Anti-corrosion countermeasures for thermal direct buried laying in coastal areas

Anti-corrosion countermeasures for thermal direct buried laying in coastal areas

【 Abstract 】 The directly buried thermal pipeline has the advantages of good heat preservation, good performance and convenient construction. However, in coastal areas, due to the high groundwater level and large salt content, the directly buried compensator and metal parts are corroded and leakage occur. Thermal pipeline leakage is an important problem that affects the quality of heating and the stability of urban life. The thermal direct buried laying in coastal area is discussed in this paper.

【 Key words 】 Coastal area directly buried laying defense laying

First, the advantages of directly buried heat pipe

The directly buried insulation pipe is tightly combined with the steel pipe of the conveying medium, the outer sleeve of high density polyethylene, and the polyurethane hard foam insulation layer filled between the steel pipe and the outer sleeve. Directly buried prefabricated thermal insulation pipes have very prominent advantages over traditional thermal insulation pipes:

-- Good insulation performance, heat loss is only 25% of the traditional pipe, long-term operation can save a lot of energy, significantly reduce energy costs.

- No need to make pipeline trench, can be directly buried underground, simple and rapid construction, low comprehensive cost.

It also has good corrosion resistance and impact resistance at low temperature and can be directly buried in frozen soil.

The service life can reach 30-50 years, the correct installation and use can make the pipe network maintenance cost is very low.

Characteristics of groundwater in coastal areas and corrosion mechanism of pipelines

In coastal areas, the salt content is not only heavy in the surface layer but also high in the core layer. The salt composition is basically the same as that of sea water, and chloride is dominant. According to the actual measurement, the content of K+ ions in underground water of Qinhuangdao is 112.32mg/L. Na+ ion content 702.32mg/L; Ca2+ content 115.43mg/L; Mg2+ ion content 94.24mg/L; CI- ion content 1318.74mg/L; The ion content of SO42- is 230.54mg/L, the ion content of HCO32- is 330.25mg/L, especially the ion content of CI- is as high as 1318.74mg/L, which is a strong corrosive medium. The corrosion of surface pipe is mainly caused by atmospheric corrosion and slight electrochemical corrosion. Because the trench pipe is placed in the wet trench, the ambient temperature is large, some harmful gases are easy to dissolve in the water film on the surface of the pipeline, forming electrolyte, causing local electrochemical corrosion, which is similar to the outer wall of the cave reservoir oil tank. The uninsulated buried pipeline is in direct contact with the soil, which is in the state of blister for a long time and corrodes very seriously.

Overview of directly buried pipeline

Heat pipe directly buried installation, in general, the use of buried pipe for pu heat preservation, outside protective layer with a layer of high density polyethylene, polyethylene is waterproof material, also from the sea water corrosion, not into the water, but each buried polyethylene pipe length is 12 meters, the piping will need to have a joint, joint processing, because is not a shape in the factory, in addition to the steel pipe needs welding, The outer polyethylene protective layer also needs to be welded with plastic at the site, and the joints become the most vulnerable to water resistance, becoming the main part of the leakage. And the polyurethane protective layer will hydrolyze in hot water. Once the outer polyethylene protective layer leaks, the water penetrates into the steel tube and is heated by the hot water inside, and then causes the hydrolysis failure of the polyurethane insulation. At the same time, the outside water causes corrosion in the steel pipe, a little longer, will lead to leakage accidents.

In addition, in order to ensure that the heat pipe is not damaged by thermal expansion and cold contraction, the heat pipe is generally equipped with a stainless steel bellows compensator, in the ordinary water environment, stainless steel is corrosion-resistant. However, corrosion of stainless steel will occur in high salt chloride ion environment. The test results show that under the experimental conditions, the concentration limit of pitting corrosion of 304 stainless steel by chloride ion is about 150mg/L, and that of 316L stainless steel is about 250mg/L.

High chlorine environment will cause the leakage of bellows compensator, but also accelerate the corrosion of internal steel pipe, thermal pipeline leakage accident is an important problem affecting the quality of heating and the temperature of city life, need to focus on preventing. However, with the increase of underground pipeline years and pipeline aging, leakage accidents are on the rise. After the analysis of the leakage accidents, 20% of the accidents are caused by the construction of outside units, heavy vehicles driving on the sidewalk, and 80% of the accidents are caused by the corrosion of pipes and pipe fittings. Preventing corrosion of pipes and fittings has become a problem that needs to be solved in thermal pipelines.

Our methods are divided into two types:

Directly buried installation 1. In general on the ground, there is no pure stainless steel bellows compensator, but in the original stainless steel corrugated pipe to do outside with a layer of sleeve type compensator, two kinds of compensator, the outside of the sleeve has a high density polyethylene protective layer, the socket contact with stainless steel bellows compensator to protect groundwater. Prevent corrosion of groundwater.

2. In crossing the river bottom and other places, in order to ensure 100% non-leakage of the external protection pipe, the external protection pipe is made of steel pipe and welded, and at the same time, cathodic protection is carried out for the external protection pipe. Taking a thermal pipeline protection as an example, this paper introduces the application of sacrificial anode cathodic protection law to thermal pipeline anticorrosion, and successfully solves the corrosion prevention problem of pipeline under high chlorine river.

Qinhuangdao thermal pipeline passes through Datang River, its outer casing is 544.8 meters in length, the pipe diameter is DN1120, the material is Q235B, and the outer anticorrosive material is 800 micron thick molten epoxy powder coating. Casing outer wall area is

S = PI * D * L = 1916 m2

S -- total pipeline area

D -- pipeline diameter

L -- pipeline length

The designed cathodic protection life is 30 years;

During the operation of the cathodic protection system, it has no pollution to the external environment and no interference to other underground metal structures

4. Design of sacrificial anode anti-corrosion system

1 sacrificial anode material selection

At present, there are three kinds of sacrificial anode materials commonly used, namely magnesium anode, zinc anode and aluminum anode. Magnesium anode is mainly used in soil media because of its small specific gravity, negative potential and high driving voltage to steel. The zinc anode has low driving voltage and high current efficiency, which can be used in soil and water medium with low resistivity. The aluminum anode is usually widely used in ships and port facilities in seawater medium, but rarely used in soil medium.

Due to the high groundwater level in the environment where the protected pipeline is located, the river crossing pipeline of Datang River is buried in the soil irrigated by fresh seawater, and the content of CI-ion is as high as 1318.74mg/L, which is a highly corrosive medium. Therefore, this design scheme: the selection specifications of the river crossing protection of Datanghe pipeline are 33kg grade zinc alloy sacrificial anode and 11kg grade magnesium alloy sacrificial anode. In order to ensure the stability of the anode output current, improve the anode current efficiency, reduce the anode grounding resistance, prevent the formation of anode surface passivation layer, the anode must be filled with strictly in proportion to the filling material around the anode, each anode needs to use 50kg filling material, the two into the bag, the size of the anode packing bag is φ300×1000mm.

2. Selection of protection current density and calculation of protection current

Applied cathodic protection, metal components needed to make the metal to achieve complete protection current density for the minimum protection current density, known as the cathodic protection current density when the design, selection of the size of the cathodic protection current density is an important parameter to influence the effect of metal corrosion protective components, it has to do with the minimum protection potential (steel for 0.85 V). If the selected protection current density is low, the protection will be insufficient, and the metal components can not achieve complete protection, resulting in different degrees of corrosion; Otherwise, it will cause unnecessary waste. The current density of cathodic protection is related to many factors, such as the type of the protected metal, the surface state, the type and quality of the surface anticorrosive coating, the nature of the medium, the effective protection life and the influence of external conditions. The current density of cathodic protection can vary from A few μA/m2 to hundreds of mA/m2 depending on these factors. In this scheme, the current density of cathodic protection is selected as I =1mA/m2 according to the previous engineering experience and the actual situation of this project.

According to the protection area given in "Protection Object and Scope" and the selected protection current density, the protection current is calculated as follows:

Protection current required for buried pipeline I= I ×s=1916mA

3. Calculation of sacrificial anode dosage

The amount of zinc anode is calculated according to the following formula:

W = s

Where: W -- dosage of sacrificial anode, kg;

T -- Design protection years, years;

Im -- average maintenance current value, A; Im = 0.85 I

Q -- sacrifice the actual capacitance of the anode, Ah/kg; Take 533 for the zinc anode

1/K -- Sacrifice anode effective utilization coefficient, usually 0.75

Substituting relevant data into the above equation, the quantity of anode can be obtained as follows:

W=1071(kg)

The number of anodes required by the buried pipeline can be calculated as follows:

N = 1071/33 = 34 (a)

Four additional 11 kg magnesium anodes were installed to enable faster potential polarization of line protection.

4 Anode layout

Anodes shall be laid in groups of one, evenly distributed and horizontally laid along both sides of the pipeline.

5 Assembly of anodes

5.1 Anode surface treatment

Before assembling the sacrificial anode, the anode surface should be tested for oil and oxides. Sacrificing the oil and oxide on the anode surface can reduce the anode activity and affect the occurrence of anode current. So if there is oil and oxide on the anode surface, the anode surface should be polished clean with sandpaper.

5.2 Assembly of anode assembly

Each anode is equipped with A VV1 ×10 cable of 3 meters, and the cable is welded with the anode steel core, the welding length is not less than 50mm, and the joint is sealed by a double-layer special sealing process. The other end of the cable is welded with 60×40 flat steel, the welding length is not less than 40mm, and the joint is sealed by a double-layer special sealing process. Each special white cloth bag is filled with 50kg of filling material, and a surface treated assembled sacrificial anode is placed in the center of the packing bag. The anode must be tightly packed with filling material, and it is strictly prohibited to be eccentric.

Five, sacrifice the anode construction installation need to pay attention to the place

1 Anode installation

The output current and protection range of sacrificial anode are closely related to the distance of anode from the tube wall. When the anode side is close to the tube wall, the circuit resistance is small, the anode output current is large, and the current distribution range is narrow. When the anode side is far away from the tube wall, the circuit resistance becomes larger and the output current is smaller. In this design, according to the actual situation of the site, determine the level of buried anode, anode buried position from the outer wall of the pipeline 1-1.5 meters, the minimum is not less than 0.3m, buried depth and pipeline center line depth is the same.

Anodic bed watering

After the anode packing bag is placed in the anode pit, the pit must be watered, the water level in the pit must be completely immersed in the packing bag, and the water in the pit must be kept for a period of time to thoroughly soak the packing bag.

3 Anode welding

The anode and the protected pipeline are connected by electric welding, that is, the steel sheet at one end of the anode cable is directly welded with the leading steel bar of the protected steel pipe. The total weld length is greater than 40mm, and the solder joints are treated with epoxy coal asphalt.

4 Anode bed backfilling

Before the anode bed backfilling, standard bricks must be laid on the anode cable to prevent the cable from being broken in the future construction. When the anode bed backfilling, it is forbidden to backfill sand, cement blocks, plastics and other sundry things in the pit

Installation of equalizing line

To avoid interference corrosion, cables are connected to close parallel pipes to eliminate potential differences between the pipes in the same way as sacrificial anode connections. There are two pressure equalization points for the Utanghe pipeline.

6 Protection effect monitoring system

In order to monitor the protection effect of pipeline, the buried long life reference electrode and test pile are designed. The reference electrode is a buried long-life Cu/CuSO4 reference electrode, the specification model is McT-ii, and the service life is more than 10 years. The test pile is CS-4 type test pile with the size of 120×120×1600mm. Due to uniform distribution of sacrificed anodes, two typical locations were selected as test points in the pipeline. One point was set in the middle of the two groups of anodes, which was the test potential parameter. Another point is set at the anode mounting point for testing the anode parameters, and the two points share one test