Application of electromagnetic induction heating in the manufacture of large petrochemical vessels

Preface

  In recent years, domestic petrochemical and coal chemical industries have developed rapidly, and in order to pursue the best economic benefits, production units tend to be large. The high parameters and special mechanical properties of chemical containers bring challenges to the manufacture of products. In order to reduce welding residual stress, prevent weld cold crack, and ensure welding quality, welding process measures must be improved, including preheating, post-heating, dehydrogenation treatment, post-welding heat treatment, etc. In the manufacturing process of petrochemical container products, dehydrogenation treatment, and post-welding local heat treatment are very key processes, whose main purpose is to accelerate hydrogen diffusion, eliminate welding residual stress and improve the performance of welded joints. Due to the large thickness of the chemical container wall, it is particularly important to select the appropriate heating equipment in order to reduce the temperature gradient in the direction of wall thickness and ensure the heating effect. Medium frequency induction heating equipment adopts IGBT module design, because of its small size, light weight, energy saving, and environmental protection, good heating effect, is a more ideal welding heat treatment equipment.

1. Domestic petrochemical vessel welding heating status

  At present, the heating methods used in the welding preheating, post-heating, dehydrogenation treatment, and post-welding local heat treatment of petrochemical container products include flame heating, infrared heating, and induction heating. Induction heating includes power frequency induction heating, intermediate frequency induction heating, and super audio frequency induction heating.

1.1 Flame heating

Flame heating is the use of liquefied gas, combustible gas direct heating welding mouth, mainly used for welding preheating, post-heating, dehydrogenation processing, and other processes, this heating method is widely used because of the simple operation, less one-time investment. But the flame heating is not easy to control, the heating effect is poor, the heating temperature is uneven, the thermal efficiency is low (only about 15%), resulting in high use cost. And because the flame directly contacts the welding, because the combustion is not sufficient will accumulate in the welding joint flame fireworks, affect the welding quality. In addition, flame heating will pollute the surrounding environment, resulting in a very bad working environment for workers.

1.2 Infrared heating

Infrared heating is heating through the resistance in the heating device, emitting a large amount of infrared ray is absorbed by the workpiece, making the workpiece temperature rise. Infrared ray heating is mainly used for welding preheating, post-heating, dehydrogenation treatment, and post-welding local heat treatment. The use of infrared heating welding preheating, dehydrogenation treatment, energy efficiency can reach 60% or so because is to rely on the way of heating heater heating conduction, in the workpiece thickness direction there is a large temperature difference, for the wall thickness of the larger workpiece, the inner wall is difficult to meet the requirements of the process.

1.3 Induction heating

Induction heating is environmental protection, energy saving, and efficient heating method. Its principle is: when an alternating current passes through the coil, an alternating magnetic field will be formed inside and around it. The metal inside the alternating magnetic field will generate eddy currents due to the effect of electromagnetic induction

The temperature is rising.

Induction heating has been widely used in the field of metal smelting and heat treatment for a long time, and the equipment is mostly fixed. However, induction heating is not used for the welding of petrochemical containers for a long time. This is mainly because the welding structure of petrochemical containers is complex, and it is not easy to achieve standardization, and the welding position is not fixed.

The heating equipment needs to be moved frequently. However, with the application of IGBT technology in the inductive heating power supply, it makes the power supply small in size, light in weight, and large in power density, which are the primary conditions for mobile use. In addition, the developed flexible induction heating cable can adapt to the change of welding structure, so as to reduce the use cost.

The induction current has a high heating power density due to skin effect and ring effect. For cylindrical workpiece, the skin effect makes the induced current mainly distributed on the outer surface of the workpiece. The ring effect causes the maximum induced current density to be distributed in the inner side of the heating coil. Therefore, the metal with a certain thickness on the surface of the workpiece is heated, forming a ring of the heat source. The heat generated by the heat source is rapidly transmitted to the entire thickness of the workpiece wall. As the heat source is generated within the wall thickness of the workpiece, the heat is transferred in the way of heat conduction, and the electric heat conversion efficiency is the highest, reaching more than 90%. The same amount of heat is produced compared to other heating methods,

The minimum amount of electrical power consumed.

2.Application of induction heating in petrochemical container manufacturing

2.1 Introduction to equipment

Currently in the petrochemical container are widely used in the production of induction heating equipment for medium frequency induction heating equipment, inverter device is composed of power supply and induction heating cable, power IGBT power module design, and USES the DSP digital control precision, all empty cold reduce the loss of the system structure, and eliminated from the water system of fault, perfecting limit protection makes the equipment safe operation under various conditions to maintain continuously; Non-polar power capacitance improves the service life and safe operation capacity of the equipment. The dc side chopper circuit effectively improves the power factor of the equipment and reduces the output harmonic, so the equipment has the remarkable characteristics of high reliability, high efficiency, and high power factor.

2.2 Equipment Application

Our company used intermediate frequency induction heating equipment in the manufacture of a container in 2010. The power supply is of 80 kW and 160 kW. The 80 kW is a single type, as shown in Figure 1, and the 160 kW is split type, as shown in Figure 2.With 80 kW equipment in the specification of  1 800 mm x 80 mm on the product of preheating before welding heating experiment was carried out, the heating range is 200 mm wide, heating cable in the cylinder body wall to decorate, within 1 h products inside heated to 230 ℃, the maximum temperature difference between inside and outside wall within 30 ℃, fully satisfy the requirement of the preheating process. With 160 kW equipment on the tube body specification for  100 mm x 100 mm girth products on local heat treatment after welding test, weld on each side of the heated width is 200 mm, decorate in the walls around the tube outside the four thermocouples, on the inner wall of the cylinder body arranged on the upper position 1 a thermocouple. The test was carried out according to the following process: the temperature was raised to 400 ℃ at 180 ℃ /h, the temperature was raised to 470 ~ 500 ℃ at 50℃ /h, the temperature was held at 470 ~ 500 ℃ for 1 hour, the temperature was lowered to 400 ℃ at 65 ℃ /h, and the temperature was lowered freely below 400 ℃. The heat treatment process curve is shown in Figure 3. The test process shows that the equipment can set the technological parameters of heat treatment according to the technological requirements. The equipment is easy to operate and safe. The temperature difference in the direction of wall thickness is within 30 ℃, which fully meets the requirements of heat treatment technology. At present, the 160 KW intermediate frequency sensing application technology heating equipment has been used in the pre-welding preheating and post-welding dehydrogenation heating of large containers of our company, which has achieved the expected heating effect.

FIG. 1 80KW intermediate frequency induction heating power supply

FIG. 2 160KW split intermediate frequency induction heating power supply

3. Economic Analysis

From heating effect and the intermediate frequency induction heating technology has obvious advantages of compliance, the following specifications as  457 mm x 84 mm steel pipe girth preheating, for example, economical efficiency analysis. The total cost of welding preheating, post-heating, and dehydrogenation treatment consists of labor cost, energy consumption, equipment consumption, production efficiency, and other costs.

Calculation data: the heating width of each side of the weld is 252 mm, then the heating metal weight W =390 kg, room temperature T 1 =20 ℃, heating temperature T 2 =220℃, heat dissipation coefficient K = 1.2. The specific heat capacity of alloy steel at 220 ℃ is C, P =0.52 kJ/(kg•℃), the infrared thermal efficiency of 60%, the number of heating tablets required is 4; The thermal efficiency of intermediate frequency induction heating is 90% and a heating cable is needed. The thermal efficiency of flame heating is 15%, and the calorific value of natural gas is 35 MJ/m 3.