JP7561474B2 - Method for separating resin-coated inner surface of metal pipe - Google Patents

Description

The present invention relates to a method for separating a resin coating layer formed on the inner surface of a metal pipe from the inner surface.

For example, the applicant has introduced a method for peeling off a resin coating layer formed on the inner surface of a metal pipe when replacing the resin coating layer by peeling off the resin coating layer at one end of the metal pipe to create a gripping area, connecting the gripping area to a gripping/pulling means inserted into the metal pipe and inverting the gripping area, heating the pipe body near the peeling front with an induction heating means so that the adhesive strength is temporarily reduced, and pulling out the gripping/pulling means toward the other end of the metal pipe while sequentially moving the induction heating means in the axial direction of the pipe to sequentially move the heating position, thereby peeling off the resin coating layer while inverting it (see Patent Document 1 below).

However, in the method described in Patent Document 1, the resin coating layer after peeling off is softened by heating and has reduced strength, so it may break during the process due to the tensile force.

The applicant has also introduced another method for peeling off the resin coating layer formed on the inner surface of a metal pipe, which includes a first step of thermally decomposing the constituent resin near the interface between the metal pipe and the resin coating layer by heating the interface between the metal pipe and the resin coating layer at a temperature equal to or higher than the thermal decomposition temperature of the resin that constitutes the resin coating layer while water-cooling the surface of the resin coating layer, and a second step of peeling off and removing the resin coating layer from the inner surface of the metal pipe (see Patent Document 2 below).

However, in the method described in Patent Document 2, the resin coating layer may re-adhere to the inner surface of the metal pipe after the first step is completed, and it is extremely difficult to peel off the re-adhered resin in the second step.

In addition, it is extremely difficult to peel off the resin coating layer formed on the inner surface of a small-diameter metal pipe (e.g., an outer diameter of 27.2 to 114.3 mm) using fingers or a jig.

However, since the resin coating layer formed on the inner surface of metal pipes that have been used in nuclear controlled areas may be contaminated by radioactive materials, it is necessary to reliably recover such contaminants together with the resin that constitutes the surface area of the resin coating layer.

JP 2000-210937 A JP 2018-202797 A

In view of these circumstances, the present applicant has proposed a method for separating a resin coating layer formed on the inner surface of a metal tube from the inner surface, which comprises creating an inert gas atmosphere inside the metal tube, positioning a coil constituting a high-frequency induction heating means so as to surround the upper end of the metal tube, inductively heating the upper end of the metal tube, and, after the temperature of the outer surface of the metal tube at the upper end exceeds the thermal decomposition temperature of the resin constituting the resin coating layer, continuing the induction heating so that the temperature of the outer surface of the metal tube opposite the center of the coil in the longitudinal direction is maintained at or above the thermal decomposition temperature. The present invention proposes a method for separating the resin coating layer on the inner surface of a metal pipe, which includes a step of sequentially thermally decomposing the constituent resin near the interface between the metal pipe and the resin coating layer from the upper end side to the lower end side of the metal pipe by moving the coil along the outer surface of the metal pipe to a position surrounding the lower end of the metal pipe, and sequentially thermally melting the constituent resin other than near the interface from the upper end side to the lower end side of the metal pipe, and discharging the molten constituent resin from an opening on the lower end side of the metal pipe (see the specification of Japanese Patent Application No. 2019-133471).

According to this separation method, the constituent resin of the resin coating layer near the interface with the metal pipe is thermally decomposed in sequence from the upper end to the lower end of the metal pipe, while the constituent resin of the resin coating layer other than near the interface is thermally melted in sequence from the upper end to the lower end of the metal pipe, and the molten resin is separated from the inner surface of the metal pipe and flows toward the lower end, and is discharged from the opening at the lower end.

The inert gas used to create an inert gas atmosphere inside the metal tube is supplied from the opening at the upper end of the metal tube, flows through the tube from the upper end to the lower end, and is discharged from the opening at the lower end together with the molten resin.

However, when attempting to separate a resin coating layer formed on the inner surface of a small-diameter metal pipe, such as one having a nominal diameter of 50A or less, using the above-mentioned separation method, the molten resin may get stuck inside the metal pipe, blocking the pipe line and causing a delay in subsequent processes.
Furthermore, even if an inert gas is supplied under such a condition, the inert gas does not flow below the molten resin that is blocking the metal pipe, so there is a risk of ignition due to induction heating. Such ignition phenomena are likely to occur near the opening of the metal pipe through which the molten resin is discharged.

The present invention has been made based on the above circumstances.
An object of the present invention is to provide a novel separation method capable of easily and reliably separating a resin coating layer formed on the inner surface of a metal pipe from the inner surface.
Another object of the present invention is to provide a separation method which prevents the constituent resin of the resin coating layer, other than in the vicinity of the interface, from re-adhering to the inner surface of the metal pipe.
It is still another object of the present invention to provide a method for separating a resin coating layer formed on the inner surface of a metal pipe from the inner surface without using fingers or a peeling tool.
Yet another object of the present invention is to provide a separation method that can immediately eliminate the resin blockage in a pipe, even if molten resin clogs the inside of a metal pipe and blocks the pipe line, without causing a fire below the blocked area.
Yet another object of the present invention is to provide a separation method capable of reliably recovering such contaminants together with the constituent resin of the resin coating layer when the surface of the resin coating layer is contaminated by radioactive substances or the like.

(1) A method for separating a resin coating layer on an inner surface of a metal pipe according to the present invention is a method for separating a resin coating layer formed on an inner surface of a metal pipe having openings at both ends from the inner surface, the method comprising the steps of:
The metal tube is raised or tilted, and an opening on the upper end side of the metal tube is closed with a lid having an inert gas injection port;
A resin recovery container having an inert gas inlet is connected to the lower end side of the metal pipe,
an inert gas is injected into the interior of the metal pipe from the inert gas injection port, and an inert gas is introduced into the interior of the resin recovery container from the inert gas introduction port, thereby maintaining the interior of the metal pipe and the interior of the resin recovery container in an inert gas atmosphere;
a coil constituting a high-frequency induction heating means is positioned so as to surround an upper end portion of the metal tube, and the upper end portion of the metal tube is induction-heated;
After the outer surface temperature of the metal pipe at the upper end portion exceeds the thermal decomposition temperature of the constituent resin of the resin coating layer,
The method includes a step of moving the coil to a position surrounding the lower end of the metal tube while continuing induction heating so that the temperature of the outer surface of the metal tube facing the longitudinal center of the coil is maintained at or above the thermal decomposition temperature, thereby thermally decomposing the constituent resin in the vicinity of the interface between the metal tube and the resin coating layer successively from the upper end to the lower end of the metal tube, and thermally melting the constituent resin other than in the vicinity of the interface successively from the upper end to the lower end of the metal tube, and causing the molten constituent resin to flow down and be discharged from an opening on the lower end of the metal tube.

According to this separation method, the constituent resin of the resin coating layer near the interface with the metal pipe is successively thermally decomposed from the upper end toward the lower end of the metal pipe, while the constituent resin of the resin coating layer other than near the interface is successively thermally melted from the upper end toward the lower end of the metal pipe, and the molten resin is separated (peeled off) from the inner surface of the metal pipe and flows toward the lower end and is discharged from the opening on the lower end side, so that the constituent resin of the resin coating layer other than near the interface does not re-adhere to the inner surface of the metal pipe.
Furthermore, the molten resin can be easily separated from the inner surface of the metal pipe without using fingers or a peeling tool.

In addition, by injecting inert gas into the interior of the metal tube through the inert gas inlet provided in the lid and introducing inert gas into the interior of the resin recovery container through the inert gas introduction port, even if the inside of the metal tube becomes clogged with molten resin and the pipeline becomes blocked, the interior of the resin recovery container and the interior of the metal tube below the blocked area can be maintained in an inert gas atmosphere, so that ignition inside the metal tube below the blocked area and inside the resin recovery container can be reliably prevented.

In addition, the opening at the top end of the metal tube is closed with a lid having an inert gas inlet, and the injected inert gas or pyrolysis gas is not discharged from the opening. Therefore, if the molten resin clogs the inside of the metal tube and blocks the pipeline, the internal pressure of the metal tube above the blocked area rises. This causes the resin blocking the pipeline to form pores (through holes) that allow gas to pass through, or the resin (lumps) are crushed, or the resin flows down the pipeline while maintaining the shape of the lumps and is discharged from the opening at the bottom end, immediately resolving the blockage.

It is also possible to preheat the metal tube in advance. For example, the above method can be carried out after preheating in a heating furnace. In addition, when positioning the coil so as to surround the upper end of the metal tube, the coil positioned so as to surround the lower end of the metal tube can be moved upward while preheating the metal tube by induction heating, and installed in a position surrounding the upper end of the metal tube. In these preheating processes, it is essential that the outer surface temperature of the metal tube does not exceed the thermal decomposition temperature of the constituent resin, and it is preferable that the temperature does not exceed the melting point of the constituent resin.

(2) In the method for separating a resin-coated layer on the inner surface of a metal pipe according to the present invention, it is preferable that the metal pipe and the resin recovery container are airtightly connected to each other.
According to this separation method, it is possible to completely prevent air (oxygen) from entering the interior of the metal pipe and the resin recovery container through the connecting portion between them.

(3) In the method for separating the resin coating layer on the inner surface of a metal pipe of the present invention, it is preferable that the resin recovery container has a gas exhaust port that communicates with a dust collection device.

With this separation method, the pyrolysis gas, black smoke resulting from incomplete combustion, inert gas, etc., which are continuously discharged into the resin recovery container through the opening at the lower end of the metal tube, can be discharged outside the container from the gas exhaust port, and the black smoke resulting from incomplete combustion can be collected by a dust collector.

(4) In the method of separating the resin coating layer on the inner surface of a metal pipe of the present invention, it is preferable that a pressure gauge is attached to the lid and the resin coating layer separation operation is performed while monitoring the internal pressure of the metal pipe with the pressure gauge.

This separation method allows the operator to grasp the internal condition of the metal pipe (the occurrence and removal of blockages in the constituent resin) and take appropriate measures as necessary.

(5) In the method for separating the resin coating layer on the inner surface of a metal pipe of the present invention, it is preferable that the specific gravity of the inert gas introduced into the resin recovery container from the inert gas inlet is greater than 1.0.

This separation method ensures that the inert gas introduced into the resin recovery container from the inert gas inlet remains inside the resin recovery container.

(6) In the method for separating the resin coating layer on the inner surface of a metal pipe of the present invention, it is preferable that the constituent resin of the resin coating layer is polyethylene.

(7) In the method for separating the resin coating layer on the inner surface of a metal pipe of the present invention, it is preferable that 80% by mass or more of the resin coating layer formed on the inner surface of the metal pipe is discharged from the opening on the other end side of the metal pipe and recovered in the resin recovery container.

According to the separation method of the present invention, the resin coating layer formed on the inner surface of the metal pipe can be easily and reliably separated from the inner surface of the metal pipe.
In addition, the resin coating layer can be separated without using fingers or a peeling tool, and the constituent resin of the resin coating layer other than in the vicinity of the interface does not re-adhere to the inner surface of the metal pipe.
Furthermore, even if molten resin becomes lodged inside the metal pipe and blocks the pipeline, it is possible to reliably prevent fire inside the metal pipe below the blockage and inside the resin recovery container.
Furthermore, even if the molten resin becomes clogged inside the metal pipe and blocks the pipe line, the blockage can be immediately resolved by, for example, discharging the resin from the opening on the lower end side.
Furthermore, the resin constituting the vicinity of the surface of the resin coating layer can be discharged from the opening at the lower end of the metal pipe and reliably recovered. As a result, if the surface of the resin coating layer is contaminated with radioactive materials, the contaminants can be recovered together with the discharged resin.

FIG. 1 is a schematic diagram showing one embodiment of the present invention, in which a coil constituting a high-frequency induction heating device is positioned so as to surround the outer surface of the upper end of a metal tube, and the upper end of the metal tube is induction heated. FIG. 2 is a schematic diagram showing one embodiment of the present invention, illustrating a state in which a coil constituting a high-frequency induction heating device has been moved (lowered) to a position surrounding the outer surface of the lower end of a metal tube.

The present invention will be described in detail below.
The separation method of the present invention is a method for separating (peeling off) a resin coating layer formed (lining) on the inner surface of a metal pipe from the inner surface.

<Embodiment>
1 and 2 are schematic, partially cutaway views showing an embodiment of the separation method of the present invention.
In Figs. 1 and 2, P1 is a metal tube, which is arranged in an upright position.
Both ends of the metal pipe P1 are open, and a resin coating layer L1 is formed on the inner surface of the metal pipe P1.

Examples of the metal pipe P1 include various steel pipes and copper pipes.
The outer diameter of the metal pipe P1 is usually set to 27.2 to 114.3 mm, and a suitable example is set to 60.5 mm (nominal diameter 50A).
The thickness of the metal tube P1 is usually 2.9 to 6.0 mm, and a suitable example is 3.9 mm.

Examples of resins constituting the resin coating layer L1 include thermoplastic resins such as polyolefins such as polyethylene and polypropylene, polyvinyl chloride, polyamide, and fluororesin, and the separation method of the present invention is particularly suitable as a method for separating a coating resin layer L1 made of polyethylene.
The thickness of the resin coating layer L1 is usually set to 0.5 to 3 mm, and a preferred example is set to 2 mm.

The separation method of this embodiment is carried out using a blocking lid 10, a high-frequency induction heating device 20, a resin recovery container 30, a first inert gas supply means 41, a second inert gas supply means 42, and a dust collection device 50.

The blocking lid 10 is an upper lid that blocks the opening at the upper end side of the metal tube P1.
An O-ring (not shown) is interposed between the blocking lid 10 and the metal tube P1, thereby ensuring airtightness between the two.
The blocking lid 10 is provided with an inert gas injection port 11 and is also fitted with a pressure gauge 13 for measuring the internal pressure of the metal tube P1.

The high-frequency induction heating device 20 includes a high-frequency power source 21 and a coil 23 that is movable (raised and lowered) and surrounds the outer surface of the metal tube P1.
In Figures 1 and 2, reference numeral 25 denotes a radiation thermometer for measuring the outer surface temperature of metal tube P1 facing the center of coil 23 in the longitudinal direction, and this radiation thermometer 25 moves (rises and falls) together with coil 23.

The resin recovery container 30 is connected to the lower end side of the metal pipe 11 in order to recover the constituent resin of the resin coating layer in a molten state.
An O-ring (not shown) is interposed between the resin recovery container 30 and the metal tube P1, thereby ensuring airtightness between the two.
The resin recovery container 30 is provided with an inert gas inlet 32 and a gas outlet 34 .

The first inert gas supply means 41 is connected to the injection port 11 (at the upper end side of the metal tube P1) of the blocking lid 10 in order to supply an inert gas into the inside of the metal tube P1.
The inert gas supplied by the first inert gas supply means 41 may be any gas that can prevent the resin from igniting during induction heating, and nitrogen gas is preferably used.

The second inert gas supplying means 42 is connected to the inert gas inlet 32 (at the lower end side of the metal pipe P1) in order to supply an inert gas into the resin recovery container 30.
The inert gas supplied by the second inert gas supply means 42 may be any gas capable of preventing the resin from igniting during induction heating, but from the viewpoint of ensuring that the gas is retained inside the resin recovery container 30 (and is not discharged from the gas exhaust port 34), it is preferable that the specific gravity of the gas be greater than 1.0, and argon gas (specific gravity = 1.38) can be suitably used.

The dust collector 50 is connected to the gas exhaust port 34 of the resin recovery container 30 to suck in and collect black smoke and other emissions resulting from incomplete combustion.

In the separation method of this embodiment, first, the opening at the upper end of the metal pipe P1 is closed by airtightly attaching the blocking lid 10 to the metal pipe P1 in an upright state, and then the resin recovery container 30 is airtightly connected to the lower end of the metal pipe P1.

Next, an inert gas is injected into the interior of the metal pipe P1 through the injection port 11 of the blocking lid 10, and an inert gas is introduced into the interior of the resin recovery container 30 through the inlet 32, thereby creating an inert gas atmosphere inside the metal pipe P1 and inside the resin recovery container 30.

Thereafter, as shown in FIG. 1, the coil 23 constituting the high frequency induction heating device 20 is positioned so as to surround the outer surface of the upper end portion of the metal pipe P1, and the upper end portion of the metal pipe P1 is induction heated.
After the outer surface temperature at the upper end of the metal pipe P1 exceeds the thermal decomposition temperature of the resin constituting the resin coating layer L1 by induction heating, the coil 23 is moved (lowered) toward the lower end of the metal pipe P1.

The thermal decomposition temperature of the constituent resin refers to the temperature at which the weight of the resin decreases by 5% when thermal analysis (TG/DTA) is performed in a nitrogen atmosphere, and the thermal decomposition temperature of polyethylene resin is 400 to 450°C.

In the induction heating by the high frequency induction heating device 20, the frequency is preferably 2 to 30 kHz, and a suitable example is 20 kHz.
The input power is preferably 5 to 120 kW, and a suitable example is 10 kW.
The temperature rise rate of the outer surface at the upper end of the metal tube P1 is preferably 1° C./sec or more, and a suitable example is 10° C./sec.

The time from when induction heating of the upper end of the metal tube P1 begins until the coil 23 starts to descend (stationary heating time) is usually 1 to 60 seconds, with a suitable example being 8 seconds.

If the stationary heating time is too short, the amount of heat supplied to the upper end of the metal pipe P1 will be insufficient to sufficiently raise the temperature of its outer surface, and the constituent resin of the resin coating layer L1 near the interface with the metal pipe P1 may not be sufficiently pyrolyzed. In such a case, it will be difficult to separate the resin coating layer L1 at the upper end of the metal pipe P1.

On the other hand, if the stationary heating time is too long, the amount of heat supplied to the upper end of the metal pipe P1 will be excessive, causing the outer surface temperature to become excessively high, which may result in thermal decomposition of the constituent resin of the resin coating layer L1 other than in the vicinity of the interface. In such a case, it will be impossible to fully recover the constituent resin of the resin coating layer L1 formed on the upper end of the metal pipe P1.

After the stationary heating time has elapsed, the coil 23 is moved (lowered) along the outer surface of the metal tube P1 to a position surrounding the outer surface of the lower end of the metal tube P1, as shown in FIG. 2, while continuing induction heating so that the temperature of the outer surface of the metal tube P1 facing the longitudinal center of the coil 23 is maintained above the thermal decomposition temperature of the constituent resin.

The temperature of the outer surface of the metal tube P1, which faces the longitudinal center of the coil 23, can be controlled (maintained above the thermal decomposition temperature of the constituent resin) by appropriately adjusting the input power and the moving speed of the coil 23 (coil feed speed).

The moving speed of the coil 23 (coil feed speed) varies depending on the power input, but is preferably 1 to 30 mm/sec, with a suitable example being 3 mm/sec.

If the coil feed speed is too fast, the amount of heat supplied to the metal tube P1 from the moving coil 23 will be insufficient, making it impossible to maintain the outer surface temperature of the metal tube P1 above the thermal decomposition temperature of the constituent resin, and the constituent resin of the resin coating layer L1 near the interface with the metal tube P1 may not be sufficiently thermally decomposed. In such a case, it will be difficult to separate the resin coating layer L1 from the inner surface of the metal tube P1.

On the other hand, if the coil feed speed is too slow, the amount of heat supplied to the metal tube P1 from the moving coil 23 becomes excessive, causing the temperature of the outer surface of the metal tube P1 to become excessively high, and even the constituent resin of the resin coating layer L1 other than near the interface with the metal tube P1 will be thermally decomposed. In such a case, the efficiency of recovering the resin coating layer L1 as resin will decrease.

The outer surface temperature of the metal tube P1 facing the longitudinal center of the moving coil 23 is set to be equal to or higher than the thermal decomposition temperature of the constituent resin of the resin coating layer L1, and preferably, is set to be equal to or higher than the thermal decomposition temperature of the constituent resin and equal to or lower than the thermal decomposition temperature of the constituent resin + 100°C.
The actual temperature at the outer surface of the metal tube P1 facing the longitudinal center of the coil 23 is measured by a radiation thermometer 25 which moves together with the coil 23.

Here, if the thickness of the metal pipe P1 is about 15.9 mm or less, the interface temperature between the metal pipe P1 and the resin coating layer L1 during induction heating will be approximately equal to the outer surface temperature of the metal pipe P1, so by maintaining the outer surface temperature of such metal pipe P1 at or above the thermal decomposition temperature of the constituent resin of the resin coating layer L1, the constituent resin near the interface with the metal pipe P1 can be thermally decomposed.

On the other hand, the thermal conductivity of the resin (e.g., polyethylene) that constitutes the resin coating layer L1 is significantly smaller than the thermal conductivity of the metal (e.g., steel pipe) that constitutes the metal pipe P1. Therefore, even if the temperature of the outer surface of the metal pipe P1 is raised to a temperature above the thermal decomposition temperature of the resin that constitutes the resin coating layer L1 (e.g., about the thermal decomposition temperature + 100°C), the temperature of the resin other than near the interface with the metal pipe P1 will exceed its melting temperature, but will not exceed its thermal decomposition temperature.

Therefore, by continuing induction heating while lowering the coil 23 so that the outer surface temperature of the metal tube P1 facing the longitudinal center of the coil 23 is maintained above the thermal decomposition temperature of the constituent resin, in particular above the thermal decomposition temperature of the constituent resin and below the thermal decomposition temperature + 100°C, the constituent resin of the resin coating layer L1 near the interface with the metal tube P1 is sequentially thermally decomposed from the upper end toward the lower end of the metal tube P1, and the constituent resin other than near the interface which has not reached the thermal decomposition temperature is sequentially thermally melted from the upper end toward the lower end of the metal tube P1.
Here, the constituent resin of the resin coating layer L1 in the "near interface" with the metal pipe P1 refers to the constituent resin that is present within 0.3t, preferably within 0.1t, in the thickness direction from the interface with the metal pipe P1, where t is the thickness of the resin coating layer L1.

As the constituent resin in the vicinity of the interface with the metal pipe P1 is gradually thermally decomposed, the adhesive strength of the constituent resin in the areas other than the vicinity of the interface to the inner surface of the metal pipe P1 is lost.
In addition, as the constituent resins other than those near the interface are successively thermally melted, the constituent resins flow down the inner surface of the metal pipe P1 due to their own weight, are discharged from the opening at the lower end of the metal pipe P1, and are stored in the resin recovery container 30.
In the present invention, so long as the entire constituent resin other than in the vicinity of the interface has fluidity, a portion of the constituent resin other than in the vicinity of the interface (for example, the constituent resin in the vicinity of the surface) does not need to be thermally melted.

According to the separation method of this embodiment, the constituent resin of the resin coating layer L1 near the interface with the metal pipe P1 is successively thermally decomposed from the upper end side to the lower end side of the metal pipe P1, while the constituent resin of the resin coating layer L1 other than near the interface is successively thermally melted from the upper end side to the lower end side of the metal pipe P1, and the molten resin immediately flows down and is discharged from the opening. Therefore, without using fingers or a peeling tool, the constituent resin of the resin coating layer L1 other than near the interface can be easily and reliably separated from the inner surface of the metal pipe P1, and the constituent resin will not re-adhere.
In addition, since the constituent resin of the resin coating layer L1 other than near the interface can be reliably recovered, if the surface of the resin coating layer L1 is contaminated with radioactive substances, the radioactive substances can be reliably recovered together with the discharged resin.

In addition, when attempting to separate the resin coating layer L1 formed on the inner surface of a small diameter metal pipe P1, such as a metal pipe with a nominal diameter of 50A or less, it is conceivable that the molten resin may clog the inside of the metal pipe P1, blocking the pipeline, and the inert gas supplied by the first inert gas supply means 41 through the injection port 11 may not flow below the resin blocking the pipeline. However, in the separation method of this embodiment, not only is the inert gas injected into the inside of the metal pipe P1 from the injection port 11, but the inert gas is also introduced into the resin recovery container 30 through the inlet 32 by the second inert gas supply means 42, and further introduced into the inside from the opening on the lower end side of the metal pipe P1. Therefore, even if the above-mentioned blockage occurs, the inside of the resin recovery container 30 and the inside of the metal pipe P1 below the blocked part can be maintained in an inert gas atmosphere, and ignition can be reliably prevented inside the metal pipe P1 below the blocked part and inside the resin recovery container 30.

In addition, the opening at the upper end of the metal pipe P1 is blocked by a blocking lid 10 having an injection port 11, and the injected inert gas or pyrolysis gas is not discharged from the opening. Therefore, if the molten resin clogs the inside of the metal pipe P1 and blocks the pipeline, the internal pressure of the metal pipe P1 above the blocked area increases, and the resin (lump) blocking the pipeline forms pores (through holes) that allow gas to pass through, or the resin (lump) is crushed, or the resin flows down the pipeline while maintaining the shape of the lump and is discharged from the opening at the lower end, thereby immediately resolving the blocked state.

In addition, when molten resin clogs the inside of the metal pipe P1 and blocks the pipeline, the pressure measured by the pressure gauge 13 rises, and when the blockage is resolved, the pressure returns to the level before the rise. This allows the operator to grasp the situation inside the metal pipe P1 (occurrence and resolution of the resin clog) and take appropriate measures as necessary.

For a polyethylene-lined steel pipe having an outer diameter of 60.5 mm (nominal diameter 50A), a wall thickness of 3.9 mm and a length of 390 mm, on whose inner surface a resin coating layer (L1) made of polyethylene and having a thickness of 1.7 mm or more was formed, the resin coating layer (L1) was separated from the inner surface of the steel pipe (P1) by the method of the above embodiment under the following conditions. When the inner surface of the steel pipe (P1) after the separation operation was observed, it was found that the resin coating layer (L1) had been completely removed.
The recovery rate of the constituent resin of the resin coating layer was calculated from the weight of the (lining) steel pipe before and after the separation operation and the weight of the recovered resin.

(conditions)
Inert gas: nitrogen gas Frequency and input power for induction heating: 20 kHz, 10 kW
Fixed heating time at the upper end: 8 seconds. Outer surface temperature at the upper end after the fixed heating time: 450° C.
Coil feed speed: 3 mm/sec. Outer surface temperature of the metal tube facing the center of the length of the moving coil (23): 400 to 500° C.

(result)
Weight of the lined steel pipe before separation operation (W1): 1812 g
Weight of steel pipe after separation operation (W2): 1713g
Weight of recovered resin (W3): 90 g
Recovery rate (W3/(W1-W2)) x 100: 91%

The above results show that the constituent resin (9%) near the interface of the polyethylene resin coating layer was thermally decomposed, and the constituent resin (91%) other than near the interface was recovered.

Although one embodiment of the present invention has been described above, the separation method of the present invention is not limited to this, and various modifications are possible.
For example, in the above embodiment, the resin recovery container 30 is airtightly connected to the lower end side of the metal pipe 11, but by adjusting (increasing) the amount of inert gas supplied from the first inert gas supply means 41 and/or the second inert gas supply means 42 and making the internal pressure of the metal pipe P1 and the resin recovery container 30 somewhat higher than atmospheric pressure, it is possible to maintain an inert gas atmosphere inside the metal pipe P1 and the resin recovery container 30 without airtightly connecting them.

In the above embodiment, the outer surface temperature of the metal pipe P1 is measured by the radiation thermometer 25, but the outer surface temperature of the metal pipe may be measured by a thermo tape, a contact thermometer, or the like.
In the above embodiment, the metal pipe P1 is arranged in an upright state, but the metal pipe may be arranged in an inclined state. Here, the inclination angle (angle with respect to the horizontal plane) is preferably 30° or more.

P1 Metal tube L1 Resin coating layer 10 Blocking lid 11 Inert gas inlet 13 Pressure gauge 20 High frequency induction heating device 21 High frequency power source 23 Coil 25 Radiation thermometer 30 Resin recovery container 32 Inert gas inlet 34 Gas outlet 41 First inert gas supply means 42 Second inert gas supply means 50 Dust collector

Claims (7)

A method for separating a resin coating layer formed on an inner surface of a metal pipe having openings at both ends from the inner surface, comprising the steps of:
The metal tube is raised or tilted, and an opening on the upper end side of the metal tube is closed with a lid having an inert gas injection port;
A resin recovery container having an inert gas inlet is connected to the lower end side of the metal pipe,
an inert gas is injected into the interior of the metal pipe from the inert gas injection port, and an inert gas is introduced into the interior of the resin recovery container from the inert gas introduction port, thereby maintaining the interior of the metal pipe and the interior of the resin recovery container in an inert gas atmosphere;
a coil constituting a high-frequency induction heating means is positioned so as to surround an upper end portion of the metal tube, and the upper end portion of the metal tube is induction-heated;
After the outer surface temperature of the metal pipe at the upper end portion exceeds the thermal decomposition temperature of the constituent resin of the resin coating layer,
a step of: moving the coil to a position surrounding the lower end of the metal tube while continuing induction heating so that the temperature of the outer surface of the metal tube facing the longitudinal center of the coil is maintained at or above the thermal decomposition temperature, thereby thermally decomposing the constituent resin in the vicinity of the interface between the metal tube and the resin coating layer successively from the upper end to the lower end of the metal tube, and thermally melting the constituent resin other than in the vicinity of the interface successively from the upper end to the lower end of the metal tube, and causing the molten constituent resin to flow down and be discharged from an opening on the lower end of the metal tube. The method for separating the resin coating layer on the inner surface of a metal pipe according to claim 1, characterized in that the metal pipe and the resin recovery container are airtightly connected. The method for separating the resin coating layer on the inner surface of a metal pipe according to claim 1 or 2, characterized in that the resin recovery container has a gas exhaust port that communicates with a dust collection device. The method for separating the resin coating layer on the inner surface of a metal pipe according to any one of claims 1 to 3, characterized in that a pressure gauge is attached to the lid, and the resin coating layer is separated while the internal pressure of the metal pipe is monitored by the pressure gauge. The method for separating the resin coating layer on the inner surface of a metal pipe according to any one of claims 1 to 4, characterized in that the specific gravity of the inert gas introduced into the resin recovery container from the inert gas inlet is greater than 1.0. A method for separating a resin coating layer on the inner surface of a metal pipe according to any one of claims 1 to 5, characterized in that the resin coating layer is made of polyethylene. The method for separating the resin coating layer on the inner surface of a metal pipe according to any one of claims 1 to 6, characterized in that 80% or more by mass of the resin coating layer formed on the inner surface of the metal pipe is discharged from an opening on the lower end side of the metal pipe and recovered in the resin recovery container.

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