2.6 PAINT COSTPAINT COST
When talking to users or potential users of blast cleaning equipment, the universal question is always asked, "What is it going to cost to clean a specific surface?"
There are six obvious reasons why no one can give an accurate answer to this type of question.
1. The Type of Surface to be Cleaned - No one can look at a surface and determine what is on that surface. It might be easy to see that a green coat of paint is facing you; however, you have no knowledge of what is under the surface coat. There may be three or four mils, or 3/16" of old paint that have been applied over the many years the surface has been standing.
Beneath, may be the original rust and mill scale which caused the initial coating to fail. There is no visual way of determining these factors and, even if there were, they could vary considerably over different areas of the surface.
2. The Type of Abrasives Being Used - The type, particle size, shape and hardness have a large influence on both the rate and degree of cleaning. Another prime consideration is the delivered cost of the abrasive. Prices on abrasives can range from a locally available sand at
$4.oo per ton to products costing as high as $800.oo per ton for specially manufactured metallic types.
3. What Is Clean? - The surface preparation required must be clearly specified, for obviously, a white metal blasted surface requires a more thorough job than does a brush-off blasted surface. If you have five inspectors in a room, you could probably secure five different decisions as to what constitutes a clean surface to match the particular specification.
4. Air Pressure Available at Nozzle - The nozzle air pressure has a tremendous effect on job efficiency and the rate of production can be seriously hampered if the nozzle pressure is too low.
5. Operator Efficiency - The ability for the operator to perform an efficient job is one of the largest variables.
6. The Type and Efficiency of the Blast Cleaning Equipment - Using properly balanced blasting equipment can increase production two- or three-fold and has a great reflection on reduced costs.
Table 1-6 AVERAGE JOB BREAKDOWN Table 1-6 AVERAGE JOB BREAKDOWN
Cost % of Total
Costs Surface Preparation 15-50 Coating Material Cost 15-20
Application 30-60
Accessory Products 2-5
Clean Up 5-10
2.7
2.7 TEMPORARY PROTECTEMPORARY PROTECTIONTION
Essential features of any temporary preventive include the following: It must be easy to apply and even more important, easy to remove. While it is on the metal, it must resist the corrosive effects of humidity, fumes, fingerprints, weathering and water. These coatings prevent mechanical damage, such as nicks and burs, and preserve the metal's original appearance.
The major preventives include (1) grease types, (2) oil types, (3) solvent types, and (4) strippable plastic types. Each has special advantages or limitations.
2.7.1 Grease Preventives
Grease preventives are thick coat compounds that won't melt or flow at ordinary room temperature. Materials range from soft petrolatums to hard waxlike compounds. Softer coatings are for moderate shipping and storage temperatures. Harder coatings stand up under higher temperatures.
Dipping in heated tanks is the usual method of applying the greases and, except for some hard solvent/drying types, they require the most time and effort to remove.
2.7.2 Oil Preventives
Oil preventives include the non-drying, non-setting oils of various viscosities. Even with the heaviest of these oils, protective films won't be thicker than .0002 in. They attain final thickness by draining only, without setting or drying.
2.7.3 Solvent Cut Back Preventives
Solvent Cut Back Preventives can be subdivided according to the solvent and material dissolved in them.
Dry Type Films These are asphaltic, resin and waxy films which are thin, fairly hard films that look like varnish. They are on par with protection achieved with heavy greases and withstand abrasion and handling.
Water Displacing These usually contain substantial amounts of soaplike materials that actually remove droplets of water from metal surfaces by 'preferential wetting.' Preservatives attraction toward the metal surface is greater than that of water, displacing the water. The major reason for their use includes the reduction of time and labor required by permitting the easy preservation of wet parts in one simple dip.
Fingerprint
Removers These contain water, an organic solvent and preserving additives. After fingerprint residues (acidic organic materials, salt, etc.) are dissolved, the additives form a protecting film. For long-term storage, remove this film and replace it with a more lasting preservative.
Combination Solvent Preservatives
This solvent is combined with either an oil, grease or wax-type preservative.
If solvent content is low and it is sprayed or brushed on, the final film will be relatively thick, comparable to greases. However, more often, these films are thin.
Water Soluble
Compound A water soluble compound is a low-cost coating from which the water evaporates, leaving an oily film.
2.7.4 Strippable Plastics
Strippable plastics are a combination of special oils, plasticizers, inhibitors, synthetic resins and plastics. Coatings are thick, from .050 to .100 in, and have several unique features. They provide top corrosion as well as mechanical protection, and they can easily be removed by slitting and peeling. These coatings are applied from hot tanks.
2.8
2.8 GALVANIZINGGALVANIZING
Protective coating systems fall into two major groups. The first group includes paints and plastics which provide a barrier coating but give no protection around the edges or a points of mechanical damage. The second group includes barrier systems such as zinc coatings which protect by sacrificial action even when they are moderately damaged. Hot dip galvanizing is used very extensively for corrosion protection of structurals and exposed carbon steel in many refineries, particularly those on the coast. A thick zinc multilayer coating is metallurgically bonded to the steel substrate. The coating corrodes at a rate of about 3-10% of the underlying steel. Typical coating weight minimum is about 610 g/m2 (2 oz/ft2). In most environments the life of the coating is proportional to the weight of the coating. An even thickness of the galvanized coating is applied to edges and flat surfaces. The zinc covers corners, edges seams and rivets to give complete protection to what may be potential failure points in other protective systems. Hot dip galvanizing applied after fabrication is tough and will tolerate handling which would damage most other coatings. The galvanizing process may vary from plant to plant but basically is as follows:
1. Cleaning - In most plants, this is a two step operation. The material is first immersed in a hot caustic bath or some other similar solution to remove oil, grease and other organic contaminants. The workpiece is then rinsed and taken into a mineral acid bath to remove rust, mill scale and other inorganic contaminants. The material is then rinsed and is ready for the next step in the process.
2. Fluxing - The steel is immersed in a tank containing an aqueous preflux solution to remove any oxides which may have formed on the material during the handling process. A molten flux blanket on the surface of the molten zinc in the kettle is also often used to clean the steel surface before the part is immersed or dipped into the bath.
3. Coating - The workpiece is then submerged in the molten zinc bath where it remains until the alloying reaction is complete. Depending upon the chemical composition of the steel, configuration, and mass of the material being coated, this process can take from 30 seconds to 8 hours. When the alloying reaction is complete, the steel is removed from the kettle.
After the material is removed, it is either allowed to air cool or is immersed in a water quench tank. The quench tank operation freezes any further reaction between the base steel and the coating. The quench water may be treated with other chemicals to give the coating certain post treatments such as wash primers or zinc dust primers for eventual top coatings.
Galvanizing usually does not cause a loss of ductility problem unless the material has been
worked areas and embrittle the workpiece if it is not allowed enough time to diffuse out.
Bessemer steels or other steels that may strain age embrittle should not be galvanized due to potential embrittlement problems There are ASTM specifications that cover safeguards to prevent embrittlement, warping and distortion during hot dip galvanizing as well as a specification covering the actual hot dip galvanizing procedure. As with any coating system, hot dip galvanizing needs proper specifications and inspection so that a quality product is furnished.
3.0
3.0 PLASTICSPLASTICS
Plastics are increasingly replacing metal parts because of their light weight and good corrosion resistance to a wide range of chemicals. However, most plastics cannot be used above 250oF (121oC), and have lower strength than metals. The mechanical properties of the plastics may be increased by the addition of glass fibers to the plastic resin. The chemical properties can be altered by adding modifiers to protect it against specific environments. However, in most petrochemical operations, safety concerns limit the use of plastics in vessels and piping because of the potential for fires and unfamiliarity with the material. The primary uses for plastics in the petrochemical industry include wrapping for electrical components, housing for electrical components, hoses and protective coatings for tanks and piping.
A plastic can be defined as a material consisting of long chained, organic molecules which are formed by combining short chained, organic molecules together in a viscous state. Plastics can be divided into two types, based on the final structure of the chains in the molecules:
1) thermoplastics and 2) thermosetters.
Thermoplastics can be repeatedly heated with only a minimum reduction in their properties because the side chains of the molecules are not connected. Generally, thermoplastics are fabricated by injection molding or casting. Thermosetters have their side chains cross-linked.
When heated above their maximum use temperature, the side chains are permanently broken, thus causing the plastic to degrade. The thermosetting plastics are usually made by combining a liquid resin with a catalyst. An exothermic reaction is produced, causing the material to set into a hard plastic. With the rapid advances in technology, plastics are being produced which are both thermosetters and thermoplastic.
The selection of a particular plastic for an application is similar to choosing a metal. Design properties for the selection of a plastic include tensile strength, heat deflection point, toughness, creep modulus, specific gravity, shrinkage, expansion coefficient, resistance to the operating environment, and cost. The dielectric strength is important when a plastic will be used for an electrical application.
Plastics can undergo mechanical failure from stress corrosion cracking, fatigue, rupture, embrittlement and overload. Corrosion is usually caused by chemicals, water, heat, and ultraviolet light interacting with the polymer chains, causing their degradation. Physical signs of corrosion include bloating, hardening, softening, discoloration and elongation. Unlike metals, dissolution of material from the plastic is uncommon.
A wide range of plastics exists with each one having unique properties. Because of the complexity of plastics, coupons of the material should be tested in the operating environment before the selection is made. The following list gives the overall properties of some groups of plastics, but individual plastics within the group can behave differently. Many of the materials can be reinforced with fibers in the resin to give increased physical properties.
3.1 THERMOPLASTICS 3.1 THERMOPLASTICS
ABS (Acrylonitrile-butadiene-styrene copolymerization)
Resistant to weathering; good all around properties; should not be used above 190oF ( 88oC); used for pipes and pump impellers
Acetals Clear; excellent fatigue life; toughness and strength; high abrasion resistance;
excellent against solvents; poor with acid and bases; may be used from -40 to 220oF ( -40 to 104oC); uses include pipes, impellers, gears
Fluorocarbon Maximum continuous use temperature of 490oF (254oC); inert to most chemicals; low coefficient of friction; low mechanical properties, but can be reinforced with fibers; uses include nonlubricated bearings, pipes, gaskets, and seals
Nylon Good toughness, impact resistance, and strength; abrasion resistant; resistant to solvents and bases, but not acids; absorbs moisture, which leads to swelling; temperature limit of 250oF (121oC); used for gears, bearings, and machinery
Polypropylene Temperature limit restricted to below 200oF (93oC); good resistance to acid and bases, but poor to solvents; moderate physical properties; inexpensive;
uses include pipes, ropes, and ducts
Polycarbonates High toughness and dimensional stability, low creep; temperature use up to 270oF (132oC); transparent; resistant to acids and some solvents; uses include electrical housings.
Polyester Excellent dimensional properties; high strength, toughness, and low creep;
good chemical resistance; maximum continuous operating temperatures of 320oF (160oC); used for tanks, sinks, and pump housings
Polyethylene Stress cracks when exposed to solvents; low temperature limit (130oF) (54oC)
Polyvinyl Chlorides Maximum temperature limit of 150oF (66oC); high strength; resistant to acids and bases, but poor against solvents; used for pipes, tanks, valves, and gaskets.
3.2 THERMOSETTERS 3.2 THERMOSETTERS
Epoxies High strength, impact resistance, and toughness; maximum continuous temperature limit is 250oF (121oC); resistant to most chemicals, except for oxidizing agents; used for linings, protective coatings, adhesives and castings Silicones Maximum temperature up to 500oF (260oC); resistant to moisture; chemical
Ureas Corrosion resistance poor; good toughness and wear resistance; maximum temperature at continuous use is 150oF (66oC); used for pulleys, pump impellers, and conveyor belts
4.0
4.0 REFRACTORIESREFRACTORIES
A refractory is a non-metallic substance that is resistant to prolonged exposure at high temperatures without undergoing a reduction in its physical properties. Refractories are widely used within various process equipment in the refining and petrochemical industry to protect the vessel metallurgy from excessive heat, erosion and/or corrosion.
The type of refractory selected for installation will depend on several parameters including:
1. the design shell temperature
2. the corrosiveness/erosiveness of the process 3. the expected service life of the lining 4. minimizing heat loss from the process
Consideration must also be given to the economics and ease of installation.
4.1
4.1 MATERIAL CLASSIFICATIONMATERIAL CLASSIFICATION
Refractory materials can be classified several ways. In this section, the classification will be based on the manufacturing method used to make the material and subdivided by their common physical properties and/or chemical properties.
4.1.1 Castable and Gunning Mixes
Castable and gunning mixes are very similar types of materials and exhibit most of the properties of standard concrete. Installation of castables requires the addition of water to a dry mix within a mixer. After approximately 5 minutes of mixing, the material is removed from the mixer and poured into place between forms. Gunned refractory mixes are applied by shooting a dry or slightly dampened mix through a high pressure air hose and adding water to the material at the hose's nozzle. The major differences between gunning and casting grade mixes are the types of additives used in the product to adjust the setup time for the cement and to facilitate the application of the gunned material. Therefore, castable and gunning mixes should not be interchanged unless the manufacturer specifies it as both gunning and casting grade material.
Most of the gunning and casting mixes consist of calcium aluminate cement phases and fired alumina-silica rich aggregates. The types of cements and aggregates are changed to obtain the desired properties. The calcium aluminate cement enables a refractory to be used at higher temperatures than portland cement based concrete which is composed of calcium silicate phases and impurities. When mixed with water, the calcium/alumina cement becomes hydrated and forms a gel which bonds the aggregates together. Before the material can be used at high temperatures, the water in the concrete must be removed by a controlled curing and dryout schedule. Improper removal of the water will cause damage to the lining and vessel along with possible endangerment to human life. This is due to the high steam pressure generated within the refractory by evaporating the water at high temperatures.
Lightweight refractory mixes are used within fired heaters where erosion is low and maximum heat retention is required. Gunned, medium weight mixes are the most commonly used refractory (based on tonnage) in refinery operations. They are used in FCCU reactors, regenerators, transfer lines, cokers and other various large volume vessels with low erosion. A new product line of extra high-strength, medium-weight material has been developed to meet
more severe process conditions in these large volume vessels. Standard-weight materials are usually installed in secondary lines that have problems with erosion, but not coking. Heavy-weight materials are used in the FCCU riser lines because of their abrasion resistance and resistance to spalling by coke impregnation but they are usually vibrocast into place. The high alumina castables are mainly used in sulfur recovery units (SRU) and gasification vessels.
4.1.2 Vibration Castable
Vibration castables are similar to castables in chemistry. However, by installing the wet mix with vibrators attached to the vessel shell, the air bubbles trapped in the wet castable mix are driven out of the material. A vibrocast mix will be more dense, less porous and stronger than its cast counterpart. The vibration energy also allows the material to be placed with a much lower water content than a castable. Because less water is used to set up the concrete, the type and the amount of cement will vary between a castable and vibration castable mix. Special additives are often used to obtain better flow properties for a vibrocast material versus a regular castable mix.
Therefore, castable and vibrocast mixes are not interchangeable and the proper mix needs to be specified prior to installation. Also vibrocast material is not suited for gunning applications.
Advantages of using vibrocast refractory over gunning and castables mixes are:
1. less porosity is present, which inhibits coke penetration
2. less water is used during installation, which reduces shrinkage and increases the strength and abrasion resistance of the material; both improving refractory life
Disadvantages of vibrocast material versus gunning and casting include:
1. increased difficulty to install 2. higher installation cost
3. higher shell temperature and vessel weight
4. increased difficulty in tearing out the material for replacement or repairs
Vibration casting may be done in place or the line can be vibrocast in a shop and assembled in the field. Shop casting usually offers better quality control of material but increases the number of field joints which are considered weak links in the line. Also, shop vibrocasting can be done in advance of a Turnaround, thus reducing the amount of time necessary for installation and easing scheduling problems. In-place vibration casting reduces the number of field joints but improperly placed vibrators may cause damage to welds in the line. Most vibrocasting has been done in FCCU riser lines. Hand held vibrators are often used in casting material into gasifiers and other large diameter vessels.
4.1.3 Plastics
A plastic refractory is made at the manufacturers' plant and has a stiff consistency similar to modeling clay. No water is added to the plastic. Usually, the plastic is installed with pneumatic ramming guns onto hexmetal anchors or S-bar anchors. Plastics are mainly used in erosive
A plastic refractory is made at the manufacturers' plant and has a stiff consistency similar to modeling clay. No water is added to the plastic. Usually, the plastic is installed with pneumatic ramming guns onto hexmetal anchors or S-bar anchors. Plastics are mainly used in erosive