In recent years, many cities in China have built sewage treatment plants. In the urban sewage treatment project, most of the water pipelines are buried steel pipelines. The corrosion problem is a major problem in the wastewater treatment project. The quality and longevity of the buried water pipelines directly affect the entire project. Therefore, it is very important to do a good job of anti-corrosion of buried pipelines in urban sewage treatment projects.
1, the characteristics of buried pipeline
1.1 Corrosion characteristics
The corrosion of buried pipelines in urban wastewater treatment projects is mainly due to soil erosion, which is characterized by scattered pipeline layout and complex soil conditions, so the corrosion mechanism is complex.
Corrosion of soil includes soil corrosion, microbiological erosion caused by salt, water and oxygen in the soil and steel pipes. Due to the dense underground electrical facilities and cables in the city, the corrosion of stray currents is also severe. Even the pipelines passing through the plant may be chemically corroded by acid or alkaline media.
1.2 Stress characteristics
The underground pipelines of cities often pass through roads, buildings, rivers, etc., and the pressure is complicated. In addition, the sandstone may be impacted and squeezed during the process of backfilling the pipeline.
1.3 difficult to repair
Once the buried pipeline is corroded and damaged, it is not easy to inspect and repair, which will increase the corrosion. It often causes damage to the environment when overhauled. Due to this feature, buried pipelines and their anti-corrosion design life are generally required to be 20 to 30 years or more.
2, anti-corrosion layer requirements
According to the characteristics of urban buried sewage pipelines, the pipeline's anti-corrosion layer should be able to meet the following basic requirements:
1 has good chemical resistance;
2 good insulation properties and compactness to prevent corrosion of electrochemical and stray currents;
3 resistant to bacterial corrosion;
4 has a certain degree of mechanical strength to prevent mechanical damage during handling and burying;
5 The construction is simple, safe and meets the on-site construction requirements.
In addition, there are special requirements in special areas such as cold resistance.
3, commonly used anti-corrosion methods
There are several methods that can be used for anti-corrosion of municipal sewage pipelines:
3.1 Petroleum asphalt - glass fiber cloth coating
This method of preservation was previously used in oil pipelines and is now being phased out. Its advantages are wide source of raw materials and low cost. The disadvantage is that it is not resistant to bacterial corrosion, poor physical properties, and poor construction conditions.
3.2 Chlorosulfonated Polyethylene Coatings - Glass Fiber Cloth Coating
This method has a good anti-corrosion effect, a wide range of construction temperatures, and can be used in conjunction with cathodic protection. However, the surface treatment requires high coating thickness and low mechanical strength. Used in Shanghai Huangpu River diversion project.
3.3 epoxy coal asphalt coating - glass fiber cloth coating
This method has good chemical resistance, electrochemical corrosion resistance, bacterial corrosion resistance, high mechanical strength, and a wide range of applications. Can be used in conjunction with cathodic protection. The disadvantage is that it cannot be applied at low temperatures (below 0°C) and requires high surface treatment. At present, it is widely used in urban sewage treatment projects.
3.4 Polyethylene insulation tape coating coating
This method is easy to construct, easy to repair, and has good corrosion resistance. However, it has low mechanical strength and poor resistance to cathodic disbondment and should not be used in conjunction with cathodic protection. It has poor sealing performance on profiled parts. It is used more in oil and gas pipelines.
3.5 Fuse epoxy coating
Fused epoxy coating has the advantages of good corrosion resistance, high mechanical strength, high construction efficiency, and good coordination with cathodic protection. However, the coating construction must be carried out in the workshop, and other methods (such as liquid epoxy resin) and fusion-bonded epoxy coatings must be used in the field to repair foreign materials.
3.6 Cathodic protection
It is an advanced anti-corrosion application technology with good anti-corrosion effect, long life and predictable, and can monitor the operation and corrosion of pipelines. Must be used with anti-corrosion coating. Industrial water pipelines and long-distance pipelines are used in many applications and they work well.
4. Preservation precautions for buried pipelines
From the comprehensive evaluation of material properties, construction, cost, and use effects, the epoxy coal-tar asphalt coating-glass fiber cloth coating is an ideal method for the preservation of buried pipelines in urban sewage treatment projects. From the perspective of technical performance, the fusion-bonded epoxy coating is an anti-corrosion method worthy of promotion. In the use should also pay attention to the following points:
1 Cathodic protection
The use of cathodic protection can not only more effectively improve the corrosion resistance, but also can monitor the corrosion and operating conditions of pipelines, accurate design and Predicting the service life of pipelines is more suitable for the application of urban sewage treatment projects. When cathodic protection is implemented, the number of sacrificial anodes and the like are increased in the sections near the railway and electrical facilities.
2 In the soil with high acidity, strong alkalinity and high salt content, cross-river sections, etc., reinforced or extra-strong anti-corrosion measures are adopted.
(3) Sleeves should be protected outside the pipeline by passing through the sections of highways and railways.
4 Pipes shall be paved with fine sand in the trenches under the pipeline. When backfilling, fine sand and soft soil shall be filled first to prevent direct burying of stones and bricks.