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Case history: Replacement of existing pressure vessel

01.01.2012  |  Fearn, D.,  Fluor Canada Ltd., St. John, New Brunswick, CanadaMcKay, J.,  Fluor Canada Ltd., St. John, New Brunswick, Canada

Installing new equipment involves more processes to ensure safety and to meet new codes

Keywords: [E&C] [engineering and design] [safety] [pressure vessel] [metallury] [reliability inspection] [Cranes towers] [lift]

The replacement of pressure vessels is a common function in an operating refinery, particularly those units that have been in operation for the full design life of the vessel. From the client’s perspective, a vessel may have operated successfully for many years beyond the original design life with no issue. Its replacement should be easily accomplished given the relative success of the original design. In the real world, the replacement of existing refinery vessels in a brownfield environment is seldom replacement in kind. Some minor, yet important, activities should be addressed to ensure project success.

In the presented example, work processes used to replace a hydrogen sulfide (H2S) absorber as part of a refinery crude unit that was originally identified as replacement in kind will be discussed. In addition to working to a documented work process, there are many areas where the various design engineers must think outside of the established work practices to ensure the timely, safe and effective installation of new vessels. This article is not meant to replace existing work processes; it will identify unclear areas that exist when replacing equipment in an existing operating unit.

The project.

This example involves the replacement of an H2S absorber tower. This tower is commonly found in refinery crude units. In this particular crude unit, the removal of H2S is done by a vertical tower with three integral vessels consisting of two drums and a packed section.

The purpose of the tower is to remove H2S, and it is necessary to minimize sulfur dioxide (SO2) emissions from an adjacent atmospheric furnace where the treated offgas is burned. As the atmospheric furnace and adjacent atmospheric column operate at very low pressures, the pressure drop through the H2S absorber must be minimal to avoid excessive backpressure on an adjacent vacuum-seal drum.

H2S removal.

To do these three activities, the vessel is separated into three major components. The bottom drum contains hydrocarbons, H2S solution and diethylolamine (DEA). The drum removes the hydrocarbons from the H2S, and the DEA assists with this process. The rich-H2S stream is sent from the bottom drum up into an H2S absorber section that is filled with random packing. Treated offgas is sent into the top drum where the untreated DEA is sent back to the absorber section and the gas is forwarded to the adjacent furnace.

As the vessel operates in an H2S and rich-DEA environment, the refinery performs regular inspections as part of a risk-based inspection (RBI) program as outlined by industry standard practices, refinery specific practices and guidelines established by the American Petroleum Institute. Following an automated ultrasonic testing (AUT) and manual ultrasonic testing (MUT) inspection of the bottom drum, it was found that stress- oriented-hydrogen-induced cracking (SOHIC), resulting in step-wise cracking, and blister formation was present in the drum with concentrations higher in the lower drum region (Fig. 1). Fitness for service calculations resulted in the recommendation to replace the vessel, thus preventing a potential unplanned production interruption.

  Fig. 1. Stress-oriented hydrogen-induced
  cracking of the H2S absorber column.

Original vessel.

The original vessel was built in 1974 to ASME Section VIII, Division 1, 1971 Ed. The vessel was specified with a joint efficiency of 0.85 (Spot RT) and the material of construction was SA-285 Grade C. Although vessel materials were not post-weld heat treated (PWHT), weld hardness was limited to 200 HB. The bottom drum is 4 ft in diameter and 40 ft in height (including the 14-ft skirt). The top drum and packed section is 70 ft long and 2 ft in diameter.

The H2S absorber internals consisted of four hold-down plates and 120 ft3 of random packing. To distribute solution to the H2S absorber, tangential nozzles were used on the inlet nozzles where the solution would collect on the spray header that is then gravity-fed down through the packing. To limit direct contact (and subsequent erosion) of DEA and H2S on carbon steel, the tangential nozzles directed flow toward a 304 stainless steel (SS) clad plate that was welded to the vessel internal diameter.

Theoretical design considerations and scope.

Initially, the project requirement was the replacement in kind of the vessel; other equipment directly attached or adjacent to the vessel would also need to be examined. For example, the piping system that also processes rich DEA and H2S was potentially at risk for associated metallurgical damage mechanisms. The foundation and corresponding anchor bolts needed to be reviewed to determine if they were acceptable for continued service for the estimated design life of the replacement vessel. Electrical and control systems were also reviewed to determine if existing systems are code compliant and adhere to current refinery practices. While other disciplines face challenges similar to the vessel designer, this article will focus solely on the vessel replacement. However, noted items are considered inter-discipline related.

For the vessel designer, the scope to replace a vessel includes far more than the replacement in kind of an existing asset and ensuring that the new asset will meet the latest codes. The designer must engage operations to ensure that manway size and location, ladder and platform access, packing access, etc., are acceptable to the current and future needs. Some needs may not be identified until the piping, electrical and controls designers also do their respective design activities.

As the request from the client was to replace the vessel in kind, the existing vessel was modeled into an available simulation model to determine if revisions to ASME Section VIII between 1971 and 2008 would result in an overall design change to the new vessel. For this vessel, particular attention was paid to the internal head design for the bottom/top drum assemblies, as well as the transition (48 in. to 24 in.) between the bottom drum and random packing section. Where the new vessel was to be constructed of SA-516-70N, the greater allowable stress compared to the original SA-283 offsets any code changes that would otherwise increase the overall thickness and potentially impact the total dimensions.

An important activity of the vessel designer is to visually verify and place hands on every item of the vessel and to check its accuracy against the original design drawings. This includes additional vessel penetrations, platform loads or equipment that was not part of the original design. Depending on the level of documentation control within the existing facility, it is possible that the original drawings do not exist or are of such poor quality that new drawings must be drafted.

Whereas construction is not typically engaged until further in the fabrication process, brownfield development should include design considerations recommended by the construction team and lift contractor. Items addressed include the timing of internals installation, adding lift lugs or relocating platform clip locations to facilitate installation where space is limited.

The output from this process should include a defined and inter-discipline reviewed datasheet and general arrangement drawing (as-built or new) that will be issued to the vessel fabricator. This allows all disciplines to review potential interferences between nozzles, clips, guides, supports, girth flanges, etc.

Actual design conditions and scope development.

The original project scope basis was for a replacement in kind vessel. However, during project development, many changes were made. To identify the changes required for this vessel, each discipline input was identified separately.

Mechanical related changes. Given the presence of H2S and SOHIC, the base materials were upgraded to SA-516-70N – HIC resistant carbon steel and included for PWHT in accordance with the recommendations of NACE MR0175. The radiographic testing (RT) was increased to full 100% RT while maintaining the 200 HB harness limit.

To prevent solution entrapment between the original internal shell, it was decided to use 304L weld overlay on top of 309L. Minimum weld overlay thickness was specified to ensure adequate thickness for long-term protection. Additional NDE was specified for the overlay, such as LPI, UT (for disbondment) and ferrite testing.

This vessel is tall relative to the base diameter, and, without supplementary support, it requires additional material on the base ring, anchor bolts and girth flanges to resist buckling due to the wind and seismic overturning moment. The original vessel design included guides, as shown in Fig. 2, at three upper elevations whereby the adjacent column provided support. The inclusion of the guides permitted the redistribution of wind load (overturning moment on the vessel base) and the acceptance of the existing anchor bolts and vessel thickness.

  Fig. 2. Guide locations from the adjacent
  atmospheric column.

Per requirements by the client and construction team, hand holes were installed to facilitate inspection, removal and installation of the random packing. Lift trunions were also installed on the base of the packed section to facilitate installation. Several new pipe guides and supports were also installed on the vessel to remove loads from platforms.

Following a review of the loads placed on the ladders and platforms, in addition to occupational health and safety changes since original construction, the ladders and platforms were redesigned.

Electrical related changes. As the refinery specifications have changed over the years, the requirement was made to include for cable tray clips on the vessel to facilitate electrical cable installation and pre-dressing prior to lift.

Vessel fabrication.

For this particular project, the vessel fabricator was provided with the original as-built vessel drawing and revised datasheets. The fabricator was required to produce new drawings incorporating all of the changes. For the vessel designer, this requires attention to detail to ensure that while overall dimensions are consistent with the original design, all changes have been incorporated into the new design.

To help facilitate quality concerns between the fabricator and client, a third-party inspector was enlisted throughout the fabrication process. The scope of the third-party inspector was to ensure the agreed to inspection and test plan was being adhered to, as well as to be a client representative for any hold points during the fabrication or final assembly and test process.

As new platforms were specified for the replacement vessel, it was decided that a shop-trial fit test should be done. This ensured that the platforms would fit during installation and prevented costly rework onsite that might, otherwise, have to be performed within the turnaround window.

  Fig. 3. Trail-fit ladders and platforms
  manufactured by the vessel fabricator.

Demolition and installation of vessel.

Depending on the time available, space considerations and resource availability, the construction team may choose to pre-install as many vessel related components as possible to reduce construction costs and to prevent doing work within an operating unit. This may include pre-dressing fireproofing, process pipes, heat tracing, insulation, valves, platforms, internals, instrumentation and cable trays. Where this vessel was being removed and installed in an operating environment, these processes were followed:

• Lift contractor reviewed available space and determined maximum allowable lift capacity (Fig. 4).

• Construction determined the recommended extent of pre-dressing.

• Mechanical engineering determined the total weight and center of gravity for the vessel, complete with all pre-dressed components. Depending on the amount of materials pre-installed, and the level of certainty of equipment/weight estimates, a lift factor was incorporated into the overall maximum lift weight. A factor of 10%–30% is not uncommon to include for errors in drawings, fabrication tolerances, etc. This weight becomes very important as space availability may limit the crane type and capacity. Improper weight estimates to the lift contractor may result in too small (or too large) of a crane.

• The lift contractor verified the weight prior to the demolition/installation to confirm that crane capacity would not be exceeded.

  Fig. 4. CAD drawing for the lift contractor 
  to confirm lift plan relative to available
  working space and local obstructions.

Elevations, offsets and grouting of baseplate.

During the design and construction process, the elevation of the foundation and underside of the vessel base ring was surveyed. This ensured that the vessel would rest at a similar elevation to the original asset.

With the original vessel removed, the foundation was prepared to accept the new vessel and associated grout. Once the vessel was installed, a survey was completed at the underside of the base ring at the shim locations as well as all girth flanges on the vessel. This served as a check to ensure there was no “wobble” in the vessel sections.

Once the vessel had achieved proper alignment, the vessel was grouted to the foundation with the remaining components (ladders, internals) installed that could not be pre-dressed. HP

  Fig. 5. Engineered lift lug and space
  limitations with pre-installed platform and
  adjacent column.

The authors 

Dan Fearn, P.Eng., is a design engineer with Fluor Canada Ltd. He holds a BS degree in mechanical engineering and is a registered Professional Engineer. Mr. Fearn has more than 10 years in mechanical engineering; his expertise lies in the in the specification and selection of mechanical equipment and in the development and implementation of maintenance programs with a focus on site support and installation.  

Jeff McKay, P.Eng., is a senior design engineer with Fluor Canada Ltd. He holds a BS degree in mechanical engineering and is a registered Professional Engineer. Mr. McKay has 14 years of experience with Fluor Canada Ltd., and, at present, is the lead mechanical engineer at a client jobsite.  

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