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Featured Projects

Flow Induced Vibration

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.

CFD Analysis - 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.

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.



Deflagration Events

baghouseDeflagration 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.

RBI Program - Corrosion

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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RBWS Program Implementation

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.rbwsimplementation

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.