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Electric resistance welded (ERW) pipe is manufactured by cold-forming a sheet of steel into a cylindrical shape. Current is then passed between the two edges of the steel to heat the steel to a point at which the edges are forced together to form a bond without the use of welding filler material. Initially this manufacturing process used low frequency A.C. current to heat the edges. This low frequency process was used from the 1920’s until 1970. In 1970, the low frequency process was superseded by a high frequency ERW process which produced a higher quality weld.
Over time, the welds of low frequency ERW pipe was found to be susceptible to selective seam corrosion, hook cracks, and inadequate bonding of the seams, so low frequency ERW is no longer used to manufacture pipe. The high frequency process is still being used to manufacture pipe for use in new pipeline construction
ERW steel pipes & tubes find widespread usage across industries and fields. In addition to various engineering industries, they are used for water, oil and gas distribution, line pipes, fencing, scaffolding, etc. They are also used for agricultural purposes, drinking water supply, and thermal power, for hand pumps for deep boring wells and also as protection for cables (telecom), among others. Depending on the requirement of the end user industry, ERW steel pipes & tubes are available in various wall thicknesses, diameters, and qualities. The different types include line precision pipes, tubular poles, electric poles, lightweight galvanised pipes for sprinkler irrigation, among others. The industry has sufficient capacity to manufacture the different types of pipes & tubes. High performance ERW steel pipes & tubes possess high strength, toughness and are corrosion resistant. In the manufacturing process of ERW steel pipes & tubes, the edges to be welded are mechanically pressed together and electric resistance or electric induction is used to generate the heat required for welding. With the adoption of better welding technology, ERW pipes & tubes are now widely used in the oil & gas sector. A number of ERW steel pipes & tubes production units are in the SSI sector. Higher demand from the oil & gas industry, infrastructure and automobile industries has led to a healthy increase in production of ERW steel pipes.
TYPES OF ERW PIPE
(1) Low-Frequency-Welded ERW (LF-ERW) Pipe
(2) High-Frequency-Welded ERW (HF-ERW) Pipe
(3) Direct-Current-Welded ERW (DC-ERW) Pipe
(1) Low-Frequency-Welded ERW (LF-ERW) Pipe
ERW pipe was introduced by Republic Steel in 1929 and variations of the original process are still in use today. Cans were formed continuously as described above, and welding was done with low-frequency alternating current (typically 120 cycles per second).
Low-frequency electric resistance weld, LF-ERW is Electric resistance welded (ERW) pipe manufactured by cold-forming a sheet of steel into a cylindrical shape. Current is then passed between the two edges of the steel to heat the steel to a point at which the edges are forced together to form a bond without the use of welding filler material. Initially this manufacturing process used low frequency A.C. current to heat the edges. This low frequency process was used from the 1920s until 1970. In 1970, the low frequency process was superseded by a high frequency ERW process which produced a higher quality weld.
Over time, the welds of low frequency ERW pipe was found to be susceptible to selective seam corrosion, hook cracks, and inadequate bonding of the seams, so low frequency ERW is no longer used to manufacture pipe. The high frequency process is still being used to manufacture pipe for use in new pipeline construction.
(2) High-Frequency-Welded ERW (HF-ERW) Pipe
Between about 1960 and 1970, most manufacturers of low-frequency-welded ERW pipe either converted to high-frequency welding (450 kilocycles per second) or went out of business. The high-frequency welding process was easier to control, the equipment was easier to maintain, and it produced weld zones with better resistance to brittle fracture than the low-frequency process.
(3) Direct-Current-Welded ERW (DC-ERW) Pipe
ERW pipe made with direct current was introduced around 1930 by Youngstown Sheet & Tube Company. Individual cans were cold formed from hot-rolled plates of more than 50 feet in length. Each pipe was thus welded as a separate unit compared to the continuous process.
Physical Properties of Piping Materials
The reasons pipe and tube are made from different materials is due the to physical properties of different materials. Properties such as:
• Malleability
• Ductility
• Brittleness
• Hardness
• Elasticity
• Conductivity
• Chemical resistance / resistance to corrosion
Malleability
Malleability can be defined as the property of a metal to be deformed by compression without cracking or rupturing. This property is very useful for copper tubing systems which allow the tube to be bent to follow the required route quickly without the need for expensive and time consuming fittings:
Ductility
Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. Ductile metals lend themselves to be formed into desired cross sectional shapes easier and therefore are cheaper to manufacture.
Brittleness
The tendency for a metal to crack or break with deformation. Metals displaying this property are not readily used for pipe or tube as this is a disadvantage to a material.
Hardness Is the property of being rigid and resistant to pressure; not easily scratched. It is measured on Mohs scale. It’s presence in metals can be an advantage for high pressure systems but can be a disadvantage as it can increase machining, cutting and fabrication times.
Elasticity
The property by virtue of which a material deformed under the load can regain its original dimensions when unloaded. This property is utilized in piping system designs where pipes may expand or contract due to temperature differences. The elasticity of piping materials can help the designer cater for this.
Conductivity
The ability of a material to conduct electrical current or heat. Some piping systems high conductivity metals for high heat transfer while other piping systems use low conductivity plastic materials to prevent heat transfer.
Chemical resistance/ Resistance to Corrosion
The degree to which a material resists the corrosive action of industrial chemicals. This is probably the most significant property which affects the choice of piping material and is the biggest contributor to the price of material. Specialist alloys of stainless steel such as Hastelloy can be 10 to 20 times more expensive that standard stainless steel and can be slower to fit and weld which increase installation costs.
