News & Views, Volume 52 | Heat Exchanger Tube Sheet Reliability Analysis

By:  Kannan Subramanian, PhD, PE, FASME & Dan Parker, PE


The hot section of a waste heat boiler, also known as the hot spent boiler, is an essential component in the regeneration of spent sulfuric acid in chemical plants that process sulfur. Due to the ever-increasing demand for sulfuric acid and other sulfur compounds, this is critical equipment as its operation results in sold-out production. As a result, these boilers need maximum uptime between scheduled maintenance outages; any unscheduled shutdowns to repair and/ or replace tubes and tube sheets directly translate into lost revenues for the plant. This article addresses the reliability issues of one such boiler located in a Louisiana chemical plant. 

GOAL: Predict the minimum number of tubes to plug, minimize downtime and allow regular operation until the next planned maintenance.  

Figure 1. Hot Spent Boiler (shaded in red) in a waste heat recovery unit Flue Gas Tube Boiler.

Figure 2. Tube-to-tube sheet joint failures and tube sheet leak

The boiler being assessed was part of an arrangement (Figure 1), with two fire tube boilers in parallel with a common external steam drum. In the case of the single boiler assessed by SI, the tubes were experiencing periodic tube leaks as the boiler was approaching the end of service life where tube failure frequency increases. As typical, there may be a single tube leak or several in the same proximity (Figure 2). Some proactive plugging has been applied based on historical performance (Figure 3(a)). In SI’s experience, tubes adjacent to a plugged tube may fail a short time after the plug is installed as there is an undefined temperature/stress interaction. In addition to tube leaks, general corrosion and tube sheet thinning can be a consequence of tube leaks (Figure 2(c). Excessive tube sheet thinning is not uncommon due to the formation of sulfuric acid that exacerbates the corrosion issue. With these consequences in mind, it is critical that the proper number of tubes be plugged to stop the costly cascade of failures. 

To bring the boiler back to service, in early January of 2022, the leaking tubes and a few other tubes around the leaking tubes were plugged. In addition, to repair the leaking location in the tube sheet knuckle region, welding followed by post-weld heat treatment was performed (Figure 3(b)). Within a few weeks after this repair, additional tube leaks were discovered and required plugging. Such frequent leaks and repairs result in production loss and unplanned expenses. To minimize those, SI was contracted to develop an engineering basis for tube plugging, which would be proactive for equipment reliability, but not produce over-plugging that affects the boiler heat duty. To achieve this, the engineering assessment should involve an advanced analytical study to understand the following:

  • Tube leaks
  • Effect of repair, PWHT, and additional plugging on adjacent unplugged tubes
  • Effect of tube sheet metal loss on the integrity of the tube sheet

This article covers both the historic details of the failures and subsequent repairs and provides a comparison of the failures documented on-site with the analytical results determined from the approach implemented by SI. 

Figure 3. Hot Spent Boiler Plugging and Repair Welding


The overall approach adopted by SI: 

  • Develop finite element (FE) model to study design deficiencies, if any, using elastic analyses. That is, perform an elastic finite element analysis (FEA).
  • Develop a criterion to study the tube-to-tube sheet integrity.
  • Using the same FE model, perform elastic-plastic analyses to determine the effect of repair and PWHT. 
    • This is to determine if any additional tubes should be plugged to reduce any adverse effects.
    • This is a sequentially coupled thermal-stress analysis.
  • Calculate the minimum required thickness for the various sections of the waste heat boiler.
Table 1. Criteria Used in the Analyses

Table 1. Criteria Used in the Analyses

Table 1 illustrates the criteria considered for the work described herein. Several stress magnitudes were considered, such as the tube material allowable stress, tube-to-tube sheet joint allowable stress, and the ratcheting limit. Typically, when elastic analyses are performed, ratcheting limits are helpful. However, this work did not utilize the ratcheting limit. The tube-to-tube sheet joint allowable stress and load are calculated using Section VIII, Div. 1, Nonmandatory Appendix A. The allowable stress and load are compared against the equivalent stresses and tube axial loads, respectively, from the FEA to determine the mechanical integrity of the tube-to-tube sheet location. However, since there exists a parallel damage mechanism (general corrosion), the yield strength of the tube is set as a limit to add conservatism.

Figure 4. Hot Spent Boiler as Modeled in FEA



Since the methodology requires the use of advanced analytical methods, an FEA model (Figure 4) was built and analyzed using the commercial FEA software package – Abaqus. The model included sufficient lengths of gas inlet and outlet sections, the tube support location at the mid-section of the mud drum, the refractory, and the brackets that connect the hot section with the boiler drum. Since the nozzles are far from the area of interest, they are not included in the model. Appropriate element types were utilized for this work. One notable feature is the use of beam elements for the tubes (Figure 5). Since the model includes hundreds of tubes, incorporating a solid tube and heat exchanger model adds both geometric and numerical complexity. The use of beam elements simplifies the model while significantly minimizing the numerical convergence issues when compared with the full solid models. 

Figure 5. Hot Spent Boiler Tubes Inside the FEA Model

All the analyses performed are thermo-mechanical analyses, wherein a heat transfer/thermal analysis is performed first, and the temperature profile from the thermal analysis is imposed along with respective mechanical loads in the subsequent stress/mechanical analysis. Initially, elastic models were used to assess the design adequacy of the subject boiler and to determine the bounding operating conditions for further analyses involving the repair process. For the analyses involving the weld repair and the post weld heat treatment (PWHT) followed by operating conditions, the sequence of steps is critical. SI discussed the methodology with the client when developing the accurate sequence to be included in the FEA. As stated earlier, to capture the effect of residual stresses (after welding and PWHT processes) on the corroded tube sheet section at the bottom where the leak was discovered, an elastic-plastic FEA is essential. Temperature results from the welding process step are shown in Figure 6. After welding and PWHT, the process conditions were applied to the model along with the number of plugged tubes at the time. Figure 7 illustrates the temperature distribution in the tube sheet, boiler drum, and tubes. Since the plugged tubes do not transport flue gas, the temperature of those tubes is the same as the water temperature around those tubes inside the drum.

Since the number of tubes is significant, post-processing of results is a challenge. SI developed a procedure to overlay the von Mises equivalent stress results on a spreadsheet layout that resembles the actual tube layout in the tube sheet. It is further simplified for better visualization in this article, as shown in Figures 8 through 10. Figure 10 (a) shows a historical perspective of the tube plugging over time. In the first set of analyses that SI performed, only the locations shown with greyish blue color (tubes plugged before Jan. 2022) were considered as plugged. The thermal analysis results for this case are shown in Figure 7. After performing the mechanical/stress analysis, it was observed that the unplugged tube locations shown in Figure 8 (a) with orange and red dots are of concern. The orange dot locations indicate the locations with stresses greater than the tube yield strength. The red dot locations indicate that the stresses exceeded the allowable stress. Since the criteria are set at joint location stresses exceeding the tube allowable stresses, both orange and red dot locations require tube plugging. Figure 8 (b) shows the locations where further tube leaks were discovered within weeks of the weld repair and plugging. The tube leak locations are identified with yellow marks. This gives the confidence that such analytical methods, when appropriately applied, can predict the locations of future failures.

Figure 6. Repair Welding Simulation – Heat Transfer Analysis

Figure 7. Post Repair and Plugging Process Conditions – Heat Transfer Analysis

After the discovery of the new leaks, further plugging was undertaken. These locations are identified by light blue dots in Figure 10 (a). SI incorporated these changes in the analyses and determined that the locations shown in red and orange dots in Figure 8 (a) are still a concern, as shown in Figure 9 (a). This was later confirmed by further tube leaks (see Figure 9 (b)) found after 6 weeks of the previous plugging was completed. This further assured the value of performing such an engineering-based approach rather than a traditional grand-fathering approach which would use plugging methods adapted for similar units based on historical information. It should be noted that SI was engaged in this study at the period between weld repair and second set of plugging as shown in Figure 8. However, all the results were made available just prior to the third leak shown in Figure 9 (b). At this time, the Client utilized the results from the FEA and decided to add additional tube plugging as shown in Figure 10. SI performed analyses with the final set of plugging, and the results indicated that other tube locations around the plugged tube locations are not of concern (see Figure 10 (b)).

Figure 8. Post Repair and Plugging Process Conditions Criteria Check and Field Observation

Figure 9. Additional Plugging and Field Observation after that Plugging

The last set of analyses were performed in mid-March of 2022, and after 6 months, the boiler did not experience any further leaks. While the engineering approach predicted the issues, it is cautioned that any engineering analysis can only simulate the known degraded material thickness and properties in the analysis and not the corrosion degradation mechanism itself. The rate of deterioration and the interaction of various damage mechanisms should be monitored by the operator.

Figure 10. Tube plugging to-date and tube-to-tube sheet joint stress state


  • The original, as-installed condition did not show significant issues. It is believed that other damage mechanisms caused the initial failures leading to the plugging of tubes around the periphery of the tube sheet.
  • The study captured the recent joint issues, specifically the failure after the plugging and repair weld performed in early January 2022. 
  • Thinner regions are more prone to further failure. The minimum required thickness using the same criteria is established for the tube sheet.
  • The analysis was successful in predicting the minimum number of tubes to plug.
  • Plugging the correct number of tubes stopped the typical tube failure cascade.
  • The applied results were directly proven. Once the results were fully implemented, the waste heat boiler had no unplanned shutdowns. The analysis met its goal and provided a major business impact.
  • Analysts need sufficient information to minimize assumptions and make a robust model.
  • Clear understanding of API 579, ASME Section VIII Div. 1, ASME Section I, and FEA is necessary to develop robust, realistic, and relevant engineering solutions.

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SI FatiguePRO for Hydrogen Fueling Station Assets - Vessel Life Cycle Management

News & Views, Volume 50 | SI FatiguePRO for Hydrogen Fueling Station Assets


By:  Erick Ritter and Daniel Peters

SI FatiguePRO for Hydrogen Fueling Station Assets - Vessel Life Cycle ManagementInitial introduction of many of the hydrogen fueling stations to support this rapidly growing demand were installed around 2010. There were many designs of cylinders developed and installed at that time, many with known limitations on the life of the equipment due to the high pressures involved and cyclic fatigue crack growth issues due to hydrogen embrittlement.  The designs were often kept relatively simple to lower their costs often with little or no considerations for in-service inspection or potential end of life considerations.  Others involved innovative designs with reinforcing wrapping to try to enhance the life of the vessels, but by doing so, these designs limited the access to the main cylinder wall for in-service inspection. 

Many of these vessels are now reaching or passing the design life established by ASME.  This is resulting in problems for operators of this equipment as some jurisdictions will not allow the vessels to operate beyond the design life without inspection or re-rating of the vessels to extend the fatigue life.  SI’s FatiguePRO is a commercial software solution which has been addressing this exact concern for over 25 years.


News & Views, Volume 49 | The 4th Dimension- Lifecycle Assessment of Critical Structures

News & Views, Volume 49 | The 4th Dimension: Lifecycle Assessment of Critical Structures

By:  Dan Parker, PE

News & Views, Volume 49 | The 4th Dimension- Lifecycle Assessment of Critical Structures

By analytically simulating the steps in the construction process, including the sequence of concrete placements, and tracking the history of the material behavior starting from initial placement, the potential for cracking is evaluated by comparing the time dependent stress and strains to the concrete cracking resistance and capacity.

Aging Infrastructure Issues

The infrastructure in the United States is aging and, whether publicly or privately owned, significant resources are required to repair, replace, or modernize it.  Due to the high costs associated with these efforts, owners need to identify structures with high risk-of-failure consequences and find the most cost-effective solutions for rehabilitation.  High consequence infrastructure includes:

  • Highway and railway bridges,
  • Roadways for intra and interstate transportation,
  • Dams, locks, and levees for flood control and cargo transportation,
  • High rise business, apartment, and condominium towers, and
  • Power generation and distribution facilities for Nuclear, Fossil and Hydro utilities.

All infrastructure, is susceptible to degradation that comes with aging.  The accumulation of degradation, and a structures subsequent failure, is difficult to predict due to the numerous real-world factors that influence rates of degradation.  These real-world factors can lead to some structures failing prematurely and others lasting well beyond their original design life.  Asset owners need to be on the lookout for:

  • A structure that is nearing or has exceeded its expected design life,
  • A structure that shows signs of steel corrosion, freeze-thaw damage, or concrete degradation such as alkali aggregate reaction (AAR),
  • A structure that is overloaded due to an increase in auto, truck or rail traffic,
  • A structure with a known design deficiency when evaluated with modern design code requirements,
  • Increases in regional hazards, such as increased seismicity or increased probable maximum flood levels, and other climate change related issues.

Often, structures are kept in service beyond their original design life.  Many older structures are held to a design basis, i.e. code requirements, consistent with the time the structure was designed. Evaluating older structures using current code requirements can potentially affect original safety margins both positively and negatively. Increased capacity limits can be established for steel welded and bolted connections and utilizing actual concrete compressive strengths above original design strength that may provide added safety margin. On the other hand, identifying substandard details relative to current practice, particularly concrete reinforcement detailing will reduce originally considered safety margins. Additional factors that can affect the service life of a large infrastructure projects include environmental conditions, reliability of materials, quality of construction, and loading conditions.

Throughout the country, many structures such as bridges, dams, and power generating facilities remain in active service as they approach or exceed their design (or licensed) service life. Replacement is often prohibitive for many of these structures due to cost. However, failure of these structures could have more significant consequences beyond lost revenue, including loss of life.  Identifying structural vulnerabilities and designing retrofit modifications is essential to economically extending the service life of these structures.

Current Regulations

There is no single agency that oversees the various types of infrastructure within the United States. The following structures generally fall under the purview of these agencies:

  • Bridges, Roadways and Railways – National Transportation Safety Board, Federal Highway Administration, State Level Departments of Transportation, and some local City Departments of Transportation
  • Nuclear Facilities – Nuclear Regulatory Commission (NRC), US Department of Energy
  • High Rise Buildings – State and Local City Building Departments
  • Dams for Hydroelectric and Water Storage – Federal Energy Regulatory Commission (FERC), State Level Dam Safety Departments
  • At a high level the different regulatory bodies have a common mission to keep asset owners accountable to maintaining the mandated level of safety for the general public. Different regulations and procedures are required depending on the type of project, owner, and overseeing agency involved.

Lifecycle of a Structure

As structures reach the end of their design service lives or are in extended service, regulators typically require asset owners to demonstrate that these structures can still maintain their functionality while posing a low risk to the public safety, regardless of expense to the owner. Thus, it is beneficial for the owner to perform maintenance to ensure safe and functional assets that are profit positive, versus the potentially large costs incurred during decommissioning, removal and remediation of project sites.

Lifecycle structural health monitoring and simulation is a methodology to track changes in a structure that occur during the structures service life. Monitoring can be performed through non-destructive examination techniques. Continuous health monitoring helps owners maintain their assets by providing a warning if a sudden change or degradation accumulation is observed.  This data can feed desktop simulations which incorporate the time variable into the modeling of the asset, giving point-in-time snapshots of how the structure behaves under loading during different stages of its life.

Predicting Degradation: During Design

During the design phase, large infrastructure projects are designed for a variety of expected loads including thermal load cycles, live loads,  and operational loads. Seldom is the cumulative impact of cyclic loading considered when estimating the expected service life of the structure. Incorporating transient seismic demands or some other unexpected blast, shock or impact loading in combination with the expected stress range that occurs in structural components the lifecycle endurance limit can be evaluated that may be different from originally established design basis limits. For example, concrete degradation typically manifests itself as cracking, sometimes occurring in unexpected locations. Cracking can allow water infiltration, leading to internal corrosion of reinforcement and corrosive swelling, which can weaken the structure and accelerate degradation. In cold environments repeated freeze-thaw cycles will further damage the concrete. 

Cumulative damage not only affects the loss of static strength, but will also change the dynamic characteristics of the structure. This can lead to the poor performance of a structure supporting vibrating equipment or a structure subjected to seismic loading. By incorporating the effects of damage accumulation in a structural assessment, the time-varying dynamic characteristics of the structure can be identified. Incorporating these effects as part of a lifecycle assessment can provide the owner with a more realistic understanding of actual structural condition of their asset that can guide targeted remediations (i.e. mitigate excess equipment vibration) or alert the owner to an increased risk of failure under a postulated seismic event.  

Predicting Degradation: During Construction

During construction of mass concrete structures large temperatures develop due to concrete curing.  A Nonlinear Incremental Segmental Analysis (NISA) evaluates the thermal and static loading of young concrete to determine the potential for cracking.  The propensity for cracking depends on the concrete mix, environment, and boundary conditions imposed during construction.  The concrete temperature varies with time and depends on the volume and rate of concrete placement, the sequence and geometry of the placements, the concrete placement temperature and heat generation rate, and the ambient conditions.  The boundary conditions imposed during construction depend on the sequence and geometry of the placements, the interaction with the foundation/formwork and any adjacent or embedded structures, and the time dependent aging, creep, and shrinkage properties of the already placed concrete lifts.  To accurately account for all of these factors, the NISA must be capable of representing a coupled thermal-mechanical analysis with nonlinear material properties.  By analytically simulating the steps in the construction process, including the sequence of concrete placements, and tracking the history of the material behavior starting from initial placement, the potential for cracking is evaluated by comparing the time dependent stress and strains to the concrete cracking resistance and capacity.  The cracking resistance is constant for any mature concrete present, such as pre-cast concrete forms, but is time dependent for the freshly placed young concrete since the tensile strength and modulus are changing as the concrete hardens and ages.

Figure 1. Aging Structures and Decreasing Margin of Safety

Predicting Degradation: During Service

A concrete structure often has predictable and repeating loading patterns over the course of its service life.  For instance, a dam will reliably have high and low water levels throughout the year, though the actual levels may depend on the weather in a given year.  A bridge will reliably experience different load patterns in weekday versus weekend traffic.  A nuclear containment structure will experience thermal load cycles during power generation operation and shutdown for planned outages.  

When looking into the future, engineers make reasonable predictions of different loading events during the initial design phase of a structure. Supplementing these prediction methods with sensor data and observed damage from onsite can help predict the time where the structure goes from safe to unsafe and remedial measures need to be taken. Sophisticated concrete material models, such as SI’s proprietary ANACAP model, can incorporate all known forms of time-based concrete behavior such as creep, shrinkage, radiological degradation, cement hydration, alkali aggregate reaction, steel corrosion, scour of concrete, and freeze-thaw cycles. This can further enhance the predicted structural performance during the design basis and extended license period of critical infrastructure as part of an asset owners risk management program.

Figure 2. Concrete arch dam circa 1909, aging degradation Issues subject to increased flood and seismic demands

Time-Dependent Margin

Figure 1 shows the capacity of a structure to resist a large event (such as a flood or earthquake), and how the margin of safety changes over time. Due to safety factors built into design codes, new structures have a minimum margin of safety against failure even when accounting for small design approximations and construction errors.  That margin of safety can decrease when a transient event causes damage (e.g. an earthquake, ship impact, or large flood) and as the structure ages and degrades over time.  Further reductions in margin can occur as hazards can increase over time, such as when flood events become more common or when new earthquake faults are discovered from geologic mapping.  Over a structure’s service life, as it accumulates damage from both transient events and aging, the available margin may be much lower than what was originally intended, increasing the risk of catastrophic failure.

Figure 3. Concrete placement with active cooling to reduce concrete heat generation

Answering Tough Questions

Can an asset survive an earthquake or large flood event today? How big of an event can it survive? Can it survive the same event ten years from now?  How does the structural performance change if we put a remedial measure into place?  Without remediation, how long until the structure is unsafe? These questions can be answered with time-based structural lifecycle modeling.  

Although much of the infrastructure in the USA is already functionally obsolete – or worse: at risk of catastrophic failure – much of it is effectively operating safely beyond its original design life.  Finding assets with the highest risk allows owners to prioritize limited funding for rehabilitation and remediation.  Lifecycle modeling helps answer those important questions when the key decisions need to be made.

Figure 4. Example of Section Loss Contour using High Definition Scanning (HDS), Spectral Analysis of Surface Waves (SASW) and Acoustic Tomography (AT) Methods


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News & View, Volume 46 | Identifying Failure Mechanisms of Typical I-Section Floodwalls

News & Views, Volume 46 | Identifying Failure Mechanisms of Typical I-Section Floodwalls

By: Eric Kjolsing and Dan Parker

News & View, Volume 46 | Identifying Failure Mechanisms of Typical I-Section FloodwallsIn 2018, Structural Integrity Associates, Inc. (SI) supported the United States Army Corp of Engineers (USACE) in the structural assessment of the concrete-to-steel connection in typical I-Section flood walls. A representative flood wall section is shown in Figure 1. This effort was part of a broader scope of work in which the USACE is revising their guidance for the design of flood and retaining walls, EM 1110-2-6066.  The purpose of the structural assessment was to better understand the mechanics of load transfer from the reinforced concrete section to the embedded sheet pile. Three-dimensional finite element models of the connection were developed employing non-linear constitutive properties for the concrete, structural steel and reinforcement to achieve this goal.  A total of nine different I-Wall configurations with varying wall geometry, sheet pile embedment depth, and connection details were analyzed.  Hydrostatic load was applied incrementally to simulate the actual load distribution due to a rising water level. 


News & View, Volume 46 | Adding Value Through Test Informed Modeling- Hydro Structures

News & Views, Volume 46 | Adding Value Through Test Informed Modeling: Hydro Structures

By: Eric Kjolsing and Dan Parker

News & View, Volume 46 | Adding Value Through Test Informed Modeling- Hydro StructuresIn 2018, Structural Integrity Associates (SI) supported a utility in the structural assessment of a submerged concrete intake tower.  The tower is nearly a century old and was investigated as part of the utility’s periodic maintenance program. 

The assessment required the generation of an analysis model that accounted for both the structure and the surrounding water.  When accounting for fluid effects, a typical analysis approach is to develop a fluid-structure interaction (FSI) model that explicitly accounts for the interaction between the surrounding water and concrete tower.  However, this modeling approach is expensive both in terms of (a) cost, due to the increased effort needed in generating the model and (b) schedule, due to the increased analysis run time.  In lieu of developing an FSI model, SI implemented an alternative numerical approach to model the effects of the water and justified the approach through physical testing of the in-situ structure.