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Nonmetallic Construction for Sulfuric and Phosphoric Acid Processes

ABSTRACT

Most sulfuric and phosphoric acid plants utilize extensive nonmetallic solution in processing units. In order to address these harsh chemical environments, specific corrosion engineering and materials needs must be specified and applied correctly to prevent operational issues.

Conventional FRP (Fiber reinforced plastics), rubber linings, and acid brick towers are commonly used with success in these environments. This paper will discuss the use of composite nonmetallic materials commonly used in sulfuric and phosphoric acid facilities. In addition, the paper will go into case studies and lessons learned from successful projects covering the importance of inspection and quality assurance for reliable service life.

Key words: Nonmetallic, Rubber, Phosphoric acid, Sulfuric Acid, Acid Brick Tower, CRM, Chemical resistant masonry.

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Michael P. Yee

The Chief Executive Officer of RTConsults who also manages an engineering and 3rd-party inspection company for the nonmetallics industry. He has over 10 years of experience in the nonmetallic industry and is a NACE Level III Certified Coating Inspector with a background in chemical engineering and construction management. He is actively involved in the nonmetallic technical committees at NACE, SSPC, ICRI, and ASME. He has presented for a number of professional organizations over the years and continues to strive for excellence in providing technical services and support for many leading petrochemical companies in phosphoric and sulfuric acid process, chloro-alkali, and to the offshore oil industry.

Richard Taraborelli, P.E.

The owner of RT Consultants with over 35 years of experience in the protective coatings and nonmetallic industry. He is a registered professional engineer in the state of Texas. Performed over 2,000 failure analysis and other projects in his career and continues to provide technical services and support to many leading petrochemical Fortune 500 companies.

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Table of Contents

Introduction..........................................................................................................................4 . Material Selection................................................................................................................4

Fiber-Reinforced Polymers (FRPs)

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INTRODUCTION

Sulfuric and Phosphoric facilities face a number of issues when dealing with corrosion and harsh

chemicals. The conventional method of addressing these issues is to reach out to in house plant

personnel that have first hand experience with materials that work well for the different services.

However, plants are suffering from a shortage of subject matter experts and the facilities are

scrambling in addressing issues with proper material selection and application in order to keep

the facilities operational. This issue is very much present today in the Sulfuric and Phosphoric

industry. This paper will address the nonmetallic issues that these facilities face when dealing

with severe chemical exposure normally seen in the Sulfuric and Phosphoric processes.

Material Selection

Material selection in severe chemical service is based mainly on case studies and also

recommendations from material suppliers/engineering specifications. Material selection must

also be decided on the cost and also service life of the system before replacement. The cost of

corrosion and chemical attack is always seen as a growing capital expenditure; until it causes

the plant to shutdown. This has been the main driver of utilizing nonmetallic in these facilities

due to the fact that nonmetallic is some cases is the only viable option for these severe chemical

processes. For realistic expectations, we are always looking to achieve a 15-20 year service life

with the material choice based on value and ease of maintaining in order to come to the best

solution for the service.

Fiber Reinforced Polymers (FRP)

There have been endless amounts of case studies of FRP construction working well in these

services. The standard resin that is specified are vinyl esters or novalac resins in sulfuric and

phosphoric acid services. The corrosion liner is typically 100-250 mils depending on the

service life requirements and temperature the material is exposed to. These materials can also

be used in dual laminate configurations (with a thermoplastic liner) but it normally overkill in

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these services. The main reason to use dual laminates in these services is having excess

temperature exposure typically over 200-300F.

Figure 1: Vinyl Ester Duct Exposed to 37 years to 30% Sulfuric Acid Vapor

As humble as standard FRP is compared to dual laminate and normally half the price, it has

demonstrated its value and service life reliability in this service with the proper resin choice and

fabrication quality. Acids especially sulfuric acid tends to discolor the resin into a darker color.

For example, sulfuric acid will make the laminate have black staining and sometimes have a

burnt visual on the process side. This normally occurs in the first few months which we

consider the break in period and other defects such as hard blisters form which is normal. It is

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recommended to perform an inspection each turnaround by a qualified inspector to understand

the condition the laminate is in plus gauge if repairs need to be made.

Figure 2: Performing Peel Test to Verify Adhesion of Different Laminate Repairs.

The first step in repairing a laminate is making sure the new resin and laminate is compatible

and has adequate adhesion to the surface of the old laminate. Grinding or sandblasting the

surface to expose the actual glass fibers is the best possible surface profile when applying

additional laminates on dry chemical free areas. A peel test as shown in the figure also is used

to quantify if it works with an acceptance criteria of 80% or greater. The key in a successful

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repair is having quality assurance and proper material selection from the front end. For new

fiberglass, a properly designed, fabricated, inspected, and operated vessel or piping should

provide reliable service for 15-20 years and more depending on the environment and process.

Acid Quench Towers (up to 600F)

In every Sulfuric and Phosphoric plant there are acid quench towers that must cool vapor from

temperatures up to 800F to lower temperatures for the chemical process. These nonmetallic

systems normally have a steel structure that can withstand the weight of the brick on the walls,

a membrane system made up of either a asphaltic membrane, FRP, or thermoplastic linings,

and finally the brick and mortar system.

The brick and mortar is the first materials that see the process fluid or gas at very high

temperatures. Depending on the temperature of the vapor/fluid, the thickness of the brick is

determined since the depth will create a temperature gradient in order for the membrane to see

a lower and acceptable temperature. This thickness is typically 3-4 inches thick and reinforced

with furan mortar. There are many types of brick that will be compatible in this service but the

important part is selecting the correct mortar and membrane system. This normally requires

consulting with the material supplier/manufacturer and expert since it can vary based off of the

chemical stream.

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Figure 3. Novalac vinyl ester membrane for a sulfuric acid tower.

For reliable service life greater than 20 years (in some cases 30 years), we normally recommend

going with a novalac vinyl ester as the membrane system and using a furan mortar from a

reputable supplier. Most contractors in this arena have their proprietary and trademark systems

but the technology is very much the same for decades. Selection of the contractor is very

important since getting support is integral for reliable service. What we mainly see as the most

common failure issues are ceramic sleeves at the nozzles being cracked and poor workmanship

on membranes. These are normally done with incorrect installation or during a turnaround with

unqualified personnel working on the nozzles and walls. Once the acid gets behind the

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membrane system, the tower is normally requiring replacement in a matter of a year or be

patched up with boxes filled with membrane material.

Figure 4. Welding Boxes on a Quench Tower Improperly.

It is important to note that the mortar requires acid to fully cure for initial start up and there must

not be a hydrotest performed since water will wash away the mortar from the joints. There are

few fabricators and inspectors in the acid brick tower market thus it is important to go by

reputation and referrals since these towers can be costly.

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Rubber Linings

Rubber can be either natural or synthetic, and is a very robust material that handles impact and

abrasion very well. For large field erected tanks, rubber linings is an economical choice for mild

chemical and brine service. On average, when properly installed, it will last longer than thin paint

coatings and unreinforced linings. It is applied by first preparing the surface to a NACE 1/SSPC

SP-5 White metal blast and then applied with a primer/adhesive that normally applied direct to

metal.

After that the rubber sheets are cut and installed in sequence to provide adequate protection

with overlaps and stitching. Different systems may use natural rubber as a bonding interface

with the synthetic rubber, forming a composite. There are a number of things to observe when

inspecting the surface as well as when repairing a rubber lining, and it is important to note the

difficulty of doing so in the field. The primer must be tacky when applying the rubber sheets and

air must be rolled out to prevent air bubbles or voids. Without proper technique, the rubber sheet

will debond and allow process fluid to enter and cause premature failures.

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Figure 5: Air bubbles in the rubber lining sheets.

Normally soft natural rubbers are used for mild acid service and the harder rubbers are used for

more rigorous service conditions. Chloro-buytl is widely used in the Sulfuric and Phosphoric

plants and is a preferred material of choice when working with many types of chemicals for large

steel tanks. The applicators must be qualified in applying the rubber linings and have important

documents such as a rubber sequencing diagram and enough cure time based on the thickness

of the rubber sheets especially on corners where there can be 3 or 4 overlaps in one section.

Curing with either autoclave or atmospheric steam must be performed within the time span

specified by the manufacturer. Hardness should be inspected and verified in order to conform to

manufacturer cure specifications prior to initiating chemical service. A final spark test must be

performed before acceptance since pinhole defects are the most common defects found in

rubber lined tanks.

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Figure 6: Issue with Stitching Quality

Application and Quality Issues with Surface Preparation:

Nonmetallic systems are labor intensive to apply and about two-thirds of the cost goes into labor

overhead in the United States. This is the result of the surface preparation, application and

quality control which encompass a successful project. Surface preparation is considered the

foundation of the lining system. Steel requires the most rigorous surface preparation because

the requirement is a NACE 1/SSPC-SP5 white metal blast. Before the abrasive blast, a detergent

pressure wash on steel surfaces must be performed in order to remove salts, sulfites and nitrate

contaminants from the surface.

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Figure 7: White Metal Blast on Steel for Lining.

Once surface preparation is complete, the application process soon follows in an effort to either

protect the steel from flash rusting or to take advantage of the recoating window. The substrate

surface must be 5-degrees Fahrenheit higher than the dew point in order to prevent moisture

from interfering with the cure and adhesion of the lining system. The application process must

be initiated at temperatures in above 50-degrees because it is almost impossible to obtain a

complete resin cure without the application of external heat.

Corners and edges must be reinforced through the use of putties or flashings. The tank must

also have the shell to wall corner radius filled-in with putty or reinforcement from the bottom to

the shell in order to prevent the formation of air pockets. This holds true for weld seams and

other metal deformities as well. A comprehensive guide to this practice is found in the standard

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NACE SP0178. The surface preparation process is considered a critical quality control point. It

must have a proper surface profile and be free of contaminates prior to application.

Resin lining applications require an accurate gel time in order to provide sufficient working time

to remove air bubbles from the laminate. A resilient wetting primer is recommended in order to

provide an interface that will assist with expansion and thermal cycling performance. The primer

must be applied with a thickness of no more than 4 mils in order to prevent detrimental effects

on the adhesion. Following manufacturer product data sheets during the application process is

important and training is needed for optimal lining performance. Quality inspections must be

performed and evaluated in order to ensure proper application techniques are being used.

Measurements of dimensions and tolerances are outlined in ASME RTP1 Standard in Section 6

and mandatory appendixes.

Figure 7. Improper FRP nozzle Installation for Acid Quench Tower Dome Roof.

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While the best materials and methods can be used, the system won’t provide reliable service if

the quality is lacking. This is why 3rd-party inspections are required, in order to ensure that details

and issues are timely-addressed and corrected.

Conclusion

Sulfuric and Phosphoric chemical services demand a level of respect for the chemicals being

used. As a result of the changing service conditions, one should not rely solely on the material

service tables. Solution mixtures in a waste stream can wreak havoc on any system that was

designed for only one concentration. Lower concentrations of certain chemicals as well as

alkaline solutions can be detrimental to the construction material being used. Permeation and

temperature changes can drastically-affect any laminate performance; therefore, quality

assurance and technical specifications addressing these issues are of vital importance. There

are many plants that had to close their doors due to chemical remediation fines and regulatory

requirements after leaks and spills resulted from containment failures.

There is an important need for nonmetallic engineering with proper specifications and for the

quality inspections used to determine satisfactory results in a material’s performance. When end

users go with very expensive proposed systems that may result in premature failures without

enough due diligence, these end users will lose confidence in nonmetallic. The burnout rate at

these plants in some cases is very high due to this difficulty. It is always our goal to provide all

the facts and to use proper engineering material selection and specifications to bring the success

to any project in the Sulfuric and Phosphoric plants. To bring up the industry will also bring

confidence to nonmetallic solutions.

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References

ASME RTP-1-2015, “Reinforced Thermoset Plastic Corrosion-Resistant Equipment,” American Society of Mechanical Engineers, New York, NY, 2013.

ASTM C582, “Standard Specification for Contact-Molded Reinforced Thermosetting Plastic (RTP) Laminates for Corrosion-Resistant Equipment,” ASTM International, West Conshohocken, PA, 2009, 7 pp.

ASTM D4541, “Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers,” ASTM International, West Conshohocken, PA, 2009.

ASTM D2583, “Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impressor,” ASTM International, West Conshohocken, PA, 2001.

ICRI CSP 4-5, ICRI Committee 310, “Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair (ICRI 310.2R-2013),” International Concrete Repair Institute, St. Paul, MN, 48 pp.

NACE SPO178, “Design, Fabrication, and Surface Finish Practices for Tanks and Vessels to Be Lined for Immersion Service,” NACE International, Houston, TX 2007.

SSPC-SP 13/NACE NO. 6, “Surface Preparation of Concrete (2003),” SSPC: The Society of Protective Coatings, Pittsburgh, PA.