Becht has been involved in a number of flow induced vibration problems. The problems typically occur in heat exchangers, boilers, flue gas coolers and piping downstream of bypass valves used for pressure letdown. Vendors typically design their equipment to operate outside the range of fluid flows to avoid the problems. However, problems can occur at off design conditions, e.g., process upsets, and outages where more flow is diverted through a piece of equipment than the design rates or unit capacity "creep" where the flow has increased flow beyond original design throughput. The driving forces are fluid phenomena such as vortex shedding, acoustic resonance, and fluid elastic instability. These phenomena can cause vibration and failures in tubes, tube banks, large diameter, thin-wall piping and small branch connection attached to larger diameter piping. Vibration amplitude is magnified if the exciting force is +/- 20% of a component's natural frequency. There are a number of industry criteria to evaluate the potential for vibration. One such criterion we have used to evaluate instability in tube banks in flue gas coolers is shown on the chart. Published industry data on the regions of Stable (low probability) and Unstable (high probability) regions of tube bank instability are shown. Overlaid on the data are the data for a superheater (SH) and high pressure section (HP1) of a flue gas cooler on the back end of a Fluid Catalytic Cracking Unit. The 1250F flue is used to generate steam. The data is shown for two mass flow rates, normal operating and a higher flow rate when one of the units is offline for repair. At the higher flow rates the SH and HP1 sections move into the lower end of the Unstable region.
Becht Engineering has been awarded an Owner’s Engineering Quality Assurance Project for Inter Pipeline Heartland Petrochemical Complex in Alberta, Canada. The Heartland Petrochemical Complex is being designed to convert locally sourced, low-cost propane into 525,000 tonnes per year of polypropylene utilizing Propane Dehydrogenation (PDH), Polypropylene (PP) and Central Utilities Block (CUB) facilities. Construction of the Heartland Petrochemical Complex is in progress with completion scheduled for late 2021. Becht Engineering will conduct a series of Cold Eye Reviews focused on operability of the complex.
Becht Engineering provides a wide array of engineering services to the refining, petrochemical, and power industries – including continuing services to 90% of the refineries in the United States and Canada.
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Recently, Becht Engineering assisted a client in their consideration of a novel licensed technology process to upgrade a black waxy crude oil to a synthetic crude with improved transportation properties. This proposed upgrade would assist the client to process crude oil. The client requested that Becht Engineering conduct a review for the proposed upgrade.
Recently, Becht Engineering assisted a client in their consideration of a novel licensed technology process to upgrade a black waxy crude oil to a synthetic crude with improved transportation properties. This proposed upgrade would assist the client to process crude oil. The client requested that Becht Engineering conduct a review for the proposed upgrade.
Upon notification, Becht Engineering immediately went to work by preparing a technical due diligence study that consisted of a process technology feasibility review of the proposed novel technology. Utilizing a seasoned technology specialist (SME) in Fluidized Catalytic Cracking, our expert reviewed the process flow diagram, unit configuration, theoretical and lab-based yields, feedstock and other relevant information.
The Becht SME reviewed the data with the objective of identifying deficiencies that would impact the operational success of the novel technology. He then worked with the client to obtain the necessary data, information, and documentation to conduct the review.
After a thorough technical review, feasibility review, and looking closely at the technology’s operation, pressure and heat balance, utilities, catalysts, and chemicals, Becht Engineering concluded five important findings previously unknown to the client that were included in our final report.
Becht Engineering found that while proposed yields for the upgrade were reasonable, the operation of the technology was not feasible without further modification to the system design. Becht then provided cost-effective and practical guidance that enabled the novel technology to be implemented.
Becht Engineering’s Process Technology Subject Matter Experts are available to provide technology feasibility reviews, as well as many other process technology services. Please see our broad range of capabilities at Becht Process Consulting.
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Last April Becht Engineering worked with an Arkansas-based refinery client to evaluate the reliability and operational challenges with a Naphtha Hydrotreater (NHT). Becht Engineering was requested by the client to investigate the NHT’s series of reliability failures that included everything from stripper overhead corrosion and recycle compressor repairs, to feed/effluent exchanger leaks and stripper feed/bottoms exchanger repairs.
- And all of these issues after the NHT catalyst had been replaced multiple times -
The client had been actively pursuing internal Root Cause Analyses (RCA) before requesting Becht Engineering to look into the problem. That this client came directly to Becht Engineering for solutions speaks to our success and experience conducting Root Cause Analyses, providing hundreds of clients worldwide with recommendations for the improvement of bad actors and mal-design.
Becht outlined a straightforward, three-pronged approach to tackle this problem. First, we utilized a senior hydroprocessing specialist with over 30 years of experience in the industry to review and analyze data and documents we requested from the client.
Second, this senior engineer was deployed to the site for a 3-week visit to engage with the client personnel to better understand what was happening with the NHT and the operations that interface with that piece of equipment. Additional remote Subject Matter Experts (SMEs) were kept on call to provide any support the client needed.
Finally, after a thorough investigation of the NHT, our senior engineer was able to pinpoint the issue and began developing a report and presentation that was delivered to the client. This detailed report included consolidated data, analysis of that data, contributions to the problem and technical recommendations to eliminate and/or mitigate the failures.
The process is completely streamlined to support our clients to manage their problems quickly and efficiently. Our SMEs are always available to consult on problems such as these – and that’s why Becht Engineering continues to be a leader in solving some of heavy industry’s most unique challenges.
Please see our broad range of capabilities at Becht Services.
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During turnaround inspection of a Reactor, High Temperature Hydrogen Attack (HTHA) damage was found on the 18” nozzle and the inclusion/planar flaw was found on the 18” nozzle to head weld. Figures 1(a) and (b) show the locations of these flaws. Figures 1(a) and (b) show the locations of these flaws.
Besides these two flaws, an inclusion/planar flaw was also found at the head to shell weld. This inclusion was 0.2” from the inside surface, 0.12” in depth and 9.3” in length.
Becht was contacted to conduct the analysis of the HTHA damage and used some of the following in its methodology:
Becht provided a detailed report and presentation for the client, and delivered its conclusions that were among the following:
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Last March, Becht Engineering was asked by a client to inspect and analyze the development of oxidation and associated failures in a steam preheat coil mounted in a stack of a reformer heater. Because the client intended to replace the steam coil during an upcoming shutdown, they requested that Becht tailor an analysis to identify more robust design options that could be incorporated during that time.
Becht Engineering deployed three Subject Matter Experts (SMEs) that conducted work in four stages: 1) Design Review; 2) Operational Review; 3) Root Cause Analysis (RCA), and then 4) offered recommendations to the client. The SMEs consisted of a Furnace Engineering Specialist, a Fire Heater Process expert, and a Fitness for Service (FFS) Specialist in Fatigue and Fracture Mechanics and Non-linear Stress Analysis.
Our SMEs found that oxidation of the coil creates tube cracks and other failures. With their many years of experience in heavy industry and ability to conduct empirically-sound testing and analysis, our SMEs found that oxidation and blistering of the tube was caused by increased flame extension and/or secondary combustion of unburned combustibles in and around the coil.
Becht Engineering provided three detailed recommendations to the client in order to mitigate the situation. After reviewing a variety of data sheets from the client, Becht provided a robust 18-page report that outlined methodology, analysis, conclusion, and recommendations to solve the problem. The entire scope of work was completed in less than two weeks after receiving the proper documents and data sheets from the client.
As a direct result of Becht Engineering’s failure analysis, these issues that were previously unknown to the client were mitigated in a safe, timely, and cost-efficient manner.
Recently, Becht Engineering mobilized a Piping Subject Matter Expert to a client site in Oceania, to provide technical assistance with regards to damage assessment, fitness for service evaluations, modification, replacement of piping components and pipe supports which sustained damages from a recent earthquake. Becht’s SME spent 3 weeks onsite where he was a member of a multidisciplinary engineering team to help bring the earthquake damaged process unit back online. He assisted in the damage assessment, fitness for service, repair, and replacement of piping, pipe support components and other mechanical equipment of the process unit. The process unit returned to normal operation after the repair.
Becht Engineering is continuously working on challenging and innovative projects for our clients. Listed below is a select sampling of recent projects from our portfolio.
Coke Drum Skirt Replacement - No Downtime
A major refinery in the Middle East requested Becht Engineering to evaluate the problem of their coke drums walking and tilting, and the baseplate/skirt/anchor bolts corroding. After an initial site visit by Trevor Seipp, Division Manager of Becht Engineering Canada Ltd., we determined that the lower portion of the skirt as well as the entire baseplate needs to be replaced for all eight coke drums. In addition, some of the coke drums needed to be re-leveled, and all needed to have shims and slide plates installed. Finally, all of the anchor bolts need to be replaced. And all of this needs to be done without any downtime. Building on the success of our world’s first in Texas, Becht Engineering developed a window methodology to complete the work whereby after we temporarily jack and re-level the drums and place them on temporary shims and slide plates, entire windows of the skirt (~1m x 1m) would be cut out and replaced with new material. While the window is cut out, however, we will have excellent access to the concrete deck to enact the anchor bolt replacement. What will make this project especially challenging is that all of this work needs to be completed while the drum remains in operation. Plus, there are 24 windows per drum and eight drums. All of this work needs to be developed using first-class engineering skills. Since the internal pressure, high temperature, and widely fluctuating weight due to the hydrocarbons, coke, and quench water all have to be evaluated during the repair process, Becht Engineering applied its expertise in finite element analysis (FEA) to the task. Our analysts are among the best in the business, and have performed evaluations on some of the most challenging problems.
Becht Engineering recently reviewed a utility company's deaerators which experienced a through-wall, circumferential crack at the toe of the fillet weld attaching the saddle to the shell and a through-wall crack was found at the head-to-shell junction at the steam inlet end of the drum.
The saddle to shell cracking was attributed to restrained axial thermal expansion of the shell at a tightly bolted sliding saddle support. The crack was ground out, welded and the support modified to permit sliding.
The head to shell crack cause was attributed to corrosion fatigue, a common occurrence in deaerators. The crack was most likely initiated at a weld surface defect on the I.D. of the drum and grew with time. The daily operating cycles of the drum during periods of reduced steam demand and thermal stresses which we attributed to a poorly designed steam inlet nozzle were the main contributors to the crack growth.
A large diameter superheated steam inlet nozzle extended through the head of the drum terminating 18” into the vessel. The steam exited through a rectangular shaped slot opening on the underside of the pipe which directed the flow of superheated steam directly into the condensate on the bottom of the drum near the shell-to-head weld.
Becht’s computational fluid dynamics model (CFD) indicated that there was little dispersion of the steam exiting the nozzle and that the velocity of the steam mixing with the condensate was relatively high. The superheated steam contact with the cooler condensate resulted in a violent reaction with localized heating and cooling of the vessel shell. This cycling can cause thermal stresses which can result fatigue cracking. Generally, fatigue cracking occurs at welds and heat affected zones adjacent to the weld. Notably no such weld cracking occurred at the opposite end drum where there is no steam inlet nozzle. Figures 1a and 1b demonstrate that the flow trajectory of the incoming steam acts more like a high velocity jet of steam directed at the condensate in the bottom of the drum.
To provide better dispersion, Becht recommended that the inlet pipe be modified with multiple and wider distributed holes. In this configuration, the nozzle acts as a sparger which better disperses the steam in all directions and at lower velocities as can be seen in Figures 2a and 2b.
Focus on Corrosion – A Refinery-Wide Risk-Based Inspection (RBI) Program
Becht Engineering has completed development of a Refinery-wide Risk Based Inspection Program (RBI) at a Caribbean Refinery with a focus on corrosion damage mechanisms. The Refinery has observed accelerated corrosion in recent years, resulting in a higher than expected equipment replacement rate and commissioned the study in order to develop a plan to mitigate the corrosion.
Becht used its proprietary Risk-Based Equipment Reliability Planning work process which is embedded in the our software program STIER© (Strategy Tool for Improving Equipment Reliability) to develop the Risk-based Inspection and Maintenance Plan to address corrosion-related failure scenarios for fixed equipment, rotating equipment and piping circuits. Our work process is compliant with API RP 580, Risk-Based Inspection and ASME PCC-3-2007 Inspection Planning Using Risk-Based Methods.The subject matter expert (SME) based approach was employed to develop failure scenarios. Once the relevant equipment data were collected and loaded into STIER, a Senior Metallurgists, James McLaughlin, reviewed the information and pre-developed damage mechanisms and failure scenarios for each item. Becht also employed the advice of a Senior Furnace SME, Robert Dubil, to develop failure scenarios, and inspection and maintenance plans for the furnaces. The facilitation team for the RBI Work Process was Dr. Eileen Chant and James McLaughlin.
During a routine inspection of a Delayed Coker Bottom Flange, a ½” deep, 360-degree crack was found by a refinery client the location of the crack is shown in Figure 1. At the time of discovery, the flange was set to be in service until the next turnaround, so a crack-like flaw analysis was performed to evaluate the flange integrity.
Becht Engineering was contacted to perform a detailed analysis. Once the client provided Becht Engineering with all requested material data, Becht conducted a detailed investigation and analysis and presented the client with a report with conclusions and recommendations.
Becht performed a variety of unique methodological analysis during for this project, some of which included:
Becht also conducted a Transient Heat Transfer Analysis, a Stress Analysis, and a Crack Stability and Crack Propagation Analysis for the client. After a thorough analysis for the client, Becht provided the following conclusions:
Becht found that if the crack propagates into the bolt holes, the crack should stop once it reaches the bolt hole.
Deflagration is a combustion that propagates through a gas/dust mixture at a rapid rate. Dust collection systems often use bag houses to enclose long tubular filters to remove the dust from a gas/air before it is vented. The National Fire Protection Association (NFPA) have rules for suppressing the combustion or venting the excess pressure that occurs in a deflagration. In this case the client had a bag house and wanted to determine if it was structurally adequate to contain the rapid rise in pressure without a catastrophic failure. Although the baghouse had a suppression system, it takes time for the system to detect a deflagration and for the suppressant to be injected into and baghouse., i.e., there is a lag time between the very rapid pressure rise and slower suppressant activation time. In the Finite Element Analysis (FEA), the dynamic pressure rise was applied internally to the sides, top and bottom of the baghouse as shown in the pressure-time history graph. The peak pressure occurs in less than 50 milliseconds and reaches a level of nearly 3 psig, the maximum unsuppressed pressure that the baghouse must sustain without failure. The figure shows the stresses in the baghouse with the grey areas exceeding two-thirds of the yield stress of the material. Structural modifications were recommended to meet the required NFPA requirements.
Recently a client installed a flue gas expander at refinery in Colombia. The refinery had been experiencing problems with excessive vibration and expander trips that was due to the accumulation of catalyst on the expander blades. The client contacted Becht Engineering for support on the design and analysis of the steam injection piping, and requested support in the evaluation of the erosion/corrosion potential on the injection of steam into the flue gas line.
Becht Engineering sat down with the client and reviewed the drawings provided by the refinery. Based on the data, Becht found that a conflict between the original design intent and the current design. Additionally, the discharge velocity at the design steam rate was found to be very high, and Becht was asked to provide a recommendation to solve the problem.
Utilizing Subject Matter Experts with many years of experience in the industry, Becht Engineering conducted a process analysis and resulting report that detailed findings for the normal expander operation, the operation during the thermal cycle, and the potential for erosion and corrosion.
Becht Engineering closely went over the data with the client and provided recommendations to mitigate the high velocity at the steam injection point by replacing the single radial injection nozzle with two angled nozzles. Becht developed a concluding report that detailed all the recommendation for mitigation and delivered it to the client.
Becht Engineering recently conducted a Fitness for Service analysis of an FCC Regenerator for a major refining client. The client contacted Becht after they discovered that the Regenerator had a small region near its riser nozzle with compromised material strength.
Becht conducted a hardness test during the client’s March 2018 turnaround and found that the Ultimate Tensile Strength (UTS) at the particular location was approximately 20% lower than the minimum UTS of SA-516 Gr. 70 material. After a detailed analysis, Becht Engineering found the material UTS was compromised because of the previous hot spot at the location, so the strength reduction was likely due to creep damage.
The client requested Becht conduct an analysis in order to figure out exactly what was happening. Becht Engineering did an elastic plastic finite element stress analysis per API 579-1/ASME FFS-1 requirements to determine whether the Regenerator is fit for service with the compromised material property.
Becht’s analysis concluded that the FCC Regenerator was fit for service and recommended that the refractory in the hot spot region be repaired to prevent future hot spots. If stud welded anchors were used, Becht recommended a variety of testing options to determine the quality of the weld.
ASME FFS-1 / API-579 Level 3 Assessment Avoids Unnecessary Downtime
In 2011, Becht Engineering was approached by a major refiner on the US West Coast to assist with the evaluation of two vacuum towers. Due to a process change, corrosive products were occurring at elevations in the towers where there was insufficient protection against corrosion. As a result, significant and extensive corrosion was measured. In many places, the wall thickness was less than the tmin established by conventional engineering assessments.
Trevor Seipp, P.Eng. from Becht Engineering’s office in Calgary, Canada took on the job of evaluating the towers’ fitness-for-service. Typical evaluations of local thin areas in pressure vessels and piping are performed frequently and easily by skilled engineers. However, this particular situation was unique in that the corrosion was extensive – the corrosion map data for each tower had over 20,000 data points, and the normal operating condition for the vessel was external pressure (vacuum).
Trevor was able to quickly write a custom-made subroutine for our finite element analysis software, ABAQUS, that used the corrosion map to modify a finite element model to accurately simulate the corrosion extent with the actual remaining wall thickness. Using this tool, we were also able to simulate future uniform corrosion forecasts. Finally, we performed an elastic-plastic stress analysis that also accurately simulates buckling failures. It is vitally important to evaluate for buckling failures because that is the primary failure mechanism of vessels that operate under external pressure (vacuum).
Each tower was evaluated for fitness-for-service at the current corrosion state. Each tower was fit-for-service. Future uniform corrosion forecasts were also evaluated from the current time to the next scheduled maintenance outage. One tower was fit-for-service at the next maintenance outage. The other tower would not meet the required design margin at the next maintenance outage, but was not predicted to actually fail. However, state law and good engineering practice require compliance with the required design margin. Accordingly, a fillet-welded lap patch repair was designed. This repair was also simulated in the analysis to determine if it would be suitable; and it was.
All of this work was completed in less than 3½ weeks; on-time and under-budget. Becht Engineering’s report was presented to the State Pressure Vessel Regulator and as a result the refinery was permitted to continue operating. The scheduled maintenance outage has since occurred and there were no further incidents with the vacuum tower.
Flue Gas Piping Design
Becht engineers did the analysis and design of the inlet and exhaust piping for a high temperature (1350F) 20 MW flue gas expander. The expander extracts energy from the overhead flue gas from a Fluid Catalytic Cracking Unit. The work included weight and thermal stress analysis of the system, expansion joint specifications and the detailed design and fabrication drawings of the specialized supports for the expander inlet, exhaust and bypass piping.
Fired Heater Tubesheet Design Review
Prompted by the failure of a convection section tubesheet in fired heater, the client wanted assurance that a similarly designed tubsheet in a new heater would not fail. The Finite Element Analysis (FEA) and review of the material properties showed the structural adequacy of the vendor's design and materials selection for the severe operating temperatures and corrosive flue gas environment.
Becht engineers performed this analysis on a high temperature valve for a jet engine test facility. The valve had an operating temperature of 1850°F with short term excursions to 2075°F. The valve is exposed to thermal shock heating due to the jet engine gas being forced through it at a high velocity. The work included a finite element thermal transient analysis, coupled with a stress analysis, to determine the stresses on the valve during the heat up and cool down during operation. We determined that the highest stresses were experienced only a few seconds into the transient cycle, something that a steady state analysis would not have captured. Using the information from the stress analysis, a fatigue life assessment was performed on the valve.
Last April, Becht Engineering completed a walk-down of a client’s refinery jet fuel treating system from the MEROX unit down through the polishing unit operations, including the salt and clay treating towers. Becht worked closely with the client to continue the investigation around root causes of recent severe operating difficulties meeting the ASTM Jet Fuel Thermal Oxidation Test (JFTOT), and to provide further technical analysis toward ranking considerations in the previously submitted RCFA Report.
The Becht team was accompanied by a chief area operator and other supporting process engineers knowledgeable of the jet fuel treating system. Together, multiple 1-liter sample bottles were filled consecutively at various existing sample point locations along the jet fuel treating sequence, including low-point drains between process vessels where no routine sample point exists.
All of the jet fuel treating vessels and interconnecting pipework were externally inspected for piping configurations that include horizontal sections followed by vertical risers that could act as liquid traps for caustic, as Bob had encountered this phenomenon in other jet fuel processing units.
Based on our SME’s experience in evaluating numerous other jet fuel treating systems, it was Becht Engineering’s impression that client’s system was in relatively robust shape, but with some separation units undersized. Several retrofit improvements had been made on the client’s side, and the Becht team concluded that the MEROX Unit section was performing well, but dealing with ongoing contaminant issues destabilizing the JFTOT test which compounded the challenges of undersized separation equipment.
The Becht team concluded that the client’s jet fuel treating system was suffering JFTOT reliability issues due to high levels of surfactants, likely entering the facility with the crude oil, and leading to hazy jet fuel which increases the likelihood that contaminants will lower JFTOT reliability. Several recommendations were submitted to the client including improved sampling and monitoring procedures, new analytical procedures to generate leading indicators of JFTOT quality deterioration, cutting edge technology in filtration and coalescing technology, optimized drying techniques, and finally the SME performed a small-scale tabletop analytical demonstration confirming that the prescribed improvements would greatly lower moisture and haze content.
Pressure Vessel Fitness for Service
A major oil company revitalizing an Iraqi oil field called on Becht Engineering to review several hundred pressure vessels containing thinned regions to determine their fitness for service (FFS) per the API-579-1/ASME-FFS-1 Part 5 process for analyzing Local Thin Areas (LTAs). Using ultrasonic testing (UT) and visual inspection data, Becht Engineering customized its proprietary API-579-1/ASME-FFS-1 compliant General Metal Loss and Local Thin Area evaluation software to create a program capable of evaluating the maximum allowable working pressure (MAWP) of these pressure vessels in an efficient manner. The software is very easy to use and presents the results in a user-friendly format for evaluation by engineers and managers. Because of the extensive nature of the corrosion, large files containing UT scan data needed to be evaluated, and the software was enhanced to parse and analyze the LTAs in the large datasets. This highly efficient process allows rapid evaluation of LTAs, and most vessels were completed in less than one week’s time. Becht’s software modification has also included the COMPRESS vessel design software output as an input to these FFS assessments. Becht has performed Level 1 and Level 2 LTA analyses on various vessels, evaluated sections of vessels for total replacement, analyzed external damage caused by shrapnel from previous wars, and developed repair procedures within the means of the maintenance team for this remote oil field.
Pump Trips and Check Valve Closure
Systems running multiple pumps in parallel can undergo serious equipment and piping damage during a pump trip caused by a power outage or pump mechanical failure. Uncontrolled reverse flow in the system can occur and if improperly selected check valves are used it can result in pumps running backwards or transient pressure spikes in the system, i.e., water hammer. Water hammer will occur if reverse flow occurs prior closure of the check valve and the effect increases with higher reverse flow velocity. Becht has worked with clients on analysis of the design of their systems, e.g., a water treating facility running multiple 52,000 gpm pumps in parallel, a seawater pump station pumping cooling water through a several mile pipeline to an inland facility and a boiler feed water circulation system for a 3000 psig forced circulation boiler.
Becht’s team of Reliability Specialists rolled out a risk-based work selection (RBWS) program at a number of refineries for a major Refining Company. Becht’s methodology utilizes RBWS, which is an industry Best Practice for Turnaround Work Scope development. Our RBWS methodology utilizes Becht’s proprietary STRAITS© software to enable risk-based decision making in equipment operations, inspection and maintenance.
The user-friendly Becht tool, STRAITS©, integrates the RBWS process and was optimized to be used in a group setting to facilitate. The software, which has an embedded S/H/E and financial risk-calculator, was delivered along with supporting documentation and training.
Our expert facilitators worked with plant personnel engaged in TA Work Scope development. A comprehensive assessment of the Work List was done to assess the drivers (Process or Equipment Integrity) for TA work. A structured risk-based process, using Becht’s software, was used to challenge the work list items and determine whether the TA work is justifiable, or whether deferments are permissible. For TAs, requiring 200,000 to 1,000,000 mhrs, the Becht process identified potential savings of 15-30% on estimated Direct Costs for the planned work scope.
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Becht Engineering is performing a seismic analysis to assess the structural integrity of the tanks subjected to a postulated earthquake. The motivation for performing the soil-structure interaction (SSI) analysis in the time domain is to capture the behavior of several contact interfaces present in the tanks including the interface between the tank concrete and the surrounding soil. The presence of this contact interface helps to establish a realistic initial geostatic stress state under gravity loading.
Before the SSI analysis was conducted, a site-response analysis was performed to determine the strain-compatible soil properties. Boundary conditions on the model were prescribed to enforce shear beam behavior of the soil column surrounding and supporting the tank. The seismic input was applied at the base of the SSI model as a force time series corresponding to the known acceleration record.
The model includes the tank waste and the effects of concrete degradation as illustrated below. The soil and concrete are modeled using linear elastic material properties with concrete degradation simulated through the use of equivalent degraded elastic properties. When linear elastic material properties are used to model soils, there is potential for developing artificial soil arching. Excessive arching behavior will result in underestimating the vertical loads on the concrete dome and tank sidewalls. To mitigate the potential for soil arching above the dome, vertical contact surfaces are inserted into the soil above the dome to create annular rings of soil that are free to displace vertically consistent with the tank dome, but allow the load to be transferred laterally during horizontal motion. This effectively creates a nonlinear yield mechanism that acts in the vertical direction only and allows for horizontal load transfer from one ring to the other ring. A low coefficient of friction is used, thereby ensuring that the soil load is carried by the tank structure.
The response history analysis results in large amounts of information generated during the simulation. To efficiently manage this information, post-processing routines are written to extract the maximum concrete demands in the tank and envelope those demands both spatially and temporally. This approach allows the analyst the flexibility to review and present results that are enveloped in both space and time, or that are enveloped in either space or time. The complete results that provide the response history for each point of interest in the model are also available, although the file sizes are much larger.
The seismic demands on the tank are combined with thermal and operating load demands and the combined demands are evaluating per the provisions of the American Concrete Institute Code 349 (ACI 349) entitled Code Requirements for Nuclear Safety Related Concrete Structures. When the updated analysis of record for the SSTs is complete it will provide a defensible technical basis for operating and maintaining the tanks and performing waste retrieval activities for the SSTs.
Special Purpose Computer Program for Large Diameter (10-30ft)Gas-Gas Heat Exchanger Design
Specialized equipment oftentimes do not lend themselves to standard design tools or even Finite Element Analysis which can be expensive for complex equipment and require individuals with background in FEA. In this case the client's design methodology used a combination of experience, hand calculations, spreadsheets and subcontracted analysis using proprietary software. Their desire was to have all the design done in one comprehensive in-house software package with the capability of investigating multiple design options rapidly. Over an elapsed time of about one-year Becht developed Windows based application with a user friendly interface for the design of large diameter, fixed tubesheet, gas-to-gas heat exchangers. The program, uses the method of finite differences supplemented with built in subroutines, e.g., materials allowable stresses, perforated tubesheets, etc. based on ASME Code Section VIII, Div. 1 rules. The program was validated using more elaborate FEA methods.
A steam cracking furnace for a major gulf coast petrochemical plant was severely distorted, with walls bulging out as much as 18 inches, due to an internal explosion. While conventional wisdom was that the furnace needed to be torn down and rebuilt, due to the extremely tight tolerances the wall burners had to be with respect to vertical, Becht Engineering designed a repair approach that enabled the furnace to be put back in production, with the walls restored to their original position. The photo shows the furnace with red jacking towers, used to jack the convection section back in place.
New Process Technology Scale-up
Becht has worked closely with the a number of our clients' process research and engineering groups on the on the scale-up of new process technology. One example of such an activity is the development of a process and mechanical design specification for a pyrolysis-based fluid bed biomass pilot plant. Becht engineers in the areas of fluid-solids process design, process simulation, analytical methods and mechanical design worked with a joint venture company to develop a process design specification of sufficient detail to provide an EPC contractor information needed to develop the detailed design. The specification included the preliminary design of the three major vessels (see reactor sketch) and solids transfer lines. In addition, Becht worked on the development of the process model (PROII simulation), the characterization of liquid product from the reactor overhead system and recommended the analytical laboratory program and methods needed to characterize the reactor overhead products
Vessel Remaining Life Assessment
An incident in a process used to remove contaminants from a process waste gas required the shutdown of the unit. The process contains multiple similar vessels that follow the same pressure and temperature cycle daily. The vessels had been in service for more than 40 years. (A typical pressure cycle is shown in the sketch.) The client wanted to determine a safe remaining life for the vessels. As an initial step, Becht conducted a Finite Element Analysis (FEA) using the operating pressure and temperature cycles. The focus was on those areas of the vessel with high peak stresses where a crack may already exist or develop in future operation. The remaining fatigue life was based on the methods in ASME Code Section VIII, Div. 2. Working with our affiliated company (Sonomatic) an estimate was developed of the minimum detectable crack size in the vessel wall. Becht recommended a fracture mechanics crack growth analysis be conducted to determine the number of cycles for a crack of the minimum detectable size to grow/propagate through the vessel wall. This information is used to set the maximum interval between inspections and to set a safe margin below the number of cycles for a through-wall crack to occur. Based on the outcomes of the above a long-term inspection program for the vessels could be established,
A cryogenic piping system was found to have a numerous instances of weld misalignment of circumferential butt welds joining sections of the piping system. Becht was asked to evaluate the effect of such misalignment and if repairs were required. The photo shows a typical cross section through the pipe showing the misalignment. Becht evaluated the misalignment using the methods of API 579 Fitness for Service that address weld misalignment and recommended where repairs were required. In a similar case on a larger diameter chilled water system Becht used its affiliated company Sonomatic to conduct a UT inspection to define the misalignment. Since internal inspection was not feasible, a special UT setup was used to conduct a 360 degree scan from the outside diameter of the pipe. A similar API 579 evaluation based on the UT data was used to determine which welds required repair.