Discussion on the New Anti corrosion and Thermal Insulation Structure

1. Structural design

Many pipelines in China have adopted a composite structure consisting of anti-corrosion layer insulation layer outer protective layer. The end face of the anti-corrosion insulation layer requires sealing and waterproofing. Due to the addition of anti-corrosion layer on the surface of the pipe and waterproofing at the pipe end, the reliability of anti-corrosion can be greatly improved. Even if the protective layer and insulation layer are permeable, it will not cause rapid corrosion and perforation of the pipeline.

2. Material selection

(1) Anti corrosion coating. The commonly used anti-corrosion coatings on long-distance pipelines currently include coal tar enamel, polyethylene three-layer structure (3PE), and epoxy powder. The first two belong to thick anti-corrosion coatings, while epoxy powder belongs to thin anti-corrosion coatings. Below is a comparison of these types of anti-corrosion coatings.

Coal tar enamel anti-corrosion coating can be used for pipelines with working temperatures between -30 and 95 ℃. It has good chemical stability, good adhesion, low water absorption, resistance to soil bacteria and marine life erosion, resistance to plant root penetration, and resistance to cathodic stripping. It does not require high surface treatment requirements for steel pipes and is easy to repair on site. It is particularly suitable for environments with multiple bacteria, reed beds, low-lying, humid, and marine conditions. The price of coal tar enamel anti-corrosion coating is relatively low. But its mechanical properties are relatively low, with a handling temperature of -15 ℃, and high requirements for environmental and labor protection during coating operations.

The 3PE structure utilizes the strong bonding force between epoxy powder and the surface of steel pipes to improve adhesion, and utilizes the excellent mechanical properties, chemical stability, insulation, resistance to plant root penetration, and water penetration of extruded polyethylene to enhance its overall performance. At present, China has the ability to produce 3PE anti-corrosion coatings on a large scale. The disadvantage is that the outer layer of polyethylene shrinks and expands with temperature changes, making it easy to detach from the surface of the steel pipe. Due to its high electrical insulation, it provides some shielding for cathodic protection; And the price is relatively high.

Epoxy powder coating is relatively thin, generally between 400-500 μ m, with high strength and strong adhesion to the surface of steel pipes. It has good chemical resistance, temperature resistance, and cathodic stripping resistance. Epoxy powder coating has poor water and light resistance. Compared with other anti-corrosion coatings, it requires stricter surface pretreatment and coating processes for steel pipes. If defects such as pinholes occur, high temperatures will cause the coating to absorb water faster from the defect and cause peeling; It is necessary to use the same material for joint filling, which incurs higher costs; The coating is thin and brittle, with poor impact resistance and easy damage during transportation. It is suitable for coating near the construction site to reduce transportation links. In recent years, some key pipeline projects in China have also used epoxy powder coating, such as the Zhongwu Line and Zhongwu Branch Line, which have higher prices.

Anti corrosion coatings can be selected according to different corrosion environments.

In the wet dry alternation zone, groundwater is the main threat to pipeline corrosion. The anti-corrosion coating is required to have good adhesion, moisture and heat resistance, salt alkali resistance, mold resistance, and low water absorption. The soil contains a large amount of salt and is highly corrosive when dissolved in water. It is recommended to use coal tar porcelain paint to strengthen the anti-corrosion structure in this area.

In the perennial dry period, the water that the pipeline may come into contact with mainly comes from rainfall, and the time is relatively short. The risk of water ingress and corrosion after the outer protective layer is damaged is relatively low. The technical requirements for the anti-corrosion coating mainly include strong adhesion to the pipe body and temperature resistance (pipeline operating temperature). It is recommended to use thin film epoxy coatings, such as epoxy powder coatings, for this type of section.

(2) Insulation layer. Polyurethane stearate foam has a very low thermal conductivity, which is incomparable to most other thermal insulation materials. Thermal conductivity<0 03 W/m · k. Secondly, it also has lightweight and high-strength properties, with a bulk density of 40-70 kg/m3 and a compressive strength>0 2MPa, low water absorption rate<0 03 g/cm3. It can be foamed on site, and can be poured or sprayed. Widely used in fields such as petrochemical pipelines, storage tanks, oil tanks, and aircraft and ships.

(3) External protective layer. According to the leakage accident analysis of foam jacket anti-corrosion and thermal insulation pipeline, the main reasons for pipeline corrosion out of control are as follows: failure of joint coating, cracking and damage of external protective layer; Groundwater enters the insulation layer, creating a corrosive environment outside the pipeline; The high insulation of the polyethylene outer protective layer has a shielding effect on the cathodic protection current, and the cathodic protection system cannot provide the necessary protection. Therefore, in addition to meeting the requirements of transportation conditions, construction environment, and operating conditions in terms of mechanical performance, waterproofing and compatibility with cathodic protection systems play a crucial role in effective corrosion control of pipelines. The outer protective layer mainly includes high-density polyethylene jackets, coal tar enamel coatings, fiberglass and other materials.