What Is a Seamless Pipe?
A seamless pipe is a pipe with no joints in the longitudinal direction of the pipe.
Steel pipes are generally manufactured by rounding steel plates into a cylindrical shape and welding them together. However, the presence of joints in pipes can lead to serious accidents in terms of strength and reliability, such as crude oil leaking from pipes depending on the intended use.
Seamless pipes are manufactured using the Mannesmann method or other methods that do not produce joints that could cause a reduction in strength. Since there is no risk of defects occurring at joints, seamless pipes are used when high strength and reliability are required.
Uses of Seamless Pipes
Seamless pipes are widely used to transport fluids such as gas, oil, and water. There are also many types of seamless steel pipes, as there are also pipes for construction applications.
- Seamless Pipes for Construction
- Seamless Steel Pipes for Fluid Conveying
- Seamless steel pipes for high-pressure boilers
- Cold-drawn precision seamless steel pipe
Since the materials and standards of seamless pipes vary depending on the application, it is necessary to select the seamless pipe that best suits each purpose. Examples of specific applications include oil and natural gas facilities, boiler tubes for thermal power plants, and high-pressure piping for industrial machinery.
Principle of Seamless Pipes
The Mannesmann process is the typical manufacturing method for producing seamless pipe. It is the most productive way to produce seamless pipe. Simply put, the Mannesmann method forms pipes from round bars, not steel plates. Since the pipe is formed from a round bar, no joints are created.
In the mannequin method, the material for the round bar-shaped pipe, called billet, is first heated to a high temperature (about 1,300°C) until it becomes bright red. When the billet is ready to be rolled, a tool called a plug (for seamless pipe bore forming) is pressed against the center of the billet to form it into a pipe.
If the plug is just pressed against the billet as it is, the billet will be pushed out to the outside and will not be formed into a pipe shape. Therefore, the billet is formed by holding the outer circumference with rolls such as cone and barrel molds. The material pushed out by the plug is pushed forward, so the outside shape can be formed while the inside diameter is formed.
In general, severe rolling of hot billets inevitably results in deterioration of surface properties. Similarly, thick-walled products, which are subjected to a lighter rolling process, are relatively easier to produce. However, there are now companies that specialize in thin-wall processing despite seamless piping. They range from ultra-thin small-diameter seamless pipes with a thickness of 0.08 mm to large-diameter seamless pipes with a diameter of 426.0 mm, the largest diameter ever manufactured in Japan.
Other Information on Seamless Pipes
1. The Difference Between Seamless Pipe and Welded Pipe
The difference between seamless pipe and welded pipe is the use of welding in the pipe manufacturing process. The reason why seamless pipe is needed in the first place is that “grooved corrosion” occurs at welded joints.
This groove corrosion is a V-shaped corrosion that occurs on the weld (inside) of the pipe. Because welding is generally accompanied by high temperatures, changes in the metallurgical structure of the joint are inevitable. The difference in microstructure between the weld and the base metal causes a potential difference, which in turn causes corrosion. Once corrosion occurs, the formation of grooves accelerates corrosion, which eventually reaches the surface of the pipe or leads to fluid leakage due to lack of strength. This is the mechanism of grooved corrosion.
The primary reason for selecting seamless pipes is to prevent groove corrosion, but there is also a type of welded pipe called groove corrosion-resistant steel pipe. Groove corrosion-resistant steel pipes are made by adjusting the molecular composition of the base material (reducing sulfur content) and adding special elements to the weld zone. They are more expensive than ordinary pipes, but not as expensive as seamless pipes, and are widely used for liquids that pose no risk of leakage, such as water.
2. Price Difference Between Seamless Pipe and Welded Pipe
This section describes the price difference between seamless pipe and welded pipe (ERW pipe is used here as an example). Here, as an example, we consider SUS304 as the material. The price of a seamless pipe is about 1.5~2 times higher than that of an ERW pipe. The price difference is small when the pipe diameter is small, but the larger the diameter, the larger the price difference.
When considering a whole plant, the price of piping alone is 1.5~2 times higher than that of a single pipe, so the price difference is not that great when replacing a part of the piping, but when constructing a new plant, the overall cost changes significantly. Therefore, it is important to select appropriate piping according to the fluid to be handled to reduce costs.