The construction methods of the inner and outer coating of ductile iron pipes are different, and the following is a specific introduction:

Inner coating application

Cement mortar inner coating construction

Preparation: First of all, the inner wall of the ductile iron pipe should be cleaned to remove oil, rust, dust and other impurities. Mechanical sandblasting or chemical cleaning can be used to achieve the specified cleanliness and roughness of the inner wall. Then, according to the specifications and construction requirements of the pipeline, prepare the cement mortar spraying equipment, such as spray gun, air compressor, mortar mixer, etc., and debug the equipment to normal operation. At the same time, prepare cement, sand, additives and other materials according to the design mix ratio to ensure that the material quality meets the requirements.

Spraying construction: The mixed cement mortar is evenly sprayed on the inner wall of the pipeline through the spray gun. When spraying, it is necessary to control the pressure, Angle and moving speed of the spray gun to ensure uniform coating thickness. Generally speaking, the pressure of the spray gun is controlled between 0.3-0.5MPa, and the moving speed is about 1-2 meters per minute. For pipelines with large pipe diameters, automatic spraying equipment can be used to improve construction efficiency and coating quality.

Curing treatment: After spraying, the inner coating of cement mortar should be cured in time. The curing method is usually to maintain a certain humidity in the pipeline, which can be used to inject water or cover wet sacks in the pipeline, and the curing time is generally 7-14 days to ensure that the cement mortar is fully hydrated to meet the design strength and performance requirements.

Epoxy resin inner coating construction

Surface treatment: The inner wall of the pipeline is also strictly cleaned and derusted first, requiring high cleanliness and roughness to ensure good adhesion between the epoxy resin coating and the pipe wall. Generally, sandblasting is used to make the surface of the pipe wall show a uniform pockmarked surface, and the roughness reaches Ra 50-100μm or so.

Coating construction: There are many coating methods of epoxy resin coating, common brush coating, roll coating and spray. Spraying method is commonly used for coating construction in ductile iron tubes. Pour the epoxy paint into the spray equipment and spray the paint evenly on the inner wall of the pipe through the spray gun. When spraying, pay attention to the ambient temperature and humidity, the general temperature is controlled between 5-35 ° C, and the humidity does not exceed 80%. At the same time, according to the characteristics of the coating and coating thickness requirements, control the pressure and spraying times of the spray gun, usually need to spray 2-3 times, each spraying interval according to the drying time of the coating, generally 2-4 hours.

Curing treatment: After coating is complete, the epoxy coating is cured under specified temperature and humidity conditions. Curing time varies according to the type of paint and environmental conditions, generally 24-72 hours. During the curing process, the pipeline should be avoided from external pollution and damage to ensure that the performance of the coating reaches the best state.

Coating application

Asphalt paint external coating construction

Surface pretreatment: First of the outer wall of the ductile iron pipe for rust and oil removal treatment, can use shot blasting or chemical rust removal method, so that the outer wall surface cleanliness of Sa2.5 or more, that is, completely remove the metal surface of oil, rust, oxide and other impurities, so that the surface shows a uniform metal luster.

Painting construction: the asphalt paint is heated to the specified temperature, generally 150-180 ° C, so that it has good fluidity and coating performance. Then, the asphalt paint is evenly coated on the outer wall of the pipeline by spraying or dipping. When spraying, it is necessary to control the pressure and distance of the spray gun to ensure uniform coating thickness, and the general coating thickness is 0.2-0.5mm. When dipping, make sure that the pipe is soaked in the asphalt paint for enough time to ensure the integrity and thickness of the coating.

Cooling and drying: After painting is completed, let the pipe cool naturally or use air cooling to cool it quickly, so that the asphalt paint solidifies quickly. In the drying process, to prevent the coating by rain, dust and other pollution, the general drying time is 24-48 hours.

Polyethylene outer coating construction

Surface treatment: Rust removal, oil removal and coarsening treatment are carried out on the outer wall of the ductile iron pipe to improve the adhesion between the polyethylene coating and the pipe wall. Sandblasting is usually used to make the surface roughness of the tube wall reach Ra 30-50 μm.

Coating process: The common construction methods of polyethylene outer coating are extrusion winding method and spraying method. The extrusion winding method is to heat polyethylene particles to a molten state, extrude them into bands by an extruder, and then wind them on the outer wall of the pipe to form a coating. The spraying method is to spray polyethylene powder on the outer wall of the preheated pipe through a special spray gun, and the powder is melted at high temperature and attached to the pipe wall to form a coating. No matter which method is used, it is necessary to control the construction temperature, speed and coating thickness. The general construction temperature is between 200-250 ° C, and the coating thickness is determined according to the engineering requirements, usually 1-3mm.

Quality testing: After the construction is completed, the quality of the polyethylene outer coating should be tested, mainly to check the thickness, adhesion, pinhole and other indicators of the coating. The coating thickness was measured by magnetic thickness gauge, the adhesion was measured by grid method, and the pinhole was detected by high voltage EDM tester. If the problem is found, timely repair or rework treatment to ensure the quality and protective performance of the outer coating.

The durability of ductile iron pipe network can be improved from the aspects of improving the performance of the pipe itself, strengthening external protection, controlling construction quality and optimizing operation and maintenance. The specific methods are as follows:

Improve pipe performance

Optimized material: In the production of ductile iron pipe, the content of carbon, silicon, manganese and other basic elements is precisely controlled, so that the cast iron structure is more dense and uniform. At the same time, the appropriate amount of nickel, chromium, molybdenum and other alloying elements can enhance the strength, toughness and corrosion resistance of the pipe, such as nickel can improve the fatigue resistance of the pipe, molybdenum can improve the pitting resistance.

Improved manufacturing process: Advanced spheroidization and inoculation process is adopted to ensure that the graphite balls are small, round and evenly distributed, thereby improving the mechanical properties and corrosion resistance of the pipes. In addition, by precisely controlling casting temperature, speed and other parameters, casting defects, such as pores, sand holes, etc., are reduced to avoid these defects becoming corrosion sources or stress concentration points, affecting the durability of the pipe network.

Strengthen protective measures

Internal and external coating treatment

Inner coating: non-toxic, corrosion-resistant inner coating materials, such as epoxy resin, cement mortar, etc. The epoxy resin coating has good adhesion and chemical corrosion resistance, which can effectively isolate the contact between water and the inner wall of the pipe and prevent the corrosion of the water quality on the pipe. Cement mortar coating can not only prevent corrosion, but also smooth the inner wall and reduce the resistance of water flow.

Outer coating: Usually use asphalt paint, polyethylene coating, etc., as the outer coating. Asphalt paint has good water resistance and soil corrosion resistance. The polyethylene coating has high mechanical strength and weather resistance, which can protect the outer wall of the pipe from the erosion of water, oxygen and acid, alkali and salt in the external soil.

Cathodic protection: For the buried ductile iron pipe network, cathodic protection technology can be used. By installing sacrificial anode or impressed current cathodic protection device in the pipe network system, the surface of the pipe becomes the cathode, so as to inhibit the corrosion reaction of the pipe. For example, in areas where the soil is highly corrosive, the installation of a magnesium alloy sacrificial anode can effectively protect the ductile iron pipe from electrochemical corrosion.

Strict construction quality control

Pipeline installation: During the installation process, ensure that the pipeline is laid in accordance with the design requirements to avoid excessive bending, stretching or compression stress in the pipeline. At the same time, strictly control the installation quality of the pipeline interface, ensure that the interface is sealed well, and prevent the corrosion of the pipe caused by the leakage of the interface. For example, when installing the socket interface, ensure that the rubber ring is installed in place and the interface gap is uniform.

Foundation treatment: Do a good job of pipeline foundation treatment, according to the geological conditions to choose the appropriate foundation form, such as sand cushion foundation, concrete foundation, etc. Ensure that the foundation has sufficient bearing capacity and stability to prevent pipeline rupture or interface damage due to uneven settlement of the foundation, affecting the durability of the pipe network.

Optimized operation and maintenance

Water quality management: For the pipeline network of water transmission, the water quality should be strictly controlled to ensure that the water quality meets the relevant standards. Through softening, desalting, disinfection and other treatment processes, the content of calcium and magnesium ions in water is reduced, and corrosive substances such as dissolved oxygen and hydrogen sulfide are removed to reduce the corrosion of pipes.

Regular inspection and maintenance: Establish a regular pipe network inspection system, and use ultrasonic detection, magnetic flux leakage detection and other technical means to detect the wall thickness, corrosion and interface status of the pipeline. Timely detection and treatment of pipeline problems, such as the repair coating treatment of slightly corroded parts, the replacement of seriously damaged pipeline sections, in order to extend the service life of the pipe network.

There are mainly the following ways to improve the pressure resistance of ductile iron pipes:

Optimize material composition

Adding alloying elements: In the production of ductile iron pipes, some alloying elements such as nickel, molybdenum, copper and so on are often added. These alloying elements can refine the grain, improve the strength and toughness of cast iron, and thus enhance the pressure resistance of the pipeline. For example, nickel can improve the toughness and fatigue resistance of cast iron, molybdenum can enhance the strength and hardness of cast iron, in some high-pressure pipeline, add the right amount of nickel and molybdenum, can make ductile iron pipe withstand higher pressure.

Control of carbon and silicon content: carbon and silicon are important elements in ductile iron, appropriate increase in carbon content can increase the degree of graphitization of cast iron, reduce the tendency of white, improve the fluidity of hot metal, is conducive to the casting process. Silicon can promote graphitization and improve the strength and hardness of cast iron. Reasonable control of carbon and silicon content to achieve the best combination of the two can improve the comprehensive performance of ductile iron pipe, and then improve its pressure resistance. In general, the carbon content is 3.5%-3.9%, and the silicon content is 2.0%-2.8%.

Improve manufacturing process

Advanced spheroidizing treatment process: Spheroidizing treatment is the key link in the production of ductile iron. By adding spheroidizing agent to the hot metal, the graphite is transformed from flake to ball, which significantly improves the mechanical properties of cast iron. Using advanced nodularization process, such as punching method and subcontracting method, and strictly controlling the amount of nodulating agent and treatment time, good nodularization effect can be obtained, and the compressive strength of ductile iron pipe can be improved.

Optimization of heat treatment process: Appropriate heat treatment of ductile iron pipe, such as annealing, normalizing, tempering, etc., can improve its organizational structure and improve mechanical properties. For example, annealing can eliminate casting stress, reduce hardness, improve toughness and plasticity; Normalizing treatment can improve the strength and hardness of cast iron. By optimizing the process parameters of heat treatment, the compressive capacity of ductile iron pipe can be further improved.

Increase pipe wall thickness: In the design and manufacture of ductile iron pipe, according to the pressure required by the project, the wall thickness of the pipe is reasonably increased. When the wall thickness increases, the bearing area of the pipeline increases and it can withstand greater pressure. For example, in some high-pressure water transmission projects, the wall thickness of the ductile iron pipe is increased from the standard 8mm to 10mm or thicker, which can effectively improve the pressure resistance of the pipeline. However, increasing the wall thickness will increase the cost and pipe weight, and it is necessary to consider the actual needs of the project and cost factors.

Improved pipe interface design

Use high performance sealing materials: Use high performance sealing materials, such as rubber rings, at the pipe interface to ensure the sealing and stability of the interface. Good sealing can prevent the medium from leaking from the interface under pressure, avoid the local pressure reduction and pipeline damage caused by leakage, so as to improve the pressure resistance of the entire pipeline system.

Optimize the interface structure: design reasonable interface structure, such as socket interface, mechanical interface, etc., and optimize the size, shape and connection mode of the interface. For example, in the socket interface, increasing the matching length and taper of the socket and the socket can improve the connection strength and compressive capacity of the interface; The mechanical interface is connected with high-strength bolts and nuts, and is equipped with an anti-loosening device, which can effectively resist the axial and radial forces generated by the pipeline under pressure.

Various pipe diameter specifications: ductile iron pipe has a variety of pipe diameters to choose from, as small as DN80mm, as large as DN2600mm or even larger. In the urban water supply network, the large-diameter ductile iron pipe can be used for the main pipe to transport water, which can meet the demand of large-scale water supply. In the water supply network within the community or factory, smaller pipe sizes can be used for branch pipes to accurately deliver water to each user terminal.

Different pressure levels: they can withstand pressures ranging from 0.2MPa to 2.5MPa. For some areas with flat terrain and low water supply pressure requirements, ductile iron pipes with lower pressure grades can be selected; In some high-rise urban centers, or in projects requiring long-distance and high-drop water transmission, it is necessary to use high-pressure ductile iron pipes to ensure that the pipeline will not break due to excessive pressure during the water supply process.

Flexible connection methods: common connection methods are socket, mechanical and so on. Socket connection is easy to install, good sealing, suitable for all kinds of terrain pipeline laying, especially in some areas with relatively soft soil, can better adapt to the slight settlement of the pipeline; Mechanical connections are faster, easier to disassemble and repair, and have advantages in some projects that require frequent pipeline repairs or modifications.

Excellent material performance: ductile iron pipe has high strength, high toughness and corrosion resistance. High strength and high toughness make it able to withstand greater external pressure and impact force, not easy to break and deformation, and ensure the safe operation of the pipeline when crossing complex areas such as roads and railways; Corrosion resistance makes it suitable for a variety of different water quality and soil environment, whether it is in salty soil and other corrosive soil, or in industrial water transportation containing certain chemicals, it can maintain good performance and extend the service life of the pipeline.

Strong customization: According to the special needs of different projects, ductile iron pipes can be special coating treatment or add special alloying elements. For example, in some drinking water projects with high water quality requirements, non-toxic inner coating can be used to prevent pipeline pollution to water quality; In some projects with special fire protection requirements, special ingredients such as flame retardants can be added to improve the fire protection performance of pipelines.

Convenient construction: The weight of ductile iron pipe is relatively light, easy to transport and install. At the construction site, there is no need for large lifting equipment and complex installation tools, and the construction personnel can easily install the pipeline in place, thereby shortening the construction period and reducing the construction cost. This is especially important for some projects with a tight schedule.

Low maintenance cost: Due to the stability and corrosion resistance of its material, ductile iron pipe does not need frequent anti-corrosion treatment and maintenance during use, and the maintenance cost is low. This enables the project owner to save a lot of maintenance costs during the whole life cycle of the project, and also reduces the impact of pipeline maintenance on normal production and life.

Ductile iron pipe is widely used in municipal water supply, drainage, gas transmission and other fields because of its excellent mechanical properties, corrosion resistance and economy. However, in the actual use process, the pipe network system may be affected by periodic load, temperature change, soil settlement and other factors, resulting in fatigue damage, which will affect the service life and safety of the pipeline. Therefore, improving the fatigue resistance of ductile iron pipe is the key to ensure the long-term stable operation of pipe network. The following aspects of material selection, design optimization, construction technology, maintenance management and so on to discuss how to improve the fatigue resistance of ductile iron pipe.

First, material selection and optimization

Chemical composition control of ductile iron

The fatigue properties of ductile iron are closely related to its chemical composition. By optimizing the content of carbon, silicon, manganese, phosphorus, sulfur and other elements, the toughness and strength of the material can be improved. For example, appropriately increasing the silicon content can improve the toughness of ductile iron, while reducing the phosphorus and sulfur content can reduce the formation of brittle inclusions, thereby improving the fatigue resistance.

Spheroidizing treatment and graphite morphology control

The fatigue properties of ductile iron mainly depend on the morphology and distribution of graphite. By spheroidizing treatment (such as adding magnesium or rare earth elements), the graphite exists in a spherical or near-spherical form, which can effectively improve the toughness and fatigue resistance of the material. In addition, controlling the size and distribution uniformity of graphite to avoid local stress concentration is also the key to improving fatigue resistance.

Heat treatment process optimization

The microstructure, strength and toughness of ductile iron can be improved by heat treatment processes such as normalizing, annealing or tempering. For example, normalizing can refine grain and increase fatigue limit of material. Annealing treatment can eliminate the internal stress and reduce the initiation of fatigue cracks.

2. Design optimization

Reasonable selection of pipe wall thickness

According to the design pressure, buried depth, soil conditions and other factors of the pipe network system, the wall thickness of ductile iron pipe is reasonably selected. Proper increase of wall thickness can improve the flexural rigidity and fatigue resistance of the pipeline, but the balance between economy and construction convenience should be paid attention to.

Optimized interface design

The joint of ductile iron pipe is a high incidence area of fatigue damage. The use of flexible interfaces (such as T-type interfaces or K-type interfaces) can effectively absorb the stress caused by temperature changes or soil settlement of the pipeline, reducing the generation of fatigue cracks. In addition, the tightness and tensile strength of the interface parts also need to be considered in the design.

Reduce stress concentration

In pipe design, sharp corners, holes or irregular shapes should be avoided as much as possible to reduce stress concentration. For example, the use of arc transition design can reduce local stress and improve the fatigue resistance of the pipeline.

Third, construction technology improvement

Strictly control the installation quality

During the construction process, the flatness and straightness of the pipeline should be ensured to avoid additional stress caused by improper installation. At the same time, the installation of the interface part should be in strict accordance with the specifications to ensure tightness and tensile strength.

Quality control of backfill

The quality of backfill has an important effect on the fatigue resistance of pipeline. Appropriate backfill materials (such as sand or gravel) should be used and rammed in layers to ensure the compactness and uniformity of the soil around the pipeline and reduce the additional stress caused by soil settlement.

Pipe support and fixation

When the pipeline turns, branches or crosses obstacles, reasonable support and fixing devices should be set up to avoid fatigue damage caused by pipeline displacement or vibration.

Iv. Operation and maintenance management

Regular testing and monitoring

Through regular detection (such as ultrasonic detection, magnetic particle detection, etc.) and real-time monitoring (such as stress monitoring, vibration monitoring, etc.), the fatigue crack or damage of the pipeline is found in time, and the corresponding repair measures are taken.

Control operating condition

During the operation of the pipe network, frequent pressure fluctuations, temperature changes or overload operation should be avoided as far as possible to reduce the accumulation of fatigue damage. For example, by installing pressure regulating valves or temperature compensation devices, the operating conditions of pipelines can be effectively controlled.

Timely repair and replacement

For the pipeline that has fatigue damage, it should be repaired or replaced in time. For example, the use of repair welding, coating or local reinforcement methods can extend the service life of the pipeline.

5. Technological innovation and application

New coating technology

Coating the surface of ductile iron pipe with anti-corrosion, wear-resisting or anti-fatigue coating can effectively improve the anti-fatigue performance of the pipe. For example, the use of epoxy resin coating or polyurethane coating can reduce the impact of corrosion and wear on the fatigue performance of the pipeline.

Intelligent pipe network system

Through the introduction of intelligent pipe network system (such as Internet of Things technology, big data analysis, etc.), real-time monitoring and early warning of pipe network operation status can be realized, and fatigue damage problems can be found and dealt with in time.

Research and development of new materials

With the development of material science, the development of new ductile iron materials with higher strength, toughness and fatigue resistance is an important direction to improve the fatigue resistance of pipe network.

Conclusion

Improving the fatigue resistance of ductile iron pipe is a systematic project, which needs to start from materials, design, construction, maintenance and other aspects, and use technical means and management measures comprehensively. Through continuous optimization and innovation, the service life of the pipe network can be effectively extended, to ensure its safe, stable and efficient operation, and to provide a strong guarantee for the sustainable development of urban infrastructure.

Advantage

1, for ductile iron and other iron pipes, has a high ability to withstand water pressure, but also to withstand the external load, but also to adapt to various geological conditions. The strength of the pipe is very high, and it also has certain advantages in hardness, corrosion resistance, flexibility and so on. In addition, this iron pipe can also be used in coastal areas and other poor geological conditions.

2. In terms of sealing performance, ductile iron pipe also has certain advantages, which is not easy to leak after installation, which can effectively reduce the leakage rate of pipe network and reduce the related costs of daily maintenance.

3, ductile iron pipe is connected by flexible joints, the construction is relatively simple, convenient, and can improve work efficiency, improve construction conditions, reduce construction costs.

Shortcoming

1, the installation of ductile iron pipe, will be affected by human factors, for example, the installation requirements are relatively high.

2, if the high-pressure pipe network installed ductile iron pipe, its pressure resistance will be relatively low. At the same time, because the tube body is relatively bulky, it must be mechanically installed.

Nodular cast iron and gray cast iron have great differences in solidification characteristics, mainly in the following aspects:

(1) The eutectic solidification range of ductile iron is wide. During eutectic solidification of gray cast iron, the end of flake graphite is always in contact with liquid iron, so the eutectic solidification process is faster. Because the graphite ball is surrounded by austenitic shell in the later stage of growth, its growth needs to be carried out through the diffusion of carbon atoms, so the solidification process is slow, so that it requires the formation of new graphite nuclei on the new graphite heterogeneous core at a greater degree of supercooling to maintain the eutectic solidification. Therefore, there is a wide liquid-solid coexistence area on the cross section during the solidification of ductile iron, and its solidification mode has the characteristics of congee solidification. This makes the feeding of nodular cast iron difficult during solidification.

(2) The graphite core of ductile iron is much. After spheroidization and inoculation treatment, the graphite core of nodular cast iron is much more than that of gray cast iron, so the size of its eutectic clusters is much finer than that of gray cast iron.

(3) ductile iron has a large eutectic expansion force. Due to the fact that the graphite is quickly surrounded by austenitic shell during the eutectic solidification process of ductile iron, the expansion caused by the volume increase during the growth of graphite cannot be transferred to the liquid iron, resulting in a larger eutectic expansion force. When the mold stiffness is not high, the resulting eutectic expansion will cause shrinkage defects.

(4) The volume change of ductile iron in the solidification process can be divided into three stages: the liquid shrinkage of liquid iron after pouring into the cast and cooling to the eutectic temperature, the volume expansion caused by the precipitation of graphite spheres during the eutectic solidification, and the volume shrinkage during the cooling process after the solidification of liquid iron.

1, first clean the nozzle, clean up all the residue on the plug.

2, next, clean the rubber ring cover and the rubber ring cover, wipe off any adhesive dirt, put the rubber ring cover in the socket in the figure 8 groove, and along the rubber ring cover compacts around it.

3. Then coat a layer of lubricant on its rubber ring and rubber ring, insert the push tube into the socket and check again.