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Computational Fluid Dynamic (CFD) Analysis Resolves Deaerator Cracking

 

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.

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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.

 

Soil-Structure Interaction Analysis of Buried Tanks

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.

sst1sst2The 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.

Corrosion – Risk-Based Inspection (RBI) Program

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 Engineering - RBI Corrosion ProjectBecht 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.

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Pressure Vessel Fitness for Service

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).PVD1 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. PVD2Because 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.

 

Fitness-For-Service of a Vacuum Tower with Wall Thinning

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).

ffsmainpicEach 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.