Xia Weidong: Comparison of the application of three main pipeline cover layers, oil and gas storage and transportation, 2000, 19 (2) hydrocarbon cover layer, the application and application scope of these three cover layers are analyzed and compared. It analyzes many factors that should be considered when selecting pipeline coatings for specific pipeline engineering projects, and comprehensively evaluates the coating characteristics of these three coatings.
The main application of Quanquan's pipeline coating is to ensure that the pipeline will not leak during the service life. The most effective way is to choose the ideal coating material. For specific pipeline anti-corrosion projects, comprehensive consideration should be given to factors such as pipeline operating conditions, soil conditions, construction environment and process, pipeline design life, environmental protection requirements, and economic rationality. The following is an analysis and discussion of the advantages and disadvantages of coal tar coatings, sintered epoxy coatings and polyolefin coatings, and based on the pipeline coatings and corrosion control of Mobil Oil Company (Lead Northworth) for nearly 20 years. Experience (including Datong ’s experiments, preparation of technical specifications, overburden detection, on-site application and evaluation, research and factory applications, etc.), combined with the characteristics of these three pipeline overburdens, developed its scoring criteria, divided into 110 points , The best score is 10 points.
1. The main characteristics of the cover layer The main characteristics of the pipeline cover layer are as follows.
Soil stress (SS). After burial, there is a certain pressure or stretching on the surface of the cover layer, which will cause the cover layer to twist, sag or crack. The most cover layer is little affected by soil stress.
Adhesion (AD). The better the adhesion of the cover layer to the steel pipe, the higher the score.
Surface pretreatment requirements (SP). The more the requirements for the surface pretreatment of the steel pipe, the lower the score.
Cathodic protection shield (CPS). If the covering layer loses its adhesion to the steel pipe and the electrolyte penetrates into the gap between the covering layer and the steel pipe, the covering layer will shield the cathodic protection current. The greater the current shielded by the covering layer, the lower the score.
Current required for cathodic protection (CPR). The lower the current required for cathodic protection, the higher the score.
In the process, the cover layer is very susceptible to damage. The smaller the damage suffered by the cover layer, the higher the score.
Repair (RP). After the repair, the more it matches the material of the main cover layer, the higher the score.
On-site supplement (F). The on-site patch covering is as good or better than the covering of the tube, and the higher the score.
The better the compatibility of the piece with the main cover layer, the higher the score.
Ease of application (AP). Easier application usually means lower cost. The simpler the application, the higher the score (OA). The total score of the cover layer is determined by many factors such as the above score and the operation of the cover layer. The better the cover layer, the higher the score.
(1) Characteristics of coal tar coating Coal tar enamel can be applied to pipelines with operating temperature between 3095C, and on subsea pipelines with concrete weights, the operating temperature can reach 115Cm. It has durable corrosion protection and is resistant to soil bacteria Corrosion with marine organisms, resistance to plant root penetration, low water absorption rate, electrical insulation, good chemical stability, erosion of petroleum products, anti-cathode stripping and other characteristics, especially suitable for multi-bacterial soil environment, reed clusters, low depressions , Humid and marine conditions. The damaged coal tar coating can be easily repaired by applying coal tar enamel, coal tar epoxy or wrapping tape on the spot (see Table 1).
Table 1 Coal tar coating: characteristics and application scores The coal tar coating is applied together with the fiberglass inner and outer entanglement belts. The resistance to soil stress is better. The adhesion depends on the bonding strength of the primer to the steel pipe. The surface pretreatment is good, the adhesion is good. The requirements of coal tar on the tube surface pretreatment are not as strict as the requirements of sintered epoxy. Unless there is enough cracking to allow current to enter the tube surface, the cathodic protection shield becomes the entire cover layer system. One problem is that the initial cathodic protection current demand is relatively small, and as the age of the cover layer increases, the thick cover layer will hide, such as cracks or peeling damage. If the kraft paper or white paint is damaged, the ultraviolet ray can make the coal tar coating deteriorate. Coal tar enamel or epoxy is used for repair, which is very compatible with the main coating. When selected and used properly, repairing with tape and shrink sleeves is very successful. Coal tar enamel or epoxy is used for on-site repair and matches the main cover very well. When the right choice is used, the use of winding tape and shrink sleeves is very successful. It is possible to coat elbows, valves and other components with coal tar enamel, epoxy or matte resin. Compared with FBE, the coating of coal tar is relatively easy. The application of tar coating and cladding is successful. The failure usually occurs because of poor coating process, and the temperature or soil stress coal tar coating has also failed in the application. It is more common in the coating applied in the trench, mainly on the surface The treatment is not strict or caused by coating under undesirable climatic conditions. When the cover layer is peeled off, if water penetrates, cathodic protection and shielding may occur. Generally, the cathodic protection current increases with the age of the coating layer. W. Coal tar enamel must be hot-coated during construction, and the fumes emitted during cooking and pouring are harmful to human health. Care must be taken during construction.
(2) Application of coal tar coating layer Coal tar coating layer is one of the first successful application in factory prefabricated pipeline coating layer. It is made by mixing or preparing various fillers with coal tar. There are four types of coal tar paint, suitable for different geographical environments and operating conditions.
Coal tar enamel can be applied either in a prefabrication plant or on-site using mobile equipment. Because the coating process of the prefabrication plant is easier to control, the coating environment is stable and not affected by the atmosphere, it is recommended to apply the prefabrication plant m. Generally, the coal tar coating used in the prefabrication plant is a plasticized enamel. When coal tar enamel is applied in the prefabrication plant, the dirt, salt, grease and other impurities on the surface of the pipe must be removed, and then the surface of the steel pipe is preheated and blasted to remove rust to reach Sa2.5 level, and the anchor depth is 50100pm. Available Brushing, spraying or rolling methods apply the synthetic quick-drying primer to the steel pipe. Then, the hot coal tar enamel is poured on the pipe rotating on the working line, and then the inner and outer winding tapes are wound to enhance the strength of the covering layer. Generally, in order to prevent ultraviolet radiation, a long-lasting sunscreen can be applied to the outer layer.
2. The performance of the fusion-bonded epoxy coating layer. The 14th pipeline cover layer was adjusted. In the application of buried gas pipelines, the amount of melted epoxy (FBE) has ranked first in the list for 10 consecutive years. The operating temperature is a 6011 (TC range) The inner can be used normally, the most usable temperature is even 138CW. FBE and steel pipe have excellent bonding properties. According to the performance comparison of various materials in Canada, FBE is resistant to chemical corrosion, bending and soil stress , Adhesion, anti-cathode peeling, wear resistance and other properties are excellent, and its service life is generally more than 60 years. When the cover layer is peeled off, the FBE cover layer absorbs. Water can make enough cathodic protection current reach through the paint film On the tube under the cover layer. Therefore, as long as there is sufficient cathodic protection, although the FBE peels off, the tube body rarely shows corrosion. There has never been any stress corrosion cracking on the FBE cover layer. Once FBE coverage occurs If the layer is damaged, it can be repaired with two-component liquid epoxy or epoxy rods. The FBE cover layer is suitable for swamp areas, unmanned areas, areas where pipeline repair is difficult, river crossing, etc. The FBE cover layer is used as a compound The underlying capping its application is very successful.
Compared with most other factory prefabricated coatings, FBE requires stricter pipe surface pretreatment and coating processes. Small cracks or other defects in the tube body can cause pinholes in the cover layer. If the on-site repair port adopts the same FBE as the main covering of the pipe body, the overall covering quality is good, but the cost is very high. If the tube surface is poorly pretreated, the temperature will make the cover layer absorb water faster and cause peeling. For 95X: the film temperature above should be attained. 81mmQ3. As the bottom layer of the composite cover layer, FBE performance is excellent, but the cost is relatively low. Because the FBE coating is thin and brittle, it is susceptible to damage caused by third parties and handling, and is sensitive to moisture absorption (see Table 2).
Table 2 Application characteristics of fused ring coating characteristics Scores Because of excellent bonding to the steel pipe and its smooth outer surface, FBE is not affected by soil stress. FBE has the strongest bonding force to the steel pipe.
It is one of the main reasons why it is used as the bottom layer in the composite cover layer. FBE is the most stringent requirement for surface pretreatment in the factory prefabricated cover layer. FBE allows sufficient current to protect the initial and long-term cathodic protection current of the steel pipe under the bubble or stripping zone. Pipes with lower FBE coatings that require FBE are used well during transportation, storage, and construction. Damage is easy to detect and repair. If stored for too long, ultraviolet light will deteriorate FBE. Various repair materials can be used for FBE covering, including repair strips, wrapping tape, shrink sleeves and 8 (two-component epoxy) two-component epoxy 6 (other) FBE site The cost of repair is very high, but it can provide the same cover as the main cover. Tape, shrink sleeves and two-component epoxy can also be used as on-site patching of 8 (two-component epoxy) 6 (other) FBE or brushable epoxy coated elbow and other components 8 (double Component epoxy) FBE coating process is very strict FBE has become one of the most commonly used pipeline coatings (especially in the United States) (2) Application situation of fused epoxy coating layer FBE successfully coated It has been used in pipeline anticorrosion industry for more than 30 years. A typical FBE formulation is composed of epoxy resin, curing agent (hardener), catalyst, basic pigment and reinforcing pigment, control agent (flow agent and stabilizer) and special components. Initially, FBE was applied to the primer, but now it can be applied directly to the steel pipe. The thickness of the FBE previously coated is more strict than the one currently prescribed in the pipeline anti-corrosion industry. The FBE coating process is the most stringent and complicated. Because its thin paint film is easy to absorb water, the surface pretreatment requirements of the tube are relatively neat. First, preheat shot blasting to remove rust, remove impurities such as oil, rust and scale on the surface of the pipe, and blast to near white level.
Some users, especially many companies in Europe, require intensive cleaning with phosphoric acid or sodium phosphate solutions. After that, it was heated to 23226 (TC with an induction heating coil, and FBE powder was sprayed on the heat pipe with dry air spray equipment. As the tube was turned on the work line, the FBE powder completed the melt gelation and curing process. Finally, Then use water quenching system for cooling.
The factory prefabricated polyolefin coating has been used in pipeline corrosion protection since 1960. Because the adhesion between the polyolefin cover layer and the steel pipe is not good, it is generally coated on the adhesion layer or other products. Polyethylene (PE) is the preferred outer extrusion coating for most polyolefins, and polypropylene (PP) is usually used for higher operating temperatures above 65C. Most of the extruded polyolefin coating and steel tube bonding rely on the adhesive, usually modified rubber asphalt binder or butyl rubber compound tl. Three-layer coating system is to use the characteristics of epoxy and polyolefin development Up. The three-layer covering layer adopts epoxy or bamboo-quality FBE as the bottom layer of the steel pipe, and then uses an ethylene copolymer or terpolymer adhesive as the bonding layer with the polyolefin outer layer. At present, the composite cover layer system uses a chemically modified polyolefin cover layer as the outer layer or adhesive layer of the anticorrosive layer, and they are mechanically bonded to the bottom layer of the FBE through chemical bonds.
Two-layer polyolefin cover layer (POA) usually has two methods to apply polyolefin on the adhesive layer. One is to use a cross-head extrusion molding system. As the pipe moves on a non-rotating line, the molten polyolefin is extruded onto the pipeline coated with modified rubber asphalt binder. Another method is to extrude the molten butyl rubber adhesive side onto the tube as the tube rotates on the conveyor line, and then extrude the molten polyolefin side onto the adhesive. This process allows bonding The agent and the polyolefin cover layer are melted together to form a theoretically seamless tubular cover layer.
The epoxy powder as the bottom layer of the three-layer covering layer is sprayed on the rotating heat pipe on the working line, and the copolymer / terpolymer is extruded on the upper side as the bonding layer of the epoxy bottom layer and the poly outer layer. Finally, the The polyolefin side is extruded out of the adhesive layer. Overlay systems have been used in Europe and the Middle East for more than 20 years and have excellent performance. The use of epoxy primer improves the cathodic stripping characteristics of these coatings.
The current trend of coating polyolefin coatings (POFBE) on FBE multilayer coatings is to use thicker (.
300.64mm) of FBE as the basic cover layer, on which the chemically modified polyolefin cover layer (CMP) is applied as the outer layer or the intermediate layer of the three-layer anti-corrosion layer. Therefore, it has a greater improvement than the traditional three-layer cover layer. CMP is chemical and mechanical bonding to FBE, so the bonding force to FBE is greater than that of copolymer / terpolymer to FBE. At the same time, the adhesion of the hot CMP to the outer polyolefin cover layer is also excellent, making the entire cover layer system excellent.
(4) Polycarbonate cover layer characteristics The characteristics and application of polyolefin cover layer are shown in Table 3. Table 3 Polyolefin cover layer w characteristics Application situation score The factory-made polyolefin cover layer is not affected by soil stress. The binding force depends on The type of cover layer used The standard for surface pretreatment depends on the type of cover layer used. All polyolefin cover layers shield cathodic protection. It mainly depends on the lower quality of the bonding of the bottom layer to the steel pipe and the outer covering layer or bonding layer to the bottom layer. If the coating of the on-site repair port and other components is not good or aging, the demand for the current amount will increase with the increase of the service life (when FBE / CMP is used for repair ports and elbows), it can effectively prevent handling damage to the POA Most of them use shrink sleeves and wrapping tapes for repairs, and POEP and POFBE covering layers use various hot coatings to repair the POA layer.
FBE / CMP can provide better on-site patch shrink sleeves, wrapping tape, matte fat and liquid epoxy coating various components, especially POA and POEP coatings. FBE / CMP is used for bending pipes and other components, providing a matching coating system for the POFBE coating layer. The POA coating layer requires less coating process than the other two coating layers. POF-BE has a coating process The type of the most demanding overlay system determines the score.
When FBE / CMP is used for repairs, on-site repairs, elbows and other components, FBE / polyolefin coatings have the best performance among all factory prefabricated coatings. Factory prefabricated polyolefin coatings have excellent performance and can be applied to working temperatures. It is a 208 (TC pipeline. When using FBE / adhesive / polypropylene cover system (developed by Montell laboratory in Italy), it can be applied to operating temperatures of up to 12 tombs. The three-layer cover system has adhesion Strong, good insulation performance, low water absorption rate, high temperature resistance, aging resistance, chemical corrosion resistance, high mechanical strength and other characteristics, almost not affected by soil stress, has excellent resistance to any damage caused by brutal handling, and, use Long life expectancy, anticorrosion life expectancy up to 100 years. The three-layer composite structure can be applied to pipe sections in various environments, especially in sections with high mechanical strength, strong soil stress and strong corrosion.
The main disadvantage of POA and POEP coatings is that the polyolefin outer layer shrinks and expands with temperature. If the polyolefin outer layer is damaged, water will penetrate between the polyolefin outer layer and the pipe (POA) or the polyolefin outer layer and the epoxy bottom layer (POEP). Due to the high electrical insulation of the polyolefin coating, the cathodic protection current will be shielded, and corrosion under the paint film will become a serious problem.
A major issue with all polyolefin coatings is what materials are used for pipeline repairs, field repairs, valves and other components.
Most tape wraps and shrink sleeves have soil stress problems, poor adhesion to polyolefin coatings, and coating elbows and other components is very difficult and prone to repair and repair failures. Similarly, the coatings such as liquid epoxy and maqi grease have poor adhesion to polyolefin coatings, and water easily penetrates into the junction of the two coatings. POFBE can be used to coat elbows and other components, but it costs less than the prefabricated coatings of other factories.
In the world, a variety of pipeline anticorrosion coatings with excellent performance are currently in existence. They are suitable for different soils, different construction environments and different operating conditions. However, it is impossible to rank them, and there is no universal anticorrosive paint that can "take over the world", which can only be comprehensively considered for specific engineering projects. First consider the coating performance that can meet the geographical environment of the pipeline section and meet the design requirements, and then consider the design life of the pipeline, the ease of construction (coating process and on-site repair method), and economic rationality (construction, operation and maintenance ), Meet the local environmental protection requirements to protect the surrounding environment and public facilities and other comprehensive factors. Only by selecting the appropriate pipeline anti-corrosion material can the reliability and integrity of the safe operation of the pipeline be improved.
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