JPS586764B2 - Heat treatment method for large tanks - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a heat treatment method for large tanks such as spherical tanks.

Pressure vessels such as LPG tanks and liquid ammonia tanks, chemical storage tanks, and oil storage tanks have welded structures.

Therefore, impurities mixed in the stored gas or liquid often cause localized corrosion or stress corrosion cracking in the vicinity of the weld.

In order to prevent this, post-weld heat treatment, ie, stress relief annealing is performed to remove residual stress due to welding.

Depending on the material and thickness of the structural material, stress relief annealing is required to ensure safety.

For small containers, in-furnace annealing is carried out in an annealing furnace that can heat uniformly, but for large tanks, annealing is carried out outside the furnace. Conventionally, the following three methods have been adopted.

First, an electric heater is installed on the outer or inner surface of one tank along the weld line of the tank, and each facing side is covered with an appropriate heat insulating material to prevent heat dissipation and perform local heat treatment.

In this method, in the case of large tanks, the welding line length is several hundred meters, so the electric heating equipment and power supply capacity are large, and a lot of setup time is required, making the heat treatment cost very high.

Moreover, since it is a local heating method, it is difficult to heat each part evenly.
, distortion occurs due to thermal stress and deformation occurs.

Further, residual stress due to bending of the outer plate other than the welded portion is not removed at all.

The second method is to heat the internal atmosphere by installing a large number of electric heaters in multiple stages inside the tank, and then annealing the entire tank by covering the entire outer surface of the tank with a heat insulating material.

In this method, no matter how well the electric heater is installed, because the atmosphere is heated by natural convection, the temperature rises due to heating and the temperature distribution, such as the part receiving the radiant heat from the electric heater first, then the middle and upper part. becomes non-uniform and deformation occurs due to thermal stress.

In addition, in order to install electric heaters, there are safety issues such as removal after heat treatment after welding the pedestal support to the inner surface, pulling out power lines due to simultaneous use of multiple electric heaters, and generation of leakage current. Moreover, it is not improved in terms of economy compared to the first method.

The third method is to insert a gas burner into the tank and burn it inside, cover the outer surface of the tank with a heat insulating material, and perform annealing on the entire tank.

Although this method significantly reduces fuel costs and labor costs compared to the above method, it also has the following drawbacks.

In other words, the strength of the structural material decreases as it is held at a high temperature around 600°C, which is the annealing temperature, and in spherical tanks etc., pedestals are attached, and these structural discontinuities prevent expansion and contraction caused by heat treatment sufficiently. There are disadvantages such as macroscopic deformation occurring after heat treatment due to inability to be absorbed by the tank and collapse due to the tank's own weight.

In particular, in this deformation, the error in the lengths of the horizontal and vertical diameters and the error in the lengths of the northern and southern radii are a problem.

In order to prevent this macro deformation, it is possible to install a reinforcing stay to prevent deformation inside the tank prior to annealing, but since the stay itself is exposed to a high temperature of 600°C, there may be problems with the strength at that temperature. There is a problem with the way the stay is installed.

The present invention aims to solve the above-mentioned drawbacks of the conventional method by uniformly heating the combustion gas through forced convection using a gas burner and performing heat treatment under pressure.

Hereinafter, in the drawings for explaining the present invention with reference to drawings of embodiments, 1 is a spherical tank, 2 is a pedestal, and the pedestal 2 is movable by providing a roller on its lower surface, and the expansion of the tank 1 due to heating is It is made to be absorbed by the movement of the pillar 2.

Openings 3 and 4 are provided at the top and bottom of the tank 1, and the top opening 3 is provided with a damper 5 for adjusting air volume and its operating handle 6, and a pressure gauge 7 is arranged to measure the pressure inside the tank. has been done.

A burner 8 and its injection port 9 are attached to the lower opening 4, and the gap at the attachment portion of the burner 8 is filled with heat-resistant reinforced cement to form an airtight structure.

Furthermore, the temperature is measured using thermocouples or the like at appropriate locations within the tank, and the burner is adjusted in accordance with this temperature.

10 is a blower, and 11 is a fuel tank.

Further, the entire outer surface of the tank 1 is kept warm by a heat insulating material (not shown).

To perform stress relief annealing of the tank 1, compressed air is blown into the tank through the lower opening 4 and the damper 5 is gradually closed.

Then, the damper is maintained to maintain a predetermined pressure inside the tank, and then the burner 8 is ignited to send hot air containing a mixture of combustion gas and air into the tank.

The combustion gas is ejected spirally from the injection port 9, rises along the inner surface of the tank 1, and is discharged from the upper opening 3.

This increases the atmospheric temperature within the tank, and the tank 1 itself is evenly heated by the combustion gas moving along its inner surface.

Further, the damper 5 is adjusted by measuring the internal pressure with the pressure gauge 7 so that the internal pressure is kept constant while the temperature is rising.

In this way, the N tank 1 is heated to a predetermined temperature (for example, 600° C.) while the internal pressure is kept constant, and after the stress is removed by holding it for a certain period of time, combustion by the burner is gradually reduced to gradually cool it down.

When using the above method, when the strength of the material is reduced due to high temperature heating, the internal pressure exerts a force to hold the tank in a spherical shape, which not only prevents the tank from falling due to its own weight, but also prevents the tank from falling due to its own weight. It is also possible to correct errors in roundness.

Example: JIS G3106 (rolled material for welded structures) was used as the material for the spherical tank, and the inner diameter was 3600 mm and the design pressure was 20.
A kg/cm2 spherical tank was used.

The plate thickness t was set to 18 mm using the following formula.

t=P×D/400σXη-0.4p p: Maximum working pressure (20kg/cm2) σ: Tensile strength of material (42kg/cm2) Efficiency (Loo) This tank was heated to a heating temperature of 600°C using the third conventional method described above, that is, the method of heating the inside of the tank with a gas burner.
When heat treated for a holding time of 2 hours, the following dimensions were obtained.

Do=3,620mm} Difference=40mm D1=3,580mm r1=1,750mm (deformation amount -40mm) r2=1,
830 mm (deformation amount + 40 mm) This tank was then heat treated according to the method of the present invention at a heating temperature of 600° C. and a holding time of 2 hours, resulting in the following dimensions.

Note that in this heat treatment, the internal pressure was set to 700 mmG.

D0=3,610mm} Difference=20mm D1=3,590mm r1=1,780mm (deformation amount -10mm) r2=1,
810 mm (deformation amount + 20 mm) As is clear from this result, the difference between the equatorial diameter D0 and the north-south diameter D1 is 40 mm using the conventional method.
By applying the method of the present invention, the 2 mm
The radius of the north pole r1 and the south pole radius r2
30 mm and 20 mm, respectively, which shows that the method of the present invention not only prevents depression due to its own weight, but also corrects the roundness.

As explained above, in the present invention, heat treatment is performed by pressurizing the inside of the tank and sending combustion gas along the inner surface of the tank to uniformly heat the tank, and the processing cost is very low. Moreover, it is extremely excellent in preventing deformation and the like.

In the embodiment, the damper was provided in the upper opening and the burner was provided in the lower opening, but the arrangement may be reversed.

It goes without saying that the present invention can also be applied to cylindrical tanks in addition to spherical tanks.

[Brief explanation of drawings]

The drawing is a sectional view of a tank in which the method of the invention is carried out. 1... Tank body, 3, 4... Opening, 5... Damper, 8... Burner, 9...
...Injection port.

Claims (1)

[Claims] 1 In a large tank, the entire outer circumferential surface is insulated with heat insulating material, air is supplied from the bottom or top opening of the tank, combustion gas is swirled and blown in by a burner, and air is pumped by a damper in the top or bottom opening of the tank. A heat treatment method for a large tank, characterized by heating the tank to a predetermined temperature by swirling combustion gas along the inner circumferential surface of the tank while maintaining a constant pressurized state inside the tank by adjusting the discharge amount of the tank.