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Reliability and Integrity Analysis of an Air-Cooled Heat Exchanger Case Study02.2.2012Air-cooled heat exchangers are used extensively throughout the entire oil and gas industry from upstream production to refineries and petrochemical plants, under difficult conditions including high pressures and temperatures, as well as corrosive fluids and environments. This article presents a reliability and integrity process analysis of an air-cooled heat exchanger for hydrocarbon service. This process as part of an AIMP (Asset Integrity Management Program) including, Risk Based Inspection Strategy (RBIS), Acoustic Pulse Reflectometry (APR) inspection and degradation analysis to perform the remaining life assessment (RLA), helping to engineers, maintenance manager and plant manager to makethe right decision even in an uncertainty environment.Some defects and discontinuities can be introduced during the heat exchanger manufacturing process and may not necessarily be detected by non-destructive testing processes. Other damage mechanisms, like erosion-corrosion, sulfide stress corrosion cracking, thermal fatigue, and pitting corrosion due to CO2 or because of chloride content in the process stream could come up while heat exchangers are in-service.Due to the equipment complexity, an asset integrity management programme should be developed and applied on-site. An asset integrity management programme seeks to ensure that all equipment and particularly those physical assets that are subject to internal pressure, are designed, constructed, inspected and maintained to the appropriated standards and best practices. This is in order to pursue the maintenance efforts, optimization and cost-effective maintenance decisions, guaranteeing a sustainable and safe operation.Figure 1. Asset Integrity Management Model.Figure 1. Asset Integrity Management Model.The issue of ageing plant, leading to an increased risk of loss of containment and other failures due to plant and equipment deterioration, has been shown to be an important factor in incidents and accidents. Recent research shows that 50 % of European major hazard “Loss ofContainment” events arising from technical plant failures were primarily due to ageing plant mechanisms such as erosion, corrosion and fatigue. This data analysis for HSE (Ageing Plant Study Phase 1) has shown that across Europe, between 1980 and 2006 there have been 96 major accidents and potential loss of containment incidents reported in the EU Major Accident database (MARS), which are estimated to be primarily caused due to ageing plant mechanism. This demonstrates the significant extent and impact of ageing plantrelated failures on safety and business performance. Ageing is not about how old your equipment is; it is about its condition, and how that is changing over time. Ageing is the effect whereby a component suffers some material degradation and damage (usually, but not necessarily associated with time in service) with and increasing probability of failure over the lifetime.The grade of deterioration and damage relates to the potential effect on the equipment’s functionality, availability, reliability and safety, and it is for this reason that a good asset integrity programme should be developed for that equipment that can be susceptible to ageing mechanisms.Asset Integrity Management ProgrammeThe objectives of an Asset Integrity Management (AIM) system are the delivery of business requirements maximizing return on assets whilst maintaining stakeholder value and minimizing business risks associated with accidents and loss of production.Asset Integrity is the ability of an asset to perform its required function effectively and efficiently whilst safeguarding life and the environment. The related management activities ensure that the people, systems, processes and resources that deliver integrity are in place, in use and fit for purpose over the whole lifecycle of the asset.AIM ensures that the assets stay fit for purpose – safe and operational –under all circumstance.Asset integrity management delivers:∙safety improvement∙reliability improvement∙optimization of maintenance and inspection activities to meet safety and business targets.A complete Asset Integrity Management programme incorporates design, maintenance, monitoring, inspection, process, operations, and management concepts, since all these disciplines impact the integrity of infrastructure and equipment. Figure 1 shows a typical AIM model. In this article an air-cooled heat exchanger case study shows how just two sections of this model (Risk assessment and Inspection strategy) are applied in an effective and easy way.Asset Integrity Strategy for Air-Cooled Heat ExchangerThe outline process in figure 2 is the risk management process developed for an aircooled heat exchanger for a hydrocarbon process. This process is based on the standard continual improvement cycle PDCA (Plan – Do –Check – Act), similar to the cycle presented in the specification PAS-55 1:2008. The seven steps process shown could be applied to any physical asset whose relative risk value is considered high for the organization. For this particular case, an aircooled heat exchanger has been considered critical for the process, not only for the risk of loss of containment, but for the business performance impact in case of equipment failure.Gathering InformationAll information related to P&ID, layout process, process data (ph, gas type, contaminant content, stream temperature, cool and heat cycles), mechanical design data and construction details, material type, welding process etc. This kind of information will be useful to understand the heat exchanger behaviour during operation and what type of flaw could have been introduced during the manufacturing process and what damage mechanisms could be developed in-service.The information gathered for the ACHE (Air-Cooled Heat Exchanger) case study presented in this article is summarized in the Tables 1 and 2. The purpose of this equipment is to cool down the gas stream that come from the gas regeneration process (removing water content in the gas stream), taking gas at 285 °C and cooling it to 35 °C at every 16 hours. Figure3shows the mechanical design details of the ACHE.RBI assessmentA RBI (Risk Based Inspection) is a risk assessment and management process that is focused on inspection planning for loss of containment of pressurized equipment in processing facilities, which considers both the probability of failure and consequence of failure due to material deterioration. These risks are managed primarily through inspection in order to influence the probability of failure.Risk = Likelihood * ConsequenceThe probability assessment is in accordance with API 580 and is based on all forms of damage that could reasonably be expected to affect equipment in any particular service. Additionally, the effectiveness of the inspection practices, tools, and techniques used for finding the potential damage mechanisms is evaluated.The consequence of a release is dependent on the type and amount of process fluid contained in the equipment. The consequence assessment is in accordance with API 580 and considers the potential incidents that may occur as a result of fluid release, the size of a potential release, and the type of potential release (includes explosion, fire, or toxic exposure). The assessment should also determine the potential outcomes that may occur as a result of fluid release or equipment damage, which include: health effects, environmental impact, and process downtime.After an RBI assessment is conducted, the results may be used to establish the inspection plan and better define the following:∙the most appropriate inspection and NDE methods, tools andtechniques∙the extend of NDE (percentage of heat exchanger tubes forinspections)∙the interval for internal, external and on-stream inspections∙the prevention and mitigation steps to reduce the probability and consequence of the air-cooled heat exchanger.Table 1. Process stream data.Table 3 shows the most probable damage mechanisms that could be expected in the ACHE equipment.Figure 4 shows the risk matrix results from the RBI assessment for theACHE (Air- Cooled Heat Exchanger). For the ACHE under study the risk is “Medium High”, so a good inspection strategy should be developed to reduce the probability of failure.Table 2. Design construction information.Inspection of Air-Cooled Heat ExchangersThe primary purposes of inspection are to identify active deterioration mechanism and to specify repair, replacement, or future inspections for affected equipment. These purposes require developing information about the physical condition of the heat exchanger, the causes of any deterioration, and the rate of deterioration. These actions should result in increased operation safety, reduced maintenance costs, and more reliable and efficient operations.Tubes that are enclosed in fins cannot be inspected from the exterior. The best methods for inspecting the tubes are internal-rotary, UTthickness-testing devices, Eddy current, remote field ET or APR (Acoustic Pulse Reflectometry). These methods work from the interior of the tubes. With competent operators and clean tubes, thickness and defects can be found with these methods. The external fins of the tubes should be checked for cleanliness. If the fins need cleaning, washing with clean water alone or clean water with soap may be sufficient. The fins are made of aluminium and they could be harmed if the wrong cleaning medium is used.Figure 3. Plug header design construction details. Table 3. Damage mechanisms and NDE techniques for ACHE.Figure 4. Risk result from RBI assessment for the ACHE under study.Figure 5. Remote Visual Inspection of ACHE header.Figure 6. APR inspection results.For the ACHE under study, internal inspection was carried out fulfillingthe RBI outcomes. Remote Visual Inspection (RVI) has been performed using a fibre optic device on the header, looking for erosion-corrosion signal and thermal fatigue indication on the header weld seam. The header of the air-cooler has been inspected using the same techniques as recommended for a pressure vessel. In addition, the sharp change direction caused by its rectangular construction was carefully checked for cracking. Figure 5 shows some pictures of the internal condition of the header. From pictures, a good physical condition can be appreciated.Following the RBI assessment for the ACHE, 10 % of finned tubes (113 tubes) have been inspected using the APR (Acoustic Pulse Reflectomery), looking for localized corrosion and Stress Corrosion Cracking. APR is based on the measurement of one-dimensional acoustic waves propagating in tubes. Any change in the cross sectional area in the tubular system creates a reflection, which is then recorded and analyzed in order to detect anomalies. The benefits of this NDE technique is that it utilizesnon-invasive APR technology, which can navigate bends, coils, elbows, fittings, etc. without difficulty. This technology allows inspection personnel to test any tube from a single point outside the tube in less than 10 seconds, saving considerable time and resources.After the inspection, 22 tubes out of 113 tubes were reported with thickness reduction. Figure 6 shows the tubes map inspection result. The blue colour represent tubes with thickness reduction.Tube degradation analysisWith the purpose of estimating the “tubes life”, a degradation analysiswas carried out using Weibull commercial software. The Degradation Analysis allows you to estimate the failure of a product based on its performance over time. This kind of analysis uses basic mathematical models to extrapolate the performance measurements over time to the point where failure is said to occur.For this case, degradation analysis had permitted to extrapolate to an assumed failure time based on the tubes thickness measurement, using a linear model. Considering that maximum degradation permitted in thetubes is 0,59 mm (1,65 mm–1,06 mm being the minimum requiredthickness to support internal pressure) the tubes MTTF (Mean Time To Failure) data were extrapolated. Table 4 shows wall thickness reduction in tubes and MTTF data extrapolated from the degradation analysis. The remaining life is acquired by subtracting the MTTF value the heat exchanger service time (11 years).It is clear that tube M1F1T32 is below the minimum thickness required, and a fitness for services assessment for this tube should be performed. In this case, the tube was blocked. This table is very useful in developing future inspection and in developing a plan to block those tubes that are very close to reaching the minimum allowable thickness.Reliability Analysis of Air-Cooled Heat ExchangerWith time to failure data, a reliability analysis of heat exchanger was performed using the mixed Weibull distribution. The mixed Weibull distribution (also known as a multimodal Weibull) is used to model data that do not fall on a straight line on a Weibull probability plot. Data of this type, particularly if the data points follow an S-shape on the probability plot, may indicate more than one failure mode at work in the population of failures time. Depending on the number of subpopulation chosen, reliability is defined by the following equation.R1 .,,,. s (t) = sΣ i = 1 Ni / N e - (T /ni) βiwhere:R(t) = Reliability valueT = Mission time (hours)b i = Weibull Shape parameterh i = Weibull Scale parameter (hours)S = Number of subpopulation (2,3 or 4)N = Identical components from a mixed populationNi = Number of components that failed from each population.Table 4. APR inspection results. Thickness reduction data.Figure 7. Weibull probability plot with confidence bound at 90 %.Figure 7 shows the Weibull probability plot for life data extrapolated from degradation analysis for the air-cooled heat exchanger.From the Weibull probability plot, it is clear that there are at least two damage mechanisms acting on the tubes (two slopes). These could be one of two corrosion mechanisms, one being “localized corrosion” (pitting)and the other “Chloride Stress Corrosion Cracking”. Both have the same behaviour; wear out. This is confirmed by the beta value that is much bigger than 1. For the subpopulation 1 the beta value is 5 and for the subpopulation 2 the beta value is 2,5; these values are typical for a corrosion process. This type of plot is very important in terms of knowing what failure mechanism could be acting on the equipment with such little data. The red line shows the confidence bound at 90 %. Figure 8 shows the aircooled heat exchanger reliability curve.The reliability curve is a very important plot that can be used to know what the probability of failure is over time. This is particularly useful to developstrategy inspection based on the heat exchanger risk. Multiplying both the POF (Probability Of Failure) and the consequence of failure (in term of cost), a risk graph can be developed to know what the risk optimum value is for the next inspection.Figure 8.Heat exchangers reliability curve with confidence bound at 90 %.Figure 9. Inspection strategy for ACHE based on business risk.Based on the Figure 9, the best optimum frequency for the next internal inspection should be at 17 years to keep the business risk below the business risk target.ConclusionsA good mechanical integrity programme for air-cooled heat exchangers and other pressure vessels is crucial for those plants that need to reduce turnaround time and inspection costs within safety standards. Due to the world financial crisis, companies are forced to reduce maintenance costs, so this type of strategy presented in this article is a powerful tool for engineers and managers to optimize inspection, turnarounds and maintenance cost through risk and reliability strategies.For this particular case study, the two most probable mechanisms are acting on the internal side of tubes, pitting corrosion and Chloride Stress Corrosion Cracking, due to gas stream that condensates water with chlorides contaminant. Both damage mechanisms should be corroborated through radiography inspection. The APR methodology is a very useful NDT to assess thickness reduction in an air-cooled heat exchanger, reducing time and inspection resources.The gas plant data has been explored using new tools to solve reliabilityproblems and to help managers to make the right decisions from the business point of view and applying the risk management decision support. This kind of reliability tool is very important and useful for the top management because the problem can be described in terms of business risk ($) and can be explained on a piece of paper. This makes the process to make the right decisions easier and efforts can be focused on avoiding important and critical problems that OEM’s and End-Users could be facing in their own process plant.»»References [1] API RP- 580: Risk Based Inspection, 1st edn, Washington, D.C, May 2002 [2] API PUBL.581: Risk Based Inspection Based Resource Document, 1st edn, Washington, D.C, May 2000 [3] API 510: Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration, 9th ed, Washington, D.C, June 2006. [4] API 661: Air-Cooled Heat Exchangers For General Refinery Services, 6th edn, Washington, D.C, February 2006. [5] Life Data Analysis, Weibull++7. Reliasoft.。