1. Ageing of Industrial Plant
(The Case for RBI)
Jonathan Lloyd B.Sc. Ph.D. M.I.M. C.Eng.
CEO, MPT-Matcor Pte Ltd, Singapore
Nick Laycock B.Sc. Ph.D.
MPT New Zealand
ABSTRACT
The high costs associated with construction of large capital
plant projects demands that these assets be effectively
managed. Reliability centred maintenance (RCM) and risk
based inspection (RBI) are have been developed to improve
long-term plant availability and reduce the frequency and
impact of failures. However, the key to enhanced reliability and
failure reduction is to build a comprehensive understanding of
the damage mechanisms that relate to individual components
of the plant in question. Optimum management of ageing
industrial plant assets beyond design life necessitates a risk-
based approach. The benefits of RBI and RCM vastly outweigh
their costs of implementation. Where operating conditions are
diverse and unpredictable the use of probabilistic techniques to
assess the likelihood of failure within a given period may
enable plant to be operated less conservatively.
Introduction
The costs associated with construction of large capital plant projects (such as
power stations, petroleum and petrochemical facilities) are immense. Tens or
hundreds of millions of dollars of precious investment capital are required. In
order to protect this investment and derive the optimum return it is essential that
these assets be effectively managed. Terms like reliability centred maintenance
(RCM) and risk based inspection (RBI) are bywords for methodologies that
have been developed to improve long-term plant availability and reduce the
frequency and impact of failures. However, the key to enhanced reliability and
failure reduction is to build a comprehensive understanding of the damage
mechanisms that relate to individual components of the plant in question.

2. The 3 Main Damage Mechanisms
There are three main damage mechanisms that cause capital equipment to
deteriorate over time in service these are:
1. Corrosion
2. Fatigue
3. Creep
Electrical control systems may also deteriorate with time. Some analogue
electronics may be relatively straightforward to repair by replacing faulty relays,
diodes and capacitors. However, systems with multi-layer printed circuit boards
may have to be replaced by new parts if available. If not, then major capital
expenditure may be required to correct the situation. Fortunately, newer digital
control systems based on standard system architectures promise to reduce the
costs associated with upgrading control systems.
For owners and operators of industrial plant the big issues will be how to
manage pressure vessels, pipework, and rotating equipment. Unlike electrical,
electronic and electromechanical spares for control systems, major boiler and
turbine components and heat exchangers are not “stock items”. The
manufacturing and delivery lead times are often many months, and so it is
essential that management of these assets is optimal. Hence for control
systems and instrumentation RCM methodologies are most appropriate,
whereas for pressure vessels and pipework RBI is appropriate.
Where Losses Occur
Data published in a recent publication [1]
revealed that pressurised equipment
has accounted for approximately 80% of large industrial property losses. Of
these, pipelines account for the largest fraction, followed by tanks, then
reactors, drums, heat exchangers, towers boilers & fired heaters, see Figure 1.
It is interesting to note that although boilers and various pressurised equipment
containing either steam or compressed air attract most attention from many
statutory bodies (e.g. MoM, WorkSafe/WorkCover, DOSH/JKKP etc.) that these
only account for a relatively small share of losses. This was not always the
case, and before the advent and implementation of today’s stringent design
codes and improved operating and maintenance practices, boilers in particular
accounted for a large number of catastrophic failures and casualties. However,
to improve matters further, there is a case for the regulatory authorities to
consider improving legislation to encourage the application of risk based
inspection (or RBI) type methodologies. The implementation of such an

3. approach has economic benefit as well as improving safety, since reliability and
availability improvements are a logical outcome of effective RBI.
What “Ages” Industrial Plant?
When considering a projected asset life it is essential to consider the factors
that will reduce the value and likely reliability of equipment over time. These can
be classified as follows: -
1. Obsolescence – newer designs may be more efficient making existing
plant uncompetitive even if it has been “written-off” by accelerated
depreciation (e.g. consider the “re-powering” of some Singaporean
steam power plants into new combined cycle units due to the 50% gain
in overall thermal efficiency this offers).
2. Corrosion – still the most significant damage mechanism by far.
3. Metallurgical damage mechanisms – long-term exposure to high
temperatures is likely to transform the microstructure of many carbon and
low-alloy steels. This may make such materials vulnerable to creep and
creep-fatigue damage.
4. Creep damage – where a component is exposed to high temperatures,
depending on the grade of material used for construction, it may suffer
from deformation under constant load, eventually leading to failure.
5. Cyclic (fatigue) damage – when a piece of plant is exposed to severe
load cycles due to an excessive number of start-stop cycles. Defects
(cracks) may initiate at stress concentrations or minor pre-existing
defects until sudden, and possibly catastrophic, failure occurs.
Corrosion
General corrosion is usually well understood, and appropriate methods of
inspection and monitoring have been established (e.g. within API 510 [2]
).
However, pitting corrosion and stress corrosion cracking are much more difficult
to predict both in terms of whether or not they will occur, and how fast they will
propagate. In large plant items over long periods of time, even nominally
general corrosion can produce a wide distribution of damage.

4. Metallurgical Damage Mechanisms
Microstructural degradation and the effect this has in reducing the long-term
creep strength of steels is well understood by metallurgists but the wider
engineering community often struggles to understand this phenomenon.
Creep Damage
Creep is the deformation of materials under constant load. It is easiest to
explain creep to the non-technical by describing it as a thermally activated
mechanism, people easily relate to the concept of things “softening” at high
temperatures.
Fatigue Damage
Fatigue can be simply explained as the initiation and growth of cracks from
stress concentrations (including pre-existing defects) under fluctuating load
conditions. Eventually the crack reaches a critical size and the component fails.
On a micro level fatigue is a highly complex process but on a macro level its
effect is easy to explain, as is the negative effect of excessive cycling (starting-
up and shutting down) of plants.
How to Manage Ageing Plant Effectively and Improve Reliability
and Availability?
Risk Based Inspection (RBI) and Reliability Centred Maintenance (RCM) have
become “buzzwords” in recent years but what do these terms really mean?
Simply put both these methodologies focus on quantifying risk and allocating
inspection and maintenance appropriately.
Risk is defined as the product of the likelihood and possible consequences of a
given event (failure) should it occur, see Figure 2. If there are minimal
consequences of a specific component failing it may even call for its necessity
as part of the process. If it is highly unlikely (or improbable) that a failure will
occur it may be possible to eliminate a specific inspection or maintenance task,
or at least reduce the frequency at which such tasks are performed. The
objective being the optimum balance between expenditure on
inspection/maintenance and safety/reliability, see Figure 3.
Unfortunately, the frequency and severity of failures often increase as the plant
ages. The concept of the “bathtub curve” describing frequency of failures
against the age of a plant is easily understood, see Figure 4. Early in the life of
a plant the management and owners will have a greater incentive to investigate
the cause of failures and commission professional failure analysis to identify
remedies that will eliminate such problems. However, as a plant ages, and the

5. capital cost has been written off or more efficient technologies and processes
create obsolescence, the incentive to spend money on such measures declines.
There may also be a perception that continued operation of old equipment is
somehow less safe than new equipment. In some countries high labour costs
may make rehabilitation or life extension of old plant uneconomic. For example
retrofitting of new instrumentation and control systems might cost as much or
more than a new (more efficient) plant.
Need to Assess the Life of a Plant to Optimise the Life of Plant
Assets
Unfortunately many plant owners are very reluctant to spend money on
assessing the life their plant until it’s too late. Effective management of large
capital assets like power plants, petroleum and petrochemical refineries,
requires detailed scientific analysis to identify which components are most at
risk of failure. Provided such an approach is applied in a disciplined and
rigorous manner, the plant may be reliably operated well beyond its design life.
A risk based approach to identify the critical components and what damage
mechanisms may limit their lives can allow costly failures to be pre-empted and
the plant operated well after the capital cost has been repaid.
Plant designs are often quite conservative, based upon average materials
property data plus a safety factor. So life extension is usually possible. In order
to plan for life extension, life assessment is essential, see Figure 5.
A Risk Based Approach to Life Assessment
Having established that life assessment is an essential component in an
effective asset management plant it is essential to consider how such
assessment should be implemented. It is too expensive to inspect the whole
plant. Hence, a risk based approach can be applied on a component-by-
component basis. We need to consider three primary factors:-
1. What material is it made from? (Carbon steel, low-alloy steel, stainless
etc.)
2. What process fluids is it exposed to in and out of service?
3. What temperatures and pressures/stresses is it exposed to?
Once these three key pieces of information have been gathered it will be
possible to determine the most likely damage mechanisms, and from there to
determine what inspection is necessary to detect damage, and the inspection
frequency. If damage is very unlikely then it may not be worth inspecting it very

6. thoroughly (if at all). However, if damage is highly likely, and the consequences
of failure extreme, then regular detailed inspection may be necessary.
The Three-Phased Approach to Risk Based Plant Life
Assessment
Ideally it is best to approach plant life assessment in three phases, see Figure
6:
Phase 1: Review the plant design and history and identify those areas at
risk, perform inverse design calculations on a worst case
(minimum materials property and design conditions basis). Define
an inspection workscope.
Phase 2: Perform the inspection workscope defined in Phase 1 and input
actual field data into calculations. The outcome facilitates definition
of future inspection plans and/or the necessity for Phase 3.
Phase 3: If Phase 2 indicates life is less than required, then more complex
analysis (e.g. sophisticated probabilistic calculations, stress
analysis and fracture mechanics) and removal of field samples for
laboratory testing may be considered necessary.
Over the life of a large power plant or refinery it may be necessary to assess the
remaining life of the primary assets several times over their life. This iterative
process underwrites safe and reliable operations, see Figure 7.
Benefits of RBI
The benefits of effective RBI are obvious:
• Ensures the safety of employees and the public.
• Assists in ensuring plant reliability.
• Optimises inspection resources.
• Assists in programming maintenance, repairs and modifications.
• Basis for extending inspection intervals.
• Provides information for life assessment studies.
• Input to failure analysis and performance assessment.

7. • Comply with standards and regulations.
For low risk items inspection is targeted at worst case locations, and the period
between inspections can be extended leading to reduced maintenance costs.
For higher risk items inspection is again targeted at worst case locations whilst
ensuring that inspection methods will: -
a) Detect the “expected” damage modes;
b) Produce sufficiently accurate data.
To calculate the expected remaining life it may be necessary to: -
a) Identify on-line monitoring methods (when appropriate), and
b) Identify damage control methods (where possible).
If effectively and successfully applied this will: -
a) Increase operational life;
b) Increase plant availability; and
c) Minimise risk of failure.
The Singapore authorities are now accepting this approach for statutory
equipment (i.e. pressurised equipment containing steam or compressed air).
This allows extension of inspection intervals from 2 to 4 years, or possibly more.
The benefits are enormous, and it is expected many tens of millions of dollars
worth of additional productive capacity will be released. In future it may be
mandatory on a plant wide basis. In Malaysia the authorities are considering a
similar approach.
Where and When Does the Worst Kind of Damage/Failure
Occur, and How to Manage the Problem?
RBI is based on identifying the likely damage mechanisms, and allocating
priority for appropriate inspection at suitable intervals. Ideally the inspection
interval should be set such that failure is highly unlikely to occur within a period
equivalent to twice the maximum inspection interval.

8. Consequences of a Failure
The actual consequences of a catastrophic failure are usually difficult to predict.
However, simple risk scoring systems have been developed that provide the
basic tools needed to derive approximately what the worst outcome might be.
For example the likely proximity of personnel and public in harms way, the
relative hazard of the contents of a pressure vessel, its temperature, pressure,
toxicity and quantity.
Likelihood of a Failure
Having considered the possible consequences it is necessary to consider the
likelihood of damage that might lead to a failure.
Heat Exchanger Tubes
General corrosion is something that is relatively easy to tolerate. Measures to
monitor control general corrosion are straightforward. We expect general
corrosion to occur everywhere, even if it is at such a low rate that it is barely
measurable. Pitting corrosion is much less predictable and more difficult to
monitor, and yet a single isolated pit may perforate a heat exchanger tube (a
failure).
Heat exchanger tubes are commonly inspected in-situ through NDT methods
such as ECT, IRIS or LOTIS, and it is usually desirable to inspect only a fraction
of the tube population (e.g. 10%) so as to minimise the cost and duration of
inspection. However, when pitting is found within the inspected tubes, it is then
necessary to estimate the extent of damage in the remaining (i.e. not-inspected)
tubes and decide whether or not the exchanger will fail before the next
scheduled shutdown. In these situations, extreme value (EV) statistical
techniques can be used to provide robust assessments of the present condition
of the entire tube bundle. For example, ASTM G46 describes a simple EV
method for estimating the maximum pit depth in such situations. However, this
method involves assumptions that are inherently conservative. Modern
computing power now makes it feasible to implement much more advanced EV
methods which yield more realistic estimates and allow for prudent
consideration of possible measurement errors [3,4]
.
High Temperature Components
Creep and creep-fatigue tend to occur when materials are exposed to high
temperatures. For steels the following rough guide can be applied:
• Carbon steels up to 400o
C creep is unlikely
• Low-alloy steels (e.g. 2.25%Cr1%Mo etc) good up to about 500o
C

9. • Stainless steels up to 600o
C or more
• Inconel etc. even better
In-situ metallography (i.e. replication) and hardness tests, together with
dimensional checks can effectively detect indications of long-term creep
damage. Creep-fatigue is likely where such components are exposed to a
combination of high temperatures and plant cycling. A example is the effect of
“two-shift” operation on high temperature (superheater and reheater) headers
and steam pipelines in power plants.
Hydrogen Damage and Metal Dusting
Hydrogen damage and metal dusting tend to occur where carbon and low alloy
steels are exposed to hydrogen service. Hydrogen damage may also occur in
boilers where water chemistry is poorly controlled. Sophisticated NDE (UT)
techniques may detect the latter. However, metal dusting is often detected too
late and the equipment may be written off.
Stress corrosion cracking (SCC)
SCC is to be expected where austenitic stainless steels are exposed to
chlorides (even in low concentrations), and can also affect carbon steels
exposed to extreme caustic conditions. Dye penetrant tests can detect cracking
where the surface is freely accessible, the best means of controlling the
problem is to prevent exposure or select alternative materials (if practicable).
Case Studies
1. Singapore Petrochemical Complex
The plant contains numerous steam heat exchangers. None of the
vessels is operated above the creep temperature, or in conditions where
significant corrosion was likely. Previously, regulations dictated costly
inspection every 2 years. Detailed risk assessment, supported by years
of corrosion data enabled the acceptance (by the MoM) of proposals to
double the inspection interval to 4 years or more. During the process the
possibility of stress corrosion cracking of stainless steel equipment
(under insulation) was highlighted, together with other corrosion under
insulation issues. This had not been previously considered. Appropriate
improvements in inspection procedures improved the confidence of the
authorities, thereby allowing granting of extensions for statutory
inspection intervals

10. 2. Power Plants
Life assessment of boilers, steam pipelines and turbines follows a risk-
based approach built on years of experience and sharing of basic
research by organisations like EPRI, ERA Technology and CEGB.
Where such knowledge has been applied correctly the improvements in
reliability have been dramatic. Thermal power plants in the USA, UK, and
Australia are amongst the most reliable anywhere. The frequency of
boiler tube failures (which can cost on average at least US$1 million
each in lost production opportunity for a 500MW unit, leaving aside the
wider national economic implications) are reduced to minimal levels.
Conclusions
1. The optimum management of ageing industrial plant assets
beyond their original design lives requires an ongoing risk-based
approach
2. .The benefits of RBI and RCM vastly outweigh their costs of
implementation.
3. The application of probabilistic techniques is a necessity where
operating conditions are diverse and unpredictable, e.g. pitting
corrosion in condensers, or creep in high-temperature heat
exchanger tubes.

11. References
1. “Large Property Losses in the Hydrocarbon-Chemical Industries, a
Thirty Year Review”, M&M Protection Consultants, 1992, 14th
Edition,
Marsh & McLennan.
2. “API 510 - Pressure Vessel Inspection Code: Maintenance
Inspection, Rating, Repair and Alteration”, American Petroleum
Institute, 1997.
3. “Condition Assessment and Life Prediction Methods for Heat
Exchangers”, D. Krouse and N.J. Laycock, Corrosion and Materials,
26, No. 2, S-1 (2001).
4. “Pitting of Carbon Steel Heat Exchanger Tubes in Industrial Cooling
Water Systems”, N. Laycock, S. Hodges, D. Krouse, D. Keen and P.
Laycock, in the web-based Journal of Corrosion Science and
Engineering, special issue containing proceedings of Corrosion
Science in the 21st
Century, UMIST, Manchester, UK, 6-11 July
(2003).

12. 0 5 10 15 20 25 30 35
Piping
Tanks
Reactors
Drums
Heat Exchangers
Towers
Heaters & boilers
Others
%
Figure 1 Equipment involved in large industrial property losses 1962 to 1992.
0 1 2 3 4 5
C onsequences Of Failure
0
1
2
3
4
5
ProbabilityOfFailure
High Risk
Low R isk
(Acceptable Risk ?)
Figure 2 Basis of a risk matrix.

13. costcost
levelofmaintenancelevelofmaintenance
operatingcostoperatingcost
maintenancecostmaintenancecost
O&MO&M
costcost
optimumoptimumundermaintainedundermaintained overmaintainedovermaintained
Figure 3 Obtaining the optimal balance between maintenance and operational cost.
Time
FrequencyofFailures
Commissioning
Phase
OldAge
Figure 4 The “bath-tub” curve describing expected failure of failures over the life
of a plant.

14. time (hours)time (hours)100,000100,000
design lifedesign life
absolute lifeabsolute life
life extensionlife extension
economic working lifeeconomic working life
remnant life assessment requiredremnant life assessment required
Figure 5 Necessity for plant life assessment occurs where the plant life is
extended beyond the design life.
Phase IPhase I --calculationalcalculational approachapproach
-operational and design data-operational and design data
-worst case material properties-worst case material properties
Phase IIPhase II - field inspection- field inspection
-input of condition assessment data-input of condition assessment data
PredictivePredictive
assessment ofassessment of
componentcomponent
integrityintegrity
IsIs
predicted lifepredicted life
greater than targetgreater than target
YESYES
Define optimum futureDefine optimum future
inspection scheduleinspection schedule
andand
life extension capacitylife extension capacity
NONO
Phase IIIPhase III - refined analysis- refined analysis
- material sampling/testing- material sampling/testing
- detailed surveillance- detailed surveillance
- complex stress analysis- complex stress analysis
Define optimum inspection andDefine optimum inspection and
refurbishmentstrategyrefurbishmentstrategy
Figure 6 The 3 Phase approach to Risk Based Plant Life Assessment

15. Identify Consequences
of Failure
Estimate Probability
of Failure
Calculate the Risk
Prepare RBI Plans:
•Plant-Wide
•Item by Item
InspectEstimate Remaining Life
Identify:
•Control Measures
•Monitoring Methods
•Remedial Actions
Implement
•Control Measures
•Monitoring Methods
•Remedial Actions
Figure 7 Simplified flowchart describing the risk based approach to plant life
assessment