JPH0610517B2 - Anticorrosion coating method for buried pipe joints - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an anticorrosion coating of a joint portion welded on site, particularly a direct steel pipe, when performing buried piping such as a steel pipe having an anticorrosion coating formed in advance at a factory. The present invention relates to a method of coating an anticorrosive film that can withstand the burial method of propelling soil into the soil.

[Conventional technology]

A pipeline is laid to transport gas, water, fuel, etc., or to pass a cable, and the underground digging work at that time generally uses an open-cut method. In the open-cut method, a groove is dug in the laying place of the pipe, the pipe is installed, welded and contacted, and the groove is filled later. However, it may be difficult to dig a groove due to environmental conditions such as ground conditions and traffic volume. Recently, a method of directly propelling a pipe directly into the soil without digging a groove is often practiced. 500m for direct propulsion method
A guide hole is first made in the soil without digging a groove over a certain distance, and then a hole-expanding bit is attached to the tip of the pipe to be laid, and the pipe diameter is increased, and the pipe is directly pushed into the soil. At this time, every time the length of one pipe (9 to 12 m) is advanced, the pipes are welded and connected, and gradually become longer and laid.

Pipes buried in the soil are coated with resin or asphalt to prevent corrosion, but especially in the direct propulsion method, the resistance of earth and sand during propulsion causes resistance to this friction. The resin is thickly coated (hereinafter, this film is referred to as a factory coating film).
However, the welded part has no choice but to be coated on site, and after welding, the joint part is wrapped with a tape or a heat-shrinkable tube.

FIG. 3 is a cross-sectional view of an example in which a heat-shrinkable tube is used to prevent corrosion. 11 is a steel pipe and 12 is a weld. 13 is the factory coating film, which is cut in steps at the end of the tube, so that the heat-shrinkable tube fits well. The end portion is placed on the first step of this staircase, the first layer of heat-shrinkable tube 14 is covered, and the second layer of heat-shrinkable tube 15 is placed thereon.
Cover. The pipe is pushed from right to left in FIG. 3, but the second layer is covered from the middle of the first step of the stairs in the traveling direction to the surface of the factory coating film 13 on the opposite side in the traveling direction. The third layer, the outermost layer, of the heat-shrinkable tube 16 covers from the second step of the stairs in the traveling direction to the front side of the stairs on the opposite side.

In the open-cut method, a tape that is easier to handle and does not require heating is often used rather than a heat-shrinkable tube, but in the direct propulsion method, the tape peels off due to the resistance of the earth and sand during the process, so the adhesive force with the steel pipe However, a large heat shrinkable tube is used.

In the heat-shrinkable tube, the surface of the anticorrosion coating is made as flat as possible by performing the above-mentioned stacking method. In addition to this, in the heat shrinkable tube, adhesive resin or rubber bituminous thread glue is attached to the inside,
When heated at the time of construction, the tube shrinks and the glue melts and adheres to the base, dealing with friction due to earth and sand. (For example, Saiki, "Piping and Equipment" October, 1985, P.3) Conventionally, such a heat-shrinkable tube has been used as an anticorrosion coating for a buried pipe joint.

[Problems to be solved by the invention]

However, when the welded joint is covered with the heat-shrinkable tube, the joint has a weld fill, and the outer peripheral surface is
It is not flat and the tube is not thick enough to suck it up. In addition, there are some areas where the tube is thickened due to the overlapping of tubes, and there is a step between the tube and the factory coating.
In this way, the heat-shrinkable tube film has a limit in the flatness of the surface layer, and in the direct propulsion method, when the pipe is advanced,
Occasionally, there was a phenomenon in which the film was rolled up due to friction with gravel in the soil. When a slight burr is generated, the resistance of this portion increases more and more, and it may develop into a defect of the anticorrosion coating. When a defect occurs, the defect is found by measuring the electric insulation resistance of the coating film at the time of completion inspection after embedding, but it is very difficult to repair the defect. In order to prevent the progress of corrosion of the steel pipe, it is necessary to take new measures such as cathodic protection, and the damage caused by the blistering of the film is extremely large.

The present invention has been made to solve the above problems, and an object thereof is to obtain a coating that reduces friction with sand and gravel and prevents the phenomenon of blistering.

[Means and Actions for Solving Problems]

The method according to the present invention forms a coating by coating a heat-shrinkable tube on the outer periphery of the joint portion in a pipe construction in which a steel pipe having a corrosion-resistant coating formed on the outer periphery of the pipe is sequentially welded and buried under a pipe, Next, a plastic coating having good adhesion is formed on the surface including the boundary portion between the anticorrosive coating and the coating made of a heat-shrinkable tube by spraying plastic powder to prevent blistering of the coating made of a heat-shrinkable tube at the joint portion. The purpose of this is to enable a corrosion-resistant coating method for a buried pipe joint portion which is prevented and has good durability.

After the joint is welded, a heat-shrinkable tube is first covered on the outer circumference, and the tube is shrunk by heating the tube so that the tube is brought into close contact with the joint. This tube may have a single layer, but if two or more layers are stacked, airtightness and watertightness are ensured, and if there is no damage due to gravel, etc., this will fully fulfill the role of anticorrosion. As the heat-shrinkable tube, a cross-linked polyethylene resin is commercially available and has a thickness of 1.5 to 2.5 mm.
A shrinkage of 50% is common, and a high shrinkage of 75% is readily available.

In this invention, in order to prevent blistering due to friction with sand and gravel, a resin is further sprayed on the heat-shrinkable tube. In thermal spraying, it is extremely easy to control the thickness of the thermal sprayed layer, and by spraying thinly on the convex portion of the heat-shrinkable tube and thickly on the concave portion, a thermally sprayed resin layer with a smooth outer surface can be obtained. It is formed. In particular, by gradually reducing the thickness toward the coating edge, it is possible to cover the factory coating film with the sprayed resin layer and coat it evenly, so there is a step between the coating film and the factory coating. Nor.

As the thermal spray resin, polyethylene resin, epoxy resin,
Various other materials such as polybutene resin can be used, and may be selected according to the type of heat-shrinkable tube and factory coating film. For example, at present, polyethylene resins are widely used for these, and in view of stable adhesiveness with them, polyethylene resins and their systems, for example, resins such as ethylene-vinyl acetate copolymer are preferable.

The coating method and the strength of the obtained coating film against blistering will be described below with reference to specific examples.

Example of Invention

Polyethylene resin coated to a thickness of 6 mm in the factory
A 400 A steel pipe was welded and joined, and the joint portion was provided with an anticorrosion coating. The coating method will be described with reference to FIG.

FIG. 1 is a sectional view showing a part of a joint portion.

FIG. 1 (a) shows a place where a corrosion-resistant coating is to be applied after welding. Reference numeral 1 is a steel pipe, and 2 is a welded portion. 3 is a factory coating film (polyethylene resin film), which is cut in steps at the joint. The both ends of the heat-shrinkable tube 4 of the first layer are placed on the first part of the stairs and covered with this joint part, and heated with a torch,
Contracted. Furthermore, both ends of the second layer of heat-shrinkable tube 5 were placed on the second stage, and similarly heated and shrunk. The heat-shrinkable tube used was stretch-crosslinked polyethylene, thickness 2.0 mm,
The shrinkage was 50%. Shrinkage after coating is about 30
%Met.

FIG. 1 (b) shows a situation in which the second layer of heat-shrinkable tube 15 was coated, but at this stage, the outermost layer surface was not necessarily flat. Polyethylene resin 6 was sprayed onto the second layer of heat-shrinkable tube 5 as shown in FIG. 1 (c). In the thermal spraying, the thickness of the coating was easy to control, and it was possible to cover the factory coating coating 3 and spray it so as to be flat. A thermal spray gun for plastics was used for thermal spraying, and 250 g / min of polyethylene powder (pneumatic pressure
Sprayed in an amount of 1.5 kg / cm ² .

In the coating thus obtained, the thickness of the sprayed layer could be controlled freely, so that the irregularities of the heat-shrinkable tube were sufficiently absorbed, and the outer layer surface was extremely flat.
In addition, the outermost layer of the thermal spray coating 6 is the heat-shrinkable tube 5 of the base.
Also, when the film is strongly adhered to the factory coating film 3, the strength against blistering increases. The results of examining this adhesion are shown in Table 1.

The test is a measurement of the force required to separate (peel off) the coating material and the substrate.

According to the conventional method shown in FIG. 3, the adhesion strength of the outermost heat shrinkable tube 16 to the factory coating film 13 is 8.0 to 18.0 kg / cm ^2.
On the other hand, in the case of the thermal spray coating 6 according to the present invention, 25.0
A stable strength of ~ 30.0 kg / cm ^{2 is} almost doubled, which also enhances the strength against blistering and peeling.

Further, the strength of the coating film obtained in the above-mentioned examples against blisters was examined by a soil sliding test. For comparison, the heat-shrinkable tube according to the above-mentioned conventional method was also measured for a coating covering three layers.

In the soil sliding test, the test material is slid in the sand and the change in the test material caused by friction with the sand is examined.
The pressure of the earth and sand at this time can be adjusted. The actual soil pressure in the case of the construction work because 0.1~0.3kg / cm ^2, but Sen'n the 0.5kg / cm ² of stringent than conditions in the test. The test results are shown in Table 2.

The coating by the conventional method caused the outermost layer tube to curl up, but the coating according to the present invention showed no sign of curling up, and it was confirmed that the coating had extremely strong durability.

In this embodiment, as shown in FIG. 1, a plastic coating was formed on the entire surface of the heat-shrinkable tube, but as shown in FIG. 2, only around the boundary between the anticorrosion coating and the heat-shrinkable tube. Similar results can be obtained by forming a plastic coating on the.

[Effects of the Invention] As described above, according to the present invention, as a method of anticorrosion coating of a joint portion, a resin is sprayed onto a heat-shrinkable tube, so that a flat coating on the surface layer surface can be obtained. At the same time
The adhesion between the skin covering film and the base was also increased, and even in the direct propulsion method, the coating did not curl up to cause defects. As a result, the addition of cathodic protection work can be avoided and its effect is great.

[Brief description of drawings]

FIG. 1 is a sectional view of an anticorrosion coating portion of an embodiment for explaining the present invention. FIG. 2 is a sectional view of an anticorrosion coating portion of another embodiment for explaining the present invention. FIG. 3 is a cross-sectional view of an anticorrosion coating portion for explaining the conventional technique. 1 ... Steel pipe, 2 ... Welded part, 3 ... Factory coating film, 4 ...
… Heat-shrinkable tube for the first layer, 5 …… Heat-shrinkable tube for the second layer, 6 …… Sprayed coating, 11 …… Steel pipe, 12 …… Welded part, 13 …… Factory coating, 14… … First layer heat shrinkable tube, 15 …… Second layer heat shrinkable tube, 16 ……
Third layer heat shrinkable tube

Continuation of the front page (56) Reference JP-A 64-40788 (JP, A) JP-A 61-21494 (JP, A) JP-A 57-195993 (JP, A) JP-A 53-77324 (JP , A) JP-A-50-35713 (JP, A)

Claims (1)

[Claims] 1. A method of corrosion protection coating of a joint portion in a direct propulsion construction method in which a steel pipe having a corrosion protection coating formed on the outer periphery of the pipe is sequentially welded and buried in a pipe, wherein a heat shrinkable tube is provided on the outer periphery of the joint portion. An embedded pipe characterized in that it is covered and contracted, and then a plastic anticorrosive coating having a smooth outer surface is formed on the surface including the boundary portion between the anticorrosive coating and the coating comprising a heat-shrinkable tube by powder spraying of plastic. Anticorrosion coating method for joints.