Pipe inner coating technology

Cui Tao, Song Xuqiang: Pipeline inner coating technology, oil and gas storage and transportation, 2000, 19 (12) caliber derusting and process technology prefabricated in the spraying plant, pointed out the gap with foreign coating technology The aim is to accelerate the research process of coating technology in domestic long-distance natural gas pipelines, improve the manufacturing level of domestic coating construction equipment, and promote the promotion and application of natural gas pipeline inner coating drag reduction technology in China.

Pipe inner coating application construction technology At present, natural gas pipeline inner coating drag reduction technology has been widely used in the developed countries of the European and American pipeline industry. For example, large diameter gas pipelines built in Canada and Germany are sprayed with epoxy on the inner wall of the pipeline. Base coating to reduce friction during gas delivery. In order to make the inner coating drag reduction technology reflect its excellent characteristics in the actual operation of the pipeline, it is necessary to pay attention to the inner coating of the pipeline, because the inner coating of the pipeline is a key technology in the entire inner coating construction, the inner coating The quality will directly affect the technical performance and service life of the inner coating. Only by adopting advanced and reliable technology can we obtain high-quality pipeline inner coating, effectively extend the service life of the pipeline and greatly improve the operating efficiency of the pipeline after it is put into production.

The research on the inner coating technology of pipelines started late in China, especially in the long-distance, large-diameter natural gas pipeline engineering. There is no need to strengthen the research in this area. With the construction of long-distance natural gas pipelines such as Suininglan, Zhongwu, and West-East Gas Transmission, the internal coating drag reduction technology will be greatly promoted. Therefore, it is necessary to carefully study and master the coating technology of the inner coating and strictly control the coating. The quality of the coating process promotes the application of the inner coating technology in China.

I. Development level of coating technology at home and abroad In-pipe coating technology and equipment are continuously developed along with the development and application of various internal anticorrosive coatings. In 1955, the Tennessee Natural Gas Pipeline was put into practical use for the first time. After decades of development, various new technologies and new technologies with practical value have emerged. Many well-known foreign pipe manufacturers and pipeline construction engineering companies have accumulated a wealth of practical experience, such as Mannesmann in Germany, NKK in Japan, Selmers in the Netherlands, NOVA in Canada, CRC in the US, and BrederoPrice in Singapore. They have a complete set of surface pretreatment and spraying lines for different pipe diameters and pipe lengths, and have many years of construction experience and good performance. In general, the technical level of the inner coating line of foreign companies is higher than that of the domestic ones. The automation of the equipment is high, the failure rate is low, the trouble-free operation time is long, the applicable pipe diameter range is wide, and the overall coating quality is reliable. A flow chart for coating the inner coating of Japanese steel pipe (NKK).

Although China started late in the coating of pipeline coatings, in recent years, various oilfields have set up technical teams for pipeline internal coatings, which have deepened the coatings, internal surface pretreatment and spraying equipment used for the inner coating. People's research has achieved many gratifying results. For example, the Daqing Oil Construction Technology Research Institute successfully developed a medium- and small-diameter pipe inner coating film-forming technology, which can improve the work efficiency by 34 times; China National Petroleum Corporation Construction Technology Research Institute The successful electrostatic spraying technology of the inner surface of the steel pipe ensures the quality and safe production of the spraying in the pipeline; the steel pipe inner coating machine and the coating detection and recoating machine successfully developed by Zhongyuan Oilfield and Shengli Oilfield can realize the mouth filling The integration of testing and recoating has filled the gap in the domestic pipe coating technology. Recently, Shengli Oil Construction Pipe Prefabrication Plant introduced a liquid epoxy inner coating coating line from the US CRC to complete the inner coating operation of 60 920mm pipe diameter. These research results have greatly shortened the gap with developed countries and promoted the promotion and application of oil and gas pipeline coating technology in China.

Pipe cleaner, from extrusion m. For the characteristics of projects such as Suininglan and West-East Gas Transmission, the factory pre-coating process for new pipelines will be highlighted.

1. Process flow of factory pre-coating method See the typical process flow of inner wall coating of the factory.

Figure Japanese steel pipe (NKK) inner wall coating flow chart II. Pipe inner coating process The inner coating process of the pipe can be generally divided into two types, namely, the factory coating method and the on-site coating method. The former is used for the construction of new pipelines, and the latter is used for the repair of existing pipelines. When repairing the old pipeline, first use the scraper pig and the wire brush pig to carry out the pigging operation, then carry out the cleaning with the rubber isolation ball carrying the chemical agent several times to make the inner surface quality of the pipe meet the coating requirements, and then use the double The pig squeezes the coating for coating. When coating, the liquid coating is sandwiched between two pigs, the front one is called the guiding pig, and the latter is called the coating process in the coating plant. The main process steps of the inner coating (1) Pretreatment of steel pipe inner surface The inner surface pretreatment of steel pipe is the most important part of the inner wall coating technology. The surface pretreatment quality is a very important condition to ensure the quality of the coating. The inner surface of the steel pipe will directly affect the coating. Adhesion to the surface of the tube. Commonly used steel pipe inner wall pretreatment has two methods of chemical cleaning and mechanical descaling.

Chemical cleaning is an early method of using a large washing tank to sequentially soak the tubes in hot acid (usually sulfuric acid), hot water, phosphoric acid, and then rinse with hot water. This method is relatively inefficient and costly. If a small amount of acid remains in the treated tube wall, the curing agent in the coating will be invalidated, and it is difficult to ensure that the inner surface of the steel tube forms a certain anchor depth (generally coated with internal coating). The anchoring depth of the inner surface of the steel pipe of the layer is preferably 3060fx mM). Therefore, the chemical cleaning method is not suitable for large-scale steel pipe rusting construction work.

Mechanical derusting is a widely used method for pretreating the inner surface of steel pipes. It includes mechanical wire brush derusting and spraying, and shot blasting (sand) derusting. In the past, steel pipes for domestic oil and gas transportation used wire brush to remove rust, but the effect was far less than that of sandblasting (pill). It was difficult to reach the Sa2/2 grade of rust removal specified in GB8923, which affected the coating and the degree to different extents. The bonding quality of the pipe body. In addition, the power consumption of the mechanical wire brush for rust removal is higher than that of the sandblasting (pill) method, which does not meet the technical development of the new coating and cannot meet the requirements of the construction schedule.

Spraying and shot blasting (sand) descaling is a pretreatment technology for steel pipe surface commonly used at home and abroad. There are two main methods for derusting the inner surface of steel pipes. One is to use a compressed high-pressure air to accelerate the abrasive of a certain shape onto the pipe wall; the other is to use the high-speed rotation of the centrifugal impeller to hit the abrasive to the surface of the steel pipe. See the sum of the two methods. The steel pipe can be rotated or fixed as needed, and the nozzle moves in the direction of the tube axis (either horizontally or vertically).

According to foreign construction experience, for small-diameter steel pipes, sandblasting and rust removal are usually adopted, and for large-diameter steel pipes, internal shot blasting is generally used.

The internal shot blasting method has high utilization rate of abrasive, fast rust removal speed and low cost.

Pneumatic internal sandblasting (pill) device 1 - steel pipe; 2 - nozzle (horizontal movement or rotation); 3 - sandblasting (pill) and airflow mixture centrifugal impeller blasting device 1 - centrifugal impeller throwing head; 2 - steel pipe; Roller of rotating steel pipe (2) Inspection of inner surface pretreatment effect of steel pipe As early as the 1950s, the United States first formulated the Steel Surface Treatment Specification (SSPC) and proposed five kinds of sandblasting treatment standards. Since then, the United Kingdom, Japan, Sweden and other countries have successively formulated their own steel surface treatment specifications. For the steel pipe for inner coating construction, the internal surface pretreatment effect is not the same, but the rust quality level is required to reach Sa2y2.

China also stipulates some corresponding standards and specifications for the surface pretreatment of steel before painting. In order to ensure the quality and technical performance of the coating. The method for pretreatment of the inner surface of oil and gas pipelines shall comply with the provisions of SY/T0407*97, and the derusting grade shall reach the Sa2/2 grade specified in GB8923-88. The inspection shall be based on the photographs in GB8923 or the standard test specimens. After the inner wall of the steel pipe is sandblasted (pill), the anchorage depth should be around 3060jum, and it is selected by design.

The inner wall of the pipe should be sprayed immediately after sandblasting, rust removal and drying.

At present, the air spraying device has been basically eliminated in foreign countries, and replaced by high-pressure airless spraying, electrostatic powder spraying, high-temperature ion spraying and other new processes, of which high-pressure airless spraying accounts for more than 80%.

The US Standard API*5L2 "Recommended Practice for Non-Corrosive Gas Pipeline Coatings" and the British Gas Engineering Enterprise Standard GBE/CM1 stipulate that (intermediate) air-injection equipment should be used for internal coating operations. For the spraying of liquid epoxy coating on the inner wall of large-diameter gas pipeline, the method of high-pressure airless pressure-feeding coating is mainly adopted. The principle of this spraying method is that after the compressed air is injected into the jet machine, the pressure of the coating chamber is raised to a certain level (78 MPa) by the principle of the piston, and the coating is re-polymerized by molecular cracking under the action of high pressure to produce an interpenetrating network cross-linking reaction. After the paint is injected into the high-pressure spray gun, it is pushed by the screw feeder to the spray head to form the ball facing the pipe wall. The working principle is as follows.

High-pressure airless spray schematic - compressor f2 - piston; 3 - paint chamber; 4 spray gun; 5 high pressure airless spray compared with air spray, the paint film after spraying is of good quality, uniform coating, without pinhole vent It can reduce the pollution of the atmosphere by the droplets of paint and solvent. High-pressure airless spray devices are available in both traction and cantilever versions. The traction type is to install the nozzle on a traction trolley with rollers. The trolley moves axially along the inner wall of the tube, and the nozzle rotates at a certain speed for spraying, and the tube is fixed, and the device is seen. The cantilever type is to mount a plurality of nozzles on the cantilever spray gun beam, and the beam drives the spray gun to move along the axial direction of the pipe, and the pipe rotates, and the device sees it.

Traction type internal spraying device For epoxy powder coating, the commonly used method in foreign countries is electrostatic spraying method. The coating utilization rate of this method is 80% 95%. The basic principle is high-voltage electrostatic induction adsorption, that is, the spray head and high-voltage electric negative electrode. Connected, the tube is grounded to the positive pole, creating a strong electrostatic field between the gun head and the tube. When the paint is ejected from the lance, the corona discharge is generated due to the sharp edge of the tip, and the powder particles are negatively charged and adsorbed on the inner surface of the tube by the electrostatic force of the electric field. As the tube rotates and heats, the paint Uniform melting, and then coated cantilever spray device 1* paint storage tank; 2* spray gun slide; 3* spray gun; 4* spray head; 5* inlet tube; 6-rotary fixture Currently, many countries and companies have developed pipelines Standards for inner coatings, such as API*RP5L2, Recommended by the American Petroleum Institute (API), Recommended Guidelines for Inner Coatings for Gas Pipelines, GasCauncil Standard GIS/CMI-1968 French Standards R03 and 20S50, Netherlands Standard CS*1*N, and Canada's Alberta Dry Pipe Company Standard C-1. The domestic standards mainly include the petroleum industry standard SY/T0442-97 "Technical Standards for Steel Pipe Fusion Epoxy Powder Coating" and SY/T4057-93 "Technical Conditions for Anticorrosive Coating Steel Pipe in Liquid Epoxy Coatings".

For pipelines coated with liquid two-component epoxy coatings, the quality inspection of the inner coating can refer to the relevant provisions of SY/T4057-93 "Technical Conditions for Anticorrosive Coating Steel Pipes in Liquid Epoxy Coatings". The standard states in its preparation instructions that the general-grade anti-corrosion coating regulations apply to the reduction of the resistance and the corrosiveness of the conveying medium. The inner coating of the pipeline is coated with imported paint and can be inspected according to the technical data provided by the manufacturer. Generally, BS (British Standard) and ASTM Standard (American Material Standard) are mainly used. The owner can refer to the foreign internal coating construction standards (such as API*5L2, GBE/CM1 and GBE/CM2) to agree with the coating manufacturer on the quality testing method and evaluation criteria of the epoxy coating. Table 1 shows the quality indicators of the inner coating of the drag reducing natural gas pipeline.

It should be noted that the general steel pipe inner coating is divided into two types: drag reducing type and anti-corrosion type according to the purpose of use. Since the natural gas purified by dehydration, desulfurization and the like has very little corrosion to the pipeline during the transportation process, the effect of the inner coating is mainly to improve the inner surface state of the steel pipeline, reduce the consumption of the transmission power, and reduce the transportation cost. Resistive inner coating. According to foreign data, the original roughness of steel pipelines is usually below 40 (20 jum in the United States and 30/xm in the former Soviet Union). For the purpose of drag reduction, the thickness of the inner coating should be greater than the original roughness of the steel tube. US API-RP5L2 "Recommended Method for Non-Corrosive Pipe Inner Coatings" stipulates that the dry film thickness of the inner coating of steel pipe is 50.8±5.08/nm, and the thickness of the drag reducing inner coating specified in SY/T0442-97 For 50 jum, this thickness can achieve both drag reduction and a more economical coating thickness margin.

Project quality standard test method Appearance surface smooth, smooth, bubble-free visual inspection (or endoscopy) pinhole number <3 / m2 (refer to the requirements of ordinary grade anti-corrosion layer) coating thickness > 50pm (or according to design requirements) attached Efforts to Approve Appendix 1 Hardness 12H Pencil) Scratch-free steel pipes shall be repaired and overcoated according to the provisions specified in the standard. The steel pipe after overcoating is unqualified by quality inspection and can only be used separately. It should not be coated for the third time.

Third, the joint construction process of the inner and outer coatings of the pipeline. Because the long-distance natural gas pipeline also involves an external anti-corrosion problem in the actual construction, in the pre-fabrication process of the pipeline, the coating process of the inner and outer coatings should be fully considered. The problem is to make the construction progress of the inner and outer coatings of the pipe as synchronous as possible, which can save investment and speed up the construction progress. It is a flow chart of the joint construction process of “inside and outside” of the inner and outer coatings of the pipeline. The "first outside and inside" process is the same as the "inside and outside" process, but in reverse order.

For the external anti-corrosion, the structure of the melt-bonded epoxy powder (FBE) or the three-layer PE and the inner coating layer using the liquid epoxy coating (resistance reduction type) adopts the process of “introspectively inside and outside” or “inside and outside”. The process is technically feasible. Investigating several large foreign coating equipment manufacturers found that there are more “first in and outside” processes. For example, British BG has at least several thousand kilometers of natural gas pipelines coated with “inside and outside” processes. It has been running well so far. The so-called "first inside and outside" process, that is, after the inner coating is finished, a protective cover is added to both ends of the pipe, and then the outer anti-corrosion layer is applied. It should be specially stated that when foreign companies heat the steel pipe body at medium frequency, how to control the power frequency of the intermediate frequency heating is the key technology, because the heating is only a certain depth on the outer surface of the pipe, and there is no deep and inner surface coating. It is over, so it will not damage the inner coating and fully guarantee the overall quality of the inner and outer coatings.

Pipeline internal and external coating factory prefabricated joint construction process flow chart At present, the domestic units with medium and large diameter pipe inner coating application lines mainly include Shengli Oilfield Oil Construction Company and Daqing Oilfield Oil Construction Company. The construction of the inner coating line introduced by Shengli Oilfield Oil Construction Company from the US CRC company is relatively fast, calculated at 8h per shift, and 11 tubes with a length of 12m, which can apply an average of 081 5km per shift. Singapore's BrederoPrice Company The construction progress of the line is as follows: for the 0711 pipe, the inner coating can be applied for 2.4km per shift, and the outer coating can be coated with 3.65km per shift, which are all finished pipes.

Since the inner coating cures for a long time, the other is similar to the outer coating progress. Therefore, in the flow operation, a certain amount of the inner coated tube is usually applied first, and then the inner and outer coatings are jointly constructed.

IV. Problems and Suggestions The Research Group of Natural Gas Pipeline Coating Reducing Technology of China Petroleum Pipeline Pipeline Technology Research Center conducted extensive research on well-known domestic coating construction companies (units) at home and abroad, and inspected the victory on the spot. The oilfield oil construction company recently introduced the inner coating line from the US CRC company. Through investigation, it found the following problems.

At present, in addition to a few oilfields in Daqing and Shengli, the oil construction company can undertake the inner coating construction of medium and small diameter pipelines, and no unit has carried out the inner coating construction of long distance and large diameter natural gas pipelines, especially It is a joint construction operation combined with external anti-corrosion.

For the design and manufacture of shot blasting (or sandblasting) descaling lines and spraying lines on the inner wall of pipes, and the layout structure in the factory, it is a problem that needs to be studied and solved. Some key equipment, such as sand blasting mechanism, nozzle (including sandblasting and spraying), hydraulic drive motor for spray gun and pipe, air compressor, sand tank, paint mixing tank, spray gun rotation and automatic control mechanism for painting amount, etc. All need to cooperate with foreign units to develop or import.

According to the implementation schedule of the West-East Gas Pipeline Project and the gap between the domestic and foreign coating construction equipment design and manufacturing technology level, relying on the current domestic technical strength and equipment, it is necessary to reach the foreign technical level in a short period of time. Great difficulty and risk.

In order to ensure the quality and construction progress of the West-East Gas Transmission Project and save investment, it is recommended that Baoji, Qingxian, Liaoyang, Ziyang and other pipe factories first introduce several key lines of equipment from abroad, and manufacture several molding lines. Then digest and absorb, and develop and produce several lines of work. It is also possible to adopt a method of cooperation with the foreign party. This can not only train the team, but also accumulate on-site construction experience, and can purchase foreign equipment with less capital after the project ends. The start of the West-East Gas Transmission Project is about to begin, and the manufacturing technology and construction process of the inner coating construction line should be mastered as soon as possible to meet the needs of the project.

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