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Abstract

An effective tube management program can reduce equipment costs at a medium-size refinery by one million dollars per year or more. ERA Technology’s Heater Tube Life Management Program is currently being applied at Koch Refining's Pine Bend, MN, and Corpus Christi, TX refineries. Results-to-date have demonstrated a savings of more than 20 times the cost of the program in 12 months at the Pine Bend refinery alone. Savings are expected to increase as the benefits of the program are achieved throughout each refinery. The program consists of heater prioritization (ranking) by a modified API 530 assessment, heater tube inspection to establish baseline conditions of diametral strain, hardness, and microstructure, and remaining life assessment by a proprietary computer program based on probabilistic analysis. The technical basis for the program is outlined, and examples of the program output are reviewed. Equipment cost savings achieved by this program primarily result from delayed tube replacement made possible by a more realistic assessment of remaining life and quantification of risk. Risk factors previously assumed by Koch were necessarily more conservative in the absence of specific economic risk factors as supplied by ERA's program. The ERA risk factors were based on individual probability of failure curves derived for each heater. Specific results of applying the program to a coker charge heater and a platformer intermediate heater at Pine Bend are discussed.

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Introduction The timing of furnace retubing can be critical in minimizing equipment costs and maintaining productivity. Consistently replacing heater tubes on a premature basis can cost a medium-size refinery up to one million dollars per year or more in unnecessary capital costs. The optimization of heater retubing schedules can only be achieved by the implementation of a set of criteria that are realistic and based on sound engineering analysis. Specific tube replacement criteria are required for each type of heater and tube material, e.g., criteria for crude heaters may not be appropriate for coker charge heaters. When replacement criteria have been established, the implementation of a comprehensive baseline condition assessment and heater inspection program to support the criteria will ensure the success of the program. What is required is an overall heater tube life management program that allows tube life to be optimized, tube replacement to be scheduled well in advance, and productivity interruptions to be minimized. In addition, the program should identify heaters that are consuming their service life at a high rate so that inspection priorities and schedules can be set. Finally, the program should have the capability of predicting the effect of higher tube skin temperatures, corrosion rates, and operating pressures on tube remaining life and equipment reliability. The prediction of tube remaining life is not an easy task. While simplistic equations are available that will give a “first-attempt” approximation, many pitfalls must be overcome if a realistic estimate of remaining life is to be achieved. Obvious pitfalls include selection of a representative or “effective” temperature, accurately determining the pressure drop along the length of the coil, and the assumption of average vs. minimum creep-rupture strength. Difficulties that may not be obvious include allowing for mean-diameter drift, using an appropriate time segment for life calculation when high rates of corrosion take place, and assessing the reduction in creep strength when embrittling reactions take place in the tube metal. An accurate assessment of tube remaining life requires not only accurate calculations but a full evaluation of all available evidence including diametral strain, tube hardness, and the condition of the tube microstructure. Tube life can be shortened by flame impingement, coke buildup, or short-term high rates of corrosion. These events cannot be predicted. Estimated tube remaining life will therefore be the maximum potential life for a given set of operating conditions. In other words, the calculated life will be that life that can be achieved in the absence of local life-limiting conditions. Achieving the estimated or maximum tube life requires a coordinated effort that includes frequent visual and infrared inspection of flame patterns, NDE for detection of coke buildup, and comprehensive UT wall thickness measurements. ERA Technology offers a Heater Tube Life Management Program that is designed to aid plant and maintenance engineers in determining the optimum time for furnace retubing. The program consists of the following:

? ? Prioritization (ranking) of heater tubes by initial estimates of remaining life

? ? Inspection prioritization/identification of critical heaters

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? ? Heater inspection/condition assessment ? ? Creep testing of tube samples (not required but highly recommended) ? ? Accurate determination of remaining life by probabilistic analysis

? ? Recommendations for optimum tube replacement dates consistent with

plant operational and productivity goals

? ? Economic analysis of retubing costs vs potential costs associated with increased profits at a higher risk level

It is important that the tube replacement criteria are consistent, rational, and based on detailed engineering analysis as well as recent advances in remaining life assessment technology. ERA Technology is currently managing over 250 heaters in the U.S., Canada, and Europe based on life assessment technology developed over the last 20 years at ERA’s Metallurgical and Creep Testing Labs in England and Houston, TX. The savings to the typical plant in optimizing heater retubing schedules is inevitably 10-20 times more than the cost of the program in terms of equipment and installation costs alone. Program Goals The major goals of the Heater Tube Life Management Program are to:

? ? Reduce risk of tube failure and resulting production losses ? ? Determine the optimum time for heater tube replacement

? ? Quantify risk of operating tubes given past consumption of life and

anticipated future operating conditions ? ? Quantify damage done to the tube, in terms of life consumption, by past

overheating, low flow, or over-pressurization incidents ? ? Predict life consumption based on anticipated future operating parameters

including crude slate composition

? ? Prioritize heaters in terms of rate of life consumption so that inspection and engineering resources can be focused on those heaters approaching the end of life

? ? Identify the primary failure mode and therefore the optimum method of tube

inspection

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? ? Identify those heaters in which the tube skin temperatures can be reliably increased from their present levels in order to increase productivity without a large increase in life consumption

? ? Determine permissible levels of tube skin temperatures to allow a given heater

to reliably reach the next scheduled shutdown

? ? Allow plant and maintenance engineers to reduce risk and improve profitability

Tube Replacement Criteria Heater tubes have a finite life that is governed by the tube material, process environment, and tube operating conditions (primarily temperature and pressure). All heater tubes in refinery service have either failed or been replaced, or will fail sometime in the future. The longer that a tube has been in service the greater is the risk of failure. Tubes will generally fail by creep or corrosion or a combination of the two. Tube swelling (Figure 1) is evidence that significant consumption of creep life has taken place. Other failure modes are possible such as metal dusting, erosion-corrosion, as well as interactions between failure modes. Thermal stresses are negligible for most heater tubes but must be taken into account for thick-wall tubes. Environmental reactions such as carburization (Figure 2), or metallurgical reactions such as the formation of sigma phase in stainless steel, can embrittle tubes and reduce creep strength and fracture toughness well below design values. Despite the several ways in which tubes can fail, many tubes are replaced on the basis of corrosion significantly before reaching the tube replacement thickness recommended by API 530. The overall goal of ERA's program is to provide a series of decision-making tools that allow plant engineers to optimize tube replacement recommendations to their plant management. Each plant has existing procedures for tube replacement, either by habit or by design. Tube replacement criteria run the gamut from life consumption calculations by deterministic (single values for temperature and pressure) API 530 methodology to a fixed value for wall thickness. Replacement criteria in the latter category generally range from fixed values several times greater than the API 530 tube replacement thickness on the lower end to the minimum thickness required by the purchase specification on the upper end. Other plants, recognizing that fired heater tubes fail overwhelmingly due to either creep or high-temperature corrosion employ dual criteria consisting of fixed diametral strain values and tube replacement thickness. Tube replacement occurs when either criterion is reached. The forecasting of strain or thickness consumption from one shutdown period to the next is obviously required. Tube replacement strain values generally range from 3 to 8 percent depending on the tube material. A criterion of 3 percent diametral strain for all tube materials in all applications tends to be unnecessarily conservative most of the time. Even modest improvements in the realism of replacement criteria can have a dramatic effect in reducing equipment costs. In addition, criteria that are applied across the board to all heaters tend to be overly conservative for most heaters while unconservative for heaters that fall into special categories. Coker charge heater tubes that have been embrittled by through-wall carburization, for example, may fail at less than 3 percent diametral strain and at a wall thickness barely less than when they were installed. In this

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instance, the hardness at the tube O.D. is the controlling factor rather than diametral strain or corrosion rate. It is obviously important therefore to have criteria for tube replacement that not only are based upon consistent and realistic engineering analysis but that also recognize differences in heater applications. The most critical decisions regarding heater tube life management are the following:

? ? When should a heater be inspected? ? ? When should a heater be retubed? ? ? Is life-extension of a given heater possible? ? ? What impact on heater service life will result from crude slate changes?

Heater Tube Inspection and Prioritization All heater tubes should be inspected, preferably early in life, to establish base-line conditions for tube diameter, wall-thickness, microstructure, and metal hardness. For plants that have not established base-line conditions, prioritization (ranking) of heaters can be done on a calculational basis to determine which heaters should be inspected at the next T/A. Heater prioritization can be based on a modified remaining life assessment per API 530. While API 530 Appendix E gives life estimates that are overly conservative, for reasons that will be discussed later, that document can be used for prioritization purposes. Prioritization is best accomplished based on actual operating conditions rather than design values. In general, the actual creep rupture strength of the tubes in a given heater will not be known (without creep testing of a tube sample) so that the lower-bound creep strength values must be assumed by the prioritization. As long as all heaters are evaluated by the same method, however, prioritization can be a useful manner of comparing heaters. After the heaters have been ranked on the basis of remaining life it is necessary to decide which heaters are critical and therefore should be inspected during the next T/A or as quickly as possible. Typical criteria for criticality include less than 20,000 hours of remaining life, a remaining life shorter than the time until the next T/A, and the importance of a given heater in achieving specific productivity goals. While a modified approach based on API 530 is adequate for heater prioritization, it is unrealistically conservative in estimating remaining life when the primary failure mode is creep. The basic reason for this is the assumptions inherently required by a deterministic analysis of a fired heater. In order for the analysis to be conservative, it is necessary to calculate the remaining life on the basis of the maximum temperature and maximum pressure, and maximum rate of corrosion. In addition it is usually also required to assume minimum creep strength since the actual creep strength for a given set of tubes will not generally be known. The assumption of "worst-case" operating parameters and lower-bound creep strength can underestimate tube remaining life by 5 to 15 years. For a deterministic assessment, not only is it necessary to assume maximum values for temperature and pressure within a given time period, it is also necessary to assume (by virtue of conducting only one calculation) that the maximum

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temperature and pressure occur at the same location in the heater. In fact this is hardly ever the case since the pressure is always greatest at the heater inlet where the temperatures are lower. In situations where there is significant variation in operating parameters with location in the equipment or with time, a deterministic assessment will simply not provide a realistic assessment of remaining life and therefore will not be of assistance in deciding when a heater should be retubed. This limitation of deterministic assessments is always the case whether the remaining life assessment is time-based or strain-based. API 530 remaining life calculations can be improved by using the actual creep rupture strength, ensuring that the life periods are broken down into segments that are sufficiently short such that the stress and therefore the LMP are more or less constant in that time period, and that realistic estimates of operating temperatures and pressures are utilized. Even with these refinements, however, a single-valued approach to heater life assessment is simply not adequate to describe the physical reality of what is taking place inside of a heater, and therefore not adequate to accurately define remaining tube life. For that task, probabilistic assessment is required. Probabilistic Life Assessment The advantage of using a probabilistic approach to heater tube life assessment is that typical variations in tube skin temperatures and pressures can be taken into account whether the variations occur with location within the furnace or with time, or both. ERA's proprietary probabilistic life assessment program is based on statistical analysis of all temperature, pressure, and wall thickness measurements, combined with a Monte Carlo simulation to derive the probability of failure. Probabilistic assessment is similar to a deterministic assessment in that a remaining life is calculated on the basis of a tube temperature, pressure, corrosion rate, year of installation, and tube material. The major difference is that a deterministic assessment can be conducted on the basis of one calculation while a probabilistic assessment requires anywhere from 1000 to 10,000 calculations in order to utilize all possible combinations of temperature and pressure. After all probabilistic calculations have been completed and tabulated, the results are presented in terms of probability (cumulative failure probability) rather than remaining life. The remaining life is obtained by deciding what level of failure probability is acceptable and entering the probability of failure curve at that point (Figure 3). Even at a failure probability of 0.1 percent or less the estimate of reliable life obtained from a probabilistic assessment can be 5-15 years greater than the life estimate from a deterministic assessment. The underlying reason for the longer life is that probabilistic assessment gives equal input to minimum and maximum temperatures, pressures, and material properties while a deterministic assessment is based only on the maximum value of each parameter. An additional characteristic of probabilistic assessment is its' greater flexibility. Each plant, or unit within a plant, can decide what level of failure probability is appropriate for the equipment that they are operating. In other words, the criteria can be modified to suit individual plants or units. ERA can provide suitable criteria if required.

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Heater Inspection Methods Thorough heater inspections are essential to a tube life management program. Baseline condition assessment should be undertaken of all heaters to establish current serviceability. Inspection should consist of diametral measurements to determine accumulated creep strain, field metallography to evaluate the progress of thermal aging and potential metallurgical reactions, and hardness measurements. The latter inspection is important since tubes will soften with long-term exposure to temperatures in the creep range. Databases are available that provide hardness-compensated evaluation of creep life consumption for some tube materials. In-situ metallography and replication should be routinely conducted on the O.D. surface of fired heater tubes to establish the extent of thermal aging, to verify heat-treatment, for general confirmation of the material condition (excessive inclusions, acceptable grain size, etc.), and most importantly, to determine if metallurgical reactions are taking place. While creep cavitation (small voids indicative of high-temperature creep located at the grain boundaries of the material) is not frequently seen in fired heater tubes, the value of field metallography in providing critical information about the metallurgical condition of the tubes cannot be overestimated. If cavitation is present in a typical thin-wall fired heater tube, it will appear first at, or near, the O.D. surface of the tube since the temperature and stress are highest at that location. Replicating the microstructure of new tubes is particularly useful since the starting condition of the microstructure is clearly defined for the purpose of future comparison. Accelerated Creep Rupture Testing Every comprehensive heater tube life management program should include removal of tube samples from heaters in which the primary mode of failure is creep. The purpose of tube removal is to provide a sample for creep rupture testing to determine the actual creep strength of the material. Testing can be accelerated by selecting a test temperature higher than the operating temperature in order to ensure that the test is completed on a timely basis. The value of knowing the actual creep strength of the material is that the tube life assessment does not need to be based on the lower-bound creep strength. The life of the tubes in a given coil can then be more realistically determined. The increase in estimated life between the life based on the assumed lower-bound creep strength and the life based on the actual creep strength can be as much as 7-12 years longer when the actual creep strength is known. In other words, knowing the actual creep strength gives more confidence that the tubes can remain in service for a longer time. The cost of creep testing is insignificant in comparison to the cost of furnace retubing. Another advantage of creep-rupture testing of tube samples is that it allows an accurate determination of remaining life when there is no information available concerning past tube temperatures and pressures. In other words, each unique set of past temperatures and pressures will reduce the remaining creep strength of a given tube dependent on the magnitude of each parameter and the time over which the tube was exposed. The reduction in creep strength, however, can be determined by creep-rupture testing and the remaining life calculated based on the creep-strength and future operating conditions. Consequently, though creep-rupture testing is always useful, it is particularly helpful when past tube temperatures and pressures are not known or when there has been short-term or long-term instances of overheating.

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Program Output The output of ERA's Heater Tube Life Management Program can be divided into 4 sections: Heater Prioritization Heater Ranking in terms of deterministic life Electronic spreadsheet of prioritization results Identify critical heaters for subsequent probabilistic assessment Heater Inspection Present condition of heater tubes Evaluation of diametral strain, extent of thermal aging, microstructure, and hardness Grain size can be determined when critical (Incoloy 800H, etc.) Probabilistic Life Assessment Cumulative Failure Probability (CFP) Curves Tube Remaining Life at a given CFP Identify tube thickness to be replaced at this T/A in order for the heater to reach the next scheduled T/A with very little chance of failure Determine heater utilization Define potential increase in temperature and corrosion rate that a heater can tolerate and still achieve a given remaining life (temperature and corrosion rate sensitivity) Evaluate effect of past instances of overheating on remaining life (when appropriate) Optional Optimize tube metallurgy for given future set of operating conditions

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Case Studies The ERA heater tube life management program is currently being applied to the Koch Refinery in Pine Bend, Minnesota. The first round of heater assessments has been completed. All heaters have been prioritized and critical heaters identified. A few heaters have been inspected and a tube removal program for accelerated creep rupture testing has been established. Tube samples have been removed from 6 heaters. Of the 5 completed creep tests at this point in time, all 5 tubes have displayed creep strengths significantly above the API 530 minimum values. Creep testing of new tubes has also been conducted to establish the baseline creep strength prior to tube installation. Baseline creep testing obviously reduces the need for subsequent tube sample removal unless an incident of significant overheating occurs that could reduce creep strength. ERA's program has allowed Koch to significantly reduce the overall lifecycle costs in the first 12 months by postponing tube replacement costs (purchase and installation). The annual savings are expected to increase as the benefits of the program (optimized tube replacement, minimization of unplanned shutdowns/replacements and optimized performance to meet efficiency and production targets) are achieved throughout the refinery. The key result or output of the program is assigning a calculated risk of failure value to previously non-quantified risks, i.e. elimination of uncertainty. As done in Risk Based Inspection (RBI) programs, the defining of the likelihood of an event (tube failure) occurring, coupled with the consequence, both economic and safety, allows for better decision making to occur relative to maintenance costs and equipment utilization. In the past Koch has assigned arbitrary risk values to these types of events to facilitate economic analysis. In most cases the assumed risk values were conservatively high. The identification of critical heaters that are consuming their life at a higher rate has allowed reliability resources to be focused on those heaters. Consideration is now being given to the potential for increasing process throughput in those heaters that are being underutilized with respect to tube skin temperature. Furthermore, the program is also currently analyzing the effect of running crude slates of increased corrosivity on heater reliability and maintenance costs. On the basis of the program's success, an identical program is now underway at Koch's refinery in Corpus Christi, TX. Two specific examples of program achievement are provided below. Example 1 Remaining life analysis of a platformer heater indicated, on the basis of the maximum temperature/pressure and the minimum API 530, creep strength that the tube life had expired. A probabilistic life assessment of this heater revealed that the tubes had significant tube life remaining (Figures 4 and 5). Eliminating the tube replacement from the turnaround plan postponed a significant expenditure in the heater’s lifecycle. The analysis went further, by

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including the consequence of a tube failure, to allow for optimizing the eventual tube replacement around the opportunity loss from an unplanned event occurring (Figures 6 and 7). The life analysis also included a tube temperature sensitivity plot, showing the life impact from changes to the temperature distribution (Figure 8). This figure defines the Unit utilization, allowing for Unit optimization planning to occur. During the recent turnaround, a full tube inspection, encompassing ultrasonic thickness measurement, tube OD gauging, replication and the removal of tube samples was conducted. The tube samples were selected based on the Infrared tube temperature data from the hottest areas. These samples are currently being tested to determine the actual tube material properties from which a Refined tube life assessment can be done. Based on preliminary sample results, the tube material properties are significantly above the API 530 values, indicating the remaining life most likely exceeds that determined in the first probabilistic analysis. Koch benefits from this Refined assessment, not only from maintenance cost savings, but also from Unit utilization, by allowing for higher production rates. Example 2 Remaining life analysis of a coker charge heater indicated, on the basis of the maximum temperature/pressure and the minimum API 530, creep strength that the tube life had expired. A probabilistic life assessment of this heater revealed that indeed some of the radiant tubes have exceeded their economic remaining life (Figures 9-12). The key result of the study was the establishment of the economic life for the coker tubes, divided into operating zones. For this heater the study determined that the floor tubes’ economic life ranged between 9 to 12 years, while the roof tubes’ economic life ranged between 15 to 16 years. Koch was able to match the results of the life assessment with operating history by comparing the historical tube replacement schedule (based on inspection parameters for wall thickness and tube OD growth and based on past tube failures) with the calculated tube lives. In the past the timing of the tube replacements was determined during these annual or more frequent internal inspections. Now, with a more accurate tool to assess tube life impact, coupled with an aggressive tube temperature measurement program, Koch can increase the time interval between internal inspections, thereby, increasing the Unit’s utilization. This change in inspection philosophy is expected to result in a 2% increase in the heater’s availability per year. This increase has a large positive effect on profitability. Another interesting aspect of this life assessment was the utilization of post exposure tube properties in calculating the economic life. Due to the nature of the process, significant internal corrosion and through wall carburization was found on the tube samples. These conditions, as well as the post exposure accelerated creep rupture testing, were included in the calculated life. Providing a tube sample for metallurgical examination also allowed the potential for metal dusting to be evaluated.

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Figure 1 Bulged heater tubes from a charge heater. Tube swelling, as shown by the

increase in diameter, is evidence of creep.

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Figure 2 Cumulative failure probability plot. (No carburization)

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Figure 3 Typical microstructure of failed tube near perforation (500x magnification).

Through-wall carburization was found at this location.

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Figure 4 Cumulative failure probability vs. time (hours of tube life) for the X-pass. Figure 5 Cumulative failure probability – Y Pass

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Figure 6 Risk exposure due to full tube replacement. Figure 7 Expected spend on replacement of tubes.

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Figure 8 Temperature sensitivity analysis. Figure 9 Cumulative failure probability plot. (Carburization)

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Figure 10 Cumulative failure probability plot. (Carburization) Figure 11 Cumulative failure probability plot. (Carburization)

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Figure 12 Cumulative failure probability plot. (Carburization)

heater tube life management

Engineering