Spiral welded pipe main uses: widely used in housing, bridges, roads and other civil engineering construction. Main origin: Spiral pipe manufacturers in China are mainly located in north and northeast China, North China such as Shougang, Tangshan Iron and Steel, Xuanhua Steel and Chengde Steel Co., Ltd., and Northeast China like Xilin, Beitai and Fugang. These two regions account for more than 50% of the total output of rebar. Spiral welded pipe is widely used in natural gas, petroleum, chemical industry, electric power, heat, water supply and drainage, steam heating, penstock for hydropower stations, thermal power generation, water sources and other long-distance transmission pipelines and piling, dredging, bridges, steel structures and other engineering fields. The horizontal ribs of spiral welded pipe are thin and low in quality. The reason is that in order to achieve a large negative tolerance, the pressure of the finished products is large, the iron type is small, and the hole is full.
The difference between spiral welded pipe and longitudinal welded pipe
- Metallurgical properties of materials
The LSAW pipe is made of steel plate, while the spiral welded pipe is manufactured by hot-rolled coil. For example, a water cooling system is installed on the output table to accelerate cooling, which allows the use of low alloy compositions to achieve special strength grades and low temperature toughness. This improves the weldability of the steel, but this system is not available in the steel plate production plant. The alloy content (carbon equivalent) of the coiled sheet tends to be lower than that of similar grade steel, which also improves the weldability of the spiral welded pipe.
What is more, because the rolling direction of the spiral welded pipe is not perpendicular to the pipe axis (the clamping solution depends on the spiral angle of the pipe), The rolling direction of the steel plate of the straight seam pipe is perpendicular to the steel pipe axis, so the cracking resistance of the spiral welded pipe material is better than the straight slit pipe.
- Welding process
In terms of welding technology, the welding method of spiral welded pipe is the same as that of straight welded pipe, but it is inevitable that there will be a lot of T-weld. Therefore, the probability of existence of welding defects is also greatly increased, and the residual stress in the T-weld is large, the weld metal is often in a three-dimensional stress state, increasing the possibility of cracks.
Moreover, according to the technical regulation of submerged arc welding, each weld should have arc starting and extinguishing, but each straight seam pipe in welding ring seam, can not reach this condition, so there may be more welding defects in the extinguishing.
- Strength characteristics
The spiral angle of spiral welded pipe is generally 50-75 degrees, so the combined stress of spiral weld is 60-85% of the main stress of straight welded pipe. At the same working pressure, the wall thickness of spiral welded pipe with the same diameter is smaller than that of longitudinal welded pipe.
According to the above characteristics, we can see that:
3.1 spiral welded pipe blasting, due to the weld by the normal stress and the synthesis stress is relatively small, the blasting hole generally does not originate in the spiral weld, its safety is higher than the straight seam welded pipe.
3.2 When there is a parallel defect near the spiral weld, the spiral welds have less force, so the danger of its expansion is not as big as the straight weld.
3.3 Because radial stress is the maximum stress existing in the steel pipe, so the weld in the direction of vertical stress bear the maximum load. That is to say, the load of straight seam is the biggest, that of circumferential weld is the smallest, and that of spiral weld is between them.
- Static burst strength
The yield pressure of spiral welded pipe and longitudinal welded pipe is in good agreement with the measured value and theoretical value of burst pressure. However, the yield pressure or burst pressure of spiral welded pipe is lower than that of longitudinal welded pipe. The blasting test also showed that the hoop deformation rate of spiral welded pipe was obviously higher than that of longitudinal welded pipe. It is proved that the plastic deformation ability of spiral welded pipe is better than that of longitudinal welded pipe. The burst hole is usually confined within one pitch, which is caused by the strong constraint of spiral weld on the expansion of crack.
- Toughness and fatigue strength
The development trend of pipeline is large caliber and high strength. As the diameter of the steel pipe increases and the grade of steel used increases, the tendency of the ductile fracture tip stable extension becomes greater. Spiral welded pipe and longitudinal welded pipe are of the same grade, but spiral welded pipes have higher impact toughness.
Because of the change of transportation capacity, the pipe is subjected to random alternating load in actual operation. It is important to know the low cycle fatigue strength of steel pipes for judging the service life of pipelines.
According to the results, the fatigue strength of spiral welded pipe is the same as that of seamless pipe and resistance welded pipe. The test data are distributed in the same zone as seamless and resistance pipe, but higher than that of general submerged arc longitudinal welded pipe.
- Field weldability
The weldability in the field is mainly determined by the material of the pipe and the size tolerance of the end fit.
Taking into account the requirements of steel pipe installation construction, the continuity of the steel pipe processing and the consistency of the geometric dimensions is particularly important.
The production of spiral welded pipe is a continuous process which is basically stable under the same working condition. The manufacturing process of longitudinal welded pipe consists of several stages, including the whole plate / indenter / precoiling / spot welding / welding / finishing / group etc. This is the spiral welded pipe production is different from the production of longitudinal welded pipe an important feature.
The stable production condition is very convenient for the control of welding quality and the guarantee of geometrical size. Compared with the longitudinal welded pipe, the spiral pipe has very good ellipticity of the nozzle and verticality of the end face, which ensures the assembly precision of the welding in the field.
- Influence on flow characteristics of conveying medium
The pressure drop in the pipeline is directly proportional to the length of the pipe, the coefficient of fluid viscosity, fluid velocity, and fluid resistance. The resistance coefficient of the fluid depends on both the Reynolds number and the roughness of the inner surface of the pipe. The roughness of the inner surface of the pipe has a ten times greater effect than the area of the local bulge, such as spiral or longitudinal welds, or even inner girth welds.
- Production and Management
The production of spiral welded pipe can embody the advantage of high quality and high efficiency. The output of a spiral welded tube unit is equivalent to 5 – 8 units of longitudinal welded pipe equipment. It is a heavy work to make the production lines of multiple coiled pipe equipment reach the same production standard, that is, to meet the welding quality requirements and pipe manufacturing grade according to the unified production process specification and quality assurance system.
The multi-head production ratio increases the amount of engineering management and quality supervision. There are many difficulties in production management, planning schedule, inspection and acceptance, delivery coordination and other aspects due to the differences of operation skills, quality awareness, distribution points and control procedures of the operators. Easy to cause management and coordination of the confusion and the quality of the manufacturers and construction units buck-passing.
- Price analysis
Because of the material technical performance and production technology of hot-rolled coil is higher, so on the one hand, domestic manufacturers in line with the standard than the plate manufacturers to less, on the other hand, its production process and quality level determine its market price is higher than hot-rolled steel plate. This is the main reason why the price of spiral welded pipe is higher than that of longitudinal welded pipe.