In our engineering career, we use the words pipe and tube interchangeably, and many people think that both are the same. However, there are significant differences between the pipe and tube.
The short answer is: A Pipe is a round tubular to distribute fluids and gases, designated by a nominal pipe size (NPS or DN) that represents a rough indication of the pipe conveyance capacity; a Tube is a round, rectangular, squared, or oval hollow section measured by outside diameter (OD) and wall thickness (WT), expressed in inches or millimeters.
This article explains the various type of Pipes-we hear with their short-form but are not familiar with those terms.
What is the Difference Between Seamless and ERW Stainless Steel Pipes?
Electric Resistance Welding (ERW) pipe is manufactured by rolling metal and then welding it longitudinally across its length. Seamless pipe is manufactured by extruding the metal to the desired length; therefore ERW pipe has a welded joint in its cross-section, while seamless pipe does not have any joint in its cross-section throughout its length.
In Seamless pipe, there are no welding or joints, and is manufactured from solid round billets. The seamless pipe is finished to dimensional and wall thickness specifications in sizes from 1/8 inch to 26 inch OD. Applicable for High-pressure applications such as Hydrocarbon Industries & Refineries, Oil & Gas Exploration & Drilling, Oil & Gas Transportation, and Air and Hydraulic cylinders, Bearings, Boilers, Automobiles, etc.
ERW Pipe VS. Seamless
The question “Shall I use ERW or seamless pipes for my project?” is coming up over and over. Each of the two has different advantages and disadvantages, which should be pondered to take a proper decision:
Advantages & disadvantages of Seamless Pipes
- Seamless pipes are manufactured out of a solid block of steel and do not have any weld seam, which may represent a weak area (subject to corrosion, erosion and, general failure)
- Seamless pipes have more predictable and precise shapes, in terms of roundness and ovality, compared to welded pipes.
- The main disadvantage of seamless pipes is that their cost per ton is higher than the cost of ERW pipes of the same size and grade (Seamless vs ERW pipe compete in the range 2 to 20 inches)
- Delivery times may be longer, as there are fewer manufacturers of seamless pipes than welded pipes (lower entry barriers exists for welded pipes vs. seamless pipes)
- Seamless pipes may have an inconsistent wall thickness across their length, indeed the general tolerance is +/- 12.5%.
Advantages & disadvantages of ERW Pipes
- Welded pipes are cheaper than seamless (ERW HFI type), as they are manufactured using steel coils as feedstock in less complex manufacturing plants
- Welded pipes have shorter lead times than seamless pipes, as the manufacturing base is larger
- Welded pipes have a consistent wall thickness, as they are manufactured using coils (ERW) or plates (LSAW), both subject to tight tolerance control
- The major “defect” attributed to welded pipes is that the presence of a weld seam constitutes a weakness factor. Whereas this may have been true in the past, this is becoming less and less true with the advances of welding technologies in the last ten years.
Conclusion: modern ERW-HFI welded pipes are absolutely a valid alternative to seamless pipes and help end-users reduce prices and lead times between 20 and 25%.
What is ERW Pipe?
ERW steel pipes are manufactured by low-frequency or high-frequency resistance “resistance”. They are round pipes welded from steel plates with longitudinal welds. ERW pipes are manufactured using steel coils: the coil is first uncoiled, then smoothed, cut, and, finally formed into a pipe shape by joining its two extremities electrically.
During ERW pipe welding, heat is generated when current flows through the contact surface of the welding area. It heats the two edges of the steel to the point where one edge can form a bond. At the same time, under the action of the combined pressure, the edges of the tube blank melt and squeeze together.
Usually ERW pipe maximum OD is 24” (609mm), for larger dimensions pipe will be manufactured in SAW.
What kind of pipes (standards) could be made in ERW processes？
There are a lot of pipes could be manufactured by ERW process. Here below we list for the most common standards in pipelines.
Carbon steel pipe in ERW
ASTM A53 Grade A and B (and Galvanized)
ASTM A252 pile pipe
ASTM A500 structural tubing
ASTM A134 and ASTM A135 pipe
EN 10219 S275, S355 pipe
Stainless steel ERW pipe/Pipe standards and specifications
ASTM A269 stainless pipe
ASTM A270 sanitary tubing
ASTM A312 Stainless pipe
ASTM A790 ferritic/austenitic/duplex stainless pipe
API ERW Line pipe
API 5L B to X70 PSL1 (PSL2 shall be in HFW process)
API 5CT J55/K55, N80 casing, and tubing.
What is HFW Pipe?
It should be noted that HFW pipe is a type of ERW pipe.
High-frequency welding (HFW) is a solid resistance heat energy. Through high-frequency current welding resistance, the heat generated in the contact objects, so the objected surface is heated to the plastic state, then with or without forging to achieve a combination of steels.
The high-frequency current pass through the metal conductor will produce two peculiar effects, skin effect and proximity effect. And HFW process is to uses the skin effect to concentrated on the steel object surface, use proximity effects to control the position and the power of the high-frequency electric current flow path. Since the speed is very high, the contacted plate edge could be heated and melted inshore time, then extruded through the docking process. The productivity of HFW steel pipe is high, the welding speed can reach 30m/min, and the welding current frequency is equal to or greater than 70 kHz. It is mainly used to make pipes with welded longitudinal seams or spiral seams. The welded pipe is welded by high-frequency welding technology and is mainly used for the transportation of pressurized fluids such as water, oil, and natural gas.
Wall Thickness: 1.65-20mm
Standard: ASTM 5L, ASTM A53, ASTMA178, ASTM A500/501, ASTM A691, ASTM A252, ASTM A672, EN 10217
Advantage of HFW pipe
In the welding process, HFW steel pipes do not need to add filling meta. So it has fast welding speed and high efficiency in production. HFW pipe is widely used in the fields of oil and gas transportation, oil well pipeline, building structure, and various kinds of mechanical pipe. However, HFW steel pipe quality is affected by many factors, such as raw material and process. And the production quality control becomes difficult. So the yield and welding process still need to be improved continuously.
What is LSAW Pipe (Longitudinal Submerged Arc Welding)
An LSAW pipe (“submerged arc welding”) is manufactured by cutting, bending, and welding steel plates (JCOE process).
LSAW pipes compete with seamless and ERW pipes in the size range between 16 and 24 inches but are “must-go” options for pipelines above 24 inches (as 24 inches is the maximum size for commercial seamless pipes).
The two main types of LSAW pipes are the longitudinal (with a single or double straight seam weld, DSAW) and the spiral type (called, HSAW, SSAW or SAWL pipe). Therefore, the difference between DSAW vs. LSAW is that DSAW pipes have a seam weld in the inside and outside of the pipe, whereas LSAW pipes have a single seam weld on the outer surface. The difference between LSAW and ERW pipes is that LSAW pipes are produced using steel plates, ERW pipes are manufactured starting from steel coils.
What is EFW Pipe?
Electric Fusion Welding (EFW) steel pipe refers to an electron beam welding, the use of the high-speed movement of the electron beam directed impact kinetic energy is converted to heat the workpiece so that the workpiece leaving the melt, the formation of the weld. It is mainly used for welding dissimilar steel welding sheets or which high power density, metal weldment can rapidly heat to high temperatures, which can melt any refractory metals and alloys. Deep penetration welding fast, heat-affected zone is extremely small, so small performance impact on the joints, the joint almost no distortion.