Becht Engineering - We Solve Problems

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.

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.

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’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 engineers performed this analysis on a high temperature (1250F) valve for a jet engine test facility. 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.

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.

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.

Becht Engineering :: Flue Gas Demo
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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.

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.

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.

Equipment Re-Rating

Becht has re-rated many pressure vessels and heat exchangers to higher design pressures and temperatures as well as de-rates to establish lower design pressures and temperatures as a consequence of corrosion or some other damage. As a part of any re-rate, an assessment of the vessels current condition is made looking for general corrosion and localized metal loss, environmental cracking, etc.  Ultrasonic thickness (UT) measurement is used to check the vessel thickness vs. the minimum required by the code of construction. The figure shows a plot of typical thickness data obtained from a UT scan of a reactor vessel. Where thicknesses fall below the required (including the future corrosion allowance), a Fitness-for-Service assessment is conducted using the methods in API 579 – Fitness-for-Service.

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.

Deflagration Events

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.

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,

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

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.