1. Spiral submerged arc welded steel pipe (SSAW) and longitudinal submerged arc welded steel pipe (LSAW)
In the 1950s and 1960s, spiral steel pipes were widely used in oil and gas pipelines. In the 1970s and 1980s, they were severely challenged by longitudinal submerged arc welded steel pipes (LSAW steel pipe) and ERW steel pipes. So far, spiral steel pipes are basically no longer used in oil and gas pipelines in the world. , Many major international oil companies and pipeline companies have not allowed the use of spiral submerged arc welded steel pipes for oil and gas pipelines. This is because the development of the pipeline industry has higher and higher requirements for the reliability of steel pipes, and the spiral steel pipe is difficult to meet the requirements of objective development due to the insurmountable defects of its manufacture.
Compared with LSAW steel pipes, spiral pipes have certain deficiencies in quality. Here are seven items for your reference.
(1) The manufacturing process of spiral steel pipes determines that its residual stress is relatively large. According to relevant foreign documents, some are even close to the yield limit. Due to the expansion process of LSAW steel pipes, the residual stress is close to zero.
(2) Spiral weld tracking and ultrasonic on-line detection and tracking are both difficult. Therefore, the probability of weld defects exceeding the standard is higher than that of LSAW pipes.
(3) Spiral steel pipe welded seam staggered amount is mostly 1.1~1.2 mm, according to the international practice, the staggered edge amount should be less than 10% of the thickness. No such problem.
(4) Compared with LSAW pipes, spiral welds have poorer streamlines and serious stress concentration.
(5) The heat-affected zone of spiral submerged arc welded steel pipe is larger than that of LSAW steel pipe, and the heat-affected zone is the weak link of welded pipe quality.
(6) The geometric dimension accuracy of spiral seam welded steel pipes is poor, which brings certain difficulties to on-site construction (such as matching and welding).
(7) With the same diameter, the thickness that the spiral seam welded steel pipe can reach is much smaller than that of the LSAW steel pipe.
2. The process difference between straight seam steel pipe and spiral steel pipe
Straight seam steel pipes and spiral steel pipes
are both a kind of welded steel pipes. They are widely used in national production and construction. Straight seam steel pipes and spiral steel pipes have many differences due to different production processes. The following specific discussion of the lower straight seam steel pipes and spiral steel pipes The difference.
The production process of longitudinally welded pipes is relatively simple. The main production processes are high-frequency welded longitudinally welded pipes and submerged arc welded longitudinally welded pipes. The longitudinally welded pipes have high production efficiency, low cost, and rapid development.
The strength of spiral welded pipe is generally higher than that of straight seam welded pipe. The main production process is submerged arc welding. Spiral steel pipe can produce welded pipes with different pipe diameters from the same width of blanks, and can also use narrower blanks to produce welded pipes with larger pipe diameters. But compared with the straight seam pipe of the same length, the length of the weld is increased 30~100%, and the production speed is low. Therefore, most of the smaller diameter welded pipes use straight seam welding, and the large diameter welded pipes mostly use spiral welding.
T-welding technology is used in the production of larger-diameter straight seam steel pipes in the industry, which means that short sections of straight seam steel pipes are connected to a length that meets the needs of the project. The probability of defects in T-welded straight seam steel pipes is greatly increased, and T-welding The welding residual stress at the seam is relatively large, and the weld metal is often in a three-dimensional stress state, which increases the possibility of cracks.