Repair of Subsurface Molten Sulfur
Containment Structures
Repair Of Subsurface Molten
Sulfur Containment
Structures
By:
By: Thomas
Thomas R.
R. Kline
Kline
Division
By: Thomas Kline
Division Manager,
Manager, Engineering
Engineering Services
Services
Structural
Preservation
Systems
Director, Investigate Design Build Services
Structural Preservation Systems
STRUCTURAL TECHNOLOGIES Houston,
Houston, Texas
Texas
INTRODUCTION
INTRODUCTION
Sulfurous
Sulfurous compounds
compounds generated
generated by
by the
the hydrocarbon
hydrocarbon refining
refining process
process are
are an
an environmentally
environmentally objectionable
objectionable
constituent
within
petroleum
products
(crude
&
natural
gas).
As
such,
sulfur
compounds
are
captured
constituent within petroleum products (crude & natural gas). As such, sulfur compounds are captured and
and extracted
extracted
from
from the
the process
process stream
stream via
via Sulfur
Sulfur Recovery
Recovery Units
Units (SRU’s).
(SRU’s). SRU
SRU processes
processes provide
provide refined
refined hydrocarbon
hydrocarbon fuel
fuel
products
products with
with significant
significant reductions
reductions in
in air
air pollution
pollution upon
upon combustion
combustion and
and are
are aa pivotal
pivotal part
part of
of our
our nation’s
nation’sAir
Air
Pollution
Reduction
Programs.
As
marketable
commodities,
elemental
sulfur
and
sulfurous
compounds,
once
Pollution Reduction Programs. As marketable commodities, elemental sulfur and sulfurous compounds, once
removed
removed from
from the
the process
process stream,
stream, must
must be
be conveyed,
conveyed, contained
contained and
and transported
transported to
to commodity
commodity brokers
brokers and
and
ultimately
end
users
of
these
products
in
an
environmentally
viable
and
economical
manner.
The
by-products
ultimately end users of these products in an environmentally viable and economical manner. The by-products of
of
the
the refining
refining process
process are
are contained
contained and
and transported
transported in
in aa myriad
myriad of
of different
different kinds
kinds of
of vessels
vessels and
and at
at varying
varying process
process
temperatures.
temperatures.The
The focus
focus of
of this
this Paper
Paper will
will be
be to
to discuss
discuss subsurface
subsurface reinforced
reinforced concrete
concrete molten
molten sulfur
sulfur conveyance
conveyance
Trenches,
Sumps
&
Pits
and
“opportunities”
associated
with
the
repair
and
maintenance
of
these
structures
Trenches, Sumps & Pits and “opportunities” associated with the repair and maintenance of these structures to
to
provide
provide an
an extended
extended service-life.
service-life.
DESIGN
DESIGNASSUMPTIONS
ASSUMPTIONS
Many
times,
Many times, design
design engineers
engineers develop
develop structural
structural concepts
concepts from
from aa perspective
perspective of
of limited
limited operating
operating process
process
experience.
Civil
engineering
designs
often
don’t
include
environmental
factors
inherent
in
the
operating
experience. Civil engineering designs often don’t include environmental factors inherent in the operating process
process
when
when itit comes
comes to
to aggressive
aggressive chemical
chemical contact
contact on
on construction
construction materials
materials and
and elevated
elevated operating
operating temperatures.
temperatures.
Structures
Structures that
that behave
behave consistently
consistently in
in aa standard
standard commercial
commercial environment,
environment, “move”
“move” differently
differently when
when subjected
subjected to
to
loadings
loadings not
not anticipated
anticipated during
during the
the original
original design
design process.
process.
One
One such
such effect
effect isis the
the relative
relative “growth”
“growth” of
of aa structure
structure when
when subjected
subjected to
to elevated
elevated temperatures.
temperatures. Growth
Growth effects
effects
are
particularly
important
when
structures
are
placed
below
ground,
backfilled
and
the
backfill
materials
are particularly important when structures are placed below ground, backfilled and the backfill materials densely
densely
compacted,
as
if
constructing
a
standard
commercial
structure.
Unfortunately,
these
conditions
are
a
prescription
compacted, as if constructing a standard commercial structure. Unfortunately, these conditions are a prescription
for
for “failure”
“failure” as
as irresistible
irresistible forces
forces (i.e.,
(i.e., thermal
thermal growth)
growth) meet
meet immovable
immovable objects
objects (i.e.,
(i.e., densely
densely compacted
compacted soil/rock)
soil/rock)
resulting,
in
the
case
of
reinforced
concrete
structures,
in
cracking
and
concrete
surface
spalling.
It’s
not
unusual
resulting, in the case of reinforced concrete structures, in cracking and concrete surface spalling. It’s not unusual
when
when reviewing
reviewing prematurely
prematurely deteriorated
deteriorated subsurface
subsurface containment
containment structures
structures that
that the
the original
original designer
designer omitted
omitted
provisions
in
the
building
code,
specific
to
environmental
structures.
These
code
provisions
take
provisions in the building code, specific to environmental structures. These code provisions take into
into account
account
corner
corner cracking
cracking and
and require
require an
an increase
increase in
in embedded
embedded reinforcing
reinforcing steel
steel to
to address
address these
these additional
additional movement
movement
11
characteristics
characteristics created
created by
by fluid
fluid containment
containment and
and elevated
elevated temperatures
temperatures ..
Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
MATERIALS OF CONSTRUCTION
Concrete, the most versatile and commonly used building product on the face of the earth, faces an aggressive
environmental adversary. Aggressive by nature, made worse with elevated temperatures, sulfurous compounds
attack and deteriorate most standard building materials employed in reinforced concrete construction. Specifically,
in standard concrete exposed to sulfurous compounds, the concrete mortar fraction alters and expands to over
200% of its’ original volume. This expansive reaction is “fatal” for concrete materials that are generally strong in
compression but weak in tensile strength. The integrity of the concrete fails under expansion pressures, leaving
piles of coarse aggregate where competent concrete once stood. Only by using cements and aggregates that are
resistant to sulfate attack can reinforced concrete structures be expected to provide long-term aggressive chemical
containment service. Often designers will specify specialty chemically resistant cements - only to have the
specified products replaced by lower-cost and less chemically resistant products, via a field change-order.
Metal embedments incorporated into subsurface vessel designs typically have metallurgical characteristics capable
of resisting the acidic environment associated with sulfurous compound exposure. However, polymer products,
incorporated into waterstop and waterproofing details, are almost always inadequate when placed in elevated
service temperature exposure conditions. Regardless of what products are specified, if the products are incorrectly
applied or installed, the containment will be compromised which can lead to premature deterioration via original
construction defects.
SUBSURFACE SULFUR CONTAINMENT
Containing molten sulfur and resultant sulfurous compounds below ground, as required when incorporating gravity
flows into the process stream, generally leaves the designer with three (3) containment options:
• Trenches
• Sumps (Day-Pits)
• Pits
Depending on process requirements, regardless of the containment type selected, reinforced concrete is the
construction material of choice for subsurface structures involved with direct burial. Castable building construction
products allow great versatility depending on local conditions and topography. However, as in all site-built
construction projects, the opportunity for construction defects can be significant depending on the effort exercised
with on-site Quality Control/Quality Assurance (QC/QA). Unfortunately, due to the aggressive operating service
environment, containment structures with even small construction defects (e.g. honeycomb concrete, misplaced
embedded reinforcing steel, waterstops, etc.) can greatly diminish the anticipated service-life of the subject
structure.
Subsurface molten sulfur containment structures vary in size and function ranging from Trenches to convey and
transport molten sulfur, Sumps/Day-Pits for low capacity/storage situations and Sulfur Pits for multi-day storage.
A sub-set of Sulfur Pits include small Working Pits which involve significant daily fluid level fluctuations and large
steady-level Storage Pits. As a rule, Working Pits are generally more prone to significant deterioration than Storage
Pits due to fluctuating molten sulfur levels and the extent of the vapor zone within the Sulfur Pit.
©2006 Structural Preservation Systems, Inc. • PAGE 2
Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
STRUCTURE DETERIORATION
While in operation, molten sulfur containment structures are
continuously exposed to elements that are detrimental to the integrity
of reinforced concrete which leads to shortened life-expectancy.
Numerous types of aggressive deterioration mechanisms exist within
molten sulfur containment structures and these mechanisms need to
be accurately identified and mitigated effectively. Typically reinforced
concrete exposed to a molten sulfur environment is in contact with
surface temperatures approaching or exceeding 300oF (148.9oC).
Long-term exposure of Portland cement-based concrete at these
temperatures can lead to sulfur impregnation of the concrete,
which chemically alters the paste fraction of the concrete matrix.
Exceeding the boiling point of uncombined water in the concrete
(i.e., “free” pore water within the concrete) desiccates the
concrete, subjecting the concrete mass to volumetric shrinkage,
resulting in concrete cracks.
DETERIORATION
DETERIORATION
Cracks, acting as conduits, allow acidic materials to deeply
penetrate the concrete mass. Sulfurous acids, chemically
detrimental to concrete, can be present within the structure
when molten sulfur mixes with groundwater or when process
condensate water enters through cracks and failed slab and wall
penetrations. Chemical alteration of concrete materials by contact
Figure 1 - Deterioration in constant
with sulfurous acids, discussed above, is readily apparent in
level pits (above) and pits with fluctuating
regions of cracked walls with leaking groundwater and around
levels of molten sulfur (below)
poorly sealed roof portals/penetrations. Additionally, exposed
reinforcing steel bars located above molten sulfur levels, within
the vapor zone, can corrode due to the electrochemical process of metal corrosion in the presence of oxygen and
moisture.
Another effect, reported by research in Canada, is the net corrosion reaction and formation of sulfur-deficient iron
sulfide from direct interaction of steel and solid elemental sulfur in the presence of moisture. Essentially, this type
of corrosion is caused by the reduction of solid elemental sulfur in contact with exposed steel reinforcing bars and/
or other steel embedments. This effect is becoming better understood when transporting solid elemental sulfur via
bulk train cars and barge/shipping vessels with low moisture contents (1-2% moisture by weight)2.
Of the three types of subsurface molten sulfur containment structures, Trenches are by far, the most deteriorated
due to poor maintenance around trench-top plate seals, allowing surface water to enter the cavity areas via plate/
trench interface seams and gaps. Fortunately, Trenches are far less common than Sumps and Pits, with molten
sulfur commonly conveyed via pipeline. Sumps and Pits are designed similarly with the relative size of each
defining the storage structure type. However, the larger the Sulfur Pit, the greater tendency for the Pit to be a
constant-level “Storage Pit” instead of a fluctuating “Working Pit” (Figure No.1).
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
Figure 2 - concrete repair process
SULFUR PIT REPAIR PROCESS
History and experience have shown that each subsurface molten sulfur containment structure poses unique
challenges to a Repair Contractor. Regardless of whether the required repair involves partial depth, full-depth, a
partial liner, structural liner or simply stopping water ingress, it’s imperative to utilize an engineered solution. A
proper repair strategy should consist of the following elements:
• Identifying and determining the root-cause of the failed concrete;
• Employing proper materials in construction and repair techniques; and
• Using a qualified, experienced contractor who can provide a solution, and a well-planned QC/QA
program for the repair.
These three steps will assure the Owner that the repair-failure-repair cycle is eliminated and a sound structure put
back into operation3. A more comprehensive view of how these steps translate into the Repair Process is shown in
Figure 2.
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
CONDITION SURVEY/FORENSIC INVESTIGATION
Concrete deterioration comprises both obvious and latent
characteristics that are not easily understood without gathering
further information through investigation. Unlike forensic
efforts in other process units, the molten sulfur environment is
too hostile for an in-process evaluation. However, techniques
have been developed to assess quickly causes and effects
of concrete deterioration, once the subsurface molten sulfur
containment structures has been cleaned and made available
for inspection during a short-duration outage. Employing a
combination of Non-Destructive (NDT) and Semi-Destructive
Testing (SDT) techniques, characterizations as to the
physical and chemical characteristics of the subject structure
can be determined quickly. Using cutting-edge analytical
and diagnostic tools, the evaluator establishes these repair
parameters:
• An evaluation that investigates further, and qualifies
causes & effects;
• A quantification of the problem that expresses its extent
in concrete terms (e.g. square feet, cubic feet, linear
feet, etc.); and
• Documentation describing where the distressed
conditions are located – and what it will cost to repair
them – arranged from highest to lowest priority.
Pachometer survey of
embedded reinforcing steel
Caliper measurement of corroded reinforcing
steel bars
Once adequately characterized, a thoughtful and detailed
repair approach can be developed addressing thermodynamic,
chemical and construction material properties of the structure
operating within the aggressive molten sulfur service
environment - optimally resulting in a long-term repair program.
Concrete core sampling
of support column
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
REPAIR SCENARIOS
As each, subsurface molten sulfur, containment structure
is unique in construction and service, so to, many repair
opportunities exist for structural restoration. Repair, based
on the results of the Condition Survey/Forensic Investigation
discussed above, can take many forms including, but not
limited to, repair of leaking cracks, repair of structural
components (e.g. walls, floor slabs and roof slabs)
establishment of new protective “skins” (e.g. permanent
& sacrificial protective linings) and construction of a new
structural liner (e.g. “box-within-a-box”).
Rout and sealing of leaking cracks
Crack Repair: Crack repair requires a basic knowledge as
to why reinforced concrete cracks. Modern concrete is the
end-product of an 80-year trend toward faster hydrating cements and ever-higher cement contents. This trend has
produced very strong but also very crack-prone concrete. Major reinforced concrete structures exhibit significant
distress because they are more restrained against volume change, undergo greater moisture and temperature
changes, the concrete is stronger, has a high modulus, and little creep capacity to relieve the self-stress from
thermal contraction, autogenous shrinkage, and drying shrinkage.4 Understanding the root-cause mechanisms
associated with observed cracking will assure that the repair, when implemented, will be long lasting and won’t
reinitiate under service conditions.
Standard crack repair technology to either “glue” the concrete members together and/or stop the ingress of
groundwater do not work well within molten sulfur containment structures. The concrete substrates and crack
interfaces are extensively contaminated with sulfurous products and molten sulfur temperatures are too high and
will typically volatize resinous materials generally employed in such repairs. After much “trial-and-error”, older
technology (late nineteenth-century) crack repair techniques, developed in tunnel construction, have proven to be
the most reliable and durable. Essentially, cracks are routed (i.e., grooved) with the resultant cavity mechanically
impacted with lead-wool. Lead, with a melting point of 621oF (327.4oC), is well suited for molten sulfur contact
temperatures (i.e., ~ 300oF (148.9oC)) and the metal is malleable, readily conforming to prepared crack cavity
contours when impacted with a cold-chisel.
Occasionally cracks form in molten sulfur containment structures that result from movement during service.
Often, designers place joints within structures that are designed to move, such as in the case of expansion joints.
Both moving cracks and expansion joints require special attention. Durable yet flexible construction materials
are required to address in-service movement as “rigid” repair efforts will fail from forces developed via restraint.
Typically, chemically-resistant, high-temperature tolerant membranes (e.g. Hypalon, etc.) are specified for repair
of moving cracks, mechanically fastened to crack shoulders. Expansion joints will generally incorporate metallic
plating (e.g. Stainless Steel, Aluminum-Alloys, etc.) into joint systems, allowing the structure to move, yet keeping
out moisture and retaining molten sulfur products within the containment.
©2006 Structural Preservation Systems, Inc. • PAGE 6
Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
Structural Member Repair: Aggressive deterioration
mechanisms associated with sulfate-related chemical attack can
significantly affect the structural integrity of reinforced concrete
members within a structural system (e.g. walls, base slab, roof
slab, etc.). Reduction in both concrete and embedded reinforcing
steel bar cross-sections can create conditions of impending
Structural Risk, in some cases requiring immediate action in
the form of temporary support shoring or process bypass. AtRisk structural behavior can range from slow, barely noticeable,
structural member deflections to “failure-without-notice” of
structural systems such as containment roof and wall collapse.
Should significant distress conditions be exposed during a
regularly scheduled maintenance outage, an evaluative approach,
as discussed earlier, should be initially employed:
• Locate the deterioration
• Qualify the distress mechanisms and determine
the “root-cause”
• Quantify the amount of repair to assess repair
methodology - determine whether to
Repair or Replace-in-Kind
Once a repair methodology has been selected, follow the Concrete
Repair Industry Best-Practices5:
• Demolish and remove unsound/deteriorated concrete
materials
• Prepare resultant sound/competent concrete substrate
surfaces
• Assess and augment, if necessary, deteriorated embedded
steel reinforcing systems6
• Implement corrosion control measures, if evidence
indicates significant embedded metal corrosion activity
• Select appropriate concrete repair materials that have
consistent plastic and hardened characteristics and
properties to ensure composite behavior between the
existing concrete substrate and new “cured” repair
materials
• Install the selected repair materials using placement/
application techniques consistent with the desired end
repair product that achieves adequate bond and results in
low shrinkage cracking
Railroad trench sluiceway – Note trench
covers, spreader beams and expansion joint
Deterioration of structural members beam and column top
Temporary post shoring –
beam span shortening
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
Durability Liner: Protective measures within molten sulfur
containment structures have historically ranged from concrete to
masonry to metal plating. The intent of these systems is to protect the
structural concrete either sacrificially along a specified limited lifecycle or as primary permanent barrier containment. Unfortunately,
sacrificial, masonry and metallic liners block visual examination of the
structural reinforced concrete components thus rendering an accurate
assessment unlikely during short-duration outages. This feature can
be disconcerting with regard to aging Trenches, Sumps and Pits as
the risk associated with existing conditions is hidden from view and
assumptions must be made as to the structure’s “Fit-for-Service”
condition. Additionally, these measures, unless engineered, can actually
exacerbate deterioration processes by trapping and concentrating
sulfurous by-products behind the protective liner and in direct chemical
contact with reinforced concrete surfaces.
Carbon steel plate liner – failure
along base slab and walls
When “engineered”, protective liners are known as “Durability Liners” that add significant service-life extensions
to existing Trenches, Sumps and Pits. For Durability Liners to function adequately, it’s important that the repair
materials selected be compatible with the substrate, matching as closely as possible:
• Modulus of Elasticity (Y = σ/ε)
• Thermal Expansion (∆l/l = α∆T)
• Low Material Drying Shrinkage (crack-free)
• Repair like-with-like!
Generally, the most successful Durability Liners are non-sacrificial, cementitious materials that are mechanically
anchored to the existing structural concrete substrate. These liners must be thick enough (>4 inches (>100mm))
and of consistent cross-section to function compositely when subjected to molten sulfur process loadings.
Obviously, besides matching the engineering properties for repair construction, cementitious repair materials
must be chemically resistant to sulfurous compounds associated with molten sulfur containment. This resistance
typically stems from low levels of Tricalcium Aluminate, C3A, so as to not react with sulfate ions which can
initiate expansive reactions within the concrete mass7. Chemical resistance can also be improved by the reduction
of the Portland cement fraction within the ready-mixed concrete and replacing that portion of the cement with
mineral or pozzolan admixtures (e.g. flyash, microsilica, etc.) that also have cementitious properties8. Typically, the
Durability Liner design involves one of the following construction types:
• Fiber-reinforced concrete products (FRC)
• Welded-wire-reinforced (WWF) cast-in-place concrete
• Precast conventionally reinforced concrete modular panels
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
Structural Liner: When deterioration is so advanced in a
molten sulfur containment structure that the structural integrity is
compromised and the risk of failure due to continued operation is
too great, Owners will explore various containment alternatives.
These options can include process bypass, new construction or the
construction of a new Structural Liner. Process bypasses are effective
when process piping can be assembled and molten sulfur storage
is contained using adjacent Pits, Tanks, Rail Cars or Barges. Often
this option is not a long-term practical solution. Construction of new
molten sulfur containment requires open land within the SRU which
in most cases is unavailable due to the density of existing process
equipment.
The construction of a new Structural Liner, sometimes referred
to as a “box-within-a-box”, is often the preferred method for reestablishing the structural integrity of a deteriorated containment
structure. Essentially, the existing structure’s support system is
“negated” and treated as rigid backfill. A new containment structure
is then designed to resist all loads, as if it was a stand-alone structure.
The design will characteristically include a bond-breaking interface
to allow independent structural behavior between the old and new
structures when under process loads. The only drawback to this
repair methodology is that some limited volume/capacity of the
containment is lost by the installation of a new structural system (i.e.,
walls, base slab, etc.). However, the volume/capacity reduction, in
most instances, are seen by Owners as a necessary compromise.
Formwork installed and braced
for concrete placement
Form & pump concrete placement from on-site
concrete mixing station
In conclusion, subsurface reinforced concrete structures containing
molten sulfur can be successfully repaired providing a significant
extension to their service-life. These types of repairs however, can
only be implemented once we understand:
• Owners Requirements;
• Process items specific to the Facility;
• Deterioration mechanisms in-place within the structure and;
• Securing of Repair Professionals who offer an “engineered
approach” and have the background and experience to
implement the repair successfully.
Repaired sulfur pit just prior to precast
concrete roof panel placement
©2006 Structural Preservation Systems, Inc. • PAGE Copyright 2014 Structural Group, Inc.
Repair of Subsurface Molten Sulfur Containment Structures
Thomas R. Kline, Structural Preservation Systems
REFERENCES:
1.
2.
3.
4.
5.
6.
7.
8.
Kline, T., “Sulfur Pit Assessment and Repair Strategies,” Paper Presented at Brimstone Sulfur Symposium,
Vail, Colorado, September, 2004.
Dowling, N. I., “Corrosion of materials used in storage and handling of solid elemental sulphur”, Alberta
Sulphur Research, Ltd., University of Calgary, Publication – Materials Performance : Sulphur and Energy,
pgs. 103-115.
Schwabenlander, R., Kline, T., “Sulfur-Recovery Operations Pose Formidable Challenge to Concrete
Infrastructure,” World Refining, Vol.12/No. 4, May 2002, pgs. 30 & 31.
Burrows, R. W., The Visible and Invisible Cracking of Concrete, American Concrete Institute Monograph
No. 11, 1998, pg. 1.
Concrete Repair Manual, 1999 Edition, Published jointly by the International Concrete Repair Institute,
Sterling , VA and the American Concrete Institute, Farmington Hills, MI, 1999, 861 pgs.
Manual of Standard Practice, 27th Edition (MSP-2-01), Concrete Reinforcing Steel Institute, Schaumburg,
IL, 2001, pgs. 4-4 & 4-5.
“Guide to Durable Concrete,” ACI Manual of Practice, Part 1, ACI 201.2R-92, American Concrete
Institute, Detroit, MI, 1998.
Kosmatka, Steven H., Panarese, William C., Design and Control of Concrete Mixtures, Portland Cement
Association, 13th Edition, Portland Cement Association, Skokie, IL, 1988, pgs. 68-70.
©2006 Structural Preservation Systems, Inc. • PAGE 10
Copyright 2014 Structural Group, Inc.