Relining of a Chlorine Dioxide Storage Tank – A Case History
Author:
Jeff Eisenman, P.E.
President
Maverick Applied Science, Inc.
ABSTRACT
There are many severely corrosive services in a pulp & paper mill. Chlorine dioxide storage is one of the more
hazardous and demanding services in the mill. Chlorine dioxide storage tanks are typically large diameter tanks,
which are constructed from Fiberglass Reinforced Plastic (FRP). This presentation will discussion the findings of
the initial inspection of the ClO2 storage tank after many years of service. We will discuss the process and best
practices for relining of the interior of the vessel. Corrective measures and key elements to maximize the longevity
and reliable service life of the replacement lining will be offered. Expectations for the service life for the original
vessel lining as well as the replacement lining will be examined.
INTRODUCTION
Some of the first chemical resistant installations of FRP were in pulp and paper mills in chlorine and chlorine
dioxide services in the 1960’s. Chlorine and chlorine dioxide have been and continue to be some of the most
corrosive and greatest challenges to materials in all areas of chemical processing. FRP has been the material of
choice for these applications for many years as a naturally corrosion resistant and cost-efficient material solution.
Still FRP degrades over time in chlorine dioxide storage. Assuming proper design and best practices in fabrication
and installation, an FRP tank can provide a reliable service life of 15 to 20 years in chlorine dioxide storage
applications. We will present a case history of a chlorine dioxide storage tank that was originally installed in 1990
and remains in service to this day.
BACKGROUND
In 1990 the pulp and paper mill commissioned two 329,000-gallon chlorine dioxide tanks, both of FRP construction.
We will be discussing the relining of one of these vessels to extend the life of the vessel. The following shows the
basic physical data on the chlorine dioxide storage tank.
ClO2 Storage Tank Parameters:
Description
Tank Diameter
Tank Height (Straight Shell)
Bottom Construction
Top Construction
Corrosion Barrier (Original)
Corrosion Barrier Resin
Tank Shell Construction
Head Construction
Exterior Insulation
Assembly
Physical Parameter
40 ft
37 ft
Flat (Hand Lay-up)
Dome (Hand Lay-up)
0.100” thk
Epoxy Vinyl Ester
Filament Wound
Hand Lay-up
2” thk Foam with FRP skin
Field Fabricated and Erected
Figure 1: An Overview of the Completed Tanks in 1990.
The tanks were provided by Ershigs, Inc., who has been acquired by another FRP manufacturing company. Tanks
of this size, typical take 6 to 9 months for fabrication and assembly. Since these tanks were too large to transport
over the road, these tanks where fabricated on site on the foundations that they are resting on. Figure 1, above,
shows the completed tanks with installed anchor lugs and platforms.
Equipment of this size is a substantial investment, so it is critical that the tanks are maintained in reliable condition.
With a quality maintenance assessment program, the vessels can be maintained to provide reliable service for 50
years or more.
INSPECTION AND LIFE EXTENSION
Maverick first inspected these tanks in 2005. Findings of that inspection indicated that the initial corrosion barrier
of the tank was at the end of its reliable service life. Although FRP is corrosion resistant in chlorine dioxide service,
over time the chlorine will slowly degrade the FRP corrosion barrier at the surface. Over a number of years, the
chlorine elements will slowly permeate the FRP corrosion barrier creating a residue on the surface that is commonly
called, “chlorine butter”. Figure 2 illustrates the soft nature of chlorine butter residue.
The chlorine in the chlorine dioxide reacts with the resin in the corrosion barrier to form a soft wax-like finish over
the inner surface of the tank. In early stages of chlorine butter reaction, Barcol hardness tests may be taken to gage
the depth of the chlorine butter and permeation. Over time the permeation and chlorine butter will get thicker until
Barcol readings may not be attainable. As the Chlorine Butter gets thicker, an inspector can remove a small area of
Chlorine Butter in an effort to get down to solid FRP corrosion barrier or structural underneath. Figure 3 shows an
example of the depth of degradation. When the structural underlying layers are clearly visible, it is time to starting
planning to replace or reline the vessel. In this case, due to the size and scale of the tanks, relining the vessels was a
cost-effective solution.
Currently, these two FRP chlorine dioxide storage tanks have been in service for 32 years, which provides an
indication of the durability of FRP and the effectiveness of the FRP relining solution. The first relining of the FRP
chlorine dioxide storage tanks occurred in early 2006. The service life of the initial construction was 16 years, which
is a good service life for these vessels. After the first relining, the vessels were periodically monitored. Based on
the inspection findings, it was determined that the vessels needed to be relined a second time in 2015, after 9 more
years of service. The tanks continue to be monitored since the last relining. We will discuss the remaining expected
service life of the most recent relining later in the conclusions of this paper.
Figure 2: Example of Chlorine Butter in the Storage Tank
Figure 3: Exposed Structural Layers
PREPARATION FOR RELINING
2006 would be the first time that the plant would be relining an FRP storage tank. For that reason, an FRP tank
relining procedure was developed for the project. The procedure defined the following requirements.
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Scope of Work
Contractor Requirements
Materials Required
Laminates
Applicable Codes and Standards
Surface Preparation
Application of Laminates
Inspection Criteria
Repair procedures
Quality Control Requirements
Environmental Monitoring
Postcuring
Testing
Potential vendors were qualified based on personnel, capabilities and experience. Vendor bids were required to be
in strict adherence with the project relining procedure. A financial and technical review was conducted during the
bid process, including a review of the execution schedule. Due to the criticality of the service, the most experienced
FRP manufacturer having an established field service crew was selected as the contractor for the job.
PROJECT EXECUTION
The tanks were emptied of contents, drained, washed-out, neutralized, and vented. The tanks continued to be
ventilated with fresh air blown through the tank to dry the tank walls and remove vapors from the tank. The tank
environment was continuously monitored for chlorine vapors and oxygen content for the entirety of the operation.
Site personnel were able to maintain chlorine and oxygen levels within an acceptable range for the duration of the
project. Once the interior environment was maintainable and safe for entry, scaffolding was built to access all levels
of the tank shell and roof. When scaffolding on the interior of an FRP vessel, it is important to lay down plywood as
a protective barrier, so the scaffold work does not damage the bottom of the FRP tank. Figure 4 shows a photo of
the scaffolding being erected.
The workflow would start from the dome top and moving down the side wall to the bottom. This allowed for full
access to all parts of the tank, removing the scaffolding level by level as level was completed. The roof would be
done first. Moving down the tank shell each level would be completed in 8 ft high sections, until the bottom was
reached.
SURFACE PREPARATION
The first step of the relining process is surface preparation, removing the existing degraded and contaminated
material from the interior. This is the remnants of the original corrosion barrier. The goal is to get down to solid,
noncontaminated material, which would provide the best opportunity for bonding. Sandblasting is the most efficient
method for removal of the loose and soft surface materials, given the large interior surface area of the tank.
Additionally, sandblasting provides for the most uniform and consistent bonding surface for applying new
laminations. Hand grinding can be used on a spot basis, though generally, does not provide as regular and even of a
surface profile for optimal bonding, compared to what can be achieved by sandblasting.
Figure 4: View of Scaffolding being Constructed
Figure 5: Image of the Structural Damage on the Shell
Once the initial blasting was completed, the remaining FRP structural wall of checked for chlorine contamination
and moisture using litmus paper. A non-neutral pH indicates the chemical contamination still remains in the wall.
Chemical contamination in the surface can impede the bonding process for new laminates. As sandblasting is still a
laborious process, the shell is closely examined for contamination as well as for areas of excessive wear or damage.
Figure 5 illustrates an area were the chemical damage reached the structural wall, which was exposed during
blasting. The texture of the underlying woven roving is visible. Those larger areas, where evidence of
contamination remained, were reblasted locally. Smaller areas were removed by hand grinding. Smaller flanges
cannot be effectively sandblasted, so those were replaced in total or had a custom FRP sleeve fabricated for later
insertion into the nozzle penetration.
Locally damaged or lost areas of the structural wall were mapped for replacement. The depth and size of the
damaged structural areas were documented, and a repair plan developed to locally restore the strength of the
structural wall of affected areas. A procedure for repair patches was implemented for the identified areas for
restoration. All repairs were completed prior to initiating the replacement of the corrosion barrier layers.
RELINING: A NEW CORROSION BARRIER
Although the original corrosion barrier thickness was 0.100” thick, the new replacement corrosion barrier was
specified as 0.250” thick. It is understood that chlorine will gradually degrade the surface of the FRP. The length of
the reliable service life of the corrosion barrier and tank is directly related to the quality of the work and thickness of
the corrosion barrier. The thickness of the corrosion barrier was increased to 0.250” from the original thickness in
an effort to maximize the service life of the tanks. All of the larger nozzles were relined in-place, while smaller
nozzles were removed and replaced, or relined with prefabricated FRP sleeves.
Figure 6: An Example of the Test Patch
Figure 8: Application of Glass Reinforcements
Figure 7: Example of a Successful Bond Test
Figure 9: Blast Preparation and New Laminates
Since the storage tank has been in service for many years it is important to confirm the ability to obtain a secure
laminate bond to the existing structural wall of the FRP tank. Prior to starting any laminating activities, the
contractor was required to perform a number of shear bond tests, also called peel tests, in the immediate area of
work for each day or work shift. Figure 6 illustrates a shear bond test. This is a test laminate that is attempted to be
peeled off the existing substrate. A successful test is demonstrated in Figure 7. After the laminate is cured hard, an
attempt is made to peel the laminate from the wall. This is a go/no go result. If the test laminate easily peels away
from the existing laminate without tearing the material, the test is a failure and additional surface preparation is
required. If the test patch tears and leaves substantial glass behind with the substrate, the test patch passes. Once a
successful patch adhesion has been demonstrated, relining laminations can proceed. Test patches were performed
on multiple locations of each level throughout the relining operation.
The sandblast surface preparation and laminate application on the roof is shown above in Figure 8 and Figure 9. As
mentioned earlier, the relining was performed from the top down. Work commenced on the roof section. The
laminates were applied a layer or a few layers at a time in a large overlapping patchwork to develop the continuous
lining and new corrosion barrier. Since there is still some potential for contamination of the existing laminates, a
resin primer is first applied to the existing laminates to maximize the opportunity for bonding.
Once lamination activities began in the tank, the relining effort was a 24-hour operation with a day and night shift.
Since the relining process is very labor intensive with a high level of coordination, the project had 24-hour thirdparty Quality Assurance (QA) surveillance, monitoring the laminate quality and environmental conditions. While
safety personnel monitored air quality, the QA inspector monitored environmental conditions, including temperature
and dewpoint in the workspace. For best results the temperature needed to be maintained between 60⁰F (15⁰C) and
90⁰F (32⁰C). If the humidity in the workspace gets too high, condensate may form on the prepared work surfaces
and contaminate the laminates. For this reason, the dewpoint is monitored continuously. Active environmental
monitoring is essential to maintaining laminate quality. The dewpoint must stay at least 5 degrees below the
temperature to ensure that moisture does not form on work surfaces. If the dewpoint meets the temperature,
condensation may form on the interior surfaces, which may contaminate the fresh laminates, requiring those to be
redone. Conditioned air was pumped into the tank to maintain environmental conditions within satisfactory limits.
Depending on the local climate and time of the year, the working environment may be managed with heaters, A/C
units or dehumidifiers.
The relining work continued for each 8ft level going down from the roof to the bottom. At the completion of each
level, a final postcoat was applied to the new corrosion barrier to provide a hard finish and seal of the laminates to
promote curing. As each level was completed, the QA inspector took Barcol Hardness measurements as a relative
check of curing of the laminates. The specification required that the Barcol Hardness reach a minimum of 90% of
the manufacturers recommended hardness for the specific resin for the application. Once the relining work
approached the floor, the scaffolding was removed. The tank floor was sandblasted and inspected for damage.
After any damaged material was removed and exposed structural layers were restored to original requirements, the
floor relining was then completed. Since inner surfaces of larger nozzles can be accessed by hand, those nozzles
were relined in place, while smaller nozzles were removed and replaced with new nozzles. Examples of relined and
replaced nozzles are shown in Figure 10 and Figure 11.
Figure 10: Relining a Suction Nozzles
Figure 11: Small Diameter Nozzle Replacement
Due to the severely corrosive nature of chlorine service, it is important to ensure that the resin is cured as fully as
possible. Once the relining was completed, the entire interior of the tank, all applied laminates are postcured with
hot air being circulated throughout the tank. This allows the resin to reach its maximum chemical resistance and
hardness. Hot air is circulated to maintain a constant temperature of 180⁰F for a period of 8 hours. This process
drives the cure through the laminate to maximize the cross-linking of the resin’s molecular structure.
Once the postcure is completed, the tank interior received its final inspection. There are times when deeply
entrapped air or residual stress concentrations grow at the elevated temperatures. Larger bubbles and delaminates
may come to the surface. These defects must be corrected before final hydrostatic testing and release for service.
ASME RTP-1, Table 6-1 provides acceptance criteria for laminate defects. Defects outside of the defined limits
should be removed and corrective measures developed to restore the integrity of the tank components of concern.
To ensure a water-tight seal of the liner, the tank was hydrostatically tested for 8 hours. Due to the thick laminates
of the tank wall and the insulation cover, it was agreed that a longer period for the hydrostatic test was merited. The
tank passed the hydrostatic testing without issue.
CONCLUSIONS
As mentioned previously, these tanks have been in service for over 30 years and remain fit for service going
forward. For large diameter tanks, relining is an effective way to extend the service life of process equipment that
handle very corrosive and demanding chemical service environments. We believe that the length of effective
service life of the relining is directly dependent on the efficiency and execution of the postcure at the completion of
the relining process. If the maximum required temperature cannot be achieved, then the duration of the postcure
period should be extended to achieve the maximum effect of the full postcure for best service life. If the postcure
process is compromised, the duration of the reliable service life will likely be diminished.
Field fabricated and erected FRP tanks are very costly, especially in small quantities. Each of these FRP storage
tanks was relined for about 25% of the cost of a replacement, which provides value to the mill. Additionally, a new
tank would take about 6 – 9 months to build, so the tank would be out of service for an extended period of tank,
which would be very disruptive to the mill’s operation as well. Extended mill outages may not even be practicable,
depending on the flexibility of the mill operation. Over time, different relining contractors were used. Efficiencies
were learned from the first to the second relining. Each individual relining project was an around the clock effort.
Depending on the weather, crew and unanticipated challenges, each tank relining averaged from 11 – 14 days, not
including hydrostatic testing. Due to the size of the tanks, it took about a day to fill the storage tank with water in
preparation for hydrostatic testing. From a time and scheduling perspective, relining provided further benefits, as
compared to replacing the tank in total.
Efficiencies of relining are dependent on the tank size and accessibility. In most cases, FRP tanks and scrubbers of
10ft diameter and less does not make sense to reline, as the cost and time savings may not be as substantial. For
relining to be a cost-effective option, scaffolding and work access are an important consideration. Tighter spaces
can make effective work more challenging. Above about 14ft in diameter, most tanks must be field erected, which
can be a costly effort for a single tank. Above 14ft in diameter, relining of a tank becomes more attractive from a
budget and schedule standpoint. Each case should be evaluated individually for merit to replace in kind or to reline.
REFERENCES
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American Society of Mechanical Engineers, ASME RTP-1, Reinforced Thermoset Plastic CorrosionResistant Equipment, 2021 ed.
ASTM C 582 – Standard Specification for Contact-Molded Reinforced Thermosetting Plastic (RTP)
Laminates for Corrosion-Resistant Equipment
ASTM D 2583 – Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol
Impressor