CC BY
169
SRSTI 81.33.03; 61.51.01
V. V. Ryndin1, G. G. Abdullina2, A. T. Abdul/in3
'Candidade of Engineering Scinces, professor, Department of «Mechanics and Petroleum Engineering», S. Toraighyrov Pavlodar State University, Pavlodar, 140008, Republic of Kazakhstan; 2PhD in Chemistry, associate professor, Department of «Mechanics and Petroleum Engineering», S. Toraighyrov Pavlodar State University, Pavlodar, 140008, Republic of Kazakhstan; 3Master of Science, S. Toraighyrov Pavlodar State University, Pavlodar, 140008, Republic of Kazakhstan
e-mail: '[email protected]; [email protected]; [email protected]
ANALYSIS OF PASSIVE METHODS OF PROTECTION FROM CORROSION OF MAIN OIL AND GAS PIPELINES
In this flock, «passive» methods of pipeline protection against corrosion are considered. High durability and trouble-free operation of pipelines directly depends on the quality of their anticorrosion protection. To minimize the risk of corrosion damage, the pipelines are protected with anti-corrosion coatings and additionally with electrochemical protection (ECC). In this case, insulation coatings provide primary («passive») protection of pipelines against corrosion, performing the function of a «diffusion barrier» through which access of corrosive agents (water, oxygen of air) to the metal is hampered. When defects appear in the coating, a cathodic protection system for pipelines is provided — «active» corrosion protection. For all time of anticorrosive coatings application, which constitute the core of «passive» protection of pipelines, a large number of insulating materials and methods of their application to the surface ofpipelines in factory and route conditions have been invented. In this regard, an important role was played by such indicators of the insulation coatings quality, as waterproofness, adhesion, durability, economy, continuity, mechanical strength, and other indicators that characterize the effectiveness of an insulation coating.
Keywords: active and passive corrosion, insulation coating.
INTRODUCTION
Corrosion of metals - spontaneous destruction of metals due to their chemical or electrochemical interaction with the external environment.
In most cases, corrosion of metals passes unevenly over the surface, there are areas where local lesions occur. Some corrosion products, forming surface films, inform metal corrosion resistance.
Sometimes loose friable products may appear, which have a weak adhesion to the metal. The destruction of such films causes intense corrosion of the exposed metal.
Corrosion of metal reduces mechanical strength and changes its other properties.
Corrosion processes are classified according to the types of corrosion damage, the nature of the interaction of the metal with the medium, the flow conditions.
By the nature of the interaction of metal with the environment, chemical and electrochemical corrosion are distinguished.
According to the conditions of the corrosion process, the following types of corrosion are most often encountered: gas, atmospheric, liquid, underground, biocorrosion, electrocorrosion, slit, contact, corrosion under stress, corrosion cavitation, corrosion erosion, fretting, structural, thermal contact.
MAIN PART
For external insulation of pipelines, the following types of factory coatings are most often used:
a) factory epoxy coating;
b) factory polyethylene coating;
c) factory polypropylene coating;
d) factory combined ribbon-polyethylene coating.
These types of coatings meet modern technical requirements and provide long-term, effective protection of pipelines against soil corrosion (figure 1).
Combined ribbon-polyethylene coatings are used mainly for insulation of pipes of small and medium-sized diameters with operating temperature up to plus 40 °C.
Epoxy coating
Figure 1 - Scheme of factory insulation of pipes
To date, when building domestic main and field pipelines as external protective coatings, the factory covers of pipes based on extruded polyethylene are most widely used.
The quality of the factory polyethylene coatings of pipes largely depends on the design of protective coatings and insulating materials used for their application.
There are 4 variants of constructions of factory polyethylene coatings of pipes: polyethylene coating applied on bituminous-mastic sublayer; Polyethylene coating applied on an insulating sublayer on the basis of a sticky polymer tape; a two-layer polyethylene coating consisting of an adhesive undercoat based on a hot-melt polymer composition and an outer polyethylene layer; a three-layer polyethylene coating consisting of an epoxy primer layer, an adhesive polymer undercoat and an outer polyethylene layer [1].
The first two types of polyethylene pipe coatings have a rather limited range of applications. These types of coatings are recommended to be used for external insulation of pipes of small and medium diameter (from 57 to 530 mm inclusive) at a pipeline service temperature not higher than +40 °C. The main area of application of such coatings is the construction of field pipelines, water pipes, low-pressure inter-settlement gas pipelines [2].
The application to the pipes of combined mastic-polyethylene and tape-polyethylene coatings can be carried out in conditions of stationary pipe-insulating bases. Protective coatings are applied using a simplified technology (brush cleaning, priming the surface of pipes, applying a mastic or strip underlayer, applying an outer extruded polyethylene layer).
Preliminary technological heating of pipes and abrasive cleaning is not required, which significantly reduces the cost of surface preparation and pipe insulation.
For the application of the mastic sub-layer, special modified bituminous mastics should be used, which have an increased frost resistance and good adhesion to polyethylene. For the application of the strip underlay, duplicated polyethylene tapes with a butyl rubber sublayer having a thickness of at least 0,45 mm should be used. The outer shell of extruded polyethylene with thickness up to 2,0-2,5 mm is designed to increase the mechanical strength of the coating, increase its resistance to punching and impact, which ensures long-term storage, storage and transportation of insulated pipes [3].
Factory double-layered polyethylene coatings are characterized by higher properties and a wider temperature range of operation (from minus 20 °C to plus 50-60 °C). The use as a melt adhesive of a hot melt polymer composition based on an ethylene-vinyl acetate copolymer or ethylene with an acrylic acid ester substantially increases the adhesion of the coating to the steel.
The technological process of applying a two-layer polyethylene coating includes preliminary heating of the pipes, their abrasive (shot-blasting or blasting) cleaning, heating to a specified temperature (180-200 °C), application of the adhesive «plane-slit» or annular extrusion of melts of adhesive and polyethylene, and cooling of isolated pipes with recycled water. To improve the quality of the two-layer coating at operating temperatures of 40-60°C, it is recommended to passivate the cleaned surface of pipes with a special chromate composition [4].
The design of the three-layer coating differs from the two-layer coating by the presence of an additional layer - an epoxy primer (figure 2). To apply the priming layer, it is possible to use both epoxy powder paints (the optimal thickness of the epoxy layer should be 100-200 microns and at least 40-50 microns higher than the roughness of the cleaned pipe surface) and liquid epoxy paints (dry film thickness primer should be 40-60 microns). Epoxy primer provides increased adhesion of the coating to steel, resistance to cathodic peeling and to prolonged exposure to water [5].
In addition, the epoxy layer is permeable to cathodic protection currents, which creates a good compatibility of the three-layer polyethylene coating with the electrochemical protection of the pipelines. The polymeric adhesive undercoat is an intermediate layer in the construction of a three-layer coating of pipes. Its function is to provide adhesion between the outer polyethylene layer and the inner epoxy layer. The outer polyethylene layer is characterized by low moisture-permeability, performs the functions of the «diffusion barrier» and provides a high mechanical and impact strength coating. The combination of all three layers of the coating makes a three-layered polyethylene coating one of the most effective outer protective coatings of pipelines.
Polyethylene coating has a high adhesion to steel (not less than 35 H/cm), high dielectric characteristics (more than 5 kv) and resistance to external mechanical damage [6].
Q
Figure 2 - Pipe with three-layered polyethylene coating
The coating is applied by the method of lateral («plane-slit») extrusion. To ensure high adhesion properties of insulation, high-quality blast cleaning of the pipe surface is applied, application of an intermediate adhesive layer (adhesion-active composition 300-400 mkm), further application of an outer protective layer based on a thermoset stabilized polyethylene composition. Pipes coated with extruded polyethylene have a number of significant advantages:
- the coating is environmentally safe;
- improves the service life of gas pipelines and the culture of construction;
- has increased mechanical strength;
- the quality of the coating does not depend on the ambient temperature, etc.
Advantages of three-layer polyethylene coatings in comparison with double-layer
coatings of pipes are related not only to the presence in the coating structure of an additional layer - epoxy primer, but also with the use of higher quality insulating materials for the application of an adhesive undercoat and an outer polyethylene layer. Undoubted advantages of three-layer polyethylene coatings of pipes include their increased heat resistance. The use of modern adhesion compositions and epoxy primer allowed to expand the temperature range of application of polyethylene coatings from plus 50-60 °C to plus 80 °C [7].
Polypropylene coating has increased heat resistance, high mechanical, impact strength, resistance to extrusion and abrasive wear (figure 3).
Figure 3 - Pipes with factory polypropylene coating
The main field of application of polypropylene coatings is anticorrosive protection of «hot» sections of pipelines (up to 110-140 °C), corrosion protection of offshore, offshore pipelines, underwater crossings, pipeline sections constructed by «closed» gasket
methods (punctures under roads, directional drilling, etc.). Due to the high impact strength of the polypropylene coating, its thickness can be 20 to 25 % less than the thickness of the polyethylene pipe coating (from 1,8 mm to 2,5 mm) [8]. The disadvantages of polypropylene coatings include their reduced frost resistance.
For anticorrosive protection of pipelines of small and medium-sized diameters (up to 530 mm) in recent years a combined ribbon-polyethylene coating has been widely and successfully used. The combined tape-polyethylene coating is applied to the pipes in factory or basic conditions. Structurally, the coating consists of a layer of adhesive primer, a layer of a duplicated polyethylene tape and an outer layer based on extruded polyethylene (figure 4). The total thickness of the combined tape-polyethylene coating is 2,2-3,0 mm. In the case of insulation of pipes in factory (base) conditions, additional layers of polymer tape and wrappers are applied to increase the impact strength of the coating required for transporting insulated pipes from the plant to the pipeline construction site. In this case, depending on the diameters of the insulated pipes, the total thickness of the tape coating in accordance with the requirements should be not less than 1,8-2,4 mm. The increased consumption of insulating materials significantly increases the cost of coating [2].
In the construction of the combined coating, a polyethylene insulating tape applied along the adhesive primer provides a stable adhesion of the coating to the steel, the resistance of the coating to cathodic peeling, while the outer polyethylene layer is responsible for the mechanical characteristics of the coating, providing coverage with increased impact strength, bursting strength and light aging. The combined ribbon-polyethylene coating can be used as an anticorrosion coating for pipes used in the construction of main, field pipelines and bends from them, for laying low-pressure inter-settlement gas pipelines, city gas, water supply networks, pipelines for municipal purposes [9].
Figure 4 - Polyvinylchloride sticky tape (PVCh Sticky)
The temperature range of operation of pipelines with combined coating is from minus 20 °C to plus 40 °C, and the predicted service life is 35-40 years.
In terms of properties, the combined ribbon-polyethylene coating is inferior to the factory two-layer and three-layer polyethylene coatings of pipes, but at the same time it largely surpasses the bitumen-mastic and polymeric tape coatings of pipelines [10].
Factory epoxy coatings of pipes with a thickness of 350-500 microns are used as external anticorrosive coatings of pipelines for about 50 years (figure 5). These coatings are characterized by increased heat resistance, high adhesion to steel, excellent resistance to cathodic peeling, resistance to cutting, scraping, abrasive wear. Pipes with epoxy coating, in contrast to pipes with factory polyethylene coating, for a long time can be stored in the open air. Epoxy coatings are permeable to cathodic protection currents. Under epoxy coatings there were no cases of stress corrosion of pipelines. The cost of applying epoxy coatings is much lower than the cost of factory polyethylene and polypropylene coatings of pipes [11].
Figure 5 - Pipes with epoxy coating
Glass-enamel is a fused, vitreous, inorganic mass consisting predominantly of oxides and applied to the metal in one or more layers (figure 6).
Figure 6 - Pipes with glass-enamel coating
Enamel coatings are quite widely used. The enamel coating has great continuity, good adhesion to metal and high electrical resistance, but it is quite expensive, so it is recommended to use it only in particularly critical cases, for example when pumping aggressive media or laying pipelines in such environments. Glass-enamel coatings of factory application are used to protect pipelines from underground and atmospheric corrosion [12].
The internal coating of pipes must have high protective properties, ensuring its safety during the period of transportation, storage and installation, and also have a high durability during operation.
With internal insulation of pipes in stationary factory or base conditions, it is possible to use modern technologies and equipment for cleaning, heating and insulating pipes, performing sequential process control and quality control of finished products, which ensures high quality coating of pipes with various anticorrosion coatings [13].
The process of internal insulation of pipes is a complex of consecutive completed operations, including: preheating, drying pipes (if necessary, thermal degreasing); cleaning of the internal surface with the creation of the necessary relief; Technological heating of pipes to the set temperature (if necessary); application of a protective coating (required by the technology of the number of layers) and their curing; quality control of the protective coating; repair of places of damage to the coating [7].
Internal polymer coatings of pipelines can be divided into anticorrosive and smooth.
The use of internal pipe coatings offers a number of advantages: extending the service life of pipelines; increase in the capacity of pipelines; reduction of paraffin formation on the walls of pipelines and facilitation of the cleaning process (cleaning costs are reduced by approximately 75 %); increasing the reliability of pipelines and reducing annual operating costs [14].
It is believed that an increase in the life of the pipeline by 1 % pays for the costs of coating the pipes.
To create a durable internal insulation of pipes, it is very important that the insulating material is properly selected and that the technological process for coating the pipes is observed.
The existing technological processes of internal insulation of pipes provide for the use as powder materials of powder polymers and paintwork materials, both liquid and with a solvents content of more than 30 % and high viscosity, with a solvents content below 30 % (high-solids LMC with solids) [3].
The criteria for selecting coatings for internal insulation of pipes are the operating conditions of the pipeline, the protective and technological properties of the coatings.
In all respects the most suitable for the internal insulation of pipes are paint materials based on epoxy, modified epoxy and phenol-formaldehyde resins. From powder polymers, coatings based on epoxy powder materials applied on a phenolic primer are widely used. The thickness of anticorrosive coatings, as a rule, is 300-500 mkm [15].
In order to improve the quality of insulation works to protect the welded joint zone, heat shrinkable tapes (cuffs) are used, which are a two-layer material consisting of a polyethylene radiation-modified base film that is combined with a hot-melt adhesive that has a high adhesion to the steel pipe and to the factory coating. The primer is a solvent-free composition that consists of an epoxy resin and a hardener (activator). The primer is supplied to the consumer in complete with cuffs. Polymer coating «TERMA-ST» is a heat shrinkable tape, which is designed for corrosion protection of welded joints of pipes and made of radiation-modified (cross-linked) polyethylene, which significantly improves the physical properties of the final anticorrosion coatings, and also simplifies the process of their application; The resulting insulation is not inferior in its properties to the basic factory polyethylene insulation; work on the application of anticorrosive materials «TERM» can be made both in factory and in road conditions
(both in summer and in winter at ambient temperatures from -35 °С to +45 °C); ease of application; materials provide protection against corrosion for a period of at least 30 years, since all materials of the brand «TERMA» are light stabilized and radiation-cross-linked, they can also be used for isolation of above-ground pipelines without fear of exposure to ultraviolet rays, solar radiation, temperature changes and without losing their properties [12].
CONCLUSIONS
Based on the analysis of «passive» methods of corrosion protection of main oil and gas pipelines, a number of requirements for protective coatings have been identified, such as: low moisture-permeability, high mechanical characteristics, high and stable in time coating adhesion to steel, cathodic peel resistance, good dielectric characteristics, resistance of coating to UV and heat aging. Insulating coatings should perform their functions in a wide range of temperatures of construction and operation of pipelines, ensuring their protection from corrosion for the maximum possible period of their operation.
According to the conducted researches of various anticorrosive coatings of factory and route application, on the basis of which it was found out that the most optimal for today is the use of pipes with a factory three-layer polyethylene coating, followed by route isolation of welded joint areas with thermo-shrink sleeves (of the type TERM-ST). Re-insulations of the trunk pipeline section with a modern insulating coating (heat-shrinkable tape) is expedient and economically viable.
It can be concluded that the insulation coatings used earlier in the isolation of the main oil and gas pipelines are no longer able to compete with modern insulation coatings that are more effective and durable.
In connection with the continuous improvement of insulation materials and their application technologies, the efficiency of passive corrosion protection of main oil and gas pipelines is constantly increasing, the longevity of the coating is increased, and, accordingly, the life of pipelines isolated by these materials, which leads to a significant reduction in costs on their operation.
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Material received on 15.05.18.
В. В. Рындин1, Г. Г. Абдуллина2, А. Т. Абдуллин3
Магистральдьщ мунай-газ ^убырларын пассивт коррозиядан коргау эдктерш талдау
1,2,3С. ТораЙFыров атындаFы Павлодар мемлекетлк университет^ Павлодар к., 140008, Казахстан Республикасы.
Материал баспаFа 15.05.18 тYстi.
В. В. Рындин1, Г. Г. Абдуллина2, А. Т. Абдуллин3
Aнализ пассивных методов защиты от коррозии магистральных нефтегазопроводов
1,2,3Павлодарский государственный университет имени С. Торайгырова,
г. Павлодар, 140008, Республика Казахстан. Материал поступил в редакцию 15.05.18.
Бул формада коррозияга царсы цубырларды крргаудъщ «пассивтi» эдс царастырылады. Кубырлардыц жогары берiктiгi мен проблемаларын цамтамасыз ету олардыц коррозиядан црргаудыц сапасына тжелей байланысты. Коррозия зацымдану цаупш твмендету ушт цубырлар коррозияга царсы жабынмен жэне цосымша электрохимиялыц цорганумен (ЭХК) цоргалган. Бул жагдайда оцшаулагыш жабындар коррозияга царсы агрессивтi агенттерге (су, оттег1) кедергi келтiретiн «диффузиялыц тосцауыл» функциясын жузеге асыратын цубырларды бастапцы («пассивтi») цоргауды цамтамасыз етедi. Каптамада ацаулар пайда болганда, цубырларга арналган катодты цорганыс жуйе^ — «белсендЬ коррозиялыц цорганыс царастырылган. Кубырлардыц пассивтi цорганышыныц негiзiн цурайтын коррозияга царсы жабындарды цолданудыц барлыц уацыттары ушт зауыттыц жэне маршруттыц жагдайлардагы цубырлар бетше квптеген изоляциялыцматериалдармен оларды цолдану эдiстерi ойлап табылды. Осыган байланысты оцшаулагыш жабындылардыц сапасына, мысалы, су вттзбеу, адгезия, бержтшк, унемдеу, узджЫздж, механикалыц бержтж жэне оцшаулагыш жабу тшмдшгт сипаттайтын басца кврсеткштер сияцты мацызды рвл атцарылды.
В данной стаье рассматриваются «пассивные» методы защиты трубопроводов от коррозии. Высокая долговечность и безаварийность работы трубопроводов напрямую зависит от качества их противокоррозионной защиты. Для сведения к минимуму риска коррозионных повреждений трубопроводы защищают антикоррозионными покрытиями и дополнительно средствами электрохимзащиты (ЭХЗ). При этом изоляционные покрытия обеспечивают первичную («пассивную») защиту трубопроводов от коррозии, выполняя функцию «диффузионного барьера», через который затрудняется доступ к металлу коррозионноактивных агентов (воды, кислорода воздуха). При появлении в покрытии дефектов предусматривается система катодной защиты трубопроводов — «активная» защита от коррозии. За все время применения антикоррозионных покрытий, составляющих ядро «пассивной» защиты трубопроводов, изобретено большое количество изоляционных материалов и методов их нанесения на поверхность трубопроводов в заводских, а также трассовых условиях. В связи с этим немаловажную роль стали играть такие показатели качества изоляционных покрытий, как водонепроницаемость, адгезия, долговечность, экономичность, сплошность, механическая прочность и др. показатели, характеризующие эффективность того или иного изоляционного покрытия.
