Generally speaking, the diameter of steel pipes can be divided into outer diameter, inner diameter, and nominal diameter. The outer diameter of a steel pipe made of straight seam steel pipe is represented by the letter D, followed by the size of the outer diameter and wall thickness. For example, a seamless steel pipe with an outer diameter of 108 and a wall thickness of 5MM is represented by D108*5. Plastic pipes are also represented by outer diameters, such as De63. Others such as reinforced concrete pipes, cast iron pipes, galvanized steel pipes, etc. are represented by DN. In design drawings, nominal diameter is generally used to represent it. The nominal diameter is a standard artificially set for the convenience of design, manufacturing, and maintenance. It is also the name of the specification of steel pipes (or pipe fittings) compared to the nominal diameter. The nominal diameter of a steel pipe is not equal to its inner diameter or outer diameter. For example, there are several types of straight seam steel pipes with a nominal diameter of 100MM, such as 102*5, 108*5, etc. 108 is the outer diameter of the steel pipe, and 5 represents the wall thickness of the steel pipe. Therefore, the inner diameter of the steel pipe is (108*5-5) = 98MM, but it is not equal to the difference between the outer diameter of the steel pipe and twice the wall thickness. It can also be said that the nominal diameter is a specification name of a steel pipe diameter that is close to the inner diameter but not equal to the inner diameter. The nominal diameter is used in the design drawings to determine the structural dimensions and connection dimensions of steel pipes, pipe fittings, valves, flanges, gaskets, etc. according to the nominal diameter. The nominal diameter is represented by the symbol DN. If the outer diameter is used in the design drawings, a pipeline specification comparison table should also be made to indicate the nominal diameter and wall thickness of a certain pipeline.

First, straight seam steel pipe expansion technology:
1. Initial rounding stage: the fan-shaped blocks are opened until all the fan-shaped blocks are in contact with the inner wall of the steel pipe. At this time, the radius of each point in the inner round tube of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded.
2. Nominal inner diameter stage: the fan-shaped block starts to reduce the movement speed from the front position until it reaches the required position, which is the required position of the inner circumference of the finished pipe.
3. Spring compensation stage: the fan-shaped block starts to further reduce the speed at the position of stage 2 until it reaches the required position, which is the inner circumference position of the steel pipe before springing required by the process design.
4. Pressure holding and stabilization stage: the fan-shaped block remains stationary at the inner circumference position of the steel pipe before springing for some time. This is the pressure holding and stabilization stage required by the equipment and the expansion process.
5. Unloading and regression stage: the fan-shaped block starts to shrink rapidly from the inner circumference position of the steel pipe before springing until it reaches the initial expansion position. This is the minimum shrinkage diameter of the fan-shaped block required by the expansion process.

Second, the common testing methods of straight seam steel pipes:
1. When a certain item of the test results of straight seam steel pipes does not meet the requirements of the product standards, the unqualified ones should be picked out, and double the number of samples should be randomly selected from the same batch of straight seam steel pipes for re-inspection of the unqualified items. If the re-inspection results are unqualified, the batch of straight seam steel pipes shall not be delivered. The following inspection items are not allowed to be re-inspected when the initial inspection fails.
2. For straight seam steel pipes with unqualified re-inspection results, the supplier can submit them for acceptance one by one; or re-heat treat them and submit a new batch for acceptance.
3. If there are no special provisions in the product standards, the chemical composition of the straight seam steel pipe shall be accepted according to the smelting composition.
4. The inspection and acceptance of straight seam steel pipes shall be carried out by the supplier’s technical supervision department.
5. The supplier guarantees that the delivered straight seam steel pipes meet the requirements of the corresponding product standards. The buyer has the right to inspect and accept according to the corresponding product standards.
6. Straight-seam steel pipes should be submitted for acceptance in batches, and the batching rules should comply with the requirements of the corresponding product standards.
7. The inspection items, sampling quantity, sampling locations, and test methods for straight seam steel pipes shall comply with the provisions of the corresponding product standards.