting from traditional crack detection to technical
diagnostics using the complex approach incorporating: defect parameters
determination, internal (residual) stresses distribution assessment,
determination of actual structural-mechanical characteristics of metal, is
restrained, first of all, by the low effectiveness of the current methods and
means of equipment stress-strained state control. For example, the paper 
states that at a current stage none of the tested means of stress determination
(about 10 various stress inspection instruments were tested) can provide
authentic data on the stress-strained state (SSS) of gas pipelines in real
The analysis of the known inspection means capabilities and stress
measurements in the base metal of equipment and structures weldments and welded
joints allow naming their major drawbacks. The basic drawbacks are:
impossibility to use most of methods in the plastic strain area;
control locality, their unsuitability for long structures inspection;
metal structure change is not considered;
inspection is carried out only on the surface of weldments, impossibility
to assess the depth layers of metal and welded joints metal;
the need to make graduated diagrams based on preliminarily prepared
the need for test surface and test objects preparation (dressing, active
magnetization, sensors adhesion, etc.);
complexity of testing sensors location determination related to the
direction of the action of main stresses and strains determining the structure
It was noted earlier that stress concentration zones were the main sources of
damages development. The metal structural-mechanical properties need to be first
of all investigated exactly in SCZ. The existing traditional methods of stresses
non-destructive testing (X-ray, ultrasonic inspection, Barkhausen noise and
others) do not allow solving this complex problem of SCZ determination on
equipment due to operational loads action.
Though the necessity for 100% equipment examination at life assessment is
realized, however, much time and large material and financial costs are required
for implementation of this task in practice. This task is not realized in
practice using conventional NDT methods (ultrasonic inspection, X-ray, magnetic
particle inspection). For instance, the length of pipe heating surfaces on a
modern 1000 t/h steam boiler makes more than 500km. Therefore it is practically
impossible to tap, clean and measure by ultrasonic inspection method such a
number of pipes, and none of electric power stations does this work. Similar
problems occur at inspection of gas and oil pipelines the length of which in
Russia reaches hundreds of thousands of kilometers, in petroleum and chemical
industries at inspection of a large park of vessels and pipelines as well as in
other branches of industry at inspection of ageing equipment and structures.
Let us consider further the capabilities of current (conventional) NDT
methods and means at solution of tasks occurring at equipment life
The existing conventional NDT methods and means (ultrasonic inspection,
magnetic particle inspection, X-ray) are known to aim at searching and detection
of a specific defect. Determination of the size of defects (occurrence depth,
length), located in the volume of the base metal or in the welded joint metal is
a complex practical task. However, if the size of the defect is determined
(modern crack detectors solve this task), it is necessary to determine the
extent of this defect danger and to answer the question: "Is this defect
developing or not?". To answer this question a calibration calculation of this
unit strength should be made taking into account the defect size. It is obvious
that such calculations are not carried out in general practice. Therefore the
existing norms on defects permissibility (revealed by ultrasonic inspection,
X-ray) for instance, in welded joints are mainly based on statistics and in most
instructions have conditional nature. There are no scientifically grounded norms
on defect size permissibility from the viewpoint of fracture mechanics in the
If capabilities of, for instance, magnetic particle inspection and
eddy-current control methods, aimed at surface cracks detection, are considered,
the following should be noted here. Despite the fact that the modern
instrumentation and testing technology using the indicated methods has been
significantly developed nowadays, there are till date no norms on surface
defects size permissibility for equipment in operation in many branches of
The existing norms and samples used, for example, in magnetic particle
inspection, were developed for new machine-building products. These norms are
nor suitable for equipment in operation for the following reasons: firstly, the
slag, the metal external layer corrosion do not allow applying the indicated
control means and methods without cleaning and removal of this layer, and
secondly, these norms from the viewpoint of fracture mechanics require special
grounding practically for every test object. Therefore for the critical
equipment in operation, for example, at thermoelectric power stations surface
cracks on most test units are not allowed and should be removed . Thus,
samples and norms specified in instructions for magnetic particle inspection and
eddy-current control methods are applied in general practice as a measure of
sensitivity of the instruments used.
The tasks of internal defects control in fillet, branch and T-joints, in
contact welded joints, in small-size joints (up to 6 mm), determination of
corrosion pits on pipeline internal surfaces are complex and not till date
solved by traditional crack detection methods.
Unsuitability of conventional NDT methods for defects detection at an early
stage of their development should be noted as well. More and more specialists
start realizing that "pre-defec" metal state is in many cases (especially on the
ageing equipment) more dangerous, when irreversible changes took place at a
structural level and the fatigue-assisted damage may occur all of a sudden and,
as a rule, in unexpected zones. The sensitivity level of conventional NDT
methods does not allow revealing the "pre-defect" state of a metal.
Methods and means of metal structural-mechanical properties NDT (measurement
of hardness, coercive force and of other magnetic characteristics of metal,
"replicas" taking for structural changes determination and other methods) are
widely used at equipment life assessment nowadays. Complex methods of metal’s
physical-mechanic properties NDT are developed and being applied in practice,
for example, plants for combined application of magnetographic method and
kinetic indenting method , the Moscow Power Institute’s instruments and
methods for materials testing by indentation or scratching for rapid evaluation
of mechanical properties  and others.
At present there are about 20 standards for non-destructive and partially
destructive sampling methods in Russia. All the available standards determine
the sampling mechanism, i.e. answer the question: "How to carry out sampling?".
This variety does not contain a single standard answering the question: "Where
to take a metal sample from?". Therefore at carrying our sampling on the
equipment after long operation to assess metal degradation specialists make
conclusion on the metal state only at the place of sampling. It is impossible to
extend the results of this conclusion on the entire metal of the test object
(and even of an individual element, for instance, the steam-water pipe bend).
Metal samples are taken, as a rule, from zones of the most probable development
of damages (or from zones where metal damages already existed).
It was noted earlier that SCZ, occurring at the stable dislocation slipbands
zones and caused by the action of operational loads, are the major sources of
equipment damages. According to the inspection experience, these zones on the
equipment metal surface show themselves in the form of lines with the size by
width and depth at the beginning of their development of not more than several
microns. The probability to hit these zones at metal sampling is very low. It is
obvious that such task can be solved only at a 100% metal examination on the
entire surface of the test object using highly sensitive methods. There were no
such methods enabling to solve this task till date.
In this connection it should be noted that if there is no opportunity to
determine SCZ and to carry out metal sampling, then, accordingly, the intention
to make strength calibration calculation for residual life assessment loses its
sense. Only in exclusive cases, when, for instance, the metal is affected by
corrosion with pipe wall (or vessel shell) thinning on a large area, it makes
sense to calculated strength taking into account wall thickness and corrosion
Thus, the presented brief analysis of existing methods of metal damages and
degradation NDT demonstrates their low effectiveness at industrial equipment
life assessment. The tendency of shifting from traditional crack detection to
technical diagnostics using principally different control methods and approaches
becomes clear and appropriate. More complicated tasks occurring at equipment
life assessment (as compared to conventional crack detection at normal
operation) require application of means and methods that are more difficult to
master but more effective at control of altering metal properties. First of all,
means and methods allowing practical control of equipment’s stress-strained
state should be assigned to such methods.
All leading diagnostic centers of the world are occupied nowadays by the
problem of mechanical stresses measurement in operating structures in order to
assess their state. However, no effective methods of stress control, suitable
for practical application, have been suggested till date.
Major drawbacks of traditional stresses and strains NDT methods were marked
Besides, traditional methods and means of stresses NDT based on active
interaction of the instrument signal with the structure metal obtain indirect
information on the test object’s stressed state, i.e. have insufficient
self-descriptiveness of physical fields used at control.
Indeed, the introduced into the investigated material field, interacting with
the material’s proper fields, alters its properties and the test object’s
stress-strained state characteristics. The alterations life nature, amount and
time are determined by the dynamic relationship of interacting fields’ energies.
In practice at carrying out diagnostics such alterations are simply
Thus, the above listed drawbacks of the known SSS control methods are caused
not only by metrological peculiarities, but also to a certain extent by these
methods’ physics, i.e. they are regular. Lack of metrological basis for
materials’ SSS characteristics measurement certification and calibration (there
are no unified standards and samples in Russia and abroad) leads to requirements
ambiguity and wrong methodological approach to the developed control means.
Paper  states that thermodynamic constitutive equation of solids must be
taken as a basis of equipment reliability theory and prediction. Basic physical
effects accompanying the metal fracture mechanism: mechanic, thermal,
ultrasonic, magnetic, electric and electromagnetic are determined. It implies
that using one or simultaneously several control parameters, reflecting the
listed effects, it is possible to most objectively assess the test object’s
It was stated earlier that the equipment metal work is mainly determined by
dislocations glide and shear strain. The metal fatigue damaging accumulation in
many cases occur in conditions of low- and multicycle operational load. The
question is, how the traditional methods of stress control can asses actual
structure SSS, when in the general case stress concentration zones due to shear
strain are unknown. It is obvious that only "passive" methods of SSS diagnostics
are able to answer the questions put and are the most suitable for practical
Passive NDT methods using radiant energy of structures, first of all,
These two methods are nowadays widely spread in practice for early
diagnostics of equipment and structures damages.
As it was demonstrated in practice, MMM, as compared to the AE method, gives
additionally the information on the test object’s actual stress-strained state,
which allows more objective determining of the reason for stress concentration
zones formation, being the source of damaging development. Besides, application
of MMM enables a 100% equipment examination with SCZ and defects detecting at an
early stage of their development. Having the complete information on the
revealed defects and on the possible influence of each of them on the equipment
residual life, one can easily solve the task of recovery work scope
determination necessary for improvement of units’ efficiency life to the
Paper  gives the method for metal limiting state and equipment life
determination using metal magnetic memory parameters.
1. GD 10-577-03. Standard instruction for metal control and
lifetime prolongation of boilers, turbines and pipelines main units at thermal
power stations. Moscow: ORGRES, 2003.
2. GD EO 0186-00. Method for Atomic Power Plant power unit
vessels' technical state and residual life assessment. Moscow: Concern
"Rosenergoatom", 1999, 75p.
3. GD EO 0185-00. Method for Atomic Power Plant power unit
pipelines’ technical state and residual life assessment. Moscow: Concern
"Rosenergoatom", 1999, 63p.
4. The concept of power objects technical reequipment at RAO
"Russian Unified Electric Power Systems" during the period till 2015. Document
of RAO "Russian Unified Electric Power Systems". Moscow, November, 2001.
5. Dubov А.А., Demin Е.А., Milyaev А.I., Steklov О.I. Gas
pipelines stress-strained state control // Gas industry, 2002, No.2,
6. Matunin V.М. Methods and means of rapid assessment of
structural materials’ mechanical properties without sampling. Moscow: Publishing
House of Moscow Power Institute, 2001.
7. Komarovsky А.А. Stress-strained state diagnostics //
Testing. Diagnostics, 2000, No.2, pp.22-27.
8. Dubov А.А. Means of metal limiting state and equipment
life determination using metal magnetic memory parameters. Proceedings of the
XVIth Russian Scientific-Technical Conference "Non-Destructive Testing and
Diagnostics". St.-Petersburg, September, 2002.