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 clients 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.
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.
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
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.
1. Stress-oriented hydrogen-induced
cracking of the H2S absorber
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
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
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
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
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
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
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
2. Guide locations from the adjacent
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
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.
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
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.
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
Lift contractor reviewed available space and
determined maximum allowable lift capacity (Fig. 4).
Construction determined the recommended extent
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.
4. CAD drawing for the lift
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.
5. Engineered lift lug and space
limitations with pre-installed platform and
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.