مقالات تکنولوژی و تجهیزات ابکاری

finishing equipment & plant engineering

CHEMICAL-RESISTANT TANKS

AND LININGS

BY C. E. ZARNITZ

ATLAS MINERALS & CHEMICALS INC., MERTZTOWN, PA.; www.atlasmin.com

The dominant and most economical construction materials used in the metalfinishingindustry are steel and concrete. Unfortunately, both of these materialsare highly susceptible to corrosive attack from many of the chemicals used in themetal-finishing industry. Pickling and plating chemicals are highly corrosive and,without proper protection, the life span of steel and concrete is limited. Tanks and

tank linings must be capable of:

1. resisting attack from organic and inorganic, oxidizing and nonoxidizingchemicals at varying concentrations, as well as from various solvents;

2. resisting broad thermal variances including thermal shock;

3. resisting weather extremes because economics dictate that very largestorage and waste treatment vessels be located outdoors;

4. resisting physical abuse that accompanies processing strip, heavy parts,shapes and castings; and

5. maximizing performance, value, and ease of maintenance.

TANKS AND LININGS

The type of tanks that have excelled in the metal-finishing industries includelined carbon steel; lined, precast, or poured-in-place concrete; precast or pouredin-place polymer concrete; self-supporting plastics, i.e., thermosets and thermoplastics;and alloys.

The success of steel or concrete-lined tanks is predicated on good engineeringdesign of the structural shell. The ultimate success of the lining, besides good engineeringdesign, is predicated on the finish and structural integrity of the substrate,as well as on the skills and proficiency of the applicator. If the structure cannotsustain the stress imposed by the process, lining failure is imminent. Similarly,plastic or alloy tanks will fail if good design engineering has been compromised.

Carbon Steel Tanks

When fabricating carbon steel tanks for subsequent lining, the following are important:

1. Minimum number of pieces and sufficient reinforcement must be used toprevent bulging when subjected to optimum process stress.

2. Vertical reinforcing is preferred to horizontal. Ledges are eliminated, thusminimizing potential for drag-out to hang, concentrate, and corrode thevessel from the “outside-in.”

3. Welds to receive lining are to be solid and continuous.

4. All corners are to be ground to a minimum radius of in.; no sharp rightangles.

5. Exterior reinforcing members may be skip welded.

6. All body seams must be butt welded true and flat with variation on alignmentnot to exceed 25% of plate thickness and in no case more than in.

7. All outlets to be flanged.

8. Interior of vessel must be free of weld splatter, pits, deep gouges, and all

welds ground smooth.The following typicaloutlet and weld details aresuggested when fabricatingcarbon steel that will belined with various types oflinings (see Figs. 1 and 2).

Stainless Steel Tanks

Stainless steel tanks can becompared to plastic tanksin the respect that they aresolid steel, thus eliminatingthe need to protect a vulnerable

exterior from fumesand splash. Stainless steels generally are classified as straight iron-chromiumalloys and iron-chromium-nickel alloys. In the metal-finishing industry, theiron-chromium-nickel alloys, i.e., the 300 series appear to be the most popular.Types 302, 304, 321, and 347 are considered to be generally equivalent in chemicalresistance.The stainless steel alloys exhibit excellent resistance to such oxidizing acids

as nitric and chromic. They have virtually no resistance to hydrochloric andhydrofluoric acids.The vulnerability of stainless steels to halogenated acids is easy to understandwhen you recognize that pickling solutions for stainless steel are acids such ashydrochloric and hydrofluoric and various combinations of nitric and hydrofluoric. Figs. 3-5 provide typical outlet and weld details for the fabrication of tanks.

Portland Cement Concrete Tanks

Concrete tanks are acceptable so long as good design engineering is practiced andincludes: sufficient reinforcement to prevent buckling and cracking; minimum3,000 psi compressive strength after 28 days; smooth, monolithic interior free ofridges, depressions, honeycomb,form marks,etc.; freedom from contaminantsand additives,i.e., form release agents,air entraining agents,etc.; and hydrostaticallytight and waterproofedon the exterior if locatedbelow grade.Self-supporting plasticand stainless steeltanks must comply withsimilar structural mandates

as those enumeratedfor carbon steeland Portland cement concrete.

Fig. 3. Pad outlets. Weld “D” is the same as “A,” except penetration is not required. Drill two -in.diameter holes, 180O apart, through weld for vent. Weld “E” plate must be bevelled distance equalto thickness of tank wall. Weld is to be built up above the surface of plate, peened and ground

smooth and flush with plate.

Fig. 4. Corner of rectangular tank. Weld “F”

should be burned into plate so welds meet

from opposite sides, thereby excluding air

pockets. Welds must be peened and ground

flush.

Polymer Concretes

Polymer concretes are a generation ofmaterials that have rapidly maturedbecause of their outstanding chemicalresistance and physical properties. Theyare not to be confused with Portlandcement concrete or polymer-modifiedPortland cement concrete. The onlysimilarity to Portland cement concreteor polymer-modified Portland cementconcrete is the use of properly gradedand sized aggregate in order to optimize

workability and physical properties ofthe composition. Polymer concretes utilizeinert siliceous aggregates with bindingsystems based on such resins as furan, epoxy, polyester, vinyl ester, and acrylic.(See Table I for typical physical properties of polymer concretes.)The advantages to be derived from polymer-modified Portland cement concretewhen compared with Portland cement concrete are:

1. Permits placement of concrete in thinner cross-sections.

2. Excellent bonding to existing concrete substrates.

3. Increased impact resistance.

4. Reduced porosity.

5. Faster set and cure.

6. Improved resistanceto salt. It does not improve resistance to chemicals.

Fig. 5. Butt joint. Weld “G” to be laid in V in beads not

exceeding -in. deep. After “G” is built up above plate on

outside, the inner surface must be gouged out sufficiently to

remove all scale and slag. Weld “H” is to be built up above

the surface of plate, peened and ground flush.

Polymer modifiers aregenerally based on variousresins and latexes, such asnatural rubber, styrenebutadiene,acrylic, polyvinylacetate, epoxy, and urethane.

LININGS

There are a host of lining materials available for protecting concrete and steel. The three basic types areglass-fiber-reinforced sheet and molten asphaltics; sheet rubber, plastics, and elastomers;and reinforced and nonreinforced ambient-cured synthetic resin systems.

Conspicuous by its absence from this list is protective coatings. This is not tosay they can’t be used; however, 60 mils is usually considered to be a minimumacceptable thickness for a material to be considered a tank lining. If a coating canbe economically applied (initial cost and longevity) to a minimum thickness of60 mils, free of pinholes and holidays, and can resist the process chemicals andtemperatures as well as physical abuse, consideration should be given to their use.Generally speaking, coatings are used for fume and splash protection and not

necessarily for total immersion process applications.Asphaltic linings are equally appropriate for application to concrete and steel.

The hot-applied, molten materials, as well as sheet stock can be used on concretetanks. For steel tanks, glass-fiber-reinforced sheet is the most desirable. Both typesof asphaltic linings, sheet and molten, are seldom, if ever, used without being furtherprotected with a chemical-resistant brick lining. Without further protectionfrom a brick sheathing, these linings can cold flow and be easily damaged fromimpact, abrasion, and thermal excursions. Masonry sheathings provide a rugged,chemical-resistant insulating barrier for protection of asphaltic as well as othertypes of linings. The physical properties and the chemical resistance of asphalticlinings are shown in Tables II and III, respectively.

Adhesive-bonded sheet linings, such as various plasticized plastics, rubbers andelastomers are most commonly used for steel tanks. Successful applications havebeen made on concrete; however, it is not the most desirable substrate on whichto bond and cure many of these systems. The physical properties and the chemicalresistance of sheet linings are shown in Tables IV and V, respectively.Mechanically bonded rigid plastic linings for precast and poured-in-place concrete tanksare a relatively new concept. Instead of bonding with adhesives, this system utilizesanchor studs sonically welded to the back of the sheet for locking or mechanicallybonding the sheet to the concrete.

Ambient temperature-cured, spray- and trowel-applied synthetic resin liningsystems are based on the following resins: furan, epoxy, polyester, vinyl ester, andurethane.These systems are entirely appropriate for application to steel and concrete.They have also been successfully applied to wood, certain plastics, and various metallic substrates. These lining systems utilize such filler reinforcements asflake glass and mica. Fabric reinforcements such as fiberglass are the most common;however, synthetic fabrics are used where fluorides are present.

These linings are extremely versatile and can be applied by maintenance personnelwith skills in the painting and masonry trades. Most manufacturers ofthese lining systems provide training programs for plant maintenance personnel.The physical properties and the chemical resistance of ambient-cured syntheticresin lining systems are shown in Tables VI and VII, respectively.The tables shown above all provide the design and corrosion engineer withbasic information on the various lining systems discussed. They identify specific

corrosives encountered in various metal-finishing operations. Enumerated areeach of the various types of linings and a general recommendation for its use in the particular medium. It is recommended that the acceptability of specificlinings, in specific media, be verified with the manufacturer.

Chemical-Resistant Brick and Tile Linings

Historically, chemical-resistant brick and tile linings go back approximately 100years, paralleling the development of sulfuric acid, various dyestuffs, and explosives.The use of masonry construction has grown in the basic steel, metal-working, andmetal-finishing industries. Chemical-resistant masonry sheathings are not to be con- strued as hydrostatically tight tank linings. They are, in fact, porous, and consequentlymust be considered as chemical, physical, and thermal barriers for protectingmembranes installed behind these sheathings. Brick sheathings contribute to thelongevity of tank linings by offering additional chemical, thermal, and physicalprotection. They are excellent insulating barriers and, consequently, can be consideredas energy savers.

PLASTIC TANKS AND LININGS

There are a multitude of plastics available for solving corrosion problems inthe metal-finishing industry. The more popular and cost effective are polyvinylchloride (PVC), Type I; polypropylene (PP); linear polyethylene (PE); andfiberglass-reinforced plastics (FRP). All of these plastics have been successfully used as self-supporting tanks and“drop-in” tank liners for process and storage applications. The thermoplastics

(PVC, PP, and PE) are being used for mechanical bonding to concrete for similarapplications.Polyvinyl chloride is one of the oldest proven plastics for fabricating highlychemical-resistant structures. (See Table VIII for the chemical resistance ofstructural plastics.) Type I PVC is one of the best plastics available for resistanceto a multitude of strong oxidizing environments up to its thermal limitation ofapproximately 150OF (66OC).Type I PVC has outstanding structural integrity attributable to its high tensile,compressive, and flexural properties. It is one of the easiest plastics fromwhich to construct tanks, tank liners, dipping baskets, and other storage andprocess equipment. PVC is easily thermoformed, cut, drilled, tapped, machined,and welded, consequently, making it an excellent, versatile, and cost-effectivematerial from which to fabricate corrosion-resistant equipment.Polypropylene has arrived. Its popularity is attributable to its excellent chemical

resistance and surpasses PVC because of its superior physical and thermalresistance. It is available as flame and nonflame retardant homopolymer andcopolymer sheet stock. Polypropylene utilizes fabricating techniques similarto those used for PVC. Small tanks for pickling and plating, large tanks forcontinuous strip pickling lines, and pickling line covers have been fabricated ofpolypropylene. These and similar fabrications are enjoying an enviable recordof success in challenging chemical and physical applications.Linear polyethylene fabrications haveperformed well in the small-parts metal-Table VIII. Chemical Resistance of Structural Plastics

finishing industry because of their low absorption, high chemical resistance, andoutstanding resistance to impact. They do not possess the rigidity and flexuralcapabilities of PVC or PP and, consequently, the fabrications are significantlysmaller. A new generation of PE is making significant inroads into the finishingindustry. Crosslinkable, high-density PE for rotational molding is being usedfor tanks of 5-10,000-gal capacity. These new resins exhibit excellent physicalproperties and good resistance to weathering. Applications for the most parthave been indoor and outdoor storage tanks and portable receiver tanks.Fiberglass-reinforced plastics have been successfully used for a multitude of

applications for many years. The earliest fabrications utilized furan and phenolicresin binder systems. The more popular resin binder systems in use today arepolyester, epoxy, and vinyl ester.

The success of FRP structures is substantially predicated on the proper choiceof resin and hardener system most inert to the environment to which the fabricationwill be subjected. It is not enough to request an FRP tank, any more than it isto request a flake-glass-reinforced polyester tank. It is important to either providethe fabricator with all chemical, thermal, and physical information pertinent to theprocess in order that the proper resin and hardener system might be selected, orto have in-house technical competency capable of making the proper selection of

the resin-hardener system from which the manufacturer can fabricate the desiredequipment.There are numerous polyester resins available; however, for aggressive corrosionenvironments, such as high concentrations of alkalies and a broad range ofacids, the bisphenol-A fumarate resin is the best recommendation.Vinyl esters are epoxy-resin-based, thermosetting resins that provide chemicalresistance similar to that of bisphenol A polyesters. They are considered tobe slightly better in high concentrationsof alkaline hypochlorites thanthe bisphenol A polyester. Vinylesters exhibit outstanding physicalproperties, i.e., tensile, flexural, andelongation that are normally associatedwith epoxies.The chemical resistance andphysical properties of epoxy resinsare functions of the resins, butequally important, they are verymuch functions of the curing systemsemployed.Aliphatic and aromatic aminecuring systems impart betterchemical resistance to epoxy resinsthan do polyamide curing agents.Polyamides, however, impart betterimpact resistance to epoxies thando aliphatic or aromatic curing systems.The intention of these threeexamples of the resin systems utilizedfor constructing fiberglass-reinforced tanks and tank linings is to point out the necessity of knowing thechemistry of the system, or relying on reputable manufacturers to provide thetechnology required to make the best selection to fulfill end use requirements.Where the chemistry of these various systems contributes substantially tothe physical properties of the structure, the most profound influence on physicalproperties is derived from the proper design and use of various reinforcingmediums, i.e., glass fibers, glass cloth, roving, mat, veil, etc. Consult reputablemanufacturers for proper design consistent with the end-use mandates forchemical, thermal, and physical properties.The chemical resistance of FRP is comparable to the chemical resistance datashown in Table VII. Table VIII summarizes the chemical resistance of PVC, PE, andPP.The mechanically bonded thermoplastic lining system previously describedcircumvents many of the limitations inherent in some plastics, as well as coating,and many other elastomeric and resin lining systems. The sonically weldedanchor studs are of the same plastic as the sheet and are placed at approximately2-3 in. on centers. Sheet thickness and anchor stud density provide the rigiditynecessary for a successful thermoplastic lining application. The studs, being ofthe same plastic as the sheet, ensure thermal and physical property similarity.The lining system is equally appropriate for new and existing concrete, as wellas for salvaging used steel tanks. Upon removal of the concrete forms and thewelding of all joints, utilizing thermoplastic welding techniques, spark testingis used for quality assurance of the lining.The system is available in a single- or double-wall system to ensure compliancewith the most rigid of environmental mandates. Leak detection systems are available and are integral with the lining system.