Field welding technology of aluminum alloy distillation tower

I. Overview

Shougang Oxygen Plant purchased 30,000 m from West Linde. The total weight of the complete set of equipment of the oxygen generating unit is about 2200t, more than 800,000 pieces. There are only 11 kinds of aluminum alloy pipelines. The aluminum alloy pipeline in the air separation unit cold box is of grade K, the design temperature is -195 to 150°C, and the pipeline has 98 pipelines from φ25mm to φ100mm, and the total length is 3470. M, only pipe joints up to: more than 3200.

Physical properties

Aluminum and its alloys have high thermal conductivity, large heat capacity, large linear expansion coefficient, and are prone to large welding deformation and internal stress. In addition, when aluminum and its alloys change from a solid state to a liquid state, there is no change in color, so it is not easy to determine whether the groove of the weld is melted, and it is very difficult to grasp and control the temperature in the welding operation. At the same time, the strength of aluminum and its alloys at high temperatures is small, and the formation of weld metal can often be damaged, which can easily cause the weld metal to fall down and burn through.

2. Chemical properties

On the surface of aluminum and aluminum alloy, it is easy to form dense and refractory aluminum oxide oxide film. This layer of oxide film not only hinders the fusion of the base metal, but also causes the inclusion of weld metal and causes the decline of the weld performance. In addition, the oxide film also adsorbs a large amount of moisture and causes pores in the weld seam.

It is not difficult to see that this installation is difficult and the welding quality is high. Therefore, we require all participating welders to carry out pre-job training. After passing the exam, German experts can only perform on-site welding operations. Although various relevant process assessments have been completed in advance, the actual group welding work at the site is more complicated and more difficult than the process evaluation of the samples.

Second, on-site welding technology of rectifying air tower

Oxygen generating unit heart equipment air separation unit cold box, according to its function is divided into the main heat exchanger, rectifying air tower and rare gas three parts of the box. The refined helium air tower is made of aluminum-magnesium alloy and is divided into a pressure tower (lower tower) and a low-pressure tower (upper tower). The annular seam of the tower section requires welding on site. In order to ensure the welding quality, welding analysis and various related welding process assessments were performed before welding, and corresponding welding parameters were formulated.

1. Selection of welding parameters

According to the results of the welding process tests, we have summarized the range of process parameters selected during field welding (see the table below).

In the actual welding, we will use the yttrium tungsten pole into the shape of a ball to basically meet the requirements, the use of argon purity of > 99.99% as the shielding gas when welding, and according to the weldment thickness and the actual welding of the specific circumstances To determine the size of the welding current, the test plate is simulated by the test welding operation, and the arc condition is observed to determine whether the current is appropriate (as shown in Figure 1).

When the welding current is normal, the tungsten tip is in a molten hemispherical shape (see Figure 1a). At this time, the arc is relatively stable and the weld is well formed; the welding current is too small and the tungsten tip is unilaterally arced (see Figure 1b). Easy to flutter; When the welding current is too large, easy to make the tungsten extreme heat (see Figure 1c), the melting part of the tungsten pole is easy to fall into the welding pool to form tungsten and other defects, and the arc is unstable, welding quality is poor. Only after adjusting the process parameters and confirming that there are no surface defects can the formal welding operation be performed.

2. The position and form of the weld in the Distillation Tower

Oxygen production equipment is a large-scale over-qualified equipment that needs to be transported to the site for installation and welding. The lower pressure column of the distillation tower (upper tower) is 18.68m high, the inner diameter is 3.85m, and the wall thickness is 8mm. It forms a transition with the end of the pressure tower (under tower) at the end of the reinforcement of 20mm; the pressure tower (under tower) is 7.9m high. Diameter 3.85m, wall thickness 14mm, the interface is a gradient of the form of the joints, splice joints need to be carried out in the cold box, the weld elevation of 21.84m, the formed weld length of 12.23m, the total weld of 61.15m, for distillation Titanium seams between the two sections We use AC argon arc welding two-sided symmetrical welding technology as required, the position and form of the welding seams are shown in Figure 2.

Third, on-site welding technology measures and operating points

1. Clean up before welding

We use pneumatic milling cutters to strictly remove surface oil and oxide film on both sides of the weld (including grooves and pipes). The wire is polished with stainless steel wire. The processing of welding bevels adopts pneumatic grinding wheel according to the requirements of the specification. Because the wall thickness of the lower tower body is 14mm, and the upper tower body wall thickness is 8mm, the following joints of the tower body must be thinned before the transition to be able to be welded.

2. Construction conditions

The welding of aluminum alloy pipes is divided into two stages: prefabrication outside the box and installation inside the box. Prefabricated non-lining rotary welding, single-side welding double-sided forming, requiring the inside of the welding meat high 1 ~ 2mm, can not have welding tumor, or affect the gas flow of the pipeline. When the fixed mouth welding is performed, the composite liner is to be added, and the weld liner ring is to be inserted into the interior of the pipeline. The fillet weld at the lap joint is not less than 6 points.

The technical requirements for the welding of the fixed position and the supine position are high, and the one-time position is guaranteed. The distance between the nozzle and the root is maintained at 6 to 8 mm. When performing prefabricated opening rotation welding on the outside of the box, the arc starting and ending of each welding line is the key to guarantee the welding quality. If the workpiece temperature is low during arc starting, it is not easy to weld through, and it is easy to produce unfused; if the warm-up time is long, the molten pool is not easy to observe, often due to the temperature is too high and the arc pit shrinkage hole occurs, so before the arc start 5 ~ 10s It is necessary to send gas in advance; during the preheating, the surface of the bath is gently touched with the welding wire, and the temperature change and the melting condition are observed, and the welding wire is moved forward; the short arc method is adopted during the arc closing, and the arc pit is filled with attention, and the arc extinguishing is performed. After the pool is cooled and darkened, air supply is stopped again; this can use argon gas delay protection to prevent defects.

3. Measures taken

Because the welding quality requirements for the joints between the upper and lower towers of the rectifying air separation tower are rather strict, the welding seam must be welded successfully. In the strict formulation of the welding process and careful organization of the site construction, the following measures have been taken:

(1) Adopt two-sided double-sided symmetry welding technology, one for each group of two people, and the inside and outside correspond to synchronous rapid welding, and the middle residence time is as short as possible. The 8 welders were divided into 4 groups, each with 4 inside and outside, and they were required to weld once for continuous work.

(2) Well-selected homogenous welding equipment, consistent performance, sensitive current regulation, fast arcing, and good welding synchronization (1st layer welding current 160-170A, 2nd-5th layer 220-230A).

(3) To ensure that the torch cooling water is unobstructed and the water quality is clean (filters are installed at the entrance gate) to ensure that the gun water channel is not blocked and the gun is prevented from burning.

(4) Ensure the quality of argon gas (purity not less than 99.99%) and flow rate, shorten the gas zone, and adjust the gas flow rate to 18L/min according to the specifications.

(5) At the same time, markings on both sides of the weld seam at the upper and lower towers shall be marked at 100mm, and observations of data changes at any time will be promptly made corresponding adjustments.

(6) Before starting the welding process, use oxyacetylene flame to perform preheating at 100-150°C, and install a reinforcement ring as a temporary support.

(7) When the upper and lower tower forming groups are assembled, the tack welding is performed. The process parameters of tack welding are the same as those of formal welding. Every fixed solder joint of 350-400mm, solder joint length 60-80mm, welding meat high 4-5mm, we start welding from the east, double-sided symmetrical welding, after natural cooling, the verticality of the tower was found to be biased northeast. Therefore, we re-strengthen the ring and divide the welders into two groups, starting from the northeast direction with the front group and the second group starting from the southwest later (see Figure 3). During the welding, it is necessary to thoroughly clean the solder joints at any time, and re-grind the groove with the grinding wheel and adjust the gap again until the welding is completed. After the weld is cooled, the verticality is measured again and the deviation is basically met. This time we use the characteristics of aluminum alloy with high thermal conductivity, large coefficient of linear expansion, and easy deformation to adjust the deviation of verticality.

(8) Welders in the 2nd to 5th welding groups weld symmetrically from different directions, pay attention to maintaining the height and angle of the welding torch, and always keep the air path free from contamination until the welding is completed.

Fourth, post weld quality inspection

According to the requirements of West German manufacturers, after the butt welds formed on the upper and lower towers were completed, after the appearance inspection and x-ray inspection, all the welds met the eligibility criteria, and satisfactory welding quality was obtained, which ensured the oxygen-making unit. Put into use on time.