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 capitalplant projects demands that these assets be effectivelymanaged. Reliability centred maintenance (RCM) and riskbased inspection (RBI) are have been developed to improvelong-term plant availability and reduce the frequency andimpact of failures. However, the key to enhanced reliability andfailure reduction is to build a comprehensive understanding ofthe damage mechanisms that relate to individual componentsof the plant in question. Optimum management of ageingindustrial plant assets beyond design life necessitates a risk-based approach. The benefits of RBI and RCM vastly outweightheir costs of implementation. Where operating conditions arediverse and unpredictable the use of probabilistic techniques toassess the likelihood of failure within a given period mayenable plant to be operated less conservatively.

Introduction

The costs associated with construction of large capital plant projects (such aspower stations, petroleum and petrochemical facilities) are immense. Tens orhundreds of millions of dollars of precious investment capital are required. Inorder to protect this investment and derive the optimum return it is essential thatthese assets be effectively managed. Terms like reliability centred maintenance(RCM) and risk based inspection (RBI) are bywords for methodologies thathave been developed to improve long-term plant availability and reduce thefrequency and impact of failures. However, the key to enhanced reliability andfailure reduction is to build a comprehensive understanding of the damagemechanisms that relate to individual components of the plant in question.

The 3 Main Damage Mechanisms

There are three main damage mechanisms that cause capital equipment todeteriorate over time in service these are:

1. Corrosion

2. Fatigue

3. Creep

Electrical control systems may also deteriorate with time. Some analogueelectronics may be relatively straightforward to repair by replacing faulty relays,diodes and capacitors. However, systems with multi-layer printed circuit boardsmay have to be replaced by new parts if available. If not, then major capitalexpenditure may be required to correct the situation. Fortunately, newer digitalcontrol systems based on standard system architectures promise to reduce thecosts associated with upgrading control systems.

For owners and operators of industrial plant the big issues will be how tomanage pressure vessels, pipework, and rotating equipment. Unlike electrical,electronic and electromechanical spares for control systems, major boiler andturbine components and heat exchangers are not “stock items”. Themanufacturing and delivery lead times are often many months, and so it isessential that management of these assets is optimal. Hence for controlsystems 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 equipmenthas accounted for approximately 80% of large industrial property losses. Ofthese, pipelines account for the largest fraction, followed by tanks, thenreactors, drums, heat exchangers, towers boilers & fired heaters, see Figure 1.

It is interesting to note that although boilers and various pressurised equipmentcontaining either steam or compressed air attract most attention from manystatutory bodies (e.g. MoM, WorkSafe/WorkCover, DOSH/JKKP etc.) that theseonly account for a relatively small share of losses. This was not always thecase, and before the advent and implementation of today’s stringent designcodes and improved operating and maintenance practices, boilers in particularaccounted for a large number of catastrophic failures and casualties. However,to improve matters further, there is a case for the regulatory authorities toconsider improving legislation to encourage the application of risk basedinspection (or RBI) type methodologies. The implementation of such an

approach has economic benefit as well as improving safety, since reliability andavailability improvements are a logical outcome of effective RBI.

What “Ages” Industrial Plant?

When considering a projected asset life it is essential to consider the factorsthat will reduce the value and likely reliability of equipment over time. These canbe classified as follows: -

1. Obsolescence – newer designs may be more efficient making existingplant uncompetitive even if it has been “written-off” by accelerateddepreciation (e.g. consider the “re-powering” of some Singaporeansteam power plants into new combined cycle units due to the 50% gainin overall thermal efficiency this offers).

2. Corrosion – still the most significant damage mechanism by far.

3. Metallurgical damage mechanisms – long-term exposure to hightemperatures is likely to transform the microstructure of many carbon andlow-alloy steels. This may make such materials vulnerable to creep andcreep-fatigue damage.

4. Creep damage – where a component is exposed to high temperatures,depending on the grade of material used for construction, it may sufferfrom deformation under constant load, eventually leading to failure.

5. Cyclic (fatigue) damage – when a piece of plant is exposed to severeload cycles due to an excessive number of start-stop cycles. Defects(cracks) may initiate at stress concentrations or minor pre-existingdefects until sudden, and possibly catastrophic, failure occurs.

Corrosion

General corrosion is usually well understood, and appropriate methods ofinspection and monitoring have been established (e.g. within API 510 [2]).However, pitting corrosion and stress corrosion cracking are much more difficultto predict both in terms of whether or not they will occur, and how fast they willpropagate. In large plant items over long periods of time, even nominallygeneral corrosion can produce a wide distribution of damage.

Metallurgical Damage Mechanisms

Microstructural degradation and the effect this has in reducing the long-termcreep strength of steels is well understood by metallurgists but the widerengineering community often struggles to understand this phenomenon.

Creep Damage

Creep is the deformation of materials under constant load. It is easiest toexplain creep to the non-technical by describing it as a thermally activatedmechanism, people easily relate to the concept of things “softening” at hightemperatures.

Fatigue Damage

Fatigue can be simply explained as the initiation and growth of cracks fromstress concentrations (including pre-existing defects) under fluctuating loadconditions. 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 itseffect 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 Reliabilityand Availability?

Risk Based Inspection (RBI) and Reliability Centred Maintenance (RCM) havebecome “buzzwords” in recent years but what do these terms really mean?Simply put both these methodologies focus on quantifying risk and allocatinginspection and maintenance appropriately.

Risk is defined as the product of the likelihood and possible consequences of agiven event (failure) should it occur, see Figure 2. If there are minimalconsequences of a specific component failing it may even call for its necessityas part of the process. If it is highly unlikely (or improbable) that a failure willoccur it may be possible to eliminate a specific inspection or maintenance task,or at least reduce the frequency at which such tasks are performed. Theobjective being the optimum balance between expenditure oninspection/maintenance and safety/reliability, see Figure 3.

Unfortunately, the frequency and severity of failures often increase as the plantages. The concept of the “bathtub curve” describing frequency of failuresagainst the age of a plant is easily understood, see Figure 4. Early in the life ofa plant the management and owners will have a greater incentive to investigatethe cause of failures and commission professional failure analysis to identifyremedies that will eliminate such problems. However, as a plant ages, and the

capital cost has been written off or more efficient technologies and processescreate obsolescence, the incentive to spend money on such measures declines.There may also be a perception that continued operation of old equipment issomehow less safe than new equipment. In some countries high labour costsmay make rehabilitation or life extension of old plant uneconomic. For exampleretrofitting of new instrumentation and control systems might cost as much ormore than a new (more efficient) plant.

Need to Assess the Life of a Plant to Optimise the Life of PlantAssets

Unfortunately many plant owners are very reluctant to spend money onassessing the life their plant until it’s too late. Effective management of largecapital assets like power plants, petroleum and petrochemical refineries,requires detailed scientific analysis to identify which components are most atrisk of failure. Provided such an approach is applied in a disciplined andrigorous manner, the plant may be reliably operated well beyond its design life.A risk based approach to identify the critical components and what damagemechanisms may limit their lives can allow costly failures to be pre-empted andthe plant operated well after the capital cost has been repaid.

Plant designs are often quite conservative, based upon average materialsproperty data plus a safety factor. So life extension is usually possible. In orderto plan for life extension, life assessment is essential, see Figure 5.

A Risk Based Approach to Life AssessmentHaving established that life assessment is an essential component in aneffective asset management plant it is essential to consider how suchassessment should be implemented. It is too expensive to inspect the wholeplant. 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, stainlessetc.)

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 bepossible to determine the most likely damage mechanisms, and from there todetermine what inspection is necessary to detect damage, and the inspectionfrequency. If damage is very unlikely then it may not be worth inspecting it very

thoroughly (if at all). However, if damage is highly likely, and the consequencesof failure extreme, then regular detailed inspection may be necessary.

The Three-Phased Approach to Risk Based Plant LifeAssessment

Ideally it is best to approach plant life assessment in three phases, see Figure6:

Phase 1: Review the plant design and history and identify those areas atrisk, perform inverse design calculations on a worst case(minimum materials property and design conditions basis). Definean inspection workscope.

Phase 2: Perform the inspection workscope defined in Phase 1 and inputactual field data into calculations. The outcome facilitates definitionof future inspection plans and/or the necessity for Phase 3.

Phase 3: If Phase 2 indicates life is less than required, then more complexanalysis (e.g. sophisticated probabilistic calculations, stressanalysis and fracture mechanics) and removal of field samples forlaboratory testing may be considered necessary.

Over the life of a large power plant or refinery it may be necessary to assess theremaining life of the primary assets several times over their life. This iterativeprocess underwrites safe and reliable operations, see Figure 7.

Benefits of RBIThe 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.

• Comply with standards and regulations.

For low risk items inspection is targeted at worst case locations, and the periodbetween inspections can be extended leading to reduced maintenance costs.For higher risk items inspection is again targeted at worst case locations whilstensuring 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 statutoryequipment (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 dollarsworth of additional productive capacity will be released. In future it may bemandatory on a plant wide basis. In Malaysia the authorities are considering asimilar approach.

Where and When Does the Worst Kind of Damage/FailureOccur, and How to Manage the Problem?

RBI is based on identifying the likely damage mechanisms, and allocatingpriority for appropriate inspection at suitable intervals. Ideally the inspectioninterval should be set such that failure is highly unlikely to occur within a periodequivalent to twice the maximum inspection interval.

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 thebasic tools needed to derive approximately what the worst outcome might be.For example the likely proximity of personnel and public in harms way, therelative 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 thelikelihood of damage that might lead to a failure.

Heat Exchanger Tubes

General corrosion is something that is relatively easy to tolerate. Measures tomonitor control general corrosion are straightforward. We expect generalcorrosion to occur everywhere, even if it is at such a low rate that it is barelymeasurable. Pitting corrosion is much less predictable and more difficult tomonitor, and yet a single isolated pit may perforate a heat exchanger tube (afailure).

Heat exchanger tubes are commonly inspected in-situ through NDT methodssuch as ECT, IRIS or LOTIS, and it is usually desirable to inspect only a fractionof the tube population (e.g. 10%) so as to minimise the cost and duration ofinspection. However, when pitting is found within the inspected tubes, it is thennecessary 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 nextscheduled shutdown. In these situations, extreme value (EV) statisticaltechniques can be used to provide robust assessments of the present conditionof the entire tube bundle. For example, ASTM G46 describes a simple EVmethod for estimating the maximum pit depth in such situations. However, thismethod involves assumptions that are inherently conservative. Moderncomputing power now makes it feasible to implement much more advanced EVmethods which yield more realistic estimates and allow for prudentconsideration of possible measurement errors [3,4].

High Temperature ComponentsCreep and creep-fatigue tend to occur when materials are exposed to hightemperatures. For steels the following rough guide can be applied:

• Carbon steels up to 400oC creep is unlikely

• Low-alloy steels (e.g. 2.25%Cr1%Mo etc) good up to about 500oC

• Stainless steels up to 600oC or more

• Inconel etc. even better

In-situ metallography (i.e. replication) and hardness tests, together withdimensional checks can effectively detect indications of long-term creepdamage. Creep-fatigue is likely where such components are exposed to acombination of high temperatures and plant cycling. A example is the effect of“two-shift” operation on high temperature (superheater and reheater) headersand steam pipelines in power plants.

Hydrogen Damage and Metal DustingHydrogen damage and metal dusting tend to occur where carbon and low alloysteels are exposed to hydrogen service. Hydrogen damage may also occur inboilers where water chemistry is poorly controlled. Sophisticated NDE (UT)techniques may detect the latter. However, metal dusting is often detected toolate and the equipment may be written off.

Stress corrosion cracking (SCC)SCC is to be expected where austenitic stainless steels are exposed tochlorides (even in low concentrations), and can also affect carbon steelsexposed to extreme caustic conditions. Dye penetrant tests can detect crackingwhere the surface is freely accessible, the best means of controlling theproblem 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 thevessels is operated above the creep temperature, or in conditions wheresignificant corrosion was likely. Previously, regulations dictated costlyinspection every 2 years. Detailed risk assessment, supported by yearsof corrosion data enabled the acceptance (by the MoM) of proposals todouble the inspection interval to 4 years or more. During the process thepossibility of stress corrosion cracking of stainless steel equipment(under insulation) was highlighted, together with other corrosion underinsulation issues. This had not been previously considered. Appropriateimprovements in inspection procedures improved the confidence of theauthorities, thereby allowing granting of extensions for statutoryinspection intervals

2. Power Plants

Life assessment of boilers, steam pipelines and turbines follows a risk-based approach built on years of experience and sharing of basicresearch by organisations like EPRI, ERA Technology and CEGB.Where such knowledge has been applied correctly the improvements inreliability have been dramatic. Thermal power plants in the USA, UK, andAustralia are amongst the most reliable anywhere. The frequency ofboiler tube failures (which can cost on average at least US$1 millioneach in lost production opportunity for a 500MW unit, leaving aside thewider national economic implications) are reduced to minimal levels.

Conclusions

1. The optimum management of ageing industrial plant assetsbeyond their original design lives requires an ongoing risk-basedapproach

2. .The benefits of RBI and RCM vastly outweigh their costs ofimplementation.

3. The application of probabilistic techniques is a necessity whereoperating conditions are diverse and unpredictable, e.g. pittingcorrosion in condensers, or creep in high-temperature heatexchanger tubes.

References

1. “Large Property Losses in the Hydrocarbon-Chemical Industries, aThirty Year Review”, M&M Protection Consultants, 1992, 14th Edition,Marsh & McLennan.

2. “API 510 - Pressure Vessel Inspection Code: MaintenanceInspection, Rating, Repair and Alteration”, American PetroleumInstitute, 1997.

3. “Condition Assessment and Life Prediction Methods for HeatExchangers”, 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 CoolingWater Systems”, N. Laycock, S. Hodges, D. Krouse, D. Keen and P.Laycock, in the web-based Journal of Corrosion Science andEngineering, special issue containing proceedings of CorrosionScience in the 21st Century, UMIST, Manchester, UK, 6-11 July(2003).

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 O f Fa ilure

0

1

2

3

4

5

Prob

abili

ty O

f Fai

lure

H igh Risk

Low R isk

(Acceptable Risk ?)

Figure 2 Basis of a risk matrix.

cost

cost

level of maintenancelevel of maintenance

operating costoperating costmaintenance costmaintenance cost

O&MO&Mcostcost

optimumoptimumunder maintainedunder maintained over maintainedover maintained

Figure 3 Obtaining the optimal balance between maintenance and operational cost.

Time

Freq

uenc

y of

Fai

lure

s

Comm

issioning

Phase Old

Age

Figure 4 The “bath-tub” curve describing expected failure of failures over the lifeof a plant.

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 isextended beyond the design life.

Phase I Phase I --calculationalcalculational approach approach-operational and design data-operational and design data-worst case material properties-worst case material properties

Phase II Phase II - field inspection- field inspection-input of condition assessment data-input of condition assessment data

Predictive Predictive assessment ofassessment ofcomponentcomponentintegrityintegrity

IsIspredicted life predicted life

greater than targetgreater than target

YESYESDefine optimum futureDefine optimum futureinspection scheduleinspection scheduleandandlife extension capacitylife extension capacity

NONOPhase III Phase III - refined analysis- refined analysis- material sampling/testing- material sampling/testing- detailed surveillance- detailed surveillance- complex stress analysis- complex stress analysis

Define optimum inspection and Define optimum inspection and refurbishment strategyrefurbishment strategy

Figure 6 The 3 Phase approach to Risk Based Plant Life Assessment

Identify Consequencesof Failure

Estimate Probabilityof 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 lifeassessment